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 K romme Estuary have broken down due to persistent euthalinity (salinity valu es above 28 parts per thousand) throughout the estuary. These conditions oc cur as a consequence of freshwater retention by a large reservoir. Only 2 x 10(6) m(3) (<2% of the mean annual runoff (MAR) from the catchment) is all ocated to the estuary per annum, and the present study was undertaken to ev aluate estuarine zooplankton response to a single release of the full reser ve. 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 signif icant change in distribution or abundance in any of the dominant copepod po pulations. It is concluded that the <2% of MAR released in a single pulse h ad no direct nor indirect advantage for the endemic copepods at the populat ion level. Similarly, no significant change was observed in zooplankton com munity structure after the release. Mixing of the water column and developm ent of a permanent but dynamic longitudinal salinity gradient is a key mech anism regulating estuarine zooplankton dynamics. A regular base flow in add ition to intermittent releases of freshwater pulses into the estuary is req uired. Because of freshwater attenuation, the Kromme Estuary is deprived of a key mechanism that regulates spatial and temporal variability of estuari ne endemic copepod populations.