Autotrophic picoplankton dynamics in a small shallow lake

The paper presents the results concerning seasonal fluctuation of phytoplankton abundance, biomass and the structure of communities, as well as the relative importance of size groups (i.e. autotrophic picoplankton, nanophytoplankton and microphytoplankton) in oligo-mesotrophic Lake Skrzynka (maximum depth 2.9 m; mean depth 1.4 m; area 1.7 ha; volume 2.4 x 10(4) m(3)). The abundance of ciliated protozoa, rotifers and crustaceans was monitored simultaneously. Water samples were taken from March to November 1995. The dynamics of autotrophic picoplankton (APP) was characterised by three abundance maxima. The first and highest maximum was recorded in March, the second maximum appeared in August, whereas the third maximum was observed in October. The average APP number was 1.4 x 10(6) cells ml(-)1, while average biomass was 1.1 mg l(-1). APP was dominated by cyanobacteria and accounted for 1-85% of total phytoplankton biomass (34% on average). The biomass of phytoplankton larger than 2 mu m was often dominated by chrysophytes, dinoflagellates and green algae and was usually higher than APP biomass (except on four days). Dominance of one size group of phytoplankton was observed whenever total phytoplankton biomass was high: APP in early spring, nanophytoplankton in late spring and microphytoplankton in summer. When phytoplankton biomass was low, which was observed in late summer and autumn, the proportion of the size groups was more or less the same. The highest values of APP biomass did not coincide with the maximum of the larger phytoplankton and a significant decrease in APP biomass coincided with an increase in ciliate abundance.