Unusual coccolithophore blooms in Scottish waters

Abstract

Two unusual blooms were observed in Scottish waters during summer 2021: one in the Clyde Sea and the other by the east coast of the Shetland Islands. Both had the appearance of coccolithophore blooms. Transmission electron microscopy of a sample from the Clyde Sea confirmed the presence there of the coccolithophore Emiliania huxleyi. We examine the conditions that led to these unusual blooms. In situ data are scarce, and so we draw inference from satellite data and reanalysis. For Shetland, the bloom can be seen to originate further north on the edge of the continental shelf. It is advected south and then west towards the Shetland coast by surface currents. For the Clyde Sea region, April 2021 was the coldest April of the last 30 years (National Climate Information Centre). We hypothesise that this cold weather restricted the usual spring bloom of diatoms. A restricted spring bloom would mean higher-than-usual concentrations of nutrients in the summer. It might also mean reduced numbers of grazers. These factors would provide ideal conditions for coccolithophores to flourish as temperatures and sunlight increase.

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State Planet, 1-osr7, 13, 2023
https://doi.org/10.5194/sp-1-osr7-13-2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.
Chapter 4.1 – 7th edition of the Copernicus Ocean State Report (OSR7)
Unusual coccolithophore blooms in Scottish waters
Richard Renshaw1, Eileen Bresnan2, Susan Kay1,3, Robert McEwan4, Peter I. Miller3, and Paul Tett5
1Hadley Centre, Met Office, FitzRoy Road, Exeter EX1 3PB, UK
2Marine Scotland Marine Laboratory, 375 Victoria Rd, Aberdeen AB11 9DB, UK
3Plymouth Marine Laboratory, Prospect Place, Plymouth PL1 3DH, UK
4Cefas, Barrack Rd, Weymouth DT4 8UB, UK
5Scottish Association for Marine Science, Oban, Argyll PA37 1QA, UK
Correspondence: Richard Renshaw (richard.renshaw@metoffice.gov.uk)
Received: 22 August 2022 – Discussion started: 30 September 2022
Revised: 19 April 2023 – Accepted: 2 May 2023 – Published: 27 September 2023
Abstract. Two unusual blooms were observed in Scottish waters during summer 2021: one in the Clyde Sea
and the other by the east coast of the Shetland Islands. Both had the appearance of coccolithophore blooms.
Transmission electron microscopy of a sample from the Clyde Sea confirmed the presence there of the coccol-
ithophore Emiliania huxleyi. We examine the conditions that led to these unusual blooms. In situ data are scarce,
and so we draw inference from satellite data and reanalysis. For Shetland, the bloom can be seen to originate
further north on the edge of the continental shelf. It is advected south and then west towards the Shetland coast
by surface currents. For the Clyde Sea region, April 2021 was the coldest April of the last 30 years (National
Climate Information Centre). We hypothesise that this cold weather restricted the usual spring bloom of diatoms.
A restricted spring bloom would mean higher-than-usual concentrations of nutrients in the summer. It might also
mean reduced numbers of grazers. These factors would provide ideal conditions for coccolithophores to flourish
as temperatures and sunlight increase.
1 Introduction
Coccolithophores belong to a diverse group of phytoplank-
ters (class Prymnesiophyceae) that is widespread in the
oceans. The ecological group that includes Emiliana is par-
ticularly abundant in upwelling and temperate sub-polar re-
gions (Balch et al., 2019). Most coccolithophores are not
themselves harmful or toxic, but they are of ecological
importance, particularly for carbon cycling and sequestra-
tion (Rost and Riebesell, 2004). They typically produce an
exoskeleton consisting of several calcium carbonate plates
called coccoliths (Young et al., 1999). These coccoliths are
not opaque (phytoplankton require light for photosynthesis),
but they scatter and polarise light. Coccolith shedding oc-
curs during the later stages of the bloom life cycle, when the
cells are threatened, for example by pathogen pressure (Frada
et al., 2008). During this stage, the coccoliths are shed and
accumulate in the surrounding water. The visual effect is to
turn the sea a milky turquoise colour, visible to the human
eye and in satellite imagery.
