The importance of flow regimes in assessing the impact of agricultural runoff on reservoir water quality and zooplankton abundance

Abstract

The impact of silt and clay loading from an agricultural watershed on zooplankton abundance and several physical, chemical, and biological measures of water quality was assessed during stormflow and baseflow flow regimes in Tuttle Creek Reservoir, near Manhattan, Kansas. Bacteria and suspended solids increased during the stormflow period, while water transparency, chlorophyll a, dissolved solids, and zooplankton declined. The abundance and reproductive performance of Daphnia parvula and Daphnia pulex in predator-free enclosures incubated along the longitudinal axis of the reservoir also decreased significantly during the stormflow period. Chlorophyll a, suspended solids, and dissolved solids were spatially distributed during the sampling period with highest concentrations at the riverine end of the reservoir where water transparency was lowest. Diaphanosoma sp. and calanoid copepods exhibited high abundances at the riverine end of the reservoir while Daphnia sp., Bosmina sp., and cyclopoid copepods exhibited low abundances there. The reproductive performance of Daphnia parvula in enclosures, however, was significantly greater at the riverine end, where silts and clays are most abundant. These spatial and temporal patterns in water quality, zooplankton abundance, and Daphnia reproductive performance demonstrate the importance of flow regime and the magnitude of its associated suspended sediment load from agricultural runoff on reservoir water quality and zooplankton dynamics. © 1990 Taylor and Francis Group, LLC.