Organic matter loading by hippopotami causes subsidy overload resulting in downstream hypoxia and fish kills

Organic matter and nutrient loading into aquatic ecosystems affects ecosystem structure and function and can result in eutrophication and hypoxia. Hypoxia is usually attributed to anthropogenic pollution and is rarely documented in unpolluted systems, particularly in rivers. Here we show that organic matter loading from hippopotami causes the repeated occurrence of hypoxia in the Mara River, East Africa. We documented 49 high flow events over three years that caused dissolved oxygen decreases, including 13 events resulting in hypoxia, and 9 fish kills over five years. Evidence from experiments and modeling demonstrates a strong mechanistic link between the flushing of hippo pools and decreased dissolved oxygen. This phenomenon may have been more widespread throughout Africa before hippo populations were severely reduced. Frequent hypoxia may be a natural part of tropical river ecosystem function, which directly challenges our notions of the reference state of rivers in the absence of hu...

有机质与营养物质向水生生态系统（aquatic ecosystems）的输入会对其结构与功能造成影响，并可能诱发富营养化（eutrophication）与低氧（hypoxia）现象。低氧现象通常被归因于人为污染，在未受污染的水体中鲜有记录，河流生态系统尤为如此。本研究证实，东非马拉河（Mara River）中河马输入的有机质负荷会反复引发低氧事件。我们在三年间记录了49次导致溶解氧（dissolved oxygen）浓度下降的强径流事件，其中13次引发低氧；同时在五年间观测到9起鱼类死亡事件。实验与模型分析结果表明，河马栖息池的水体冲刷与溶解氧浓度降低之间存在显著的机制性关联。在河马种群遭到大规模锐减之前，这一现象可能在非洲大陆分布更为广泛。频繁发生的低氧事件或许是热带河流生态系统自然功能的组成部分，这直接挑战了我们关于无人类干扰下河流参考状态的认知……