Significantly depleted 15N in suspended particulate organic matter indicating a strong influence of sewage loading in Daya Bay, China.

The influence of anthropogenic nutrient loading on the stable isotopic signatures (δ13C and δ15N) in the suspended particulate organic matter (SPOM) is still not fully understood. Water quality and the values of δ13C and δ15N in the SPOM were investigated in the surface water of Daya Bay during the spring of 2016 and 2017. The results indicated that the Dan'ao River is the main point source of nutrient pollution in Daya Bay. The δ15NPOM was very low in the Dan'ao River. The distribution pattern for this parameter in Daya Bay was determined chiefly by Dan'ao River discharge. Variations in δ15NPOM were ascribed mostly to the input of 15N-depleted DIN assimilated by the estuarine phytoplankton in Daya Bay. Extremely high NH4+ level in the river discharge should be responsible for the low δ15NPOM in the river water. The distribution of δ13CPOM in Daya Bay was regulated mainly by the input of 13C-depleted riverine SPOM. In the present study, the influence of phytoplankton growth on the δ13CPOM was not significant. Moreover, episodic rain events significantly influenced the temporal and spatial variations in water quality and isotopic signature in Daya Bay. The relatively depleted SPOM 15N in 2016 may have been correlated with the strong El Niño events of 2015-2016. Increases in the frequency and volume of rainfall associated with El Niño may have enhanced nutrient loading and the risk of algal red tide in the Daya Bay. In general, significant 15N depletion in SPOM could be the characteristic of hypereutrophic riverine waters. This study suggested that δ15NPOM may be an effective indicator of the strength of riverine nutrient loading in Daya Bay.