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Impacts of Urbanization and Development on Estuarine Ecosystems and Water Quality

August 2019
Estuaries and Coasts 42:1821-1838

August 2019
42:1821-1838

DOI:10.1007/s12237-019-00597-z

Authors:

Lauren Freeman

United States Naval Research Laboratory

D. Reide Corbett

East Carolina University

Allison Fitzgerald

New Jersey City University

Daniel Alan Lemley

Nelson Mandela University

Show all 6 authorsHide

Urbanization and human-led development have increased more rapidly along shorelines and in coastal watersheds than inland regions over the past century. The result of major land use changes for both urban tracts and agriculture to serve the urban areas, as well as infrastructure development is increased runoff carrying sediments, nutrients, pollutants, pharmaceuticals, and toxins downstream to estuarine systems. The increased runoff levels are only the tip of the iceberg, with human development resulting in increased fecal bacteria from urbanization and excess nutrients from agriculture leading to harmful algal blooms. Estuaries act as a natural filter between land and sea, but have been overloaded by the influx of sediments and pollutants in recent decades. As a result, there have been a variety of impacts to estuarine ecosystems and water quality including increased sediment load, eutrophication, harmful algal blooms, fecal bacteria, as well as shellfish and fisheries declines. In some estuarine systems, the reduction in light penetration to the benthos has led to the loss of seagrasses. In others, seasonal hypoxia is a visible symptom of prolonged eutrophication. There is a need to augment long-term monitoring techniques with new technologies and data processing methods to better understand the current state of estuaries and work towards mitigating human impacts on estuarine ecosystems and water quality.

a Fecal coliform (FC) concentrations found in the Newport River Estuary, NC, according to general rainfall categories of < 0.25 cm, 0.25-2.54 cm, > 2.54 cm, and then in regard to the management action plan of < 3.81 cm and > 3.81 cm. The dotted line indicates the FC standard limit of 14 MPN per 100 ml. Asterisks indicate a significant difference as compared to other categories. Error bars indicate ± 1 standard error. b

…

The Galveston Bay watershed extends from Dallas/Fort Worth to Houston to Galveston Island (see inset). Counties surrounding Galveston Bay in the lower watershed and nearby are: Matagorda, Brazoria, Galveston, Chambers, Liberty, Harris, Fort Bend, Wharton, Colorado,

…

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Content may be subject to copyright.

Content uploaded by Antonietta Quigg

Content may be subject to copyright.

Impacts of Urbanization and Development on Estuarine Ecosystems

and Water Quality

Lauren A. Freeman

&D. Reide Corbett

&Allison M. Fitzgerald

&Daniel A. Lemley

&Antonietta Quigg

Cecily N. Steppe

Received: 11 June 2018 /Revised: 24 April 2019 /A ccepted: 11 June 2019

#This is a U.S. government work and its text is not subject to copyright protection in the UnitedStates; however, its text may be subject to foreign copyright

2019

Abstract

Urbanization and human-led development have increased more rapidly along shorelines and in coastal watersheds than inland

regions overthe past century. The result of major land use changes for bothurban tracts and agriculture to serve the urban areas, as

well as infrastructure development is increased runoff carrying sediments, nutrients, pollutants, pharmaceuticals, and toxins

downstream to estuarine systems. The increased runoff levels are only the tip of the iceberg, with human development resulting

in increased fecal bacteria from urbanization and excess nutrients from agriculture leading to harmful algal blooms. Estuaries act

as a natural filter between land and sea, but have been overloaded by the influx of sediments and pollutants in recentdecades. As a

result, there have been a variety of impacts to estuarine ecosystems and water quality including increased sediment load,

eutrophication, harmful algal blooms, fecal bacteria, as well as shellfish and fisheries declines. In some estuarine systems, the

reduction in light penetration to the benthos has led to the loss of seagrasses. In others, seasonal hypoxia is a visible symptom of

prolonged eutrophication. There is a need to augment long-term monitoring techniques with new technologies and data process-

ing methods to better understand the current state of estuaries and work towards mitigating human impacts on estuarine

ecosystems and water quality.

Keywords Estuarine ecosystems .Wa ter quali ty .Urbanization .Human i mpacts .Oysters .Suspended sediments .Harmful algal

blooms

Introduction

Urbanization is transforming the world’s coastlines on timescales

of years to decades. Human population centers tend to be near

rivers and coastlines. In 2010, 123.3 million people, or 39% of

the US population, lived in counties directly on the shoreline

(Fig. 1). This population is expected to increase by about 10

million people, or 8%, from 2010 to 2020 (https://oceanservice.

noaa.gov/facts/population.html). While less than 20% of land in

the USA is in a coastal watershed county, these counties are

home to more than half of the population (53%). From 1970 to

2010, coastal shoreline counties added 125 people per square

mile, over three times the national average of 39 people per

square mile (http://stateofthecoast.noaa.gov). As human

populations continue to grow and urbanization escalates in

coastal and watershed counties, waterways and aquatic

ecosystems are often adversely affected. Estuaries, the natural

buffer zone between rivers and the ocean, are among the most

Communicated by Kenneth L. Heck

*Lauren A. Freeman

lauren.a.freeman@navy.mil

Naval Undersea Warfare Center Division Newport Code 8511,

Newport, RI, USA

Coastal Studies Institute, East Carolina University, Wanchese, NC,

USA

Biology Department|, New Jersey City University, Jersey City, NJ,

USA

Botany Department and the Institute for Coastal and Marine

Research, Nelson Mandela University, Port Elizabeth 6031, South

Africa

Marine Biology Department, Texas A&M University at Galveston,

Galveston, TX, USA

Department of Oceanography, US Naval Academy, Annapolis, MD,

USA

Estuaries and Coasts

https://doi.org/10.1007/s12237-019-00597-z

impacted waterways and aquatic ecosystems. Estuaries are

particularly vulnerable because they often have a large

watershed, much greater than their own area. Impacts of

urbanization on estuaries include increases in sediment, nutrient,

and fecal microbial loading and subsequent shifts in water quality,

potentially leading to harmful algal blooms; changes in streamflow

and salinity; and the subsequent effects of these environmental

shifts on plankton, marshes, seagrasses,shellfish,andfishwithin

the estuary (Dauer et al. 2000; Mallin et al. 2000;Alberti2005;

Carle et al. 2005; Campos and Cachola 2007; Paerl et al. 2014;

Lemley et al. 2018a).

Water quality further impacts quality of life for humans living

near estuaries as events like harmful algal blooms and high

turbidity may limit recreational use of the waters, and could incur

an economic cost as well. Despite the value of the ecosystem

services provided by estuaries, most urban design planning takes

little account for neighboring estuarine waterways. Rapid urbani-

zation in the past century has put immense stress on these systems

that has resulted in sometimes dramatic shifts in water quality and

fish/shellfish stocks, with some oyster grounds completely extir-

pated due to microbial pathogens and historical levels a distant

memory (Figs. 2and 3). Those shellfish beds that are still surviving

are often closed to fishery collections due to microbial pathogens. It

is thus critical to understand the specific effects of urbanization on

estuarine waterways and ecosystems to mitigate the potential harm

caused by this process. This is more imperative now than ever

Fig. 1 US population density by county in 2010 (a) and population change by county from 1990 to 2000 (b), courtesy US Census Bureau. Note higher

population densities in coastal counties as well as more rapid population growth along coasts and in watersheds in the twenty-first century

Estuaries and Coasts

given many estuaries are now beyond their capacity to assimilate

or process these inputs, although that the same pollutants, excess

nutrient loads, and toxic materials make their way downstream to

the oceans.

Many estuaries have monitoring systems in place in response

to the stress of human impacts and the intrinsic value of these

systems. In 1987, the Environmental Protection Agency (EPA)

National Estuary Program recognized 28 estuaries of national

significance along the US Atlantic, Gulf, and Pacific coasts and

in Puerto Rico (https://www.epa.gov/nep). Several years later, the

National Oceanographic and Atmospheric Administration

(NOAA) National Estuarine Research Reserve System, a net-

work of 29 coastal sites, was designated to protect estuarine

systems (https://coast.noaa.gov/nerrs/; Voulgaris and Meyers

2004). Globally, marine protected areas such as these serve to

protect estuarine systems as well as monitor flora, fauna, and

water quality (http://www.mpatlas.org/; Washburn and Sanger

2011). While monitoring is critical for understanding changes

over time, methods utilized today have changed little over the

past decades. Likely due to their proximity to human population

centers, many estuaries have had the luxury of scientific

monitoring for decades including measurements of nutrients

and suspended sediment loads. These historic data provide

important baseline information for studying changes to

estuaries over time. The benefit is that multi-decadal stages are

being published examining ecosystems (e.g., Thronson and

Fig. 2 Oyster ground extent in US estuaries historically (a)andpresently(b). Note significant decline throughout US coastal and estuarine regions

regardless of latitude or coast (Zu Ermgassen et al. 2012, reprint of open access material)

Fig. 3 Percentage of estuary area containing oyster grounds historically and present by US coast line (Zu Ermgassen et al. 2012, reprint of open access

material)

Estuaries and Coasts

Quigg 2008;Paerletal.2014; Steichen and Quigg 2018)which

reveal the prevailing consequences of long-term land use chang-

es on water quality, including its degradation.

Given the high flux volumes and rapid transformations occur-

ring in estuaries worldwide, there is a pressing need to incorpo-

rate modern technologies and methods alongside traditional

monitoring to improve our understanding of estuaries and human

impacts on water quality and estuarine ecosystems. Here, several

direct impacts of urbanization on estuarine ecosystems and water

quality are discussed. The breadth of topics included is represen-

tative of the wide range of human impacts, many of which are

not immediately apparent to the individuals living in population

centers adjacent to the coast. Effects are potentially only perceiv-

able downstream or after a time delay, making it even more

important to understand adverse effects of human impacts and

urban development on our natural buffer zones so that preventa-

tive action and mitigation strategies can be enacted.

