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While excessive anthropogenic nutrient loads are harmful to coral reefs, natural nutrient flows can boost coral growth and reef functions. Here we investigate if seabird-derived nutrient subsidies benefit the growth of two dominant corals on lagoonal reefs, submassive Isopora palifera and corymbose Acropora vermiculata , and if enhanced colony-level calcification rates can increase reef-scale carbonate production. I. palifera and A. vermiculata colonies close to an island with high seabird densities displayed 1.4 and 3.2-times higher linear extension rates, 1.8 and 3.9-times faster planar area increase, and 1.6 and 2.7-times higher calcification rates compared to colonies close to a nearby island with low seabird densities, respectively. While benthic ReefBudget surveys in combination with average coral growth rates did not indicate differences in reef-scale carbonate production across sites, coral carbonate production was 2.2-times higher at the seabird-rich island when using site-specific linear growth rates and skeletal densities. This study shows that seabird-derived nutrients benefit fast-growing branching as well as previously unstudied submassive coral taxa. It also demonstrates that nutrient subsidies benefit colony-scale and reef-scale calcification rates, which underpin important geo-ecological reef functions. Restoring natural nutrient pathways should thus be a priority for island and reef management.

Coral growth is an important metric of coral health and underpins reef-scale functional attributes such as structural complexity and calcium carbonate production. There persists, however, a paucity of growth data for most reef-building regions, especially for coral species whose skeletal architecture prevents the use of traditional methods such as coring and Alizarin staining. We used structure-from-motion photogrammetry to quantify a range of colony-scale growth metrics for six coral species in the Mexican Caribbean and present a newly developed workflow to measure colony volume change over time. Our results provide the first growth metrics for two species that are now major space occupiers on Caribbean reefs, Agaricia agaricites and Agaricia tenuifolia . We also document higher linear extension, volume increase and calcification rates within back reef compared to fore reef environments for four other common species: Orbicella faveolata , Porites astreoides , Siderastrea siderea and Pseudodiploria strigosa . Linear extension rates in our study were lower than those obtained via computed tomography (CT) scans of coral cores from the same sites, as the photogrammetry method averages growth in all dimensions, while the CT method depicts growth only along the main growth axis (upwards). The comparison of direct volume change versus potential volume increase calculated from linear extension emphasizes the importance of assessing whole colony growth to improve calcification estimates. The method presented here provides an approach that can generate accurate calcification estimates alongside a range of other whole-colony growth metrics in a non-invasive way.

Cross-ecosystem nutrient fluxes can influence recipient food webs, including both static measures of structure and dynamic measures of function. However, a mechanistic basis for how nutrient subsidies affect both structure and function across multiple trophic levels is still lacking. Here, we investigate how nutrient subsidies provided by seabirds influence coral reefs, focusing on the link between primary producers and primary consumers. We quantified turf algal cover and herbivorous fish biomass (static metrics of structure), as well as productivity of turf algae and herbivorous fish (dynamic metrics of function) at sites in the inner Seychelles with a range of seabird densities due to different rat invasion histories. Turf algae grew faster with increasing amounts of seabird-derived nutrients. These higher rates of primary productivity, in turn, fueled higher productivity and biomass of herbivorous fishes. In contrast, seabird-derived nutrients did not increase cover of turf algae nor did turf algal cover affect herbivores. Instead, seabird nutrients indirectly enhanced herbivorous fish productivity and biomass via effects on primary productivity, which, in turn, led to increased top-down control by herbivores to limit turf algal cover. Overall, dynamic metrics better revealed the flow and effects of seabird-derived nutrients through coral-reef food chains and revealed the mechanisms by which seabirds can enhance coral-reef ecosystem function. These findings could be used to predict the benefits of removing introduced rats from islands, which can increase seabird populations and restore nutrient connectivity, thus potentially enhancing ecosystem function across multiple trophic levels on coral reefs.

