Sources and transformations of nitrate in Qixiangcuo Lake and its inflow rivers in the northern Tibetan Plateau.

Human activities and climate change input more reactive nitrogen into alpine lakes. Alpine saline lakes are usually located in endorheic watersheds at high-altitude areas, with no other drainage methods than evaporation, and are prone to accumulate nutrients. Meanwhile, alpine saline lakes are usually oligotrophic and sensitive to reactive nitrogen inputs, and even modest reactive nitrogen inputs may have significant effects on them, such as eutrophication. Nitrate is the main form of reactive nitrogen in lakes; therefore, clarifying the sources and transformations of nitrate in alpine saline lakes is important to prevent or mitigate eutrophication in alpine saline lakes. In this study, the sources and transformations of nitrate in Qixiangcuo Lake and its inflow rivers in the northern Tibetan Plateau were identified using dual nitrate isotopes and hydrochemistry. The results show that (1) the ranges of NO3- concentrations, δ15N - NO3-, and δ18O - NO3- values were 3.6 ~ 26.1 µg/L, - 10.5 to + 6.0‰, and - 10.4 to + 9.2‰ in Qixiangcuo Lake and 194.4 ~ 728.1 µg/L, + 5.8 ~ + 8.8‰, and - 1.9 to + 2.4‰ in its inflow rivers, respectively. The NO3- concentrations and δ15N - NO3- values were significantly lower in Qixiangcuo Lake than in its inflow rivers (P < 0.05). (2) The main sources of nitrate in both surface water and bottom water of Qixiangcuo Lake were ammonium in atmospheric deposition (mean probability estimate (MPE) 41.0% and 32.2%, respectively) and livestock manure (MPE 28.9% and 21.7%, respectively). The main sources of nitrate in the inflow rivers of Qixiangcuo Lake were domestic sewage (MPE 35.7%) and livestock manure (MPE 29.6%). (3) The main nitrogen transformation process in Qixiangcuo Lake was nitrification. The conservative mixing of multiple sources controlled the nitrate concentration and isotopic composition of Qixiangcuo Lake. Improvement in grazing area planning and the installation of sewage treatment facilities are effective measures to prevent eutrophication in Qixiangcuo Lake and its inflow rivers.

Growth performance and ecological services evaluation of razor clams based on dynamic energy budget model.

Bivalve shellfish aquaculture has been proposed to abate eutrophication and increase carbon sink in integrated multi-trophic aquaculture ecosystems. An individual growth model for razor clams Sinonovacula constricta in an integrated aquaculture pond of Portunus trituberculatus- Penaeus japonicus- S. constricta was constructed based on dynamic energy budget (DEB) theory after parameter measurement and model validation. Goodness-of-fit indices (R-squared, mean difference, and absolute and relative root mean square error) showed that the DEB model accurately reproduced razor clam growth. The growth performance evaluation of razor clams under different environmental conditions showed that warming climate and food shortages inhibited the razor clam growth. The quantification results of ecological service showed that individual razor clam have the potential for nutrient (nitrogen and phosphorus) removal and CO2 fixation, but exhibit a source of CO2 in individual month. The possible applications and contribution of this aquaculture model in China are also discussed, and the assessment results can provide important support for the low carbon bivalve integrated aquaculture.