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

Dutton, Christopher L.; Subalusky, Amanda L.; Hamilton, Stephen K.; Rosi‐Marshall, Emma J.; Post, David M.

doi:10.1038/s41467-018-04391-6

Cited by 63 publications

(58 citation statements)

References 66 publications

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“…The discrepancies in estimated inputs and measured flux suggest we could be overestimating hippo feces inputs, but it is more likely that we are underestimating CPOM flux in the river, as a large amount of hippo feces is stored in the river at low and medium discharge and only flushed through the system during floods, when hippo feces transport can exceed base flow concentrations by several orders of magnitude (Dutton et al. ). Our estimate of N inputs from hippos was 99% of daily net TN flux at the Hippo site and 51% at the Hippo + WB site when carcasses were absent.…”

Section: Discussionmentioning

confidence: 99%

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

Subalusky

Dutton

Njoroge

et al. 2018

Ecology

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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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Section: Discussionmentioning

confidence: 99%

“…, , Dutton et al. ). These differences also highlight the potentially important role of other wildlife and human activities on the landscape, particularly in the Talek River sub‐catchment (Dutton et al.…”

Section: Discussionmentioning

confidence: 99%

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

Subalusky

Dutton

Njoroge

et al. 2018

Ecology

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“…By travelling further inland in response to lower donor ecosystem productivity, hippos spend a longer time feeding in the grasslands, which leads to less time spent in the river and therefore less excretion/egestion in the aquatic ecosystem (Subalusky et al, ). The magnitude of hippo inputs entering the recipient ecosystem appears to drive large differences in effects on ecosystem function, with low levels of input increasing primary and secondary production, and high levels decreasing production, particularly during low discharge when inputs concentrate on the river bottom (Masese et al, , ; McCauley et al, ; Dawson et al, ; Stears et al, ; Dutton et al, ; Subalusky et al, ). Hippo inputs can become so concentrated they lead to hypoxic conditions in the hippo pools and hypoxic floods and fish kills downstream, a condition Dutton et al .…”

Section: The Effects Of Subsidy Characteristics On Trophic Dynamics Amentioning

confidence: 99%

“…Hippo inputs can become so concentrated they lead to hypoxic conditions in the hippo pools and hypoxic floods and fish kills downstream, a condition Dutton et al . () termed ‘subsidy overload’. Hippos can also influence characteristics of the river ecosystem by increasing turbidity in the river directly, through bioturbation, but also indirectly, through decomposition of faeces into fine particulates (Dutton et al, ).…”

Section: The Effects Of Subsidy Characteristics On Trophic Dynamics Amentioning

confidence: 99%

Context dependency of animal resource subsidies

2018

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The transport of resource subsidies by animals has been documented across a range of species and ecosystems. Although many of these studies have shown that animal resource subsidies can have significant effects on nutrient cycling, ecosystem productivity, and food‐web structure, there is a great deal of variability in the occurrence and strength of these effects. Here we propose a conceptual framework for understanding the context dependency of animal resource subsidies, and for developing and testing predictions about the effects of animal subsidies over space and time. We propose a general framework, in which abiotic characteristics and animal vector characteristics from the donor ecosystem interact to determine the quantity, quality, timing, and duration (QQTD) of an animal input. The animal input is translated through the lens of recipient ecosystem characteristics, which include both abiotic and consumer characteristics, to yield the QQTD of the subsidy. The translated subsidy influences recipient ecosystem dynamics through effects on both trophic structure and ecosystem function, which may both influence the recipient ecosystem's response to further inputs and feed back to influence the donor ecosystem. We present a review of research on animal resource subsidies across ecosystem boundaries, placed within the context of this framework, and we discuss how the QQTD of resource subsidies can influence trophic structure and ecosystem function in recipient ecosystems. We explore the importance of understanding context dependency of animal resource subsidies in increasingly altered ecosystems, in which the characteristics of both animal vectors and donor and recipient ecosystems may be changing rapidly. Finally, we make recommendations for future research on animal resource subsidies, and resource subsidies in general, that will increase our understanding and predictive capacity about their ecosystem effects.

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“…Studies along the Mara River of Kenya and northern Tanzania have documented the strong role that hippos play in river nutrient and oxygen dynamics, as one individual hippo may contribute an estimated six kg/day of waste [40]. Aggregations of hippos in pools can lead to an accumulation of organic matter, that when flushed by a pulse or peak hydrologic event, can cause a large drop in dissolved oxygen, often leading to fish kills [41]. The annual wildebeest migration is an additional source of animal resource subsidies to the Mara River and also somewhat hydrologically mediated [42].…”

Section: Framework For River Conservation and Managementmentioning

confidence: 99%

Conserving Rivers and Their Biodiversity in Tanzania

Seeteram

Hyera²,

Kaaya

et al. 2019

Water

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The United Republic of Tanzania (Tanzania) is rich in freshwater resources and biodiversity. In this article, we highlight the importance of Tanzanian rivers and make a case for the conservation of the freshwater and terrestrial species that rely on these rivers. We provide an overview of current knowledge on Tanzanian rivers and discuss progress towards implementation of the National Water Policy (2002) and Water Management Act (2009), two legislative instruments that have motivated environmental flow assessments on at least six major rivers and offer legal backing for river conservation. We examine major challenges that pose significant threats to water security for river ecosystems and humans in Tanzania, among those: (1) human population growth, (2) agricultural expansion, (3) river flow alterations, (4) industrialization, (5) introduced species, and (6) climate change. We conclude by offering recommendations for future river conservation efforts in Tanzania.

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

Cited by 63 publications

References 66 publications

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

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

Context dependency of animal resource subsidies

Conserving Rivers and Their Biodiversity in Tanzania

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