Seasonal and vertical distribution of picoplankton in Llyn Padarn, Gwynedd

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Abstract
The seasonal and vertical distribution of picoplankton was recorded, using epifluorescence microscopy, from October 1985 to September 1987. This size fraction was made up of red and orange autofluorescent coccoid cyanobacteria and heterotrophic bacteria. Bacterial numbers were high and fluctuated between a winter minimum of 4·0 × 104 cells ml-1 and a summer maximum of 1·8 × 106 cells ml-1. There was no correlation between bacterial cell number and water temperature. Orange fluorescent cells were the numerically dominant phototrophic picoplankton. Their numbers were lowest during winter (158 cells ml-1) and highest in late summer (1·0 × 105 cells ml-1). This seasonal fluctuation was paralleled by the red autofluorescent cells though their overall numbers were consistently less abundant, often by an order of magnitude, than those of the orange cells. During periods of high cell numbers, the autotrophic picoplankton contributed 5–15% of total picoplankton abundance. During periods of high total chlorophyll concentration in 1986 and 1987 (21·8 mg chlorophyll a m-3 and 17·1 mg chl a m-3), picophototrophs contributed 45% and 75% respectively to total chlorophyll. During stratification chlorophyll values and cell numbers decreased with depth. Analysis of variance showed that the biomass of autotrophic picoplankton was significantly correlated with water temperature. The interaction of cell numbers with month and year after the depth effect had been taken into account was also significant.