Coral reef fishes perform essential and well‐documented ecological functions on reefs, but also contribute important geo‐ecological functions, which influence reef carbonate cycling regimes. These functions include reef framework modification (through bioerosion and breakage), and the production, reworking, and transport of reefal sediments. To explore how these functions vary across reefs and regions, we compiled a dataset of available taxa‐specific function rates and applied these to fish census data from sites in the Pacific Ocean (PO), Indian Ocean (IO), and Greater Caribbean (GC), each region displaying a gradient in fish biomass. The highest overall function rates occur at the highest fish biomass sites in the PO (Kingman Reef) and IO (Chagos Archipelago), where bioerosion dominates framework modification and sediment generation (up to 7 kg m−2 year−1). At the lowest biomass PO and IO sites, framework modification and sediment generation are driven mainly by breakage and occur at lower rates (~2 kg m−2 year−1). Sediment reworking rates are high across all PO and IO sites (~1–5 kg m−2 year−1) and higher than other function rates at low biomass sites. Geo‐ecological function rates are generally low across the GC sites, despite total fish biomass being comparable to, or even exceeding, some PO and IO sites, with sediment reworking (up to ~1 kg m−2 year−1) being the dominant function. These site‐level differences partly reflect total fish biomass, but fish assemblage size structure and species identity are critical, with a few fish families (and species) underpinning the highest function rates and regulating the “health” of the fish‐driven carbonate cycling regime. Reefs with high fish‐driven framework modification, sediment production and reworking rates define one end of this spectrum, while at lower biomass sites little new sediment is produced and sediment reworking dominates. While additional species‐level rate data are urgently needed to better constrain function rates, these transitions align with ideas about the progressive shutdown of carbonate production regimes on ecologically perturbed reefs, with important implications for reef‐building, shoreline sediment supply, and sediment carbon and nutrient cycling.

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.

Parrotfish provide important ecological functions on coral reefs, including the provision of new settlement space through grazing and the generation of sediment through bioerosion of reef substrate. Estimating these functions at an ecosystem level depends on accurately quantifying the functional impact of individuals, yet parrotfish feeding metrics are only available for a limited range of sites, species and size classes. We quantified bite rates, proportion of bites leaving scars and scar sizes in situ for the dominant excavator (Cetoscarus ocellatus, Chlorurus strongylocephalus, Ch. sordidus) and scraper species (Scarus rubroviolaceus, S. frenatus, S. niger, S. tricolor, S. scaber, S. psittacus) in the central Indian Ocean. This includes the first record of scar frequencies and sizes for the latter three species. Bite rates varied with species and life phase and decreased with body size. The proportion of bites leaving scars and scar sizes differed among species and increased with body size. Species-level allometric relationships between body size and each of these feeding metrics were used to parameterize annual individual grazing and bioerosion rates which increase non-linearly with body size. Large individuals of C. ocellatus, Ch. strongylocephalus and S. rubroviolaceus can graze 200–400 m2 and erode >500 kg of reef substrate annually. Smaller species graze 1–100 m2 yr−1 and erode 0.2–30 kg yr−1. We used these individual functional rates to quantify community grazing and bioerosion levels at 15 sites across the Maldives and the Chagos Archipelago. Although parrotfish density was 2.6 times higher on Maldivian reefs, average grazing (3.9 ± 1.4 m2 m−2 reef yr−1) and bioerosion levels (3.1 ± 1.2 kg m−2 reef yr−1) were about 15% lower than in the Chagos Archipelago (4.5 ± 2.3 and 3.7 ± 3.0, respectively), due to the dominance of small species and individuals in the Maldives (90% <30 cm length). This demonstrates that large-bodied species and individuals contribute disproportionally to both grazing and bioerosion. Across all sites, grazing increased by 66 ± 5 m2 ha−1 and bioerosion by 109 ± 9 kg ha−1 for every kg increase in parrotfish biomass. However, for a given level of parrotfish biomass, grazing and bioerosion levels were higher on Maldivian reefs than in the Chagos Archipelago. This suggests that small-bodied fish assemblages can maintain ecosystem functions, but only if key species are present in sufficiently high numbers.