Sediment Dynamics in Tidal Creek Systems

Tidal creeks act as a gateway between uplands and marshes to

estuaries and coastal waters. These smaller water bodies are in-

fluenced by both freshwater flows running off the landscape and

continuous tidally driven changes in salinity (Holland and Sanger

2008). The upland areas of these watersheds are often popular

locations for development due to the adjacent coastal landscape.

These ecosystems are important habitat for commercial and rec-

reation fishes, including seatrout, crevalle jack, flounder, spade-

fish, spot, black drum, blue crab, brown shrimp, and white

shrimp (Hackney et al. 1976; Shenker and Dean 1979;Kneib

1997; Beck et al. 2001). Tidal creeks act as a gateway of sedi-

ments, nutrients, and water to and from adjacent wetlands. The

wetlands serve vital roles for ecosystems, act as niche habitats for

organisms, act as an initial “filter”of water from the landscape,

and provide coastal protection from storms through wave atten-

uation (Daborn et al. 1993; Goodwin et al. 2001;Graneketal.

2009; Townend et al. 2011; Shi et al. 2012;Mölleretal.2014).

The physical and biological dynamics of tidal creek and wetland

ecosystems are intricately connected. Changes that occur at that

landward (e.g., stormwater runoff, shoreline hardening, land

clearing) and seaward (e.g., trawling, sea level change) ends of

tidal systems may alter the resilience of these habitats in the

future. Rapid economic development and associated land use

change in these coastal areas can increase point and nonpoint

source pollution loading into these tidal creeks and nearby estu-

aries, influencing productivity, biodiversity, and ecological func-

tioning of these systems (Vitousek et al. 1997). Previous research

on tidal creek systems has focused primarily on freshwater dis-

charge, water quality, macrobenthic communities, and biodiver-

sity (Voulgaris and Meyers 2004; Corbett et al. 2007;Sanger

et al. 2008; Corbett et al. 2009; DiDonato et al. 2009;

Washburn and Sanger 2011; Gleeson et al. 2013; Webster et al.

2013; Sanger et al. 2015). There has been much less focus on the

response of sediment dynamics associated with land use change,

particularly development, in these critical coastal ecosystems.

Suspended sediment is cited as the most prolific water pollutant

(USEPA 2004,2006) and therefore studying sediment dynamics

within coastal tributaries and estuaries is critical to better manage-

ment. It is commonly found that sediments introduced to coastal

systems are chemically and physically bound with terrestrially

based nutrients and other sediment associated pollutants (e.g.,

heavy and trace-metals, herbicides, pesticides) and ultimate deg-

radation of these water bodies (Schropp et al. 1990; Riggs et al.

1991; Phillips 1997; Paerl et al. 1998;Hylandetal.2000; Buzzelli

et al. 2001;Paerl2006;). In shallow coastal tributaries and estuar-

ies, there is continuous exchange and deposition of particle-

reactive materials between the water column and bed sediment

(Giffin and Corbett 2003). Because of this, there is a need to study

the sediment dynamics of these urbanized estuarine systems if we

are to better understand their function as sediment sources or sinks,

as well as their sedimentological characteristics. However, only

focusing on the sediments does not provide insight into the drivers

that lead to the ultimate accumulation of material. Additional focus

on the biogeochemical transformations of the material along its

route to deposition/accumulation would better link the sediments

to the biological community and productivity. Other studies have

shown the impact of development in tidal creek watersheds to

detrimental changes in the benthic biological community

(Lerberg et al. 2000; Holland et al. 2004). Linking sediment deliv-

ery, quality, and dynamics to land use change and biological

diversity/productivity would be a next logical step in better man-

aging these systems.

Remote Sensing of Suspended Sediment

Concentrations

Estuaries have long been considered a “filter”for

suspended sediments and material bound to those sediments

such as pollutants and organic debris (Schubel and Carter

1984). Classically suspended sediment study was limited

case studies using water samples and cores. Satellite and

aerial imaging of waterways provides a means of rapidly

filling spatiotemporal gaps in traditional sampling of

suspended sediment concentration; however, it does so at

the cost of spatial resolution. Satellite-based instruments

image a particular scene on a daily to weekly timescale,

with a spatial resolution of 15–250 m. Studies combining

field data collection of water samples with remote sensing

imagery evaluation have found a high degree of accuracy

even in coarse spatial resolution (250 m) MODIS imagery

(Miller and McKee 2004). The broad spatial information

offered by remote sensing is critical for examining sediment

loads in estuarine systems as transport may occur on scales

of 1000s of kilometers (e.g., Van Rijn 1993;Miller

Estuaries and Coasts

and McKee 2004). Use of remotely sensed imagery also

allowed assessment of a large spatial scale on prolonged

time scales (20+ years) from historical records (e.g., Zhou

et al. 2017;Wangetal.2018).

Many studies have explored the relationship between re-

mote sensing reflectance at the sea surface from an atmospher-

ically corrected satellite image and suspended sediment con-

centration, as early as the 1980s (Stumpf and Pennock 1989).

There is an exponential relationship between these quantities,

particularly at mid-range sediment concentrations from about

10 to 30 mg/L (Freeman et al. 2017). A wide variety of algo-

rithms have been developed to link these two quantities using

in situ data alongside concurrently collected satellite or aerial

imagery. It is critical to carefully assess the conditions under

which an algorithm was developed prior to applying it to a

new imagery dataset. Many field-based algorithms are linear

and are most applicable to the conditions under which they

were developed, and will not perform well for regions with

much higher or lower concentrations of suspended sediment

(Freeman et al. 2017)(Fig.4). That said, there is immense

information contained in remotely sensed imagery for studies

of suspended sediment loads in estuarine systems, or related

material such as organic matter, chlorophyll, and pollutants

which bind to sediments. Studies combining in situ samples

with remotely sensed imagery provide the highest degree of

accuracy and spatial extent; however, enough ground work

has been laid to date to extrapolate meaningful information

about sediment concentration from satellite imagery for nearly

any estuarine system.

While earlier studies focused on validating relationships

between remote sensing reflectance and suspended sediment

concentration, the abundance and improved resolution of re-

motely sensed imagery has allowed for more detailed studies.

Mixing over distance is now being studied remotely and on a

larger scale thanks to Landsat imagery (Umar et al. 2018).

Interplay of different physical forcing from tides, waves, and

seasonal events such as monsoon are now being tied to sedi-

ment load with a high level of fidelity utilizing a combination

of geostationary and mobile platforms (Zhou et al. 2017).

Furthermore, remote sensing has opened a window to under-

stand variability of different physical influences (such as

freshwater inflow and wave events) on regions of clear and

turbid water in both shallow and deep water (Ruhl et al. 2001).

Established relationships tend to link one band (usually red or

infrared) of remote sensing reflectance to suspended sediment

concentration. Modest performance increase has been obtain-

ed using multi-band algorithms for scenarios where a higher

degree of accuracy may be necessary (DeLuca et al. 2018).

Harmful Algal Blooms—Insight from a South

African Case Studies

The increased global incidence of harmful algal bloom (HAB)

events is closely linked to the urbanization of coastal areas

resulting in intensification of anthropogenic nutrient loading

(Paerl and Scott 2010; Havens and Paerl 2015). The primary

concern arising from such HAB events is founded in the array

Fig. 4 Comparison of suspended particulate matter (SPM) measured in

situ from water samples in a variety of water types with calculated SPM

utilizing at surface reflectance from a handheld radiometer and a suite of

algorithms relating reflectance to sediment load. Note that there is

increased spread between calculated values at higher sediment

concentrations, and that each algorithm has a “niche range”in which it

performs well. (Freeman et al. 2017)

Estuaries and Coasts

of possible consequences. Some of these include oxygen de-

pletion related to bloom decay processes (i.e., respiration),

direct toxic effects on higher trophic levels (i.e., shellfish poi-

soning and bioaccumulation), mechanical interference and

suffocation of faunal communities (e.g., mucilage produc-

tion), and habitat destruction through shading of submerged

aquatic vegetation. Further complicating the issue of HABs

are the variety of mechanisms employed by species—both

microalgae and protozoans—which enable their proliferation.

Therefore, global issues, such as the accelerated distribution,

frequency, magnitude, and variety of HABs, “require the in-

terfacing of phenomena that occur on very different scales of

space, time and ecological organization”(Levin 1992).

The dynamics of HABs, and phytoplankton communities

in general, are controlled by both local processes and physical

transport processes (Qin and Shen 2017). A combination of

bottom-up (e.g., nutrient supply, light availability, tempera-

ture) and top-down (e.g., grazing pressure and interspecies

competition) controls constitute the local processes structuring

these communities. Alternatively, external transport processes

(e.g., tidal exchange, freshwater discharge and wind-driven

circulation) serve as the physical forces driving observed pat-

terns. Therefore, given the plasticity regarding the periodic

fluctuations for each of these processes (hourly, daily, season-

ally, annually, or stochastic), the relative importance of local

and external transport drivers varies with timescales. As such,

gaining an understanding of external and internal processes

governing spatio-temporal patterns of phytoplankton commu-

nities allows for natural progressions to be discerned from

anthropogenically induced change (Hall et al. 2015).

Detailed site-specific investigations of HABs are thus required

to explain the diversity of patterns at multiple temporal scales

and to understand the ecological significance thereof (Cloern

and Jassby 2010).

Given the significant economic and social development

opportunities provided by aquatic ecosystems, the prevention

of continued urbanization and expansion of catchment activ-

ities is unlikely (Satterthwaite et al. 2010), thus necessitating

an adaptive and proactive approach to conservation of these

ecosystems. Potential management options geared at restoring

a degree of natural phytoplankton functionality at an annual or

seasonal scale are primarily catchment based, while those

aimed at disrupting short-term processes can be implemented

at the catchment scale or in situ. The Zandvlei (Cape Town)

and Swartkops (Port Elizabeth) estuaries provide two South

African examples of where increased urbanization has culmi-

nated in eutrophic conditions and the proliferation of HABs.

