Deininger A, Faithfull C L, Bergström A-K
Department of Ecology and Environmental Science, Umeå University, Umeå, Sweden.
Oecologia. 2017 Aug;184(4):901-916. doi: 10.1007/s00442-017-3921-5. Epub 2017 Jul 29.
Global environmental change has altered the nitrogen (N) cycle and enhanced terrestrial dissolved organic carbon (DOC) loadings to northern boreal lakes. However, it is still unclear how enhanced N availability affects pelagic food web efficiency (FWE) and crustacean zooplankton growth in N limited boreal lakes. Here, we performed in situ mesocosm experiments in six unproductive boreal Swedish lakes, paired across a DOC gradient, with one lake in each pair fertilized with N (2011: reference year; 2012, 2013: impact years). We assessed how zooplankton growth and FWE were affected by changes in pelagic energy mobilization (PEM), food chain length (phytoplankton versus bacterial production based food chain, i.e. PP:BP), and food quality (seston stoichiometry) in response to N fertilization. Although PP, PEM and PP:BP increased in low and medium DOC lakes after N fertilization, consumer growth and FWE were reduced, especially at low DOC-potentially due to reduced phytoplankton food quality [increased C: phosphorus (P); N:P]. At high DOC, N fertilization caused modest increases in PP and PEM, with marginal changes in PP:BP and phytoplankton food quality, which, combined, led to a slight increase in zooplankton growth and FWE. Consequently, at low DOC (<12 mg L), increased N availability lowers FWE due to mismatches in food quality demand and supply, whereas at high DOC this mismatch does not occur, and zooplankton production and FWE may increase. We conclude that the lake DOC level is critical for predicting the effects of enhanced inorganic N availability on pelagic productivity in boreal lakes.
全球环境变化改变了氮（N）循环，并增加了北方寒带湖泊中陆地溶解有机碳（DOC）的负荷。然而，目前尚不清楚在氮有限的北方寒带湖泊中，氮有效性的提高如何影响浮游食物网效率（FWE）和甲壳类浮游动物的生长。在此，我们在瑞典六个生产力低下的北方寒带湖泊中进行了原位中宇宙实验，这些湖泊按DOC梯度配对，每对中的一个湖泊在2011年（参考年）施氮，2012年和2013年（影响年）重复施肥。我们评估了浮游动物生长和FWE如何受到浮游能量动员（PEM）、食物链长度（基于浮游植物与细菌生产的食物链，即PP:BP）和食物质量（悬浮物化学计量）变化的影响，以响应氮肥施用。尽管在低和中度DOC湖泊中施氮后PP、PEM和PP:BP增加，但消费者生长和FWE降低，尤其是在低DOC时，这可能是由于浮游植物食物质量下降[碳:磷（P）增加；氮:磷增加]。在高DOC时，施氮导致PP和PEM适度增加，PP:BP和浮游植物食物质量变化不大，综合起来导致浮游动物生长和FWE略有增加。因此，在低DOC（<12 mg/L）时，氮有效性增加会由于食物质量供需不匹配而降低FWE，而在高DOC时不会出现这种不匹配，浮游动物产量和FWE可能会增加。我们得出结论，湖泊DOC水平对于预测北方寒带湖泊中无机氮有效性增加对浮游生产力的影响至关重要。