The function of the coccoliths is unclear. They are believed
to be protective, either against grazing, against viral or bacte-
rial attack, or as a refractor of light that acts as a sunshade in
excessively bright conditions (Monteiro et al., 2016). Johns
et al. (2023) show that coccoliths initially provide some pro-
tection from viral attack but once shed can mediate such at-
tacks. Müller (2019) suggests that coccolith production may
have evolved originally as an efficient mechanism for intra-
cellular Ca2+detoxification at a time of elevated seawater
Ca2+concentrations (e.g. during the Cretaceous and Jurassic
periods).
Summer 2021 saw milky, turquoise-coloured waters
caused by algal blooms in two locations off Scotland, in the
Clyde Sea on the west coast and also to the east of the Shet-
land Islands (Fig. 1). These blooms were visually striking
and so unusual that they were reported in the news (e.g. Brad-
shaw, 2021). Transmission-electron-microscope analysis of
Published by Copernicus Publications.

2 R. Renshaw et al.: Coccolithophore blooms
Table 1. CMEMS and non-CMEMS products used in this study.
Product Data
ref. no. Product ID and type access Documentation
1 NWSHELF_MULTIYEAR_PHY
_004_009
(1993–2022), numerical models
EU Copernicus Service Product
(2021a)
Quality Information Document
(QUID): Renshaw et al. (2021);
Product User Manual (PUM):
Tonani et al. (2022)
2 OCEANCOLOUR_ATL_BGC_L3
_MY_OBSERVATIONS_009_013
(1998–2021), satellite observations
EU Copernicus Service Product
(2022)
QUID: Garnesson et al. (2022);
PUM: Collela et al. (2022)
3 SST_ATL_SST_L4_REP
_OBSERVATIONS_010_026
(1998–2021), satellite observations
EU Copernicus Service Product
(2021b)
QUID: Autret et al. (2021a);
PUM: Autret et al. (2021b)
4 ERA5 atmospheric reanalysis
(1979–2022)
Copernicus Climate Change Ser-
vice, Climate Data Store (2023)
(2023)
Hersbach et al. (2023)
a water sample from the Clyde Sea (Fig. 1d) confirmed the
algae to be a morphotype of coccolithophore, Emiliania hux-
leyi (morphotype B). Blooms of this organism are common
in spring and early summer in the North Atlantic and occur
in some years in the northern North Sea and the western En-
glish Channel. However, such striking occurrences have not
been reported from the Clyde Sea for many years. Colleagues
of one of the authors (PT) recall no such event since 1983,
when they remember sampling turquoise waters and coccol-
ithophores in sea lochs of the Firth of Clyde. In this paper we
use observations and reanalysis data to look for unusual envi-
ronmental conditions in 2021 that might have allowed these
blooms to thrive.
2 Data
2.1 Ocean colour
Ocean colour (OC) instruments measure water-leaving radi-
ation at various wavelengths in the visible and near-infrared
spectrum. Satellite measurements of ocean colour are used
here in two forms. One is imagery. The enhanced colour
maps from Sentinel-3 OLCI (Ocean and Land Colour Im-
ager) and Sentinel-2 MSI (Multi-Spectral Imager) in Fig. 2
provide visual indications of algal blooms. The other is ob-
servation products, point estimates of near-surface chloro-
phyll concentration derived from multiple sensors. The
CMEMS (Copernicus Marine Environment Monitoring Ser-
vice) product used here (product ref. no. 2, Table 1) estimates
chlorophyll concentration for several distinct phytoplankton
functional types, including diatoms and nanophytoplankton.
The nanophytoplankton category includes E. huxleyi.
Several factors make chlorophyll estimation difficult for
coastal waters. Bowers et al. (2000) find that dissolved or-
ganic matter (DOM) from fresh water is usually the largest
optically active constituent in the Clyde Sea. Ocean colour
algorithms are designed to minimise errors due to suspended
sediment and DOM. The presence of large numbers of coc-
coliths would also have a strong impact on backscattered ra-
diation (Voss et al., 1998). The estimation process does mask
for cloud, sun glint, and coccoliths (OC-CCI, 2020). This
masking means there are few OC chlorophyll estimates avail-
able for the Clyde Sea during the period of the Clyde bloom
(12 June to 7 July 2021). We use the OC product in Fig. 5
not as a measure of absolute value but to show the timing of
growth and also how values in 2021 compare to other years.
2.2 North West Shelf reanalysis
The North West Shelf (NWS) reanalysis (product ref. no. 1,
Table 1) is based on the physical ocean model NEMO
(Madec and Team, 2008) at 7km horizontal resolution and
51 vertical levels, with tides represented, over a domain that
encompasses the North West Shelf (Renshaw et al., 2021).