The temporarily open/closed Zandvlei Estuary is subject to

numerous impacts associated with urbanization, including

catchment hardening (increased runoff), river canalization,

dredging, habitat loss, flow obstruction (weir and causeway

construction), and elevated nutrient inputs. The impacted na-

ture of the system, low salinity environments (< 10), and the

propensity for closed mouth conditions have facilitated peri-

odic HAB proliferations of Prymnesium parvum

(Prymnesiophyceae) since the 1970s (Whitfield et al. 2016).

In 2012, an extensive bloom of P. parvum significantly

discolored surface waters and resulted in oxygen depletion

and mass fish mortalities within the estuary. Subsequently,

an effective adaptive management approach geared at

preventing such incidents, together with continuous monitor-

ing, has been adopted by the local authorities. Current man-

agement interventions include submerged macrophyte har-

vesting (maintain nutrient uptake capacity), mouth manipula-

tion (increased salinity variability), and dredging (improved

connectivity).

Unlike the Zandvlei Estuary, management of the perma-

nently open and eutrophic Swartkops Estuary is more com-

plex given its size and multiplicity of anthropogenic activities

concentrated in the lower reaches of its catchment, including

industrial, wastewater treatment plants (WWTP), and

stormwater inputs (Scharler and Baird 2003; Lemley et al.

2017a). Of these, the WWTP inputs have the most notable

impacts, with the effects being twofold, i.e., consistent daily

inputs of freshwater and inorganic nutrients. Therefore, be-

cause the probability of reducing discharge rates from the

WWTPs is unlikely, due to the high demand placed on these

facilities, efforts should be focused on reducing the high N and

P loading. One way in which this can be achieved is through

the adoption of a dual-nutrient reduction approach to waste-

water treatment (Conley et al. 2009). Interestingly, the high

inorganic P levels in the Swartkops Estuary currently serve to

skew inorganic N:P molar ratios which in turn constrain the

magnitude of phytoplankton blooms through N-limitation. In

saying this, HABs of species belonging to the Peridinium

genus (Dinophyceae) have been documented to occur in the

stratified, occasionally hypoxic, upper reaches of the estuary

(Lemley et al. 2017a). Therefore, exacerbated eutrophication

symptoms (e.g., HABs) could be expected in the estuary by

only reducing P loads.

Thorough assessments of HAB dynamics in South

African estuaries have been largely understudied, with the

Sundays Estuary providing the only example in the literature

where this has been addressed. Linked to the urban demand

for agricultural products (Satterthwaite et al. 2010), acceler-

ated rates of urbanization are central to the expansion of

agricultural activities in the Sundays Estuary catchment. An

early study in the late 1980s (Hilmer and Bate 1991) was the

first to document the occurrence of two HAB species, name-

ly Heterocapsa rotundata (Dinophyceae) (formerly

Katodinium rotundatum)andHeterosigma akashiwo

(Raphidophyceae). As such, a recent review of microalgae

as indicators in South African estuaries (Lemley et al. 2016)

highlighted—among others—the need for (1) “hypothesis-

driven,”fine-scale experimental research, and (2) compre-

hensive autecological studies of HAB species. Geared at

Estuaries and Coasts

addressing these knowledge gaps, an intensive 3-year sam-

pling program was initiated in the Sundays Estuary. Annual

(Lemley et al. 2017a) and seasonal (Lemley et al. 2017b)

studies verified the persistently eutrophic condition of the

estuary and, more specifically, the progressive and recurrent

nature of extensive HABs of H. akashiwo and H. rotundata

(> 550 μg Chl-a/L) occurring in spring/summer and winter,

respectively. The Sundays Estuary provided an ideal ecosys-

tem to delve into the short-term processes (daily and hourly

studies) driving phytoplankton dynamics, due to its highly

regulated (interbasin water transfer scheme and irrigation ca-

nal network) and seasonal (temperature profiles, nutrient in-

puts and phytoplankton dynamics) nature. These investiga-

tions (Lemley et al. 2018a,b) identified seasonal bottom-

water hypoxia trends to be a direct consequence of collaps-

ing H. akashiwo spring/summer HABs. Additionally, inor-

ganic nutrient availability (nitrate and phosphate) was iden-

tified as the key bottom-up control promoting biomass accu-

mulation, while also limiting the magnitude and duration of

HAB events, while a vertically stratified water column con-

comitant with mesohaline surface waters (~ 10) were identi-

fied as conditions promoting the success of observed HAB

species (H. akashiwo,H. rotundata,Mesodinium rubrum,

and Karenia cf. mikimotoi). The significance of internal bi-

otic processes was revealed during the hourly investigation,

with the plasticity of diel vertical migration behavior, reli-

ance of M. rubrum on suitable cryptophyte “prey”resources,

and the allelopathic effects of H. akashiwo on co-occurring

taxa explaining phytoplankton community dynamics beyond

the influence of physicochemical variability. These findings

provided insight regarding the ecology of HAB taxa and

how they have adapted to thrive in anthropogenically manip-

ulated environments. As such, the broad- to fine-scale infor-

mation obtained for the Sundays Estuary (Fig. 5) offered the

opportunity to suggest management options specifically

geared at mitigating against the occurrence of HAB prolifer-

ations in similar ecosystems, including urbanized systems.

Potential management actions during HAB periods include

mechanical harvesting and algal flocculation techniques as a

means of biomass removal (Dai et al. 2015). However, many

HAB species are well-adapted to persisting during unfavor-

able conditions, e.g., alternating benthic-pelagic life histories

and mixotrophic capabilities. Therefore, the most feasible

approach regarding the management of HABs in estuaries

is to implement measures at the catchment scale (e.g., bene-

ficial management practices, recycling of wastewater, and

scheduled dam releases). These will serve to increase the

incidence of episodic disturbances and reduce the extent of

press disturbances, with the ultimate objective of (1) main-

taining a state of flux in the phytoplankton community, (2)

limiting the frequency and magnitude of HAB events, and

(3) disrupting the seasonal nature of bottom-water hypoxia

(Fig. 6).

The persistent occurrence of harmful phytoplankton taxa is

a new feature in South African estuaries, and as such further

research is required to assess the distribution, magnitude, va-

riety, and ecology thereof. However, given the logistical con-

straints associated with many developing regions, including

South Africa, there are numerous research gaps which need to

be addressed to facilitate effective management of HABs,

some of which include:

1. Identification of alternative resource pools (i.e., organic

nutrients and bacterial communities) that facilitate the per-

sistence and dominance of HAB species.

2. Microcosm experiments aimed at isolating the effect of

specific environmental variables on HAB species to make

predictions regarding global change scenarios.

3. Assessment of how primary consumer communities (e.g.,

copepods and larval fishes) respond during HAB periods.

4. Molecular comparisons with global strains of HAB spe-

cies to elucidate the potential dispersal mechanisms facil-

itating their global occurrence.

5. Feasibility studies of remote sensing as a tool with which

to monitor estuarine HABs.

6. Installation and implementation of continuous in situ

HAB monitoring technologies.

Pathogen Accumulation in Shellfish Due

to Urbanization Near Estuarine Watersheds

Human illness associated with bacterial pathogens has risen

alongside population density growth in major cities (i.e., New

York City) since the late nineteenth century. This time period

corresponds with the initial construction of sewage systems in

many urban areas, which delivered human wastes to the nearby

estuaries (Rippey 1994). Since then, over 400 different out-

breaks have been reported, many of these due to bacteria in

wastewater (Rippey 1994). The National Shellfish Sanitation

Program and sanitation surveys created in the 1920s (to track

effluent and point source inputs) helped to reduce the number

of human illnesses (DePaola et al. 2010). Coastal urbanization

can lead to elevated inputs of enteric bacteria (fecal coliforms)

and nutrient rich waters over subtidal/intertidal shellfish beds,

including commercially and ecologically important areas of

clams, scallops, and oysters (Mallin et al. 2000; Campos and

Cachola 2007; Maalouf et al. 2010;Camposetal.2013). Many

of these organisms live in small estuaries and creeks

surrounded by impervious surfaces and agricultural land, and

shellfish beds can be closed due increasing fecal coliform bac-

terial concentrations (Mallin et al. 2000). For example, in the

New Hanover County watersheds, NC, there are five tidal

creeks with 42.9–77.8% developed land, and in the low

Estuaries and Coasts

salinity regions of the creeks, all five had elevated fecal

coliform counts over a sampling season.

EPA standards for shellfish sanitation and shell fishing

(1986) indicate that regular fecal coliform sampling should

occur to adequately monitor conditions in the water column

that could impact the oysters on the bottom (Campos et al.

2013). If routine sampling shows a medial fecal coliform con-

centration above 14MPN/100 mL (or 10% above 43MPN/

100 mL), then water is not safe for shellfish harvesting (EPA

Water Quality Criteria 1986). The concentration of fecal indi-

cator organisms that are in the water column serve as a good

indicator of what is in the shellfish meat; however, multiple

environmental and physiological factors influence the accu-

mulation of fecal coliforms within oyster tissue (reviewed in

Campos et al. 2013). Historically, most outbreaks of bacterial

pathogens occur in the summer months (Rippey 1994). A

nationwide survey of marketed oysters in 2006 examined mi-

crobial indicators in shellfish (along with other viral patho-

gens) and found that mean levels of fecal coliforms and

E. coli during summer were above the FDA limits for con-

sumption in some regions (DePaola et al. 2010); in areas with

closed shellfish beds, illegally harvested oysters could have

much higher pathogen loads and cause illness when consumed

(DePaola et al. 2010). Maalouf et al. (2010) examined histor-

ical data looking at illness outbreaks associated with sewage

discharges over shellfish beds, and found many linkages be-

tween the two. Outbreaks of gastroenteritis due to fecal coli-

formsinsewageeffluentwerelinkedtocontaminatedoysters

in Australia, New York, and France in the past three decades

(Maalouf et al. 2010).

