delta N-15 and nutrient stoichiometry of water, aquatic organisms and environmental implications in Taihu lake, China

Nitrogen pollution has become a worldwide problem and the source identification is important for the development of pertinent control measures. In this study, isotope end members (rain, nitrogen fertilizer, untreated treated sewage), and samples (river water discharging to Taihu lake, lake water, aquatic organisms of different trophic levels) were taken during 2010-2015 to examine their delta N-15 values and nutrient stoichiometry. Results indicated that phytoplankton (primary producers), which directly take up and incorporate N from the lake water, had a similar delta N-15 value (14.1 parts per thousand +/- 3.2) to the end member of treated sewage (14.0 parts per thousand +/- 7.5), and the most frequently observed delta N-15 value in the lake water was 8-12 parts per thousand, both indicating the dominant impact of the sewage discharge. Relationship analysis between N isotope value of nitrate and nitrate concentration indicated that different N cycling existed between the algae dominated northwest lake (NW) and the macrophyte-dominated southeast lake (SE), which is a result of both impacts of river inputs and denitrification. Our nutrient stoichiometry analysis showed that the lake water had a significantly higher N:P ratio than that of algae (p < 0.05), suggesting that N is available in excess relative to the amount demanded by the algae. The long-term trend of the socio-economic development in the watershed further confirmed that the rapid population increase and urbanization have resulted in a great change in the N loading and source proportion. We suggest that although P control is necessary in terms of eutrophication control, N pollution control is urgent for the water quality and ecological recovery for Taihu lake. (C) 2018 Elsevier Ltd. All rights reserved.