Data from: Organic matter and nutrient inputs from large wildlife influence ecosystem function in the Mara River, Africa

Animals can be important vectors for the movement of resources across ecosystem boundaries. Animals add resources to ecosystems primarily through egestion, excretion and carcasses, and the stoichiometry and bioavailability of these inputs likely interacts with characteristics of the recipient ecosystem to determine their effects on ecosystem function. We studied the influence of hippopotamus excretion/egestion and wildebeest carcasses, and their interactions with discharge, in the Mara River, Kenya. We measured nutrient dissolution and decomposition rates of wildlife inputs, the influence of inputs on nutrient concentrations and nutrient limitation in the river and the influence of inputs on biofilm growth and function in both experimental streams and along a gradient of inputs in the river. We found that hippopotamus excretion/egestion increase ammonium and coarse particulate organic matter in the river, and wildebeest carcasses increase ammonium, soluble reactive phosphorus, and total phosphorus. Concentrations of dissolved carbon and nutrients in the water column increased along a gradient of wildlife inputs and during low discharge, although concentrations of particulate carbon decreased during low discharge due to deposition on the river bottom. Autotrophs were nitrogen-limited, and heterotrophs were carbon-limited and nitrogen- and phosphorus- co-limited, upstream of animal inputs, but there was no nutrient limitation downstream of inputs. In experimental streams, hippo and wildebeest inputs together increased biofilm GPP and R. These results differed in the river, where low concentrations of hippo inputs increased gross primary production (GPP) and respiration (R) of biofilms, but high concentrations of hippo inputs in conjunction with wildebeest inputs decreased GPP. Our research shows that inputs from large wildlife alleviate nutrient limitation and stimulate ecosystem metabolism in the Mara River, and that the extent to which these inputs subsidize the ecosystem is mediated by the quantity and quality of inputs and discharge of the river ecosystem. Thus, animal inputs provide an important ecological subsidy to this river, and animal inputs were likely important in many other rivers prior to the widespread extirpation of large wildlife.动物可作为资源跨生态系统边界（ecosystem boundaries）流动的重要载体。动物主要通过排遗（egestion）、排泄（excretion）与动物尸体（carcasses）向生态系统输入资源，而这些输入物的化学计量学（stoichiometry）特征与生物可利用性（bioavailability），大概率会与受纳生态系统（recipient ecosystem）的属性相互作用，进而决定其对生态系统功能（ecosystem function）的影响。本研究以肯尼亚马拉河（Mara River）为研究对象，探究河马排泄/排遗（hippopotamus excretion/egestion）与角马尸体（wildebeest carcasses）的生态影响，及其与径流量（discharge）的交互作用。本研究测定了野生动物输入物的养分溶解速率与分解速率，分析了输入物对河流水体养分浓度与养分限制的影响，并通过实验溪流以及沿河流输入物梯度的原位调查，探究了输入物对生物膜（biofilm）生长与功能的作用。研究结果显示：河马排泄/排遗会提升河水中的铵态氮（ammonium）与粗颗粒有机物（coarse particulate organic matter）含量，而角马尸体则可提升铵态氮、可溶性活性磷（soluble reactive phosphorus）与总磷（total phosphorus）的浓度。随着野生动物输入物梯度升高以及低流量条件下，水柱中的溶解碳与养分浓度均有所上升；但由于河床沉积作用，低流量下颗粒碳浓度会出现下降。在动物输入物上游区域，自养生物（autotrophs）受氮限制，异养生物（heterotrophs）则受碳限制以及氮-磷共同限制；而在输入物下游区域则未观测到养分限制现象。在实验溪流中，河马与角马输入物共同作用可提升生物膜的总初级生产力（gross primary production, GPP）与呼吸速率（respiration, R）。但在天然河流中结果有所不同：低浓度河马输入物可提升生物膜的总初级生产力与呼吸速率，而高浓度河马输入物与角马输入物共同作用则会降低总初级生产力。本研究表明，大型野生动物的输入物可缓解马拉河的养分限制并刺激生态系统代谢（ecosystem metabolism），而这类输入物对生态系统的补给程度，取决于输入物的数量、质量以及河流生态系统的径流量。综上，动物输入物为该河流提供了重要的生态补给（ecological subsidy），且在大型野生动物广泛灭绝之前，这类动物输入物可能在全球诸多河流中均发挥了关键作用。