Key features of photosynthetic picoplankton populations were compared during 1988 in ten lakes in northem England ranging from oligotrophic to slightly eutrophic; two of the three eutrophic lakes were shallow and lacked a thermocline. Measurements were made at 0.5 m depth of temperature, total chlorophyll a, chlorophyll-containing picoplankton cell density, mean picoplankton Celt volume and percentage of phycoerythrin-rich cells in the total picoplankton population. All lakes showed maxima for total chlorophyll concentration and picoplankton cell density in mid-to late summer. The maximum value for picoplankton density ranged from 3.4 x 103 (Esthwaite Water) to 1.3 x 106 cells ml-1 (Ennerdale Water).
There was a significant negative relationship (p c 0.05) between log,,, of maximum picoplankton cell density and maximum total chlorophyll, the latter being taken as an indicator of lake trophic status. The ratio of maximum to minimum picoplankton density during the year in a particular lake ranged from 39 to 2360 and showed no obvious relationship to lake type. Overall, the seasonal range in picoplankton density was about one order of magnitude greater than the range in total chlorophyll a, but there were considerable differences between lakes.
Phycoerythrin-rich picoplankton as a percentage of total picoplankton reached a maximum in summer in all lakes. Values were always very low (c 5 %) in the two shallow eutrophic lakes, but reached 97 % and over in the four most oligotrophic lakes. In two of the oligotrophic lakes, Wastwater and Ennerdale Water, phycoerythrin-rich picoplankton was a major component of the summer phytoplankton biomass.