Atmospheric forcing is from the ERA5 atmospheric reanal-
ysis (Hersbach et al., 2023). River discharge volumes for the
year 2018 onwards come from a daily climatology. The re-
analysis uses NEMOVAR (Mogensen et al., 2012) to assim-
ilate observations of physical variables (satellite sea surface
temperature, SST, as well as in situ temperature and salinity
profiles; Waters et al., 2015).
Here we use the reanalysis for sea surface temperature
and also for near-surface currents in the region of the Euro-
pean Slope Current. Reanalysis SST is strongly constrained
by high-quality satellite observations. Variations in surface
currents are driven predominantly by surface winds (Röhrs
et al., 2023), provided here by ERA5. The slope current it-
self is forced also by meridional density gradient and steep
bathymetry (Marsh et al., 2017). Validation of the reanalysis
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R. Renshaw et al.: Coccolithophore blooms 3
Figure 1. (a) Sentinel-2 MSI image of Clyde Sea on 21 June 2021 at 11:35 UTC, true colour with enhanced contrast. Processed by the
Natural Environment Research Council Earth Observation Data Acquisition and Analysis Service (NEODAAS) using ACOLITE atmospheric
correction. (b) Sentinel-2 MSI image of Shetland Islands on 1 July 2021, processed by ESA (https://www.esa.int/ESA_Multimedia/Images,
last access: 30 May 2023). (c) Bathymetry map with locations of (a) and (b) marked in red. Shelf edge is visible as transition from light blue
(less than 200 m depth) to dark blue (deep water). (d) Scanning electron micrograph of sample from the Clyde Sea, June 2021, identified as
E. huxleyi, morphotype B. Credit: Eileen Bresnan (Marine Scotland Science).
shows that it does produce a realistic slope current (Renshaw
et al., 2021).
3 Shetland bloom
Figure 1 shows a bloom on the eastern side of Shetland.
There is no information on the species present, but the
brightness and turquoise colour of the bloom suggest coc-
colithophores. Coccolithophore blooms in the North Sea are
not unusual. An unusual feature of the 2021 bloom was that it
came so close inshore. Examination of imagery for the years
2017 to 2020 from Plymouth Marine Laboratory (PML) (de-
scribed in Highfield et al., 2014) and of a dataset of coccol-
ithophore blooms for 1998 to 2016 in Kondrik et al. (2019)
finds no other examples where blooms intrude among the is-
lands and bays on the eastern side of Shetland.
Kondrik et al. (2019) show that often blooms develop fur-
ther south and east in the North Sea in spring or early summer
and are advected by an anti-clockwise circulation sometimes
reaching as far north as Shetland. Sometimes a bloom orig-
inates in the north, along the northern edge of the continen-
tal shelf, and is advected southwards. Bathymetry in Fig. 1c
shows the location of this shelf edge. The European Slope
Current flows eastward along this edge, bringing North At-
lantic water into the Norwegian Sea. Some of this water flows
south past Shetland into the North Sea. Imagery for July 2021
(Fig. 2c, d) seems to show the bloom is of the latter kind,
originating along the northern edge of the shelf.
To confirm the bloom’s origins and to understand why
2021 was unusual, we used the OceanParcels software pack-
age (Delandmeter and van Sebille, 2019) to simulate the tra-
jectory of virtual particles in the ocean. Particles were ini-
tially positioned at 1 m depth along the slope current (Fig. 3).
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4 R. Renshaw et al.: Coccolithophore blooms
Figure 2. Enhanced ocean colour satellite imagery (provided by PML) from the Sentinel-3 OCLI and Sentinel-2 MSI instruments for 12 and
19 June 2021 and 2 and 7 July 2021. The brightest pixels are indicative of high numbers of coccoliths. Land and cloud are coloured black.
These locations were chosen based on where the speed of
the current (from a reanalysis mean climatology for 1993 to
2021) exceeded 0.2 m s−1.OceanParcels modelled 3D move-
ment of the particles, advected by daily mean currents from
the NWS reanalysis (product ref. no. 1, Table 1) and using
a 3 h time step. This was done separately for each year from
1998–2022, starting with particles in initial positions on 3
April and running forward 3 months to predict positions on
3 July (Fig. 3).
In some years nearly all the particles move off beyond the
edge of the plots (1999, 2002, 2003, 2009, 2022). In most
years all the red particles (those initially at the eastern end of
the slope current) disappear in this way. In 2021 it is these
red particles that end up close to the eastern side of Shetland
on 3 July. The year 2021 was unusual, although not unique,
in that easterly winds during spring drove surface currents
that pushed particles westward for part of that time. Figure 4
shows 10 m winds (from ERA5) and surface currents (from
NWS reanalysis) for May 2021 and for a May climatology.
