The annual migration of ∼1.2 million wildebeest (Connochaetes taurinus) through the Serengeti Mara Ecosystem is the largest remaining overland migration in the world. One of the most iconic portions of their migration is crossing of the Mara River, during which thousands drown annually. These mass drownings have been noted, but their frequency, size, and impact on aquatic ecosystems have not been quantified. Here, we estimate the frequency and size of mass drownings in the Mara River and model the fate of carcass nutrients through the river ecosystem. Mass drownings (>100 individuals) occurred in at least 13 of the past 15 y; on average, 6,250 carcasses and 1,100 tons of biomass enter the river each year. Half of a wildebeest carcass dry mass is bone, which takes 7 y to decompose, thus acting as a longterm source of nutrients to the Mara River. Carcass soft tissue decomposes in 2-10 wk, and these nutrients are mineralized by consumers, assimilated by biofilms, transported downstream, or moved back into the terrestrial ecosystem by scavengers. These inputs comprise 34-50% of the assimilated diet of fish when carcasses are present and 7-24% via biofilm on bones after soft tissue decomposition. Our results show a terrestrial animal migration can have large impacts on a river ecosystem, which may influence nutrient cycling and river food webs at decadal time scales. Similar mass drownings may have played an important role in rivers throughout the world when large migratory herds were more common features of the landscape.animal migration | mass drowning | nutrient cycling | Serengeti wildebeest | stable isotope

1. Hippopotami can play a significant role as ecosystem engineers and may play an important role as carbon and nutrient vectors from savanna grasslands to aquatic systems. 2. We coupled the results of a feeding study of captive hippopotami, faeces leaching/mineralisation experiments, hippopotamus consumption estimates and the stoichiometry of savanna grasses to calculate excretion and egestion rates of hippopotami. We then used time budgets and population estimates to calculate nutrient loading by hippopotami in the Mara River, Kenya. 3. In captivity, hippopotami consumed 11.0 g dry matter (DM) kg hippopotamus À1 day À1 (110.6 C; 6.8 N; 1.0 P) and egested 5.3 g DM kg À1 day À1 . They excreted or egested 2.72 g C, 0.28 g N and 0.04 g P kg hippopotamus À1 day À1 , and urine comprised 12% of C, 70% of N and 33% of P of total excretion and egestion. 4. By integrating data from previously published work on hippopotamus digestion with the data we collected in the field, we estimated an average hippopotamus in the Mara River would excrete or egest 1.93-3.58 g DM, 0.78-1.47 g C, 0.13-0.19 g N and 0.01-0.02 g P kg hippopotamus À1 day À1 , and that half of this excretion/egestion would enter the river. 5. The hippopotamus population increased by 1500% inside the Maasai Mara National Reserve, Kenya, between 1959 and. We estimate that hippopotami egest 36 200 kg faeces day À1 into the Mara River (wet mass). Daily loading into the river by excretion and egestion equals 8563 kg DM, 3499 kg C, 492 kg N and 48 kg P, which is equivalent to 670% of CPOM, 15% of DOC, 27% of TN and 29% of TP of loading from the upstream catchment. 6. This research provides the first estimates for hippopotamus inputs to rivers that include both excretion and egestion and provides evidence that hippopotami are important resource vectors in sub-Saharan African rivers, even when compared to other sources of carbon and nutrients.

Organic matter and nutrient loading into aquatic ecosystems affects ecosystem structure and function and can result in eutrophication and hypoxia. Hypoxia is often attributed to anthropogenic pollution and is not common in unpolluted 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 3 years that caused dissolved oxygen decreases, including 13 events resulting in hypoxia, and 9 fish kills over 5 years. Evidence from experiments and modeling demonstrates a strong mechanistic link between the flushing of hippo pools and decreased dissolved oxygen in the river. This phenomenon may have been more widespread throughout Africa before hippopotamus populations were severely reduced. Frequent hypoxia may be a natural part of tropical river ecosystem function, particularly in rivers impacted by large wildlife.

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 interact 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 increases 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 colimited upstream of animal inputs but there was no nutrient limitation downstream of inputs. In experimental streams, hippo and wildebeest inputs together increased biofilm gross primary production (GPP) and respiration (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.

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The Mara River Basin in East Africa is a trans-boundary basin of international significance experiencing excessive levels of sediment loads. Sediment levels in this river are extremely high (turbidities as high as 6,000 NTU) and appear to be increasing over time. Large wildlife populations, unregulated livestock grazing, and agricultural land conversion are all potential factors increasing sediment loads in the semi-arid portion of the basin. The basin is well-known for its annual wildebeest (Connochaetes taurinus) migration of approximately 1.3 million individuals, but it also has a growing population of hippopotami (Hippopotamus amphibius), which reside within the river and may contribute to the flux of suspended sediments. We used in situ pressure transducers and turbidity sensors to quantify the sediment flux at two sites for the Mara River and investigate the origin of riverine suspended sediment. We found that the combined Middle Mara—Talek catchment, a relatively flat but semi-arid region with large populations of wildlife and domestic cattle, is responsible for 2/3 of the sediment flux. The sediment yield from the combined Middle Mara–Talek catchment is approximately the same as the headwaters, despite receiving less rainfall. There was high monthly variability in suspended sediment fluxes. Although hippopotamus pools are not a major source of suspended sediments under baseflow, they do contribute to short-term variability in suspended sediments. This research identified sources of suspended sediments in the Mara River and important regions of the catchment to target for conservation, and suggests hippopotami may influence riverine sediment dynamics.

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