Phytoplankton play a key role in supporting aquatic food webs. However, the effects of ongoing large-scale changes in the concentrations and stoichiometry of important biological compounds [dissolved inorganic N (DIN), total phosphorus (TP), dissolved organic carbon (DOC) and DIN:TP] on the development and nutritional quality of phytoplankton for higher trophic levels are unclear. We conducted lake studies and in situ bioassay experiments in two Swedish mountain regions [Abisko (north) and Jämtland (south)] with different N deposition and where lakes in each region were distributed along a similar gradient in lake DOC (2–7 mg L −1 ) to assess whether differences in nutrients, DOC and DIN:TP induced differences in phytoplankton quantity [chlorophyll a (Chl- a ) and seston carbon (C)] and quality [seston C:N:P stoichiometry and fatty acid (FA) composition]. Using long-term monitoring data from lakes in these two mountain regions, we found declining long-term trends in N deposition and lake DIN and total TP concentrations, but not in lake DIN:TP. Lakes in Abisko received lower N deposition and had lower DIN:TP than those in Jämtland. Phytoplankton was N- to NP-limited in Abisko lakes but NP dual-limited in Jämtland lakes. The N fertilization effects induced by higher DIN:TP were weak on phytoplankton quantity but strong on phytoplankton quality. The phytoplankton had lower eicosapentaenoic acid (EPA) content and higher P content (lower seston C:P) in Abisko compared to in Jämtland. In addition, the quality of the DOC (as indicated by its aromaticity and SUVA) influenced not only the light conditions and the seston C:P ratios, but also the FA composition. We found higher bacteria FA concentrations in seston in Abisko than in Jämtland, despite lower amounts of FA of terrestrial origin in Abisko. Our findings suggest that declining N deposition and enhanced colored terrestrial C loadings leads to lower nutritional quality of basal resources for higher consumers in mountain lakes.

中文翻译：

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山区湖泊氮沉降减少对浮游植物营养质量和营养限制机制的影响

浮游植物在支持水生食物网方面发挥着关键作用。然而，重要生物化合物 [溶解无机氮 (DIN)、总磷 (TP)、溶解有机碳 (DOC) 和 DIN:TP] 的浓度和化学计量的持续大规模变化对发育和营养质量的影响浮游植物对较高营养水平的影响尚不清楚。我们在两个具有不同 N 沉积的瑞典山区 [阿比斯库（北部）和 Jämtland（南部）] 进行了湖泊研究和原位生物测定实验，其中每个地区的湖泊沿湖泊 DOC 的相似梯度分布（2-7 mg L -1 ) 以评估营养物质、DOC 和 DIN:TP 的差异是否会导致浮游植物数量 [叶绿素 a (Chl- a) 和 seston 碳 (C)] 和质量 [seston C:N: P化学计量和脂肪酸（FA）组成]。使用这两个山区湖泊的长期监测数据，我们发现 N 沉积和湖泊 DIN 和总 TP 浓度的长期趋势下降，但湖泊 DIN:TP 没有下降。阿比斯库的湖泊与 Jämtland 的湖泊相比，N 沉积较少，DIN:TP 较低。浮游植物在阿比斯库湖中受 N 到 NP 限制，但在 Jämtland 湖中 NP 受双重限制。较高DIN:TP诱导的施氮效应对浮游植物数量较弱，但对浮游植物质量较强。与 Jämtland 相比，阿比斯库的浮游植物具有较低的二十碳五烯酸 (EPA) 含量和较高的 P 含量（较低的 seston C:P）。此外，DOC 的质量（如其芳香性和 SUVA 所示）不仅影响光照条件和 seston C:P 比，还有 FA 的组成。我们发现阿比斯库塞斯顿的细菌 FA 浓度高于 Jämtland，尽管阿比斯库陆地来源的 FA 含量较低。我们的研究结果表明，氮沉积的减少和有色陆地碳含量的增加导致山地湖泊中较高消费者的基础资源的营养质量降低。