Importance of accurately quantifying internal loading in developing phosphorus reduction strategies for a chain of shallow lakes

Robertson DM, Diebel MW. 2020. Importance of accurately quantifying internal loading in developing phosphorus reduction strategies for a chain of shallow lakes. Lake Reserv Manage. 36:391–411. The Winnebago Pool is a chain of 4 shallow lakes in Wisconsin. Because of high external phosphorus (P) inputs to the lakes, the lakes became highly eutrophic, with much P contained in their sediments. In developing a total maximum daily load (TMDL) for these lakes, it is important to determine how their phosphorus concentrations should respond to changes in external P loading. In many TMDLs, internal P loading is assumed to be negligible or it is estimated based on sediment release rates and dissolved oxygen conditions in the lake, and each lake is considered independently. To evaluate these assumptions, internal P loading and external P loading were quantified by developing detailed P budgets for the Winnebago Pool chain of lakes. This information was then inputted into 2 eutrophication models (BATHTUB and Jensen models), which were used to simulate the steady-state and transient effects of various P reduction strategies. The importance of internal P loading varied among lakes, from being a minor source to representing almost 60% of the summer P input. Model results indicate that each lake responds to external P reductions, but internal loading can delay the lake responses, especially in the most downstream lake, Lake Winnebago, where internal P loading was most important to its summer P budget. Accurately quantifying net internal P loading and using this information in lake models are important in evaluating how large shallow lakes should respond to P reduction strategies, setting realistic expectations from watershed P reductions, and guiding TMDL efforts.

Robertson DM与Diebel MW于2020年发表了题为《浅湖链磷削减策略制定中精准量化内源负荷的重要性》的学术论文，刊载于《Lake Reserv Manage》（湖泊与水库管理）第36卷，页码391–411。温尼贝戈湖群（Winnebago Pool）是坐落于美国威斯康星州的一处由4个浅水湖泊组成的湖链。由于该湖群接收了大量外源磷（external phosphorus, P）输入，其水体已呈现重度富营养化状态，沉积物中蓄积了大量磷元素。在为该湖群制定总最大日负荷（total maximum daily load, TMDL）方案时，明确其磷浓度对外源磷负荷变化的响应规律，是一项核心工作。在多数已有的TMDL方案中，内源磷负荷（internal phosphorus loading）通常被假定为可忽略不计，或是基于沉积物磷释放速率与湖体溶解氧条件进行估算，且各湖泊均被独立开展分析。为验证上述假设的合理性，研究团队通过构建温尼贝戈湖群的精细化磷收支模型，量化了其内、外源磷负荷。随后，研究将量化得到的磷负荷数据输入至两款富营养化模型：BATHTUB模型与Jensen模型，以模拟不同磷削减策略的稳态与瞬态效应。内源磷负荷的重要性因湖泊而异，其对夏季磷输入总量的贡献占比从微乎其微直至接近60%不等。模型结果显示，所有湖泊均会对外源磷削减产生响应，但内源负荷会延缓湖泊的响应进程，其中尤以最下游的温尼贝戈湖（Lake Winnebago）最为显著——该湖的夏季磷收支中，内源磷负荷的贡献占比最高。精准量化净内源磷负荷（net internal phosphorus loading），并将该数据应用于湖体模型，对于评估大型浅水湖泊对磷削减策略的响应、合理设定流域磷削减的预期效果，以及指导TMDL相关工作均具有关键意义。