Linking macroalgal δ¹⁵N-values to nitrogen sources and effects of nutrient stress on coral condition in an upwelling region

Land-based sewage pollution is a major threat to the health of near-shore coral reefs. Macroalgal δ¹⁵N-values are useful indicators for tracking nitrogen sources and sewage input; however, the direct link between δ¹⁵N-values of macroalgae and the coral condition, including live coral cover, species richness, and juvenile coral density, is unclear. Macroalgal δ¹⁵N-values of two species, Lobophora variegata and Tricleocarpa oblongata, were measured in Wenchang (northeast Hainan Island, South China Sea) in both the dry and wet seasons, to assess the influence of nitrogen sources and nutrient stress resulting from sewage input or upwelling on the condition of corals. Macroalgal δ¹⁵N-values showed that shallow stations between the reef and the coast were severely affected by sewage input (e.g., river discharges and aquaculture ponds), whereas deep stations on the outside of the reef were influenced by upwelling-based nutrients. Correlation and redundancy analysis demonstrated that δ¹⁵N- values of L. variegata had a stronger relation with coral condition than any other environmental parameter (e.g., water quality index, turbidity, and dissolved nutrients), with significant negative correlations in both the dry and wet seasons. Thus, macroalgal δ¹⁵N-values can be used to determine where nutrient stress is negatively affecting corals, highlighting the links of human activities and coastal upwelling with marine ecosystems.