The amount of urbanization in the surrounding watershed

can be estimated by the degree of impervious surfaces sound-

ing the waterways. Impervious surfaces mean that during pe-

riods of high rainfall, any pollutants on the surfaces wash into

the storm drains/creeks. Mallin et al. (2000) found that if

creeks were surrounded by greater than 10% impervious sur-

faces, they were periodically closed to shell fishing activities

due to increased fecal coliform bacteria. Comparison of the

fecal coliform abundance with the percentage of impervious

surface showed a strong correlation (Pearson correlation r=

0.975, p< 0.005). Impervious surfaces mean that during pe-

riods of high rainfall, any pollutants on the surfaces would

wash into the storm drains/creeks. During a thorough sam-

pling season highlighting various rainfall amounts,

Coulliette and Noble (2008) saw a significant relationship

between the quantity of rain and the quantity of fecal coliform

Fig. 5 Sundays Estuary region

discussed in the harmful algal

bloom South Africa case study

Estuaries and Coasts

bacteria (E. coli and Enterococcus) in the water columns of the

Newport River Estuary (NC, USA). Not only did rainfall

(above the recommended threshold for shellfish sanitation

safety) cause E. coli and Enterococcus levels to rise above

the regulated MPN/100 mL levels, there was also a significant

rise in fecal indicator bacteria during dry periods, possibly due

to a reservoir of cells in the sediment that were suspended into

the water column due to human activity in the creeks (Fig. 7).

Input of pathogenic bacteria into an enclosed waterway

(i.e., a tidal creek) can impact not only the water column but

the sediments as well. Fecal coliform bacteria can be deposit-

ed onto the substrate during sedimentation, adding to the sed-

iment microflora. It is highly likely that they would be resus-

pended during tidal cycles, or human interactions (wading in

the creek, swimming, etc.). Following a raw sewage spill,

Mallin et al. (2007) found that sediment bacteria counts

Fig. 6 Summary of the processes facilitating the proliferation and recurrent nature of two commonly occurring harmful algal bloom species in the

Sundays Estuary (note: the factors promoting and limiting the growth of these two species are indicated in green and red text, respectively)

Estuaries and Coasts

remained elevated for weeks, and increased yet again follow-

ing an intense precipitation event (Mallin et al. 2007). Once

resuspended, fecal coliform bacteria that were present in the

sediments (i.e., 200 CFU/cm

) showed corresponding quanti-

ties in the water column (i.e., 200 CFU/100 mL). Since sedi-

ments can accumulate viable bacterial cells rapidly (acting as a

reservoir for cells), this could cause a rapid increase in water

column bacteria concentrations above the recommended

threshold for human contact (Al Aukidy and Verlicchi 2017;

Madoux-Humery et al. 2015; Coulliette and Noble 2008;

Mallin et al. 2007). Stormwater input can cause increased

turbidity as well, due to both the input of particulates in the

runoff as well as the resuspension of existing sediments

(Coulliette and Noble 2008). Several studies in both

subtropical and temperate regions have demonstrated the

survival of fecal indicator bacteria in the sediments;

Coulliette and Noble (2008) linked periods of increased fecal

indicator bacteria even during dry times to this ‘reservoir ef-

fect’(Fig. 7).

Urbanization-Driven Land Use Changes:

Galveston Bay Case Study

The conversion of land to support growing populations is a

major component of human modification of the environment

around the world (Alberti et al. 2003; Alberti 2005;Diazand

Rosenberg 2008; Paerl et al. 2014). An examination of land

use land cover data is an important element in helping us to

understand the environmental and anthropogenic influences

affecting waterways and watersheds (Carle et al. 2005;Lee

et al. 2009). Urbanization, industrialization, agriculture,

deforestation, loss of wetlands, and several other types of land

use change have taken place in response to human population

growth in watersheds all over the world. Urban-related runoff/

stormwater is one of the largest contributors to the impairment

of river and stream water quality in most US states (USEPA

2010; Paerl et al. 2014), with high levels of eutrophication

reported in 45% of the estuaries surrounding the Gulf of

Mexico (Clement et al. 2001).

Recent efforts to understand water quality degradation

have focused on the watershed of Galveston Bay (TX), a

subtropical estuary in the north-western Gulf of Mexico

(Fig. 8). With a billion-dollar commercial and recreational

fishery, home to the US energy nexus, several large ports

and cities, the Galveston Bay ecosystem is under pressure

from a variety of sources (Lester et al. 2013). Yet unlike

Chesapeake Bay and San Francisco Bay (see e.g., Dauer

et al. 2000; Smith et al. 2003;Kempetal.2005;Glibert

et al. 2011), clear effects of the environmental pressures asso-

ciated with eutrophication or invasive species have not yet

been observed in Galveston Bay (Gonzalez and Lester 2011;

Steichen et al. 2012,2015). Ongoing efforts are focused on

understanding the tipping points in this estuarine system (e.g.,

Roelke et al. 2013;Doradoetal.2015;Pinckneyetal.2017).

The Galveston Bay watershed (72,000 km

)includes

Houston and Dallas, ranked as the fourth and ninth largest

cities and fastest growing cities in the USA, respectively

(Census 2012, https://www.census.gov). Further, the cities of

Austin and Fort Worth rank eleventh and sixteenth, so it is not

surprising that the population density in Texas is expected to

double by 2050 (Texas Water Development Board TWDB

2007). Galveston Bay (1554 km

), at the southern end of the

watershed, is the second largest estuary in the Gulf of Mexico.

Fig. 7 aFecal coliform (FC) concentrations found in the Newport River

Estuary, NC, according to general rainfall categories of < 0.25 cm, 0.25–

2.54 cm, > 2.54 cm, and then in regard to the management action plan of

< 3.81 cm and > 3.81 cm. The dotted line indicates the FC standard limit

of 14 MPN per 100 ml. Asterisks indicate a significant difference as

compared to other categories. Error bars indicate ±1 standard error. b

Mean FIB concentrations at Newport River Estuary sampling sites

categorized by distance from land (close≤0.25 km to shoreline,

distant ≥0.25 km to shoreline). Error bars indicate ± 1 standard error.

Reproduced from Journal of Water and Health Volume 6, issue number

4, page 478, with permission from copyright holders IWA Publishing

(Coulliette and Noble 2008)

Estuaries and Coasts

It has two major rivers flowing into its northwest (San Jacinto)

and northeastern (Trinity) reaches. These rivers bring in 16

and 55% of annual freshwater inflows (Guthrie et al. 2012)

into the Bay. While the San Jacinto River flows through the

highly urbanized and industrialized Houston, the Trinity River

is surround primarily by forested and grassland areas imme-

diately north of the Bay (H-GAC 2017).

An examination of land use change maps (National

Land Cover Data) between 1996 and 2011 classified

by H-GAC using the NOAA Coastal Ocean

Program’s Coastal Change Analysis Program (C-

CAP) (see H-GAC 2017) indicate an 11% increase

in area used for development in Harris County (were

Houston is located) since 1996 (Fig. 9). An exami-

nation of all the counties surrounding Galveston Bay

revealed that forest land cover experienced the

greatest loss, primarily due to development

(urbanization) during this period (Fig. 9). Forests

were also lost to grasslands and more shrubs as well

as losses to agricultural (cultivated) lands and wet-

lands (Fig. 9). In most cases, wetlands were convert-

ed into developed lands, to shrubs and grasslands

associated with urban community centers, particularly

those connected to waterways (Figs. 8and 9; H-GAC

2017).

As a result of the Clean Water Act and other pol-

icies, nutrient and chlorophyll a (often used as a

proxy for phytoplankton biomass) levels have been

declining in Galveston Bay since the 1970s, although

less so in more recent years (Gonzalez and Lester

2011). This decline and/or change may also be the

result of less agricultural lands surrounding the bay.

The losses of wetlands likely preceded the period of

record for the land use/land change maps.

Nonetheless, we know that consequences of these

losses include both habitat quality and dietary chang-

es that impact higher trophic levels. One of the larg-

est increases associated with development in the

Fig. 8 The Galveston Bay watershed extends from Dallas/Fort Worth to

Houston to Galveston Island (see inset). Counties surrounding Galveston

Bay in the lower watershed and nearby are: Matagorda, Brazoria,

Galveston, Chambers, Liberty, Harris, Fort Bend, Wharton, Colorado,

Waller, Montgomery, Waller, and Walker. The majority of freshwater

inflows (~ 81%) enter from the Trinity and San Jacinto Rivers, and

Buffalo Bayou (not shown)

Estuaries and Coasts

Galveston Bay watershed has been waterways, either

as detention ponds and lakes or canalization associat-

ed with residential developments (H-GAC 2017).

These are known to be localized zones of degraded

water quality, particularly canals that often have a

dead end, and experience increased hypoxia and/or

harmful algal blooms, predominantly in warmer

months (Gonzalez and Lester 2011). Galveston Bay

was found to be a hotspot for fish kills over a

multi-decadal period (Thronson and Quigg 2008).

Despite this, overall fish abundance and diversity

has not changed significantly (Steichen and Quigg

2018).