To the east and north of Shetland, climatological winds are
westerly, and the surface current is westerly or northerly.
For May 2021, winds are north-easterly. These winds induce
easterly surface currents (Ekman transport effect).
The years 2012, 2016, and 2019 also experienced easterly
winds in spring or early summer (based on ERA5 reanaly-
sis) and similarly show large numbers of red particles still
within the plot region on 3 July. Other years see large num-
bers of other-coloured particles come close inshore, in par-
ticular 2007, 2017, and 2020. In the satellite imagery (2017–
2020) and Kondrik catalogue (1998–2016), none of these
years show coccolithophore blooms that reach into the bays
and inlets of eastern Shetland. Imagery for 2020 (not shown)
comes the closest, with a bloom 10–20 km away from the
coast. The 2020 particle tracking has orange particles in this
region, originating from further west on the shelf edge.
We conclude that in some years blooms around Shetland
form in water coming from the shelf edge. We cannot con-
clude that bloom development is always linked to specific
locations and timings of source water along the shelf edge.
To understand how the bloom appeared so close inshore,
we examined the daily particle trajectories for 2021 (not
shown). Particles move south down the eastern side of Shet-
land during the second half of June. In late June and early
July there is a brief period of easterly winds, and the parti-
cles are driven in towards the coast. Brief easterlies are not
unusual, but these coincided with coccolithophore-laden wa-
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R. Renshaw et al.: Coccolithophore blooms 5
Figure 3. Top plot shows 3 April starting positions for particles positioned along the European Slope Current. The area around Shetland is
marked with a grey line. Other plots show particle positions in this area advected forward 3 months, using reanalysis surface currents for
each year from 1998–2022. Particles are coloured according to longitude of starting position.
ter near the coast. We suggest that coincidences of timing and
weather in 2021 created the unusual phenomenon of a visible
coccolithophore bloom on the eastern Shetland coast.
4 Clyde Sea bloom
Analysis of the 2021 bloom in the Clyde Sea is hampered
by a paucity of observations. Weather stations provide data
on the atmospheric conditions. Satellite instruments provide
estimates of SST and of chlorophyll, with caveats discussed
below. We have found no in situ measurements of conditions
within the Clyde Sea itself. Biogeochemical reanalysis data
are available (Kay et al., 2021), but their ability to accurately
simulate the Clyde Sea is hampered by a lack of data on river
discharge. Freshwater input and nutrient input from rivers are
important variables for the biogeochemistry here. In this pa-
per we analyse the data that are available and build from them
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6 R. Renshaw et al.: Coccolithophore blooms
Figure 4. Top: ERA5 monthly mean 10m winds (product ref. no. 4, Table 1) for (a) May 2021 and (b) May 1979–2020 climatology. Bottom:
surface current for May from reanalysis (product ref. no. 1, Table 1) for (c) May 2021 and (d) May 1983–2020 climatology.
a plausible storyline. That storyline starts with diatom growth
in early spring.
4.1 The Clyde Sea and the annual cycle of diatoms
The Clyde Sea comprises a large tidal estuary with several
islands and fjord-like sea lochs. It is the outlet of the River
Clyde and other rivers into the Irish Sea. It has a maximum
depth of 164 m, with a sill (the “Great Plateau”) of approxi-
mately 40 m depth where it meets the Irish Sea. Freshwater
outflow from rivers and from land drainage tends to main-
tain stable stratification in the basin (Simpson and Rippeth,
1993). This together with the sill restricts tidal mixing to
mostly near-surface waters. Edwards et al. (1986) estimate a
residence time of 2 months for surface water in the main body
of the Clyde Sea. Nutrient content in the Clyde Sea tends to
be higher than adjacent coastal waters (based on measure-
ments of nitrate in Slesser and Turrell, 2013). Tidal currents
within the Clyde Sea tend to be weak. Water in the deeper
waters below the sill can stagnate, leading to nutrient build-
up near the sea bed. Simpson and Rippeth (1993) show that
strong winds can sometimes overcome the vertical stability
and mix the water column. This would act to replenish nutri-
ents in the surface layers in the event of an algal bloom.
