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Location and bathymetry of Vavvaru, Lhaviyani Atoll. (A) Regional setting of Lhaviyani Atoll in the Maldives; (B) Atoll margin setting of Vavvaru (center of view) based on 2008 image supplied courtesy of DigitalGlobe Foundation (http://www.digitalglobefoundation.org/) and with water reflectance removed using ENVI image analysis software. White dots show position of ground control points used to quantify water depths and confirm geo-ecological zonation patterns. Yellow boxes show the location of the survey areas within central areas of each identified geo-ecological zone. (C) Platform surface bathymetry of Vavvaru based on ground truthing of remotely sensed imagery.
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A coral reefs carbonate budget strongly influences reef structural complexity and net reef growth potential, and thus is increasingly recognized as a key “health” metric. Despite this, understanding of habitat specific budget states, how these scale across reef platforms, and our ability to quantify both framework and sediment production values rem...
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Synopsis
Bioeroding organisms play an important part in shaping structural complexity and carbonate budgets on coral reefs. Species interactions between various bioeroders are an important area of study, as these interactions can affect net rates of bioerosion within a community and mediate how bioeroders respond to environmental change. Here we te...
Citations
... Furthermore, it is recognised that as a function of differing ecologies, reef sediment sources will vary within and between reef systems, with resultant divergent implications for reef accretion and landform sediment supply (Perry et al., 2011a, ). Various past studies have sought to quantify these inputs, often for individual groups of producers, for example, for foraminifera (Hallock-Muller, 1974;Fujita et al., 2008;Doo et al., 2012), calcareous green algae (Neumann and Land, 1975;Multer, 1988), and seagrass epiphytes (Nelsen and Ginsburg, 1986), and occasionally for wider suites of producers (e.g., Bosence et al., 1985;Browne et al., 2013;Perry et al., 2017). Given the increasing knowledge of, and interest in, reef sediment generation in the context of reef carbonate budget studies (Browne et al., 2021), and the development of different methodologies for estimating sediment production rates, there is now an opportunity to build on this knowledge base and start developing a standardised methodology that can be applied across reef settings. ...
Standardised methodologies for assessing reef-derived sediment generation rates do not presently exist. This represents a major knowledge gap relevant to better predicting reef-derived shoreline sediment supply. The census-based SedBudget method introduced here generates estimates of sediment composition and grain-size production as a function of the abundance and productivity of the major sediment-generating taxa at a reef site. Initial application of the method to several reefs in the northern Chagos Archipelago, Indian Ocean, generated total sediment generation estimates ranging from (mean ± SE) 0.7 ± 0.1 to 4.3 ± 1.3 kg CaCO 3 m ⁻² yr ⁻¹ . Sediment production was dominated by parrotfishes (>90% at most sites), with site-variable secondary contributions from sea urchins (up to 20%), endolithic sponges (~1–7%) and benthic foraminifera (~0.5–3.5%). These taxa-level contributions are predicted to generate sediments that at all sites are coral- (83–94%) and crustose coralline algae-dominated (range ~ 5–12%). Comparisons between these estimates and sedimentary data from proximal reef and island beach samples generally show a high degree of consistency, suggesting promise in the SedBudget approach. We conclude by outlining areas where additional datasets and revised methodologies are most needed to improve rate estimates and hope that the methodology will stimulate research on questions around sediment production, transport and shoreline maintenance.
... For example, on an atoll rim platform in Lhaviyani Atoll, northern-central Maldives, sediment production rates were highest (up to 5.18 kg CaCO 3 m −2 per year) in an outer patch reef zone that occupied only 6.5% of the reef platform area 33 . These rates are markedly higher (per m 2 ) than those reported in the present study for seagrass meadows in an atoll rim reef setting (up to 0.86 kg CaCO 3 m −2 per year in Zone 3). ...
The future vulnerability of low-lying atoll nations is inextricably linked to the production of carbonate sediments by organisms living in their adjacent marine environments. Seagrass meadows are commonly found adjacent to reef islands, but their role as sources of reef island-building sediments has been overlooked. Here, we combine field, satellite and sedimentological data to quantify rates of sediment production by seagrass epibionts in a reef island sediment supply context. Total seagrass epibiont sediment production at our study site (Huvadhoo Atoll, Maldives) was 853,000 ± 90,000 kg CaCO3 yr⁻¹ over an area of 1.1 km². Of this total sediment production, 541,000 ± 23,000 kg CaCO3 yr⁻¹ was estimated to be suitable to contribute to reef island building (sand-sized, post-agitation). Our findings highlight a valuable ecosystem service provided by tropical seagrass meadows as important potential sources of reef island-building sediment. This study, therefore, presents a compelling geomorphic argument for seagrass conservation.
... Although these studies provide empirical relationships between grazers and external bioerosion rates, snapshot observations of fish for carbonate budget assessments may overestimate or underestimate species abundance depending on season, recent local disturbance, as well as specimens fleeing the path of observing divers. Further, recent studies have shown that there may be significant spatial variation in parrotfish bioerosion activities across an individual reef platform [15,16], and that grazing may be more intense at specific times of the day, as well as between species [17]. Grazing can also intensify as a result of increased endolithic microborer and macroborer activity in response to environmental drivers (e.g., increased nutrients: [18][19][20]). ...
Bioerosion on inshore reefs is expected to increase with global climate change reducing reef stability and accretionary potential. Most studies investigating bioerosion have focused on external grazers, such as parrotfish and urchins, whose biomass is more easily measured. Yet, cryptic endolithic bioeroders such as macroboring (worms, sponges and bivalves) and microboring taxa (fungus and algae) have the potential to be the dominant source of reef erosion, especially among inshore reef systems exposed to increased nutrient supply. We measured bioerosion rates of bioeroder functional groups (microborers, macroborers, and grazers), and their response to environmental parameters (temperature, light, turbidity, chlorophyll a), as well as habitat variables (coral cover, turfing algae, macroalgae) across two inshore turbid reefs of north Western Australia. Total bioerosion rates were low (0.163 ± 0.012 kg m−2 year−1) likely due to low light and nutrient levels. Macroborers were the dominant source of bioerosion and were positively correlated with turfing algae cover, highlighting the role of turf-grazing fish on endolithic bioerosion rates. Overall low bioerosion rates suggest that despite the reduced coral cover and carbonate production, these reefs may still maintain positive reef accretion rates, at least under current environmental conditions. However, an improved understanding of relationships between environmental drivers, habitat and grazing pressure with bioeroding communities is needed to improve predictions of reef carbonate loss with future climate change.
... Notably, the site with a more even distribution of fleshy algae and reef builders (RT13) is the site most proximate to the lagoon, where sedimentation or access to higher concentrations of inorganic nutrients may have resulted in more fleshy algal cover. Dominance by reef builders at the majority of sites studied here suggests that Palmyra's reefs are in a state of net calcification and growth (Goreau 1963;Perry et al. 2017). Reef builders such as CCA promote coral recruitment and regrowth, whereas turf and other fleshy algae can prevent coral settlement, inhibit growth, or otherwise harm corals (Birrell et al. 2005;Price 2010; Barott and Rohwer 2012). ...
The prevalence of coral bleaching due to thermal stress has been increasing on coral reefs worldwide. While many studies have documented how corals respond to warming, fewer have focused on benthic community responses over longer time periods or on the response of non-coral taxa (e.g., crustose coralline algae, macroalgae, or turf). Here, we quantify spatial and temporal changes in benthic community composition over a decade using image analysis of permanent photoquadrats on Palmyra Atoll in the central Pacific Ocean. Eighty permanent plots were photographed annually between 2009 and 2018 on both the wave-exposed fore reef (FR, 10 m depth, n = 4 sites) and the wave-sheltered reef terrace (RT, 5 m depth, n = 4 sites) habitats. The El Niño events of 2009–2010 and 2015–2016 resulted in acute thermal stress and coral bleaching was observed at both reef habitats during these events. Across 10 yr and two bleaching events, the benthic community structure on Palmyra shows evidence of long-term stability. Communities on the RT exhibited minimal change in percent cover of the dominant functional groups, while the FR had greater variability and minor declines in hard coral cover. There was also spatial variation in the trajectory of each site through time. Coral cover decreased at some sites 1 yr following both bleaching events and was replaced by different algal groups depending on the site, yet returned to pre-bleaching levels within 2 yr. Overall, our data reveal the resilience of calcifier-dominated coral reef communities on Palmyra Atoll that have persisted over the last decade despite two bleaching events, demonstrating the capacity for these reefs to recover from and/or withstand disturbances in the absence of local stressors.
... The finding that parrotfish assemblages supported by the rubble -Z5 habitat produced the greatest quantity of new sediment (resulting from bioerosion of reef substrate, making up >90% of parrotfishderived sediment produced in the habitat) is an important finding because it demonstrates that naturally low coral cover habitats (see habitat summary data in Perry et al., 2017) can support species that perform important ecological functions. This is especially the case as some previous research has suggested parrotfish do not play an important role in rubble habitats in other reef building regions (Adam et al., 2015). ...
... 1 as described inPerry et al. (2017). ...
Coral reef fish perform numerous important functional roles on coral reefs. Of these, carbonate sediment production, as a by‐product of parrotfish feeding, is especially important for contributing to reef framework construction and reef‐associated landform development. However, only limited data exist on: (i) how production rates vary among reef habitats as a function of parrotfish assemblages, (ii) the relative importance of sediment produced from eroded, reworked, and endogenous sources, or (iii) the size fractions of sediment generated by different parrotfish species and size classes. These parameters influence not only overall reef‐derived sediment supply, but also influence the transport potential and depositional fate of this sedimentary material. Here, we show that parrotfish sediment production varies significantly between reef‐platform habitats on an atoll‐margin Maldivian reef. Highest rates of production (over 0.8 kg m−2 year−1) were calculated in three of the eight platform habitats; a rubble‐dominated zone, an Acropora spp. dominated zone, and a patch reef zone. Habitat spatial extent and differences in associated parrotfish assemblages strongly influenced the total quantities of sediment generated within each habitat. Nearly half of total parrotfish sediment production occurred in the rubble habitat, which comprised only 8% of the total platform area. Over 90% of this sedimentary material originated from eroded reef framework as opposed to being reworked existing or endogenously produced sediment, and comprised predominantly coral sands (predominantly 125–1000 µm in diameter). This is comparable to the dominant sand types and size fractions found on Maldivian reef islands. By contrast, nearly half of the sediment egested by parrotfish in the Acropora spp. dominated and patch reef habitats resulted from reworked existing sediments. These differences between habitats are a result of the different parrotfish assemblages supported. Endogenous carbonate production was found to be insignificant compared to the quantity of eroded and reworked material. Our findings have important implications for identifying key habitats and species which act as major sources of sediment for reef‐island systems. Our findings show that parrotfish sediment production varies between reef habitat types as a function of the species assemblages and fish sizes supported. The origins of this material (i.e., bioerosion of reef substrate, sediment reworking, or endogenous production) also varies between habitat types because of the species and sizes of parrotfish present. In addition, we show that all species studied produce sediment size fractions suitable for shoreline maintenance.
... kg m -2 yr -1 at Low Isles; Osorno et al. 2005;Tribollet and Golubic 2005). Furthermore, there is also the potential for over-estimation of grazing bioerosion rates based purely on a snapshot measure of taxa abundance and biomass (see Perry et al. 2017 andYarlett et al. 2020 for further discussion). With such low grazing rates, the relatively low bioerosion rates on these marginal inshore reefs is the result of endolithic borers. ...
Bioerosion on turbid inshore reefs is expected to increase with global climate change reducing reef stability and accretionary potential. Most studies investigating bioerosion have focused on external grazers, such as parrotfish and urchins, whose biomass is more easily measured. Yet, cryptic endolithic bioeroders such as macroboring (worms, sponges and bivalves) and microboring taxa (fungus and algae) have the potential to be the dominant bioeroders, especially among inshore reef systems exposed to increased nutrient supply. We measured bioerosion rates of three bioeroder groups (microborers, macroborers, and grazers), and their response to environmental parameters (temperature, turbidity, light, chlorophyll a, salinity, pH) across two inshore reefs of north Western Australia. Total bioerosion rates were low (0.152 ± 0.012 kg m ⁻¹ yr ⁻¹ ) compared to average global rates, but were comparable to other inshore turbid reefs. Macroboring worms were the dominant source of bioerosion and displayed a significant negative relationship with temperature (r ² = 0.24, P=0.008) and light (r ² = 0.16, P=0.037). A global assessment of environmental influences on bioerosion further identified chlorophyll-a as a significant driver of macroboring (r2= 0.60, P=<0.001). Low bioerosion rates among marginal inshore reefs is encouraging as these reefs may be able to maintain a positive carbonate budget state despite lower coral cover and gross carbonate production rates. These data also highlight the necessity of reducing local impacts such as nutrient loads, which drive increases in chlorophyll a, to support marginal reef systems with climate change. Further, the development of empirical relationships that quantitatively link bioeroder activity and rates of environmental change will improve our ability to predict reef responses to environmental change, and better manage reefs into the future.
... This material is processed in the gut along with organic matter and egested into the environment as sediment [20][21][22][23][24][25]. In some regions, parrotfish bioerosion and the resultant sand egestion has been reported to dominate reef sediment production [26][27][28]. Bioerosion and grazing thus play important roles in overall reef carbonate production and cycling processes, and act as a "top-down" influence on reef ecological and physical structure [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. ...
... In some regions, parrotfish bioerosion and the resultant sand egestion has been reported to dominate reef sediment production [26][27][28]. Bioerosion and grazing thus play important roles in overall reef carbonate production and cycling processes, and act as a "top-down" influence on reef ecological and physical structure [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. ...
... However, grazing pressure (defined here as the total surface area of parrotfish bites, expressed as a proportion of grazable substrate area per year-as an indicator of the area of substrate bitten per year) and bioerosion rates (the mass of reef substrate eroded per year) can vary significantly among species, fish size classes, and between "scraper" (where bites are restricted to the removal of substrate surface material) and "excavator" (where bites remove chunks of substrate material) species [22][23][24][25][26][27][28][29][30][31][32]. These issues become important for understanding habitat-scale parrotfish ecological functions, because parrotfish assemblages (as with other taxonomic groups) can vary markedly between habitats or along gradients of structural complexity [33][34][35]. ...
Parrotfish perform a variety of vital ecological functions on coral reefs, but we have little understanding of how these vary spatially as a result of inter-habitat variability in species assemblages. Here, we examine how two key ecological functions that result from parrotfish feeding, bioerosion and substrate grazing, vary between habitats over a reef scale in the central Maldives. Eight distinct habitats were delineated in early 2015, prior to the 2016 bleaching event, each supporting a unique parrotfish assemblage. Bioerosion rates varied from 0 to 0.84 ± 0.12 kg m −2 yr −1 but were highest in the coral rubble-and Pocillopora spp.-dominated habitat. Grazing pressure also varied markedly between habitats but followed a different inter-habitat pattern from that of bioerosion, with different contributing species. Total parrotfish grazing pressure ranged from 0 to ~264 ± 16% available substrate grazed yr-1 in the branching Acropora spp.-dominated habitat. Despite the importance of these functions in influencing reef-scale physical structure and ecological health, the highest rates occurred over less than 30% of the platform area. The results presented here provide new insights into within-reef variability in parrotfish ecological functions and demonstrate the importance of considering how these interact to influence reef geo-ecology.
... Reef platforms are important reef zones, acting as sediment factories (i.e., production and supply) for island construction and maintenance, and as sediment reservoirs [18,19]. As the 2016 survey only captured topographic LiDAR, and the 2018 topo-bathymetric LiDAR was only collected for four islands, we used an image-based approach to map the platform area for all 22 islands. ...
Reef islands are some of the most highly sensitive landforms to the impacts of future environmental change. Previous assessments of island morphodynamics primarily relied on historical aerial and satellite imagery. These approaches limit analysis to two-dimensional parameters, with no ability to assess long-term changes to island volume or elevation. Here, we use high-resolution airborne LiDAR data to assess three-dimensional reef island features for 22 islands along the north-western coast of Australia. Our primary objective was to utilize two regional LiDAR datasets to identify characteristics indicative of island sensitivity and future vulnerability. Results show reef platform area to be an accurate predictor of island area and volume suggesting larger island volumes may reflect (1) increased carbonate production and supply from the reef platform and/or (2) enhanced shoreline protection by larger reef platforms. Locations of foredune scarping (an erosional signature) and island orientations were aligned to the regional wind and wave climate. Reef island characteristics (island area, volume, elevation, scarping, and platform area) were used to rank islands according to sensitivity, using a new Island Sensitivity Characteristics Index (ISCi) where low ISCi indicates stable islands (large areas and volumes, high elevations, and fewer scarped areas) and high ISCi indicates unstable islands (small areas and volumes, low elevations, and more scarped areas). Comparison of two LiDAR surveys from 2016 and 2018 validates the use of 3D morphometrics as important (direct) measurements of island landform change, and can complement the use of 2D parameters (e.g., area) moving forward. Results demonstrate that ongoing use of airborne LiDAR and other 3D technology for monitoring coral reef islands at regional scales will enable more accurate quantification of their sensitivity to future impacts of global environmental change.
... Therefore, the relative importance of herbivory on sediment production here is largely dependent on segment age. Few carbonate budget studies have considered spatial and temporal variability in Halimeda biomass (Perry et al. 2017(Perry et al. , 2019. Most studies have focused on sand-bottom Halimeda populations, yet, sediment generation greatly differs among species and habitats where Halimeda grows. ...
ABSTRACT: Reef carbonate production and sediment generation are key processes for coral reef development and shoreline protection. The calcified green alga Halimeda is a major contributor of calcareous sediments, but rates of production and herbivory upon Halimeda are driven by biotic and environmental factors. Consequently, estimating rates of calcium carbonate (CaCO3) production and transformation into sediment requires the integration of Halimeda gains and losses across habitats and seasons, which is rarely considered in carbonate budgets. Using seasonal rates of recruitment, growth, senescence and herbivory derived from observations and manipulative experiments, we developed an individual-based model to quantify the annual cycle of Halimeda carbonate and sediment production at Heron Island, Great Barrier Reef. Halimeda population dynamics were simulated both within and outside branching Acropora canopies, which provide refuge from herbivory. Shelter from herbivory allowed larger Halimeda thalli to grow, leading to higher rates of carbonate accumulation (3.9 and 0.9 kg CaCO3 m−2 yr−1 within and outside Acropora canopies, respectively) and sediment production (2.5 versus 1.0 kg CaCO3 m−2 yr−1, respectively). Overall, 37% of the annual carbonate production was transformed into sediments through senescence (84%) and fish herbivory (16%), with important variations among seasons and habitats. Our model underlines that algal rates of carbonate production are likely to be underestimated if herbivory is not integrated into the carbonate budget, and reveals an important indirect pathway by which structurally complex coral habitats contribute to reef carbonate budgets, suggesting that coral losses due to climate change may lead to further declines in reef sediment production.
... Despite their recognized importance, carbonate budgets have been estimated for few reefs globally, most of which are located in the Caribbean [17][18][19]22,31,32 . However, recent work in the past couple of decades has expanded the growing knowledge of reef carbonate budgets to the central Indian Ocean 13,16,[33][34][35] and elsewhere in the Indo-Pacific 22,29,36,37 . A recent global analysis of carbonate budget data 12 indicates that net carbonate budgets of reefs in the Indo-Pacific and tropical western Atlantic are currently low and are sub-optimal for reef accretion potential. ...
Coral reefs experienced the third global bleaching event in 2015–2016 due to high sea-surface temperature (SST) anomalies. Declines in net carbonate production associated with coral bleaching are implicated in reef structural collapse and cascading impacts for adjacent coral reef islands. We present the first carbonate budget study of a reef platform surface (reef crest and reef flat) in the southern Maldives and the first record of upper reef flat condition in the central Indian Ocean post the 2015–2016 coral bleaching event. Scleractinian corals were the primary carbonate producers, with live coral cover averaging between 11.1 ± 6.5 and 31.2 ± 21.8% and dominated by massive corals. Gross carbonate production rates averaged 5.9 ± 2.5 G (kg CaCO3 m2 yr−1). Bioerosion was estimated at 3.4 ± 0.4 G, resulting in an average net carbonate production rate of 2.5 ± 2.4 G. Comparison of results with a study of the fore-reef slope highlights major differences in post-bleaching carbonate budget state between the fore-reef slope and the reef platform surface. The positive reef flat carbonate budget is attributed to the persistence of massive corals (Porites spp. and Heliopora spp.) through the bleaching event.