It hypothesized that until now, GalvestonBay has been resilient

to the upstream changes in land use and land cover, in terms of

eutrophication and subsequent hypoxia and invasive species. The

former has not yet been measured, and little information is avail-

able to confirm the latter. The lack of obvious signs of eutrophica-

tion may in part be facilitated by the relatively high freshwater

inflows and short turnover time in the bay (Roelke et al. 2013;

Dorado et al. 2015). This may also be due to the fact that the vast

majority of developed lands (as urbanization) flushing into the

river contributes to only one sixth of the bays freshwater inflows;

the other important river (Trinity) contributes more than half the

freshwater inflows but has had fewer alterations to its landscape,

which is primarily forest and wetlands areas. It may also be that the

watershed as a whole has not been overdeveloped, that is, water-

sheds with ≥10% altered land hit the threshold for water quality

decreases due to impervious surface effects (Walsh et al. 2005;

Zampella et al. 2007). Nonetheless, as regional planning and

natural resource management endeavors to determine the appro-

priate amount of freshwater inflows for Galveston Bay, under-

standing the balance between land use land cover change andwater

quality will be key to maintaining ecosystem services and func-

tions for future generations. This is challenging as linear responses

to land use change are unlikely given the complex and dynamic

nature of estuarine systems.

Hurricane Harvey made landfall from August 25–30 (2017)

and set a continental US record for rainfall with more than

100 cm in this short time frame (van Oldenborgh et al. 2017;

Emanuel 2017). The massive flooding in urbanized areas is

thought to be a result of prolonged changes in land use including

development in low lying areas and flood plains along with

deferred maintenance projects which results in a weakening of

protective barriers around major bayous. Along with extensive

flooding, overflowing reservoirs, and overtopping dams, >

100,000 homes were damaged along with numerous schools

and businesses (Shultz and Galea 2017). Of the more than 13

million persons directly affected by the storm, 6.5 million were

impacted by the rainfall (NOAA NCEI 2017). Highly contami-

nated Superfund sites in the Houston area were deluged by flood-

waters; these along with spills from inundated sewage and waste-

water systems released > 31 million gallons of raw sewage

(Stuckey 2017). Pollutants released by Texas facilities into

Galveston Bay include benzene, 1,3-butadiene, hexane, hydro-

gen sulfide, toluene, and xylene, at least two of which are cancer-

causing chemicals and > 700,000 pounds of sulfur dioxide,

which can lead to the formation of dangerous contaminants

(FEMA 2017). The impact on the health of the bay is the source

of many ongoing studies. While the flood waters effectively

Fig. 9 The percentage change in land use in counties surrounding

Galveston Bay (see Fig. 8). In Harris, Fort Bend, and Montgomery

counties, the grow corridor of Houston and surrounding areas, there

was a significant increase in developed lands. As a result, there was a

significant decrease in forested lands in Harris and Montgomery counties

while agricultural lands were lost in Fort Bend county. Development also

impacted the land cover in Galveston, Brazoria, Liberty, and Chambers,

but to a lesser extent. Loss of forested land was significantly in Liberty;

this coincided with increase in grass cover

Estuaries and Coasts

“flushed”out the bay, and possibly associated nutrients and pol-

lutants, the concurrent scouring of the sediment bottom, particu-

larly in the industrial areas around Houston and its ship channel

may have (re)introduced legacy contaminants such as dioxins,

PCBs, lead and others (this data is however not yet published).

Ongoing studies will be required to monitor how this massive

disruption on urban systems negatively impacts the health of

Galveston Bay.

Environmental Limits of Ecosystems: Oyster

Case Study

Urbanization has affected coastal species composition and eco-

system function in part by changing biogeochemical processes

within the water column (Levin et al. 2014). These alterations

can ultimately reduce the survival of species, particularly for

those already nearing their physiological tolerance limits. One

keystone species found throughout urbanized estuaries along

the US East Coast and Gulf of Mexico is the eastern oyster

Crassostrea virginica.C. virginica possesses a complex life

cycle that includes a benthic adult stage and a pelagic larval

phase (Fig. 10). While C. virginica adults are relatively robust

to the dynamic physical conditions observed in coastal waters

(Bauman et al. 2015), reproduction and survival of early life

stages may be disproportionately negatively affected by urban-

ization. Though factors such as pollution, sedimentation, and

competition predation by invasive species have been shown to

hurt oyster populations, here the focus is on urbanization-

driven changes in physical parameters. Specifically, the effects

of eutrophication-driven hypoxia, eutrophication-driven hyper-

capnia, and lowered salinities from altered streamflow on sur-

vival of C. virginica are explored.

As mentioned in previous sections, one component of ur-

banization, increased area of impermeable surfaces and sub-

sequent runoff, raises the supply of limiting nutrients to coast-

al waters, leading to phytoplankton blooms. These blooms are

critical to successful fisheries. However, the subsequent

bloom die-off and respiration of organic matter produces pe-

riods of hypoxia, which may be pronounced in areas of high

stratification (Baker and Mann 1992). While adult oysters can

withstand prolonged hypoxic periods of days to weeks

(Stickle et al. 1989), hypoxia and anoxia reduce both larval

set and spat survival in C. virginica (Baker and Mann 1992).

In shallow eutrophic tributaries, diel cycling hypoxia is com-

monly observed during summertime, yet urbanization in-

creases the frequency and severity of the cycling (Keppel

et al. 2015). Although normoxic conditions return daily, diel

cycling hypoxia both reduces feeding, and increases suscepti-

bility of eastern oysters to the protistan parasite Perkinsus

marinus. This potentially leads to estuarine-scale effects of

disease acquisition in the species (Breitburg et al. 2015;

Keppel et al. 2015). A recent study by Steppe et al. (in prep)

demonstrated that severe cycling hypoxia representative of

urbanization-impacted estuaries also affects oyster reproduc-

tion, both reducing the proportion of reproductive oysters

Fig. 10 Crassostrea virginica reproduction sampling locations. Circles indicate spat collector deployments; squares, reef collections; and stars,

continuous water quality sondes

Estuaries and Coasts

within a sample and lowering gonadosomatic index (the ratio

of the germinal tissue to visceral mass). Therefore,

eutrophication-driven hypoxia is detrimental to C. virginica

throughout its life cycle.

Eutrophication, often a result of urbanization and land use

changes, can also lead to the hypercapnic conditions that are

frequently found in coastal waters (Cai et al. 2011, Nixon et al.

2015;Baumanetal.2015). Reduced pH may inhibit

C. virginica shell formation (Waldbusser et al. 2010) and dis-

solve oyster shell base, reducing substrate available for spat

set (Waldbusser et al. 2011a, b). Sustained seasonal acidifica-

tion in line with predictions of severe climate change scenarios

has been shown to reduce reproductive success in C. virginica

by limiting oogenesis (Boulais et al. 2017). Likewise, constant

moderate pH (7.45) has been shown to reduce gametogenesis

in C. virginica (Steppe et al. 2019). These values (7.45) coin-

cide with values observed in early summer in Chesapeake Bay

tributaries (Steppe and Wallendorf 2017). Cycling pH, on the

other hand, does not affect oyster reproduction or disease ac-

quisition (Keppel et al. 2015, though it is associated with

increased hemocyte activity, a sign of physiological stress.

Urbanized coastlines have fundamentally changed the

physical and chemical parameters of estuaries. Along with

reduced oxygen and pH, sharply lowered salinities, such as

those caused by coastal storms and dam releases may hurt

oyster populations, particularly when these events coincide

with gametogenesis and larval development (Butler 1949).

Low salinities may reduce the buffering capability of estu-

arine water. Combined with hypercapnic conditions, this

mayreducelipidandglycogenstores,andincreasefractur-

ing of juvenile oyster shells (Dickinson et al. 2012).

Because mesohaline tributaries are often selected for oyster

restoration, due to reduced disease prevalence in these

areas (Westby et al. 2015), oysters may already be living

close to their salinity tolerance limits. Salinities below 6

may limit gametogenesis (Butler 1949), yet even when ga-

metogenesis is documented, spat recruitment, and subse-

quent growth is often low in these areas (Bergquist et al.

2006; Steppe and Wallendorf 2017). An example of this has

been observed in the Severn River, a mesohaline tributary

of Chesapeake Bay, Maryland USA, near the urban centers

of Baltimore, Annapolis, and Washington, DC. From 2011

to 2013, high-frequency water quality data (period =

15 min) were collected at multiple sites that coincided with

bi-weekly samples of oyster gametogenesis and spat.

Gametogenesis was observed each year of the study at three

reefs and in oyster grow-out cages, yet of 2760 oyster shell

valve “spat collectors”analyzed for recruitment, only 3

spat were recorded. The spat collected were all found in

2012, the only year of the three in which salinities remained

above the threshold of 6 throughout the entire spawning

period (Steppe et al. 2018).

In addition to spring freshets, salinities may fall due to

altered streamflow that cause prolonged periods of low salin-

ity. This includes dam and spillway releases that result from

severe storms (Turner 2006; LaPeyre et al. 2016). In 2011, the

Conowingo Dam was released in early September, as a result

of Hurricane Irene. The release caused the salinity to drop

from 9 to 2 at the Severn River study site, likely killing any

larvae that may have been in the water column at that time.

Similarly, a salinity drop from 8 to 3 in mid-July 2013 may

have led to increased larval mortality, and reduced recruitment

that year (Steppe et al. 2018).

Further research on the effects of urbanization on

ecosystems must be conducted to better understand the

role of anthropogenic stresses on water quality and eco-

system function. In particular, high-frequency observa-

tions of coastal physical parameters should be colocated

with biological sampling of species at each segment of

their life history. Additionally, results should be con-

veyed to managers to better time decisions (such as

dam releases) to coincide with period that are less

stressful to keystone species.

Closing Remarks

As human populations increase, and technology allows

further coastal development, there is a pressing need for

improved tools and data collection methods to rapidly

obtain meaningful data on broad spatiotemporal scales.