Marshall and Orr (1927) sampled the Clyde Sea and its
lochs extensively, finding the following:
There is a well-marked spring diatom maximum
which starts at the end of March or the beginning
of April.
A diatom bloom will consume nutrients (Stief et al., 2022),
which will tend to inhibit further phytoplankton growth
(Elser et al., 2007). Marshall and Orr (1927) also observed
a second, smaller summer maximum but noted that diatoms
near the surface were less healthy than those several metres
deeper. Tests with samples left in direct sunlight and in shade
showed that summer light levels were injurious for these di-
atoms.
Hannah and Boney (1983) assessed extensive and more
recent surveys (1976–1978) of the Inner Firth. They found
rapid growth in diatoms from late March or early April
in each year of the study, dominated by Skeletonema spp.
and Thalassiosira nordenskioldii. They also found Nitzschia
seriata (now called Pseudo-nitzschia “seriata type”) and
Chaetoceros spp. at those times present in considerable
numbers. During these spring blooms total chlorophyll was
dominated by diatoms. For 1977 they found evidence that
the Skeletonema were being grazed by microzooplankton
(Ebria). Bresnan et al. (2016) also report an intense spring
bloom dominated by Skeletonema during the monitoring pe-
riod 2005–2013.
4.2 Timing and source of 2021 Clyde coccolithophore
bloom
Figure 1 shows the Clyde bloom on 21 June 2021. There are
earlier visual reports of bright patches in the sea around the
Isle of Arran in the centre of the Clyde Sea on 12 June (Evers-
King et al., 2021). Satellite imagery (Fig. 2a) has no bright
patch in the Clyde Sea that day, but the sea around Arran is
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R. Renshaw et al.: Coccolithophore blooms 7
partly obscured by cloud. Imagery for 13–16 June is almost
wholly obscured by cloud, and the first clear satellite image
of a bright patch across the whole of the Clyde Sea is from
18 June (Fig. 2b). This patch persists until 5 July and then
fades. Figure 2c shows that by this time the bloom is apparent
even in the northernmost reaches of Loch Fyne. This sea loch
flows into the Clyde Sea but is tidal along its 65 km length.
Transmission electron microscopy of a sample from Mill-
port collected at the end of June revealed the bloom to be
comprised of liths and cells of E. huxleyi – morphotype B
(Fig. 1d), providing the first confirmation of this species in
high abundance at this site. E. huxleyi morphotypes A and
B have been recorded in the waters around Scotland (van
Bleijswijk et al., 1991; León et al., 2018). Little is known
about the seasonality of E. huxleyi morphotypes on the west
coast of Scotland. A study at the Marine Scotland Scottish
Coastal Observatory (SCObs) monitoring site at Stonehaven
on the east coast from 2010–2013 showed a distinct repeated
seasonality in the occurrence of different E. huxleyi mor-
photypes (León et al., 2018). Morphotype B was commonly
recorded in spring, with morphotype A occurring from June
to August followed by an overcalcified form of morphotype
A (type AO) in autumn and winter months. The dominance
of E. huxleyi morphotype B in the 2021 Clyde bloom differs
in timing from the seasonality recorded on the east coast.
Coccolithophores have a haplodiplontic life cycle (Keuter
et al., 2021). New cells are haploid (one set of chromosomes
in the nucleus). These haploid cells develop into diploid cells
(two sets of chromosomes in the nucleus). For the genus
Emiliania, it is only the diploid form that produces coccol-
iths. Frada et al. (2008) explain its “Cheshire Cat” strategy
for resisting viral attack. Giant phycodnaviruses (Emiliania
huxleyi viruses, EhVs) infect and lyse diploid-phase cells
and are heavily implicated in the termination of blooms. The
diploid cells transition to haploid cells that are resistant to
EhVs, shedding coccoliths as they do. Thus the bloom in the
Clyde Sea may have started some time before sufficient coc-
coliths had accumulated to make it visible.
It is possible that E. huxleyi was introduced by tidal mix-
ing into the Clyde Sea from the Irish Sea or that it was
already resident. Reverse particle tracking (not shown) ex-
cludes immediate seeding from blooms at the Malin shelf
break as a likely cause. The satellite imagery shows the
bloom mostly confined to the Clyde Sea. We conclude that
conditions within the Clyde basin in late May or early June
were particularly favourable for E. huxleyi to thrive. We aim
here to understand exactly which aspects were favourable
and what brought them about.
