The effects of a single freshwater release into the Kromme Estuary. 3: Estuarine zooplankton response

Natural patterns of zooplankton variability (temporal and spatial) in the Kromme Estuary have broken down due to persistent euhalinity (salinity values above 28‰) throughout the estuary. These conditions occur as a consequence of freshwater retention by a large reservoir. Only 2 x 10 m (<2% of the mean annual runoff (MAR) from the catchment) is allocated to the estuary per annum, and the present study was undertaken to evaluate estuarine zooplankton response to a single release of the full reserve. The experimental release was predicted to create freshwater conditions throughout the upper half of the estuary. Instead, the water column became highly stratified for about two weeks after which salinity profiles rapidly returned to prerelease conditions. The freshwater pulse elicited no significant change in distribution or abundance in any of the dominant copepod populations. It is concluded that the <2% of MAR released in a single pulse had no direct nor indirect advantage for the endemic copepods at the population level. Similarly, no significant change was observed in zooplankton community structure after the release. Mixing of the water column and development of a permanent but dynamic longitudinal salinity gradient is a key mechanism regulating estuarine zooplankton dynamics. A regular base flow in addition to intermittent releases of freshwater pulses into the estuary is required. Because of freshwater attenuation, the Kromme Estuary is deprived of a key mechanism that regulates spatial and temporal variability of estuarine endemic copepod populations.