Human populations have been experiencing rapid

growth over the past century, in addition to a migration

towards the coasts. The rapidly changing landscape has

altered the nature of estuarine ecosystems and water

quality. In general, urbanization land use changes result

in increased runoff containing a higher concentration of

pollutants, nutrients, and toxic materials, which are in-

troduced to estuarine systems (Carle et al. 2005;

DiDonato et al. 2009). Many estuaries are now beyond

their capacity to synthesize or process these inputs,

meaning that the same pollutants, excess nutrient loads,

and toxic materials make their way downstream to the

oceans. The dynamics of suspended sediments, nutri-

ents, and pollutants, effects on shellfish, harmful algal

blooms, and impacts of land use changes, all have wide

reaching effects on estuaries around the world. The re-

ality is that while estuaries provide valuable ecosystem

services including seafood, recreation, and a buffer zone

from storm energy, urbanization and human develop-

ment are putting many of these important systems at

risk. It is our responsibility to understand the risks and

impacts, and make the best use of emerging technolo-

gies and data to mitigate and reduce the adverse conse-

quences for estuaries.

Estuaries and Coasts

Acknowledgments The authors are grateful to the participants of CERF

2017 Session 1299: Ruth Carmichael, Nelle D’Aversa, Bethany

DeCourten, Naomi Detenbeck, Brooke Frohloff, Alexandria Hounshell,

Michael T. Lee, Haley Nicholson, and Guangming Zheng, as well as the

organizers of the CERF 2017 Biennial Conference in Providence, RI.

LAF appreciates feedback and support from Art Miller & Simon

Freeman. AMF is grateful for released-time support from NJCU-

College of Arts & Sciences. AQ: the Houston-Galveston Area Council

graciously provided the land use land cover data (http://www.h-gac.com/;

December 2016). Without this data and support from the Galveston Bay

Estuary Program, this work would not have been possible. CNS thanks

USACE, Baltimore District, D. Brietburg, R. Burrell, (Smithsonian

Environmental Research Center) A. Keppel, and L. Wallendorf (United

States Naval Academy). An anonymous reviewer provided constructive

suggestions that greatly improved the quality of this manuscript.

Funding Information DRC is supported by the NC Sea Grant

(R/MG-1522). DAL is supported by the South African Network

for Coastal and Oceanic Research (SANCOR) and the National

Research Foundation (NRF) of South Africa through a postdoctoral

fellowship (grant number 112650)

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Apr 2024

Major precipitation events from destructive hurricanes can change the hydrodynamics of estuaries. Indeed, water rather than wind was the major destructive force behind Hurricane Harvey, the category 4 storm that stalled over Texas in August 2017. Herein, we present a unique examination of the phytoplankton community as it responded to the changing hydrodynamic conditions in Galveston Bay. We used traditional (primary productivity, pigments, Fluorescence Induction and Relaxation System, pulse-amplitude modulation fluorometer), and modern (Imagining FlowCytobot [IFCB], 16S rRNA, and 18S rRNA gene sequencing) approaches to characterize the phytoplankton community and their physiological response. Phytoplankton communities transitioned from pre-storm estuarine to freshwater species and then began to transition back to an estuarine community with similar but not identical structure to that present before the storm. Further, phytoplankton communities were initially displaced by the flood waters, which lowered biomass and productivity despite the high nutrient concentrations and dissolved inorganic nitrogen to phosphate ratios of ~ 16. We found for cyanobacteria, factors such as time (i.e., sampling event after the storm), temperature (°C), salinity, NH4+ (µM), NO3− (µM), and turbidity (Secchi disk depths and total suspended solids) were the most critical environmental factors that explained the variation in community composition relative to the major eukaryotic groups. Only factors such as time, followed by salinity, and NO3− played a crucial role for eukaryotes, with time having the most impact on diatoms. The results herein provide a greater understanding of the succession and natural variability of phytoplankton species following large flood events in subtropical estuaries, which are predicted to occur more frequently with increasing effects of climate change.

Regional-scale assessment of eutrophic condition indicators in subtropical temporarily closed estuaries

Article

Full-text available

Jun 2024
ESTUAR COAST SHELF S

The adoption of a catchment-to-coast management approach is critical to prevent the continued loss of ecosystem functionality in small temporarily closed estuaries (TCEs). This study investigated the impact of anthropogenic pressures on the water quality (i.e., inorganic N and P, dissolved oxygen) and phytoplankton community dynamics of 55 TCEs situated along the subtropical coastline of KwaZulu-Natal, South Africa. Extensive spatial surveys were conducted in summer (high flow) and winter (low flow) to capture variability associated with open (84%; n = 46) and closed (73%; n = 40) mouth conditions, respectively. The health of the TCEs in this region are severely threatened ('High' or 'Very High' ratings) by pollution (67%) and habitat loss (53%) pressures. Wastewater effluent discharges are a key source of pollution, with 25 of the study estuaries receiving substantial combined annual inflow volumes (44 × 10 6 m 3) and inorganic nutrient loads (352,900 kg DIN; 222,200 kg DIP) from upstream wastewater treatment plants. The loss of habitat in these small TCEs stems from high levels of catchment land-use modifications (~50% transformed), with agricultural (e.g., sugarcane farming, commercial forestry) and development (e.g., built-up, informal) activities accounting for 35% and 12% of overall land-use, respectively. The impact of these pressures is highlighted by the relatively high proportion of TCEs exhibiting eutrophic levels of inorganic nutrients during the high (DIN: 25%, n = 14; DIP: 18%, n = 10) and low (DIN: 31%, n = 17; DIP: 26%, n = 14) inflow periods. Despite this, estuarine phytoplankton bloom conditions (>20 μg Chl-a l−1) are sporadic due to the limited tidal exchange (perched mouths) and low water residency (augmented baseflows) that characterise these TCEs. As such, anthropogenic perturbations serve to dampen the role of estuarine primary producers as nutrient buffers in subtropical TCEs, thus promoting the export of excessive nutrient loads to the adjacent coastal waters. Catchment-scale interventions should be geared towards effective governance, infrastructure maintenance, and implementation of restoration activities (e.g., diversion and/or reuse of wastewater discharges) to improve the health and biodiversity of small, microtidal TCEs.

Drivers of seasonal water hyacinth dynamics in permanently eutrophic estuarine waters

Article

Full-text available

May 2024
BIOL INVASIONS

Anthropogenic nutrient enrichment is one of the key global change pressures threatening the health of estuaries. This is evident at the persistently eutrophic Swartkops Estuary located along the warm temperate coast of South Africa. Augmented nutrient-rich baseflows (e.g., stormwater runoff, wastewater treatment work discharges) have resulted in the persistent growth of invasive alien aquatic plants (IAAPs), particularly water hyacinth (Pontederia crassipes), in the upper estuarine reaches. As such, the objective of this study was to investigate the temporal population dynamics of water hyacinth in the Swartkops Estuary to inform management options. Methods included five-weekly sampling campaigns in winter, spring, and summer, interspersed with monthly sampling over a year-long period. Physico-chemical variables and river inflow were measured in situ, while samples were collected for inorganic nutrients, phytoplankton biomass, and IAAP measurements (cover, biomass, and tissue nutrients). Model results indicated that increased water temperature and inorganic nitrogen (ammonium and NOx) levels promoted increased coverage of water hyacinth, while seasonal analysis highlighted reduced (p < 0.05) dissolved oxygen levels during the peak summer IAAP accumulation period. Notably, model results indicated declining IAAP tissue TN and TP concentrations with increasing water temperature, yet overall TN and TP storage was highest in summer due to the extensive water hyacinth cover observed during this period. Overall, the proportionally low and transient nature of nutrient storage by water hyacinth populations, together with the detrimental consequences they facilitate, highlights the need to integrate short-term control measures with catchment-scale management interventions geared towards mitigating the causative drivers.

Habitat characteristics and anthropogenic activities influence the distribution of macroinvertebrate traits and ecological preferences in Nigerian streams: a case study of Osun State, Nigeria

Article

Full-text available

May 2024
AQUAT ECOL

Impact of habitat characteristics and anthropogenic activities on the distribution of macroinvertebrate traits and ecological preferences was assessed in Osun State, Nigerian. Physicochemical parameters involving water depth (WD), channel width (CW), flow rate, discharge, water temperature (W.T), dissolved oxygen (DO), total dissolved solids (TDS), electrical conductivity (EC), pH, biological oxygen demand (BOD), turbidity, nitrate, phosphate and organic matter (OM), were appraised, alongside macroinvertebrates from 5 streams in different geographical locations. Sampling spanned from December 2019 to October 2021, covering early and late dry seasons, as well as early and late rainy seasons. Principal component analysis classified the sites into three impact categories: least impacted (LIS), moderately impacted (MIS), and highly impacted (HIS). Nine traits and ecological preferences displayed significant variations alongside nine physicochemical variables, such as phosphate, OM, WD, discharge, CW, TDS, EC, W.T, and nitrate, within the Fourth-corner figure. Furthermore, these traits and ecological preferences exhibited notable correlations with site impact categories, including–no turbidity preference, spherical body shape, climbers to LIS, preference for transparent water, crawlers, climbers, burrowers, small body size to MIS, and highly tolerant to ocxygen depletion to MIS.This study underscores the effectiveness of macroinvertebrate trait-based approach as a freshwater biomonitoring tool in Osun State, Nigeria, providing essential insights into how environmental factors influence macroinvertebrate traits in lotic systems in the region, with implications not only locally but also for broader eclogical studies.

