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Alternative Biogeochemical States of River Pools Mediated by Hippo Use and Flow Variability

Abstract

Hippopotami (hippos) are ecosystem engineers that subsidize aquatic ecosystems through the transfer of organic matter and nutrients from their terrestrial grazing, with potentially profound effects on aquatic biogeochemistry. We examined the influence of hippo subsidies on biogeochemical cycling in pools of varying hydrology and intensity of hippo use in the Mara River of Kenya. We sampled upstream, downstream, and at the surface and bottom of pools of varying volume, discharge, and hippo numbers, both before and after flushing flows. The product of hippo number and water residence time served as an index of the influence of hippo subsidies (hippo subsidy index, HSI) on aquatic biogeochemistry. Low-HSI hippo pools remained oxic between flushing flows and could be a source or sink for nutrients. High-HSI hippo pools quickly became anoxic between flushing flows and exported nutrients and byproducts of anaerobic microbial metabolism, including high concentrations of total ammonia nitrogen, hydrogen sulfide, and methane. Medium-HSI hippo pools were more similar to high-HSI hippo pools but with lower concentrations of reduced substances. Episodic high discharge events flushed pools and reset them to the oxic state. Transitions from oxic to anoxic states depended on water residence time, with faster transitions to anoxia in pools experiencing smaller flushing flows. Frequent shifts between these alternative oxic and anoxic states create heterogeneity in space and time in pools as well as in downstream receiving waters. In river systems where the influence of hippos on water quality is a concern, maintaining the natural flow regime, including flushing flows, ameliorates impacts of hippos.

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Acknowledgements

We thank the Government of Kenya and the National Council for Science and Technology for authorizing this research (NCST/RRI/12/1/BS-011/25). The National Museums of Kenya provided assistance with permits and logistics. Support in the field was provided by Brian Heath and the Mara Conservancy. Paul Geemi, James Landefeld and Jordan Chancellor provided field assistance. Access to the hippo pools was facilitated by Tarquin and Lippa Wood, Amani Mara Camp, and the wardens of the Maasai Mara National Reserve, the Mara Conservancy and Naboisho Conservancy.

Funding

Funding was provided by US National Science Foundation Grants to DMP and EJR (NSF DEB 1354053, 1354062, and 1753727); a Grant from the National Geographic Society to DMP; Grants from the Yale Tropical Resources Institute, the Yale Institute for Biospheric Studies and the Yale MacMillan Center for International and Area Studies to CLD; and a fellowship from the Robert and Patricia Switzer Foundation to ALS.

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Correspondence to Christopher L. Dutton.

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CLD, ALS, SKH, EJR, and DMP conceived of or designed the study; CLD, ALS, SKH, ECJ, LN, EJR, and DMP Performed the research; CLD and SKH analyzed the data; and CLD, ALS, SKH, ECJ, LN, EJR, and DMP wrote the paper.

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Dutton, C.L., Subalusky, A.L., Hamilton, S.K. et al. Alternative Biogeochemical States of River Pools Mediated by Hippo Use and Flow Variability. Ecosystems 24, 284–300 (2021). https://doi.org/10.1007/s10021-020-00518-3

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Keywords

  • Tropical river
  • Hippopotamus
  • Subsidy
  • Oxygen
  • Alternative state
  • Anoxia
  • Hypoxia
  • Pool
  • Residence time
  • Flow variability