Impacts of nutrient enrichment on coral reefs: new perspectives and implications for coastal management and reef survival

Anthropogenic nutrient enrichment is often associated with coral reef decline. Consequently, there is a large consent that increased nutrient influxes in reef waters have negative long-term consequences for corals. However, the mechanisms by which dissolved inorganic nutrients can disturb corals and their symbiotic algae are subject to controversial debate. Herein, we discuss recent studies that demonstrate how nutrient enrichment affects the heat and light stress tolerance of corals and their bleaching susceptibility. We integrate direct and indirect effects of nutrient enrichment on corals in a model that explains why healthy coral reefs can exist over a rather broad range of natural nutrient environments at the lower end of the concentration scale and that anthropogenic nutrient enrichment can disturb the finely balanced processes via multiple pathways. We conceptualise that corals can suffer from secondary negative nutrient effects due to the alteration of their natural nutrient environment by increased phytoplankton loads. In this context, we suggest that phytoplankton represents a likely vector that can translate nutrients effects, induced for instance by coastal runoff, into nutrient stress on coral reefs in considerable distance to the site of primary nutrient enrichment. The presented synthesis of the literature suggests that the effects of nutrient enrichment and eutrophication beyond certain thresholds are negative for the physiological performance of the coral individual and for ecosystem functioning. Hence, the immediate implementation of knowledge-based nutrient management strategies is crucial for coral reef survival.