4.3 Physical environment in 2021
Figure 5a shows daily values of sea surface temperature
(SST) from the NWS reanalysis averaged over the Clyde Sea
for the years 1998–2021. June 2021 values are in the middle
of the range. Values for April and May are towards the cold
Figure 5. Daily mean values averaged over the Clyde Sea basin of
(a) reanalysis sea surface temperature for individual years. The year
2021 is in black. Chlorophyll concentrations (product ref. no. 2, Ta-
ble 1) for the Clyde Sea from ocean colour products for (b) diatoms
and (c) nanophytoplankton are also given. Black dots and lines are
ocean colour estimates for 2021. Blue shows mean estimates for
1998 to 2020. Smaller purple dots are values for individual years,
showing the year-to-year spread.
end of the range. Statistics from a high-resolution satellite
SST product in Table 2 (product ref. no. 3, Table 1) confirm
this. Monthly means for April, May, and June 2021 are, re-
spectively, at the 10th, 5th, and 52nd percentiles for those
months over the period 1982–2021.
Table 2a has monthly statistics of SST from a satellite
SST product (product ref. no. 3, Table 1) for a point within
the Clyde Sea (55.27◦N, 5.11◦W), close to the lower right
intersection of grid lines in Fig. 1. April and May 2021
SSTs were unusually cold compared to climatology. June
was close to the median. The cold SST can be linked to
the weather (Table 2b, c, d). April saw anticyclonic weather
that was cold, dry, and exceptionally sunny (Weather Maga-
zine, 2021a). May 2021 had anomalously low atmospheric
pressure over the UK, bringing storms, high rainfall, and
high winds (Weather Magazine, 2021b). June was drier and
warmer than average.
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8 R. Renshaw et al.: Coccolithophore blooms
Table 2. Statistics of monthly means from (a) CMEMS European
area level 4 SST analysis at 55.27◦N, 5.11◦W, and (b, c, d) from
weather stations in the Clyde catchment area (National Climate In-
formation Centre, NCIC).
(a) Sea surface temperature
Monthly
mean (◦C) Anomaly (◦C) Percentile
April 7.53 −0.65 10 %
May 8.81 −1.20 5 %
June 12.24 0.05 52 %
(b) Air temperature
Monthly
mean (◦C) Anomaly (◦C) Percentile
April 5.5 −1.4 10 %
May 8.4 −1.4 10 %
June 13.4 1.0 83 %
(c) Sunshine hours
Percentage
Total of climatology Percentile
April 236 159 % 100 %
May 151 80 % 19 %
June 174 111 % 76 %
(d) Rainfall
Percentage
Total (mm) of climatology Percentile
April 16 20 % 5 %
May 96 115 % 64 %
June 42 47 % 12 %
Anomalies and percentiles are relative to yearly climatology: (a) SST
is CMEMS reprocessed level 4 satellite product 1982–2020 (product
ref. no. 3, Table 1). (b, c, d) Weather station data are for the Clyde
catchment area from NCIC for 1980–2021.
There were two severe storms in May 2021, one on 9, 10,
and 11 May and a stronger one on 20 and 21 May. These
appear to coincide with periods when there is a pause in the
rate of increase in SST (Fig. 5a). This suggests the strong
winds are mixing the water column.
4.4 Ocean colour estimates of chlorophyll
Figure 5 shows a time series of estimates of chlorophyll a
from the CMEMS ocean colour product (product ref. no. 2,
Table 1) for two plankton functional types, diatoms and
nanophytoplankton. Mean values for 1998–2020 (blue line)
show similar patterns for diatoms and nanophytoplankton:
concentrations rising to a peak in late March and a smaller
second peak in early May, although with considerable year-
to-year variation (purple dots). Estimates for 2021 are again
similar between both functional types. Both have strong
peaks in early April, well above the 1998–2020 mean. Val-
ues drop rapidly during April, rising again towards the end
of that month. Both types also show a fall immediately fol-
lowing the two May storms, around 10 and 20 May.
