Abstract
Hippos transfer massive quantities of trophic resources from terrestrial to aquatic ecosystems through defecation. The ramifications of the latter for the functioning of benthic ecosystems are unknown, but are dependent ultimately on rates of utilisation relative to inputs. Low input and high utilisation can strengthen bottom-up pathways and enhance consumer biomass and abundance. However, if inputs exceed utilisation rates, dung can accumulate, leading to a decline in water quality, with important repercussions for resident assemblages. Here, we quantify the consequences of hippo dung inputs on benthic assemblages in an estuarine lake in South Africa. The system supports over a thousand hippos, and during recent drought periods (extending over a decade), hippo dung has been observed to form mats over benthic habitats. Enrichment of plots using exclusion/inclusion cages with dung at naturally occurring concentrations indicated a decline in benthic chl-a by roughly 50% and macrofaunal abundance, biomass and richness by up to 76, 56 and 27% respectively. Our findings suggest that persistent inputs of hippo dung can act as an important stressor of benthic systems, leading ultimately to a loss of productivity. Accumulation of hippo dung over benthic habitats is therefore an important mechanism by which hippos indirectly structure aquatic ecosystems.
Highlights
Magnitudes of transfers exceed rates of utilisation, transfers can accumulate, leading to deteriorating water quality and physiological stresses on communities[1]
Our results indicate that persistent inputs and accumulation of hippo dung at levels recorded in the Narrows may play an important ecological role in structuring benthic ecosystems in the St Lucia Estuary, principally by depressing benthic community metrics and microphytobenthic biomass
While there is no direct evidence in the literature pointing to hippo dung causing shading of benthic environments, the latter can be inferred from studies reporting a reduction in light penetration and productivity in response to elevated suspended organic matter levels[21,22]
Summary
Magnitudes of transfers exceed rates of utilisation, transfers can accumulate, leading to deteriorating water quality and physiological stresses on communities[1]. During the latest drought period, hippo dung has regularly been observed to form dense mats on the benthos, and has frequently been recorded in benthic grab samples, in areas where hippos are common. The latter formed the core rationale underlying the central objective of the study. Based on our observations of dung forming dense mats (up to 1 cm in thickness) over the benthos in the Narrows, we hypothesized that enrichment of the benthos with hippo dung would generate biologically significant shifts in benthic communities, by depressing macro-invertebrate community metrics and microphytobenthic biomass
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