The dynamics of planktonic heterotrophic nanoflagellates in response to environmental drivers during initial reservoir filling

ABSTRACT Heterotrophic nanoflagellates (HNF) are a functionally essential group in freshwater ecosystems metabolism, mediating the transfer of nutrients from bacteria to higher trophic levels, and playing a central role in biogeochemical cycles, including the carbon balance. Despite that, it has been systematically overlooked. This study assessed changes in HNF density and biomass during initial filling phase of a neotropical reservoir, as well the factors influencing this dynamic. Abiotic and biotic variables were measured in a sample station close to the dam, at surface and bottom strata over 60 days. HNF density and biomass were higher at the middle of the filling period, especially at the surface. With impoundment, HNF growth was no longer limited by physical factors (high water flow), and became structured by abiotic (oxygen concentration) and biotic (resources and predation) factors. Over time, changes in the influence between bottom–up, such as bacteria resource, and top–down mechanisms, with the control by ciliates at the beginning of filling, while zooplankton assumed the predator role by the end. The results highlight the relevance of microbial food webs in the ecological functioning of newly formed reservoirs, emphasizing the importance of integrating HNF communities and their interactions into assessment of reservoir ecosystems.