Hippos transfer 0.4 tonnes of silicon into Kenya’s Mara River on a dry-season day; scientists explain why the element matters to aquatic life |

Representative Image of a hippo grazing on grasslands at night and transferring silicon to a river through its dung ( AI Generrated Image)

Hippos spend their nights grazing on grass and their days resting in rivers. This daily routine also moves silicon from land into water, where it can influence the organisms that form the base of aquatic food webs. In a study published in Science Advances titled Hippos (Hippopotamus amphibius): The animal silicon pump, researchers estimated that hippos transfer 0.4 metric tonnes of silicon into Kenya’s Mara River on a dry-season day by eating grass on land and depositing dung in the water. Their model estimated that hippos influence up to 76% of silicon flux along the study reach in the Maasai Mara. These estimates came from a dry-season study of one river, not a global assessment. The findings nevertheless show how large animals can connect terrestrial ecosystems with rivers and potentially affect aquatic food webs downstream.

Hippo silicon pump explained: how grazing and dung move silicon into the Mara River

The researchers conducted their field sampling in February 2014 along the Mara River in south-western Kenya, where hippos graze on savannah grasslands at night and congregate in river pools during the day. Silicon absorbed by grasses can pass through the animals’ digestive systems and enter the river through their dung, bypassing some of the processes that would otherwise retain silicon in the soil. The researchers measured biogenic silicon, a form of silicon produced by living organisms, in grass and hippo dung. Grass in the Maasai Mara National Reserve contained an average of 1.8% biogenic silicon by dry weight, while hippo dung contained 4.1%. Using earlier estimates from a 2015 study, ‘The hippopotamus conveyor belt’, that hippos consume 42.0 metric tonnes of dry grass and deposit 11.1 metric tonnes of dry dung in the river each day, the researchers estimated a daily transfer of approximately 0.4 metric tonnes of silicon. At the time of sampling, the Mara River carried daily fluxes of 7.9 metric tonnes of dissolved silicon and 2.4 metric tonnes of biogenic silicon. The researchers estimated that hippos directly accounted for 32% of the increase in biogenic silicon flux between the most upstream and most downstream sampling sites. They attributed the remaining increase partly to resuspended riverbed material, which can contain silicon from older hippo dung, as well as dust and indirect inputs such as faeces deposited on riverbanks. Some of these additional sources may also be linked to hippos.Monitoring over 24 hours along a 250-metre stretch of river where up to 80 hippos were present also showed that biogenic silicon concentrations doubled relative to upstream values. Laboratory experiments found that 2.8% of the biogenic silicon in fresh hippo dung dissolved after 24 hours in rainwater. This was 17.2 times the proportion released from undigested grass under the study’s experimental conditions. The researchers used a mass-balance model to estimate the contribution of hippos to silicon flux along the study reach in the Maasai Mara. Their calculation suggested that hippos influenced up to 76% of the flux, but this estimate depended on a model assumption that the sediment contained an effectively unlimited stock of biogenic silicon. The authors acknowledged that sedimentation and resuspension vary, making the real system more complex. They also cautioned that the estimate came from dry-season observations and could overstate the contribution of hippos under other seasonal conditions.

Silicon, diatoms and aquatic food webs: why river inputs matter

Silicon is important to diatoms, microscopic algae that use dissolved silicon to build their glass-like cell walls. Diatoms contribute substantially to aquatic primary production and support food webs by providing food for other organisms. The availability of silicon can therefore influence the composition of phytoplankton communities. The Science Advances researchers explain that a reduction in riverine silicon supply could change the balance between diatoms and other algae, with potential consequences for aquatic food webs. Their study considered the possible implications for Lake Victoria and other East African lakes, where river inputs contribute to silicon availability.The Mara River contributes about 5% of the water flowing into Lake Victoria, according to the study. However, the researchers did not directly measure how the silicon transferred by hippos affects the lake’s algal communities or food web. The downstream consequences remain a potential implication of their findings rather than a demonstrated outcome. A press release from the GFZ German Research Centre for Geosciences, whose researchers contributed to the study, explains that insufficient silicon can disadvantage diatoms relative to other algae. Some algal blooms can contribute to oxygen depletion and fish deaths, creating problems for aquatic ecosystems and communities that depend on fishing. These are possible consequences of changes in silicon availability, not effects that the hippo study directly established in Lake Victoria.

Hippo dung benefits and risks: how large animals can also affect river oxygen levels

Hippos transport more than silicon from grasslands into rivers. In a separate study, The hippopotamus conveyor belt, researchers examined how hippos move organic matter and nutrients from terrestrial habitats into aquatic systems. These inputs can provide resources for aquatic organisms, but excessive organic matter can also affect water quality. A 2018 study published in Nature Communications, titled Organic matter loading by hippopotami causes subsidy overload resulting in downstream hypoxia and fish kills, documented repeated low-oxygen events in the Mara River. Researchers recorded 49 high-flow events associated with decreases in dissolved oxygen over three years. Thirteen of these events resulted in hypoxia, or critically low oxygen levels, and the researchers documented nine fish kills over five years. Experiments and modelling linked the oxygen depletion to high flows that flush oxygen-depleted water and organic matter from hippo pools into downstream sections of the river. As this material decomposes, it can consume oxygen faster than flowing water replenishes it. The researchers suggested that similar events may have been more widespread in Africa before hippo populations declined, although this historical possibility was not directly measured.