As discussed in Sect. 2.1, chlorophyll estimation can be
difficult in coastal waters. Vertical mixing and river discharge
due to the May storms might increase levels of dissolved
organic matter and sediment in the water. Vertical mixing
might dilute plankton in the surface layers that are sensed
by the ocean colour instruments. The presence of coccoliths
and cloud mean that much of the data for June have been
masked in the estimation process. The ocean colour prod-
uct includes an estimate of root mean square error (RMSE)
following Brewin et al. (2017). For the Clyde Sea, values
of RMSE for diatoms and nanophytoplankton in both April
and May 2021 are given as approximately 0.5 mg m−3. This
is similar in size to the estimates themselves for nanophyto-
plankton, and so we avoid drawing conclusions from Fig. 5c.
Estimated concentrations for diatoms are somewhat larger,
and so we have more confidence in drawing conclusions from
the diatom time series (Fig. 5b).
4.5 Possible causes of Clyde bloom
Mayers et al. (2019) assessed coccolithophore growth and
mortality rates based on samples from the Celtic Sea in
April 2015. They identified several conditions that favour
coccolithophore blooms. These are considered individually
below.
(a) Warm, stratified waters. SST in June 2021 was close
to average for that month (Table 2a). Vertical profiles of
temperature from the reanalysis for 2021 (not shown)
are stably stratified, but this is typical for June. The fol-
lowing conclusion was reached: June 2021 temperatures
were not unusual.
(b) Sunlight. June 2021 was sunnier than average (76th
percentile, Table 2c). The following conclusion was
reached: sunshine might have been a contributory fac-
tor in 2021.
(c) Availability of nutrients. We have no direct measure-
ments of nutrients in the Clyde Sea. April 2021 was
a dry month (5th percentile, Table 2d), and so river
discharge in April would have been low. Rainfall in
May was above average. May was also a stormy month,
which may have mixed the water column, bringing
nutrients from deep water into the photic zone (Pin-
gree et al., 1977). Each of these factors could lead
to higher-than-usual nutrient availability by the end of
May. Ocean colour estimates of diatoms suggest lower-
than-usual diatom growth in April and increased growth
during May (Fig. 5b). The following conclusion was
reached: observations show low chlorophyll mass in
spring 2021. This could be due to the cold water tem-
peratures and limited nutrient input in April 2021. The
State Planet, 1-osr7, 13, 2023 https://doi.org/10.5194/sp-1-osr7-13-2023

R. Renshaw et al.: Coccolithophore blooms 9
wet and stormy conditions in May of both years likely
increased nutrient levels in near-surface layers in the
Clyde Sea. Both these factors would help produce suit-
able conditions for a bloom.
(d) Scarcity of predatory microzooplankton. Mayers
et al. (2019) found that microzooplankton exert strong
top-down control on coccolithophore populations,
grazing up to 80 % of daily production in a bloom of
E. huxleyi. We might hypothesise that fewer diatoms in
April and May led to low numbers of microzooplankton
during that time, reducing the grazing pressure on
E. huxleyi in late May and early June. However, growth
rates for microzooplankton can be rapid, sometimes
more than three doublings per day for tintinnids (Verity,
1986). The following conclusion was reached: this
could be a contributory factor, though we have no
evidence for this. The ability of microzooplankton to
multiply rapidly suggests at least that other factors were
also involved.
Advection of a bloom into the region is another possible
cause. We consider this unlikely for two reasons. The Clyde
Sea is semi-enclosed, with an estimated residence time of
2 months for surface water in the main body of the Clyde
Sea (Edwards et al., 1986). This was consistent with reverse
tracking (not shown) of a set of virtual particles placed in
the Clyde Sea in June and tracked backwards for 60 d to find
their source. The majority remained within the Clyde Sea.
Also, satellite imagery shows a bloom in the Clyde Sea but
not in the adjoining Irish Sea.
5 Discussion
Phytoplankton are of special interest in the waters around
Scotland, where aquaculture and fishing are major industries.
Through primary production, phytoplankton form the base of
a food chain that sustains marine fauna (Frederiksen et al.,
2006). Blooms of phytoplankton can be harmful to other ma-
rine life and can produce toxins dangerous for human con-
sumers of seafood (Davidson et al., 2011). There is thus in-
creasing interest from policy-makers to understand the diver-
sity dynamics of phytoplankton communities in Scotland and
other parts of the North West Shelf (NWS) and to understand
its influence on industries and diversity status assessments
(Siemering et al., 2016; McQuatters-Gollop et al., 2019).
