Data from: Watershed versus within-lake drivers of nitrogen: phosphorus dynamics in shallow lakes

Research on lake eutrophication often identifies variables affecting amounts of phosphorus (P) and nitrogen (N) in lakes, but understanding factors influencing N:P ratios is important given its influence on species composition and toxin production by cyanobacteria. We sampled 80 shallow lakes in Minnesota (USA) for three years to assess effects of watershed size, proportion of watershed as both row crop and natural area, fish biomass, and lake alternative state (turbid versus clear) on total N: total P (TN:TP), ammonium, total dissolved phosphorus (TDP), and seston stoichiometry. We also examined N:P stoichiometry in 20 additional lakes that shifted states during the study. Lastly, we assessed importance of denitrification by measuring denitrification rates in sediment cores from a subset of 34 lakes, and by measuring seston δ15N in four additional experimental lakes before and after they were experimentally manipulated from turbid to clear states. Results showed alternative state had the largest influence on overall N:P stoichiometry in these systems, as it had the strongest relationship with TN:TP, seston C:N:P, ammonium, and TDP. Turbid lakes had higher N at given levels of P than clear lakes, with TN and ammonium two-fold and 1.4-fold higher in turbid lakes, respectively. In lakes that shifted states, TN was three-fold higher in turbid lakes, while TP was only two-fold higher, supporting the notion N is more responsive to state shifts than is P. Seston δ15N increased after lakes shifted to clear states, suggesting higher denitrification rates may be important for reducing N levels in clear states, and potential denitrification rates in sediment cores were among the highest recorded in the literature. Overall, our results indicate lake state was a primary driver of N:P dynamics in shallow lakes, and lakes in clear states had much lower N at a given level of P relative to turbid lakes, likely due to higher denitrification rates. Shallow lakes are often managed for the clear-water state due to increased value as wildlife habitat. However, our results indicate lake state also influences N biogeochemistry, such that managing shallow lakes for the clear-water state may also mitigate excess N levels at a landscape scale.

湖泊富营养化研究通常会识别影响湖泊内磷（P）与氮（N）含量的变量，但鉴于氮磷比（N:P）会对物种组成及蓝藻（cyanobacteria）毒素产生造成影响，解析其影响因素具有重要意义。本研究针对美国明尼苏达州的80个浅水湖泊开展了为期三年的采样工作，以评估流域面积、流域内大田作物与自然区域占比、鱼类生物量以及湖泊交替稳态（浊水态vs清水态）对总氮总磷比（total N: total P, TN:TP）、铵态氮、总溶解磷（total dissolved phosphorus, TDP）及浮游生物化学计量特征的影响。此外，我们还对研究期间发生稳态转换的另外20个湖泊的氮磷化学计量特征进行了分析。最后，我们通过对34个湖泊子集的沉积物岩芯（sediment cores）进行反硝化速率测定，以及对4个经人工调控从浊水态转为清水态的实验湖泊在调控前后的浮游生物δ15N（seston δ15N）进行检测，评估了反硝化作用（denitrification）的重要性。研究结果显示，湖泊交替稳态对系统内整体氮磷化学计量特征的影响最为显著，其与总氮总磷比（TN:TP）、浮游生物碳氮磷比（C:N:P）、铵态氮及总溶解磷（TDP）均存在最强的关联。在磷含量处于相同水平时，浊水态湖泊的氮含量高于清水态湖泊：总氮与铵态氮含量分别较清水态湖泊高出2倍与1.4倍。在发生稳态转换的湖泊中，浊水态的总氮含量是清水态的3倍，而总磷含量仅为清水态的2倍，这印证了氮相比磷对稳态转换的响应更为敏感这一观点。湖泊转为清水态后，浮游生物δ15N升高，这表明更高的反硝化速率可能是清水态湖泊氮水平降低的重要原因，且沉积物岩芯测得的潜在反硝化速率处于已发表文献中的较高水平。总体而言，本研究结果表明，湖泊状态是浅水湖泊氮磷动态变化的主要驱动因素；相较于浊水态湖泊，清水态湖泊在磷含量固定时的氮水平显著更低，这大概率源于更高的反硝化速率。浅水湖泊通常因作为野生动物栖息地的价值提升而被管理为清水态。然而，本研究结果显示湖泊状态同时会影响氮生物地球化学循环，因此将浅水湖泊调控为清水态，还可在景观尺度上缓解氮过剩问题。