Ecology of freshwater plankton in contrasting hydraulic environments

The effects of contrasting water residence time on the planktonic community of three water bodies was studied. The sites were Rutland Water, Covenham reservoir and the River Nene, in order of decreasing retention time. The river was the only site where the residence time was sufficiently short to directly impact upon planktonic populations.;The importance of physical mechanisms of control in the river was indicated by the dominance of r-selected phytoplankton, namely diatoms and green algae, and zooplankton, namely rotifers. This pattern was in contrast to that of the reservoirs in which slower-growing cyanobacteria and cladocerans dominated. Riverine rotifers were found to have rapid population turnover, due to a combination of short pre-adult duration and high maximum growth rates. Some species also showed a shift in reproductive behaviour to the constant production of resting eggs. This was assumed to convey benefits in a system where resting stages may be less susceptible than adults to downstream displacements.;The total chlorophyll 'a' concentration achieved per unit phosphorus was significantly lower in the river than either of the reservoirs. Algal density was negatively associated with discharge in the Nene, suggesting the importance of physical control. The relationship was not a simple one, however, as periods of increased retention time during the summer were associated with low algal density. The absolute density of rotifers was found to be constrained by discharge. Animals were rare above a threshold flow velocity of 0.08ms-1.;Simple models of water flow in open channels were insufficient to account for the continued persistence of a plankton in the Nene given the level of advection. It was assumed that the overall retention time of the system was increased by the presence of dead zones within the channel which 'held' planktonic organisms for sufficient time for populations to develop. This behaviour was important during the spring when population density increased in the main channel.