Carbonate framework and sediment production across island-fringing coral reef habitats and a natural nutrient gradient

Coral reef framework production and sediment generation are key controls on vertical reef accretion and reef island maintenance. However, quantitative data on these geo-ecological functions remain scarce, particularly within shallow habitats that mediate incoming wave energy and sediment supply to shorelines. This study quantified carbonate framework and sediment budgets across shallow lagoon reef (1–2 m), shallow forereef (2–3 m) and deep forereef habitats (8–9 m) surrounding six islands within three atolls (2 islands/atoll) in the remote Chagos Archipelago, Central Indian Ocean. One of the islands in each atoll supports dense seabird colonies that deliver natural nutrient subsidies to nearshore habitats via guano run-off, enabling assessment of nutrient impacts on reef carbonate dynamics. Benthic surveys (ReefBudget and SedBudget) were combined with sediment composition analyses from shallow reefs and adjacent beaches. Variation in coral cover and composition among atolls, linked to differences in atoll size and post-2016 bleaching recovery, resulted in higher framework production at Salomon Atoll. After accounting for atoll-level variability, framework production exhibited habitat-specific patterns, with highest mean rates at lagoon reefs (5.5 kg m^⁻2 yr^⁻1), followed by deep and shallow forereefs (3.5 and 3.1 kg m^⁻2 yr^⁻1, respectively). Reefs adjacent to seabird islands showed enhanced framework production at lagoon and shallow forereef sites (by + 3.7 and + 3.1 kg m^⁻2 yr^⁻1, respectively), driven by increased coral calcification and reduced parrotfish grazing. Sediment production was dominated by parrotfish erosion, with highest rates at deep forereefs (3.0 compared to 1.3 kg m^⁻2 yr^⁻1 at shallow reefs), little variation among atolls, and a limited response to nutrients. Sediment composition was dominated by reef framework constituents (coral and crustose coralline algae), with secondary contributions from Halimeda, molluscs and benthic foraminifera. Beach sediments resembled shallow reef sediments, both showing increased Halimeda near nutrient-enriched reefs due to past calcareous algal blooms. Our findings demonstrate how different spatial scales and seabird-derived nutrients modulate carbonate production in remote reef island systems.