Impact of terrestrial erosion on coral reef health at Bonaire: a plea for nature-inclusive "watershed-to-reef" based coastal management

Samenvatting

Coral reefs are rapidly degrading worldwide, due to a combination of global and local stressors. While global stressors, such as ocean warming, cannot be managed, management of local stressors can increase the resilience of coral reefs to these global stressors. One such local stressor is overgrazing-induced terrestrial erosion causing sediment and nutrient run-off to coastal waters, which is associated with coral mortality and changes in benthic community composition. In this study we assessed the link between watershed-specific erosion hazard and coral reef health, using the coral reefs on the west coast of Bonaire, Caribbean Netherlands, as a case study. We first identified watersheds, waterflow and run-off sites into the sea based on a digital elevation model. Next, for each watershed we determined the mean erosion hazard (a proxy for the degree of terrestrial run-off based on slope and ground vegetation cover). Subsequently, we used multiple regression models to investigate whether mean erosion hazard of the nearest upstream watershed, and distance to the nearest upstream run-off point explained variation in marine benthic community composition at two depth zones (~5 m and ~10 m). We found a negative relationship between mean erosion hazard and coral cover (a proxy for coral reef health) at 5 m depth, but no such effect at 10 m depth. In addition, we found a positive relationship between mean erosion hazard and sand cover (a proxy for sediment run-off) for both depth zones, and a small but significant quadratic effect of mean erosion hazard on algae cover (a proxy for nutrient run-off) at 5 m depth. Moreover, distance to the nearest upstream terrestrial run-off point was positively related with coral cover at 10 m depth, and negatively related with sand cover at 10 m depth. Our results provide direct evidence for a negative relationship between watershed-specific erosion hazard and coral reef health, and highlight the need for a nature-inclusive “watershed-to-reef” based coastal management approach that integrates terrestrial and marine conservation to preserve the island's valuable coral reef and related ecosystem services.