Effects of nutrient enrichment and skewed N:P ratios on physiology of scleractinian corals from Weizhou Island in the northern South China Sea

Abstract

The availability of nitrogen (N) and phosphorus (P) is crucial for maintaining coral-dinoflagellate symbiosis, whereas excess nutrients and skewed N:P ratios are often associated with coral reef decline. It is thus essential to understand the general patterns of species-specific as well as dose-dependent responses of corals to elevated nutrient concentrations and skewed N:P ratios. Here, we found that the impacts of nutrient enrichment on the corals Acropora millepora and Platygyra crosslandi from Weizhou Island, South China Sea, were highly dependent on nutrient dose, N:P ratios, and coral species. Moderate nutrient enrichment (N: 19-140 µmol l-1 and P: 0.5-1.5 µmol l-1) did not lead to bleaching of either coral species, but their metabolic processes (calcification, nutrient uptake, and dinitrogen fixation) were affected. More depleted δ13C and lower dinitrogen fixation rates of A. millepora than of P. crosslandi indicated that A. millepora was more vulnerable to the disturbance of nutrient enrichment. However, N:P ratios (±SD) as high as 106 ± 10 decreased the photosynthetic efficiency and nutrient uptake rates of P. crosslandi, indicating that this species may be vulnerable to high N:P ratios, although it was one of the dominant coral assemblages at Weizhou Island. The results provide an important basis for understanding coral reef degradation triggered by nutrients and a suggestion that coastal management should focus not only on the concentrations but also on the ratios of nutrients.