Urbanization effects on Growth and Otolith Asymmetry in Chrysichthys nigrodigitatus and Oreochromis niloticus within Tropical Coastal Lagoon Watersheds

Article

May 2024
CHEMOSPHERE

In this study, we investigated the growth dynamics and otolith shape asymmetry of two fish species, Chrysichthys nigrodigitatus (CN) and Oreochromis niloticus (ON), within urbanized watersheds of the southern lagoon system, Nigeria. Using the von Bertalanffy growth model (VBGM), in addition to sediment metal concentration indices such as the average shale content, index of geoaccumulation (Igeo), contamination factor (CF), pollution load index (PLI), and potential ecological risk (PER) index, contamination levels were classified, and ecological risks were assessed. Notably, a lower growth potential (t0) was observed in CN at Ikorodu than at Epe, with similar trends for ON in the Epe during the dry season. Otolith asymmetry patterns, particularly in the CN at Ikorodu and ON in the Epe during the dry season, exhibited distinct ecological variations, indicating heightened stress levels at Ikorodu. Sediment analyses revealed moderate to strong contamination (Cd, Pb, Ni, and Cr) in both Lagos Lagoon (Ikorodu) and Epe Lagoon, with Ikorodu exhibiting notably high to moderate contamination levels according to the CF index. Elevated PLI values for Cd and Pb in Ikorodu, in addition to greater PER, indicated increased risk, with Cd posing a high risk (61.42%) and Pb posing a moderate risk (49.50%). Additionally, the reduced asymptotic length in the Epe during the dry season suggests that Chrysichthys nigrodigitatus is adaptable to seasonal variations, while divergent growth patterns in both areas indicate the existence of trade-off mechanisms in response to changing conditions. Habitat-specific otolith asymmetry and metal contamination underscore species adaptability, with wider stressor variability in Lagos than in Epe. Furthermore, multidimensional scaling analysis highlights the intricate relationship between otolith shape variables and environmental factors, emphasizing the need for tailored conservation efforts in urbanized watersheds.

Effects of spatial variability in vegetation phenology, climate, landcover, biodiversity, topography, and soil property on soil respiration across a coastal ecosystem

Article

Full-text available

Apr 2024

Coastal terrestrial-aquatic interfaces (TAIs) are crucial contributors to global biogeochemical cycles and carbon exchange. The soil carbon dioxide (CO2) efflux in these transition zones is however poorly understood due to the high spatiotemporal dynamics of TAIs, as various sub- ecosystems in this region are compressed and expanded by complex influences of tides, changes in river levels, climate, and land use. We focus on the Chesapeake Bay region to (i) investigate the spatial heterogeneity of the coastal ecosystem and identify spatial zones with similar environmental characteristics based on the spatial data layers, including vegetation phenology, climate, landcover, diversity, topography, soil property, and relative tidal elevation; (ii) understand the primary driving factors affecting soil respiration within sub-ecosystems of the coastal ecosystem. Specifically, we employed hierarchical clustering analysis to identify spatial regions with distinct environmental characteristics, followed by the determination of main driving factors using Random Forest regression and SHapley Additive exPlanations. Maximum and minimum temperature are the main drivers common to all sub-ecosystems, while each region also has additional unique major drivers that differentiate them from one another. Precipitation exerts an influence on vegetated lands, while soil pH value holds importance specifically in forested lands. In croplands characterized by high clay content and low sand content, the sig- nificant role is attributed to bulk density. Wetlands demonstrate the importance of both elevation and sand content, with clay content being more relevant in non-inundated wetlands than in inundated wetlands. The topographic wetness index significantly contributes to the mixed vegetation areas, including shrub, grass, pasture, and forest. Additionally, our research reveals that dense vegetation land covers and urban/developed areas exhibit distinct soil property drivers. Overall, our research demonstrates an efficient method of employing various open-source remote sensing and GIS datasets to comprehend the spatial variability and soil respiration mechanisms in coastal TAI. There is no one-size-fits-all approach to modeling carbon fluxes released by soil respiration in coastal TAIs, and our study highlights the importance of further research and monitoring practices to improve our understanding of carbon dynamics and promote the sustainable management of coastal TAIs.

Contamination by microplastics in oysters shows a widespread but patchy occurrence in a subtropical estuarine system

Article

May 2024

Assessing the utility of shellfish sanitation monitoring data for long-term estuarine water quality analysis

Article

May 2024

Can Multispectral Information Improve Remotely Sensed Estimates of Total Suspended Solids? A Statistical Study in Chesapeake Bay

Article

Full-text available

Sep 2018

Total suspended solids (TSS) is an important environmental parameter to monitor in the Chesapeake Bay due to its effects on submerged aquatic vegetation, pathogen abundance, and habitat damage for other aquatic life. Chesapeake Bay is home to an extensive and continuous network of in situ water quality monitoring stations that include TSS measurements. Satellite remote sensing can address the limited spatial and temporal extent of in situ sampling and has proven to be a valuable tool for monitoring water quality in estuarine systems. Most algorithms that derive TSS concentration in estuarine environments from satellite ocean color sensors utilize only the red and near-infrared bands due to the observed correlation with TSS concentration. In this study, we investigate whether utilizing additional wavelengths from the Moderate Resolution Imaging Spectroradiometer (MODIS) as inputs to various statistical and machine learning models can improve satellite-derived TSS estimates in the Chesapeake Bay. After optimizing the best performing multispectral model, a Random Forest regression, we compare its results to those from a widely used single-band algorithm for the Chesapeake Bay. We find that the Random Forest model modestly outperforms the single-band algorithm on a holdout cross-validation dataset and offers particular advantages under high TSS conditions. We also find that both methods are similarly generalizable throughout various partitions of space and time. The multispectral Random Forest model is, however, more data intensive than the single band algorithm, so the objectives of the application will ultimately determine which method is more appropriate.

Triggers of phytoplankton bloom dynamics in permanently eutrophic waters of a South African estuary

Article

Full-text available

Jun 2018

The permanently eutrophic Sundays Estuary experiences recurrent harmful algal blooms (HABs) of Heterosigma akashiwo (Raphidophyceae). This study aimed to identify the environmental variables shaping phytoplankton community composition and succession patterns during a typical spring/summer HAB period. Monitoring of abiotic and phytoplankton variables was undertaken over the period of a month in 2016. Surface water salinity corresponding to mesohaline conditions (9 to 12) was a prerequisite for site selection. During the study, two HABs (> 550 µg Chl-a l-1) of H. akashiwo occurred, each lasting for approximately a week in duration. Analyses highlighted nutrient depletion (i.e. nitrate and phosphate concentrations) as the key constraint on bloom duration. When the density of H. akashiwo decreased, the community composition became more diverse with species belonging to Bacillariophyceae and Dinophyceae becoming more abundant; albeit to a lesser degree (< 180 µg Chl-a l-1). Dissolved oxygen shifted from super-saturated conditions (> 14 mg l-1) during peak HAB conditions, to instances of bottom-water oxygen depletion (2–4 mg l-1) during the decay phase. These findings highlight the potential severity of transforming a catchment from natural to one that is highly-regulated by agricultural practices, whilst also emphasising the need for management intervention.

Attribution of extreme rainfall from Hurricane Harvey, August 2017

Article

Full-text available

Dec 2017
ENVIRON RES LETT

During August 25–30, 2017, Hurricane Harvey stalled over Texas and caused extreme precipitation, particularly over Houston and the surrounding area on August 26–28. This resulted in extensive flooding with over 80 fatalities and large economic costs. It was an extremely rare event: the return period of the highest observed three-day precipitation amount, 1043.4 mm 3dy⁻¹ at Baytown, is more than 9000 years (97.5% one-sided confidence interval) and return periods exceeded 1000 yr (750 mm 3dy⁻¹) over a large area in the current climate. Observations since 1880 over the region show a clear positive trend in the intensity of extreme precipitation of between 12% and 22%, roughly two times the increase of the moisture holding capacity of the atmosphere expected for 1 °C warming according to the Clausius–Clapeyron (CC) relation. This would indicate that the moisture flux was increased by both the moisture content and stronger winds or updrafts driven by the heat of condensation of the moisture. We also analysed extreme rainfall in the Houston area in three ensembles of 25 km resolution models. The first also shows 2 × CC scaling, the second 1 × CC scaling and the third did not have a realistic representation of extreme rainfall on the Gulf Coast. Extrapolating these results to the 2017 event, we conclude that global warming made the precipitation about 15% (8%–19%) more intense, or equivalently made such an event three (1.5–5) times more likely. This analysis makes clear that extreme rainfall events along the Gulf Coast are on the rise. And while fortifying Houston to fully withstand the impact of an event as extreme as Hurricane Harvey may not be economically feasible, it is critical that information regarding the increasing risk of extreme rainfall events in general should be part of the discussion about future improvements to Houston's flood protection system.

Assessing the present and future probability of Hurricane Harvey’s rainfall

Article

Full-text available

Nov 2017

Kerry A. Emanuel

Significance Natural disasters such as the recent Hurricanes Harvey, Irma, and Maria highlight the need for quantitative estimates of the risk of such disasters. Statistically based risk assessment suffers from short records of often poor quality, and in the case of meteorological hazards, from the fact that the underlying climate is changing. This study shows how a recently developed physics-based risk assessment method can be applied to assessing the probabilities of extreme hurricane rainfall, allowing for quantitative assessment of hurricane flooding risks in all locations affected by such storms, regardless of the presence or quality of historical hurricane records.

Quantitative Assessment on Multiple Timescale Features and Dynamics of Sea Surface Suspended Sediment Concentration Using Remote Sensing Data

Article

Full-text available

Oct 2017

Wind-waves, tidal currents, and some other dynamic factors dominate the suspended sediment concentration (SSC) variations in shallow seas and it is difficult to quantitatively evaluate the effects of individual dynamic factors on SSC modulation. This work used the long-term Moderate Resolution Imaging Spectroradiometer (MODIS) and the high temporal-resolution Geostationary Ocean Color Imager (GOCI) remote sensing data to quantify the sea surface SSC variations on multiple timescales (intratidal, spring-neap, seasonal, and long-term timescales) in the Bohai Sea, and further quantitatively evaluated the effects of corresponding dynamic factors on the SSC modulation. The results indicated that the monsoon associated wind-waves and stratification played the most important role in modulating SSC, with seasonal SSC variation of 8.1 mg/L in the Bohai Sea. The intratidal current variations played the secondary important role, causing SSC variation of 5.8 mg/L. The spring-neap tidal current variations led to SSC variation of 3.1 mg/L in the Bohai Sea. In the long run (2003–2014), the SSC of the Bohai Sea decreased slightly with SSC variation of 2.8 mg/L (decline rate: 0.23 mg/L/year), which may be caused by the weakening wind, decreasing sediment load from the Yellow River or the massive reclamation in recent decades. Probably due to the topography, sea bed sediment grain size and river plume, SSC variations in the southern Bohai Sea were more pronounced than those in the northern Bohai Sea.

Fish species as indicators of freshwater inflow within a subtropical estuary in the Gulf of Mexico

Article

Feb 2018
ECOL INDIC

As anthropogenic pressures on fresh water continues, a balance must be achieved to ensure estuaries receive sufficient riverine inputs to sustain ecosystem health. With increased demand on freshwater resources upstream, salinities in the coastal systems may increase, providing an opportunity for encroachment or invasion by species with a higher salt tolerance. These events compromise ecosystem function, integrity and sustainability, particularly of native estuarine habitats. Estuarine dependent teleost fish species may be considered appropriate bioindicators of abiotic stressors. While estuarine fish have a wide tolerance of many environmental conditions, each species has its own tolerance range and/or sensitivity, and at specific phases during their life cycle. A statistical analysis was conducted to better understand the relationship of Ictalurus furcatus (blue catfish), Brevoortia patronus (Gulf menhaden) and Lagodon rhomboides (pinfish) to abiotic parameters associated with freshwater inflows (i.e. salinity, nutrients, turbidity, etc). Data for this study were collected in Galveston Bay, Texas (USA) by various state agencies and spanned 124 seasons between 1979 and 2010. Distance based linear models were run to determine correlations between the selected indicator species and water quality parameters known to be related to freshwater inflows. These relationships were visualized with Spearman vectors overlain on principal coordinates analysis plots. Blue catfish and Gulf menhaden displayed a significant inverse correlation with salinity and significant positive correlation with NO3⁻ +NO2⁻ (μM), DIN:DIP and turbidity (NTU). During periods of drought, blue catfish were found closest to the river mouth, where salinity was lowest when considering other locations within Galveston Bay. Gulf menhaden maintain a wider distribution across the Bay as they have a wider salinity range tolerance. Pinfish display a significant positive correlation with salinity and temperature within Galveston Bay; as bioindicators this species is expected to be more tolerant to higher salinities. These relationships support the potential use of these species as beneficial indicators of changing estuarine conditions, namely freshwater inflows. Further study is essential to determine if these and/or other species can be applied to other bays along the Texas coast as well as other sub-tropical estuarine systems in which they are found.

Unwinding a Tangled Web: a Fine-Scale Approach towards Understanding the Drivers of Harmful Algal Bloom Species in a Eutrophic South African Estuary

Article

Feb 2018

A permanently eutrophic South African estuary provided an ideal model ecosystem from which to unravel the drivers of recurrent accumulations of harmful algal bloom (HAB) species. Designed to encapsulate broad- to fine-scale variations, seasonal in situ bihourly monitoring of abiotic and phytoplankton components took place at a fixed location over a 24-h period on four sampling occasions. Four known HAB species were recorded at bloom concentrations (> 20 μg Chl-a L⁻¹) during the study, including Heterosigma akashiwo, Heterocapsa rotundata, Mesodinium rubrum, and Karenia cf. mikimotoi. Model results identified temperature as a key driver, with distinct community shifts between winter (~H. rotundata and M. rubrum) and spring/summer (~H. akashiwo and K. cf. mikimotoi) conditions. Evidence of niche overlap between all four HAB taxa was highlighted by their predilection for elevated nitrate levels, a vertically stratified water column and mesohaline (ca. 10) surface waters. As such, internal biotic processes such as plasticity of diel vertical migration patterns, reliance of M. rubrum on suitable ‘prey’ resources, and the suppressive pressure of H. akashiwo on co-occurring taxa-explained phytoplankton community dynamics beyond the influence of physico-chemical variability. These findings provide novel insight regarding the ecology of HAB taxa and how they have adapted to thrive in anthropogenically manipulated environments.

Comparison of remote sensing algorithms for retrieval of suspended particulate matter concentration from reflectance in coastal waters

Article

Dec 2017
J APPL REMOTE SENS

Suspended particulate matter (SPM) is a key environmental indicator for rivers, estuaries, and coastal waters, which can be calculated from remote sensing reflectance obtained by an airborne or satellite imager. Here, algorithms from prior studies are applied to a dataset of in-situ at surface hyperspectral remote sensing reflectance, collected in three geographic regions representing different water types. These data show the optically inherent exponential nature of the relationship between reflectance and sediment concentration. However, linear models are also shown to provide a reasonable estimate of sediment concentration when utilized with care in similar conditions to those under which the algorithms were developed, particularly at lower SPM values (0 to 20 mg/L). Fifteen published SPM algorithms are tested, returning strong correlations of R2>0.7, and in most cases, R2>0.8. Very low SPM values show weaker correlation with algorithm calculated SPM that is not wavelength dependent. None of the tested algorithms performs well for high SPM values (>30 mg/L), with most algorithms underestimating SPM. A shift toward a smaller number of simple exponential or linear models relating satellite remote sensing reflectance to suspended sediment concentration with regional consideration will greatly aid larger spatiotemporal studies of suspended sediment trends. © 2017 Society of Photo-Optical Instrumentation Engineers (SPIE).

Use of Multispectral Satellite Remote Sensing to Assess Mixing of Suspended Sediment Downstream of Large River Confluences

Article

Nov 2017
J HYDROL

Although past work has noted that contrasts in turbidity often are detectable on remotely sensed images of rivers downstream from confluences, no systematic methodology has been developed for assessing mixing over distance of confluent flows with differing surficial suspended sediment concentrations (SSSC). In contrast to field measurements of mixing below confluences, satellite remote-sensing can provide detailed information on spatial distributions of SSSC over long distances. This paper presents a methodology that uses remote-sensing data to estimate spatial patterns of SSSC downstream of confluences along large rivers and to determine changes in the amount of mixing over distance from confluences. The method develops a calibrated Random Forest (RF) model by relating training SSSC data from river gaging stations to derived spectral indices for the pixels corresponding to gaging-station locations. The calibrated model is then used to predict SSSC values for every river pixel in a remotely sensed image, which provides the basis for mapping of spatial variability in SSSCs along the river. The pixel data are used to estimate average surficial values of SSSC at cross sections spaced uniformly along the river. Based on the cross-section data, a mixing metric is computed for each cross section. The spatial pattern of change in this metric over distance can be used to define rates and length scales of surficial mixing of suspended sediment downstream of a confluence. This type of information is useful for exploring the potential influence of various controlling factors on mixing downstream of confluences, for evaluating how mixing in a river system varies over time and space, and for determining how these variations influence water quality and ecological conditions along the river.

The spatial and temporal variation of total suspended solid concentration in Pearl River Estuary during 1987-2015 based on remote sensing

Article

Oct 2017

The movement and migration of total suspended solid (TSS) are the essential component of global material cycling and change. Based on the TSS concentrations retrieved from 112 scenes of Landsat remote sensing imageries during 1987-2015, the spatial and temporal variations of TSS concentration in high flow season and low flow seasons of six sub-regions (west shoal, west channel, middle shoal, east channel, east shoal and Pearl River Estuary Chinese White Dolphin National Nature Reserve and its adjacent waters (NNR)) of Pearl River Estuary (PRE) were analyzed and compared by statistical simulation. It was found that TSS concentrations in east and west shoals were about 23mg/L and 64mg/L higher than that of the middle shoal, respectively. There was a significant decreasing trend of TSS concentration from the northwest (223.7mg/L) to southeast (51.4mg/L) of study area, with an average reduction of 5.86mg/Lperkm, which mainly attributes to unique interaction of runoff and tide in PRE. In high flow season, there existed a significant and definite annual cycle period (5-8years) of TSS concentration change primarily responding to the periodic variation of precipitation. There were five full-fledged period changes of TSS detected in west shoal and west channel (the years of changes in 1988, 1994, 1998, 2003, 2010, 2015), while there were the last four cycle periods found in middle shoal, east channel, east shoal and NNR only. TSS concentrations in shoals and channels of PRE showed a significant decreased trend mainly due to the dam construction at the same time, with an average annual TSS concentration decrease of 5.7-10.1mg/L in high flow season from 1988 to 2015. There was no significant change trend of TSS concentration in NNR before 2003, but the TSS concentration decreased significantly after the establishment of the NNR since June 2003, with an average annual decrease of 9.7mg/L from 2004 to 2015. It was deduced that man-made protection measures had a great influence on the variation trend and intensity of TSS concentration in PRE, but had little effect on the cycle of TSS changes, indicating that the cyclical change is a very strong natural law. In low flow season, there was no significant change trend of TSS concentrations in PRE except that TSS concentrations in west channel and middle shoal showed a weak increasing trend (2.1mg/L and 2.9mg/L, respectively), which is probably because of controlled discharge for avoiding the intrusion of saltwater in PRE. Evidently, the change trend and cycle periods of TSS concentration in high- and low-flow seasons in six sub-regions of PRE had significant difference. The decreasing trend and cycle periods of TSS concentration mainly occurred in high flow season. The change trend and cycle periods of TSS concentration in low flow season was relatively small in PRE. The study shows that long series mapping of Landsat remote sensing images is an effective way to help understanding the spatial and temporal variation of TSS concentrations of estuaries and coasts, and to increase awareness of environmental change and human activity effects.

Impacts of Urbanization and Development on Estuarine Ecosystems and Water Quality

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