The effects of artificial mixing on phytoplankton growth and periodicity.

Continuous, low intensity mixing using a perforated pipe system prevented a lowland, eutrophic reservoir - Staunton Harold - from stratifying in 1982 and 1983. Environmental variables were measured and related to the responses of individual phytoplankton species and assemblages. These responses were compared to those from Staunton Harold during a year when natural mixing occurred (1981) and Foremark - a nearby, eutrophic reservoir - which was allowed to stratify in all three years. Diatoms and small Chlorophytes had higher growth rates under mixed conditions whilst colonial Chlorophytes and filamentous Cyanobacteria were favoured by stable water columns. Stratification of phytoplankton communities did occur during mixed years when water column stability was high and/or motile species were present. Nutrient availability and the stability of the water column were found to be the dominant factors affecting the seasonal changes in assemblages. Artificial mixing and a lowering in the stability of the water column in Staunton Harold during 1982 and 1983 did not have major effects upon the periodicity but the perturbations caused by changes in stability could be explained using a matrix whose axes were these dominant factors. It was also shown that populations of phytoplankton which were mixed throughout the water column were able to tolerate wide ranges of underwater irradiance whilst stratified populations became light-shade adapted. This difference in the photosynthetic physiologies led to the mixed populations having a lower assimilation number and assimilation rates at higher intensties. The stratified populations had a higher assimilation number and assimilation rates at higher irradiances. Phytoplankton sedimentation was measured in both reservoirs but no differences were found under stratified and mixed conditions. Although phytoplankton assimilation was lowered by mixing, the mean total biomass was lower in one year of continuous mixing (1982) and higher in another (1983) when compared to the mean biomass in the naturally mixed year and those in Foremark.