This paper presents hypotheses to explain two unusual
blooms. We suggest that the bloom on the eastern side of
Shetland originated in Atlantic water brought north of Shet-
land by the European Slope Current. The water’s passage
eastward was retarded by a period of anomalous easterly
winds in May, and it was later steered towards the Shetland
coast by a shorter period of easterly winds. The timing was
such that there were abundant coccoliths present when this
water was close inshore. The bloom within the Clyde Sea ap-
pears to have developed in place. We hypothesise that envi-
ronmental factors may have combined to create suitable con-
ditions in the Clyde Sea. A cold and dry April could have re-
stricted spring growth of diatoms, leaving nutrients available
for a summer bloom of coccolithophores. A wet and stormy
May might also have added to the nutrients.
Our explanations are based on limited evidence (SST and
chlorophyll estimates from satellites, modelling by the re-
analysis). We do not have in situ measurements from within
the blooms to confirm our hypotheses.
For both blooms, we propose the weather as a key fac-
tor. Other studies also identify the importance of the weather
for algal blooms. Whyte et al. (2014) looked at unusually
strong blooms of the biotoxin-producing dinoflagellate Dino-
physis on the western side of the Shetland Islands in the sum-
mers of 2006 and 2013. They found these blooms coincided
with periods where the winds, usually more southerly, be-
came westerly. They suggested the westerly winds advected
Dinophysis populations onshore, resulting in an increase in
diarrhetic shellfish toxin levels in farmed mussels (Mytilus
edulis).
There is evidence that the distribution of coccolithophores
has expanded polewards in recent decades (Beaugrand et al.,
2013; Winter et al., 2013; Rivero-Calle et al., 2015), due ei-
ther to changes in ocean temperature or dissolved inorganic
carbon. Growth of E. huxleyi is also known to be impacted by
major changes in ocean pH (Riebesell et al., 2017). Changes
in pH observed in seas around the UK (Findlay et al., 2022)
are not large enough to explain recent variability in coccol-
ithophore abundance in Scottish waters.
Changing weather patterns have the potential to influence
the occurrence of unusual phytoplankton blooms in coastal
waters. These changes have the potential to impact higher
trophic levels in the marine ecosystem. A better understand-
ing of the processes and dynamics involved will help in fore-
warning, preparation, and development of adaptation mea-
sures for these changes.
This paper shows how use of satellite data and model re-
analysis can help to meet the challenge of assessing major
events in UK waters, despite a sparsity of in situ observa-
tions. However we could be more confident in our findings
if we had more information about environmental conditions.
More complete data on river discharge would help in simulat-
ing biogeochemical and ecosystem variables in the Clyde Sea
and other inshore water bodies. Widespread routine monitor-
ing of nutrient levels and phytoplankton components could
help greatly in understanding future blooms.
Data availability. The data products used in this article, as well
as their names, availability, and documentation, are summarised in
Table 1.
https://doi.org/10.5194/sp-1-osr7-13-2023 State Planet, 1-osr7, 13, 2023

10 R. Renshaw et al.: Coccolithophore blooms
Author contributions. RR processed the reanalysis data and
wrote much of the text. EB produced the micrograph of the E. hux-
leyi sample. PM produced the ocean colour images. Every author
contributed to discussion and development of the hypotheses pre-
sented. Every author also added to and reviewed the text.
Competing interests. The contact author has declared that none
of the authors has any competing interests.
Disclaimer. Publisher’s note: Copernicus Publications remains
neutral with regard to jurisdictional claims in published maps and
institutional affiliations.
Acknowledgements. We gratefully acknowledge use of the
OceanParcels code (Delandmeter and van Sebille, 2019) in cal-
culating backward trajectories. We also acknowledge use of col-
lated station statistics from the UK National Climate Informa-
tion Centre and ERA5 reanalysis data (Hersbach et al., 2023)
downloaded from the Copernicus Climate Change Service (C3S)
Climate Data Store. We thank ESA for the satellite image in
Fig. 1, made available under Creative Commons License BY-SA
3.0 IGO (https://creativecommons.org/licenses/by-sa/3.0/igo/, last
access: 30 May 2023). Transmission electron analysis of the Mill-
port water sample was performed at the Microscopy Unit, Institute
of Medical Sciences, University of Aberdeen.
Review statement. This paper was edited by Griet Neukermans
and reviewed by David Bowers and two anonymous referees.
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