GROWTH, PRODUCTION AND LOSSES OF PHYTOPLANKTON IN THE LOWLAND RIVER SPREE .1. POPULATION-DYNAMICS

The seasonal and longitudinal dynamics of dominant phytoplankton populations were observed in the lower Spree (Germany), a lowland river draining several lakes. Centric diatoms and cyanobacteria were the chief components of the total phytoplankton biomass of 16 mg l-1 at the end of the river course (average March-October, 1988-89). The cyanobacteria required a retention time of several weeks in a drained lake to establish stable populations, whereas the other algal populations originated from upstream impoundments and slow-flowing river stretches. In spring, the dominant populations increased (the diatoms Nitzschia acicularis, Asterionella formosa, Stephanodiscus hantzschii, Cyclostephanos dubius, Cyclotella radiosa, C.meneghiniana) or kept their abundances nearly constant (cyanobacterium Limnothrix redekei, diatom Synedra acus), but the biomass of the 'summer species' (diatom Actinocyclus normanii, cyanobacteria Planktothrix agardhii and Aphanizomenon sp.) was halved along the river course. These seasonal differences in algal growth are probably caused by the changed mixing pattern in the regions of origin and thus different pre-adaptation to turbulent,mixing. Pronounced clear-water phases occurred