Reefs in the remote Chagos Archipelago (central Indian Ocean) were severely affected by sea surface temperature warming and coral bleaching in 2015–2016. Here we assess the impacts of this event on community composition and reef carbonate production at twelve fore reefs sites across three atolls. Bleaching caused a 69% decline in coral cover, mostly driven by mortality of tabular Acropora spp. and a 77% decline in mean coral carbonate production (2015: 13.1 ± 4.8; 2018: 3.0 ± 1.2 kg CaCO3 m2 yr−1). Changes were accompanied by a major shift from competitive to stress-tolerant coral taxa, with magnitudes of decline comparable to those reported elsewhere in the Indian Ocean, despite inter-site differences in dominant coral species. These trends differ from those on reefs already dominated by stress-tolerant taxa, which experienced minor declines in production post-warming. The study highlights the potential for different suites of functional coral groups to drive divergent post-bleaching budget responses.

Bioerosion of calcium carbonate is a fundamental process that impacts net coral reef accretion. Besides large grazers, endolithic organisms play a major role in carbonate removal. Here we provide the first rate data for both macro- and microendolithic bioerosion in the remote Chagos Archipelago, Indian Ocean. Based on analysis of experimental blocks using computer tomography, we show similar macrobioerosion rates at 5 m (0.086 ± 0.026 kg m−2 yr−1) compared to at 10 m depth (0.066 ± 0.016 kg m−2 yr−1) after three years of exposure, with a succession from worm to sponge bioeroders over time. Microbioerosion rates analysed with scanning electron microscopy were 2–5 × higher than macrobioerosion rates at 5 m (0.187 ± 0.028 kg m−2 yr−1) and 10 m depth (0.313 ± 0.049 kg m−2 yr−1), but the microborer community was dominated by cyanobacteria in all samples. Total endolithic erosion was small compared to external erosion by parrotfishes, which increased over time, but did not show significant differences between 5 m (0.74 ± 0.11) and 10 m depth (1.12 ± 0.16 kg m−2 yr−1) after three years of exposure. These erosion rates are indicative of the oligotrophic and remote reef setting in the Chagos Archipelago (clear water, low nutrients, high fish biomass). The data will help to improve local carbonate budget estimates and provide a context for wider regional and environmental comparisons.

In a time of unprecedented ecological change, understanding natural biophysical relationships between reef resilience and physical drivers is of increasing importance. This study evaluates how wave forcing structures coral reef benthic community composition and recovery trajectories after the major 2015/2016 bleaching event in the remote Chagos Archipelago, Indian Ocean. Benthic cover and substrate rugosity were quantified from digital imagery at 23 fore reef sites around a small coral atoll (Salomon) in 2020 and compared to data from a similar survey in 2006 and opportunistic surveys in intermediate years. Cluster analysis and principal component analysis show strong separation of community composition between exposed (modelled wave exposure > 1000 J m−3) and sheltered sites (< 1000 J m−3) in 2020. This difference is driven by relatively high cover of Porites sp., other massive corals, encrusting corals, soft corals, rubble and dead table corals at sheltered sites versus high cover of pavement and sponges at exposed sites. Total coral cover and rugosity were also higher at sheltered sites. Adding data from previous years shows benthic community shifts from distinct exposure-driven assemblages and high live coral cover in 2006 towards bare pavement, dead Acropora tables and rubble after the 2015/2016 bleaching event. The subsequent recovery trajectories at sheltered and exposed sites are surprisingly parallel and lead communities towards their respective pre-bleaching communities. These results demonstrate that in the absence of human stressors, community patterns on fore reefs are strongly controlled by wave exposure, even during and after widespread coral loss from bleaching events.