Seabird nutrients increase coral calcification rates and boost reef carbonate production

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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.