Award: OCE-1547952

Coral reefs are currently imperiled from a variety of human-induced threats from climate change, coral diseases, overexploitation of important fish species, and enrichment with excessive amounts of nutrients. These threats can result in the decline in corals and fishes and the rise in seaweeds, turning coral reefs into seaweed reefs. Our research on the coral reefs of Moorea, French Polynesia examined how fishes provide important natural nutrients to coral reefs and how disrupting these natural nutrient sources by removing fishes or by increasing nutrient pollution from humans can negatively impact coral reefs. Our research suggests that fishes may be one of the most important sources of nutrients on reefs via their daily excretion and egestion (feces). We have shown that maintaining the biodiversity of fishes on coral reefs is important because different species of fishes recycle nutrients in different ways. Importantly, the nutrients that are provided by fishes can help corals grow faster under normal conditions as well as withstand stressful events, like marine heatwaves. Corals that have fishes sheltering within their branches are more likely to survive and thrive during marine heatwaves than are corals without fishes. However, we have also shown that when natural nutrient cycles are disrupted by nutrients from human-derived sources such as runoff from agriculture or sewage discharge that the health of coral reefs can suffer significantly. During the 2016 marine heatwave that hit the coral reefs of Moorea, we showed that on reefs with higher levels of nutrient pollution that key habitat-forming corals were more likely to bleach and bleach severely than on reefs with low levels of nutrient pollution. These data suggest that nutrient pollution actually makes coral more sensitive to heatwaves, causing them to expel their symbiotic algae and possibly making them more likely to die from these heatwaves. In a complementary experiment, we exposed corals in the field to nutrients that mimicked human pollution and compared them to corals exposed to nutrients that mimicked those from fishes (while also keeping corals exposed to no extra nutrients as controls. This experiment showed that the nutrients that mimicked human pollution were much more harmful to corals and made them bleach and die more frequently when subjected to a marine heatwave. A consequence of disrupting the natural nutrient cycles of coral reefs may be the death of corals and the increase in seaweeds (also often called harmful algae). We showed that corals in areas with more nutrient pollution are more likely to bleach and die during marine heatwaves. We also showed that areas of the lagoon around Moorea that have had historically higher levels of nutrient pollution also have less coral and more seaweeds. Importantly, these patterns were not related to the abundance of herbivorous fishes, which eat the seaweeds, suggesting that the abundance of nutrient pollution is likely a trigger that changes reefs from being dominated by corals to being dominated by seaweeds. The shift in these reefs from being dominated by corals to being dominated by seaweeds also impacts the abundance and species diversity of fishes on these reefs. We showed that reefs that are dominated by seaweeds have a different community of fishes on them and that these fishes recycle nutrients in different quantities and ratios as compared to fishes on reefs that are dominated by corals. Thus, our work suggests that human nutrient pollution reduces the abundance of corals on reefs, increases the abundance of seaweeds, and disrupts the natural nutrient cycles that fishes provide to these ecosystems. Last Modified: 11/14/2022 Submitted by: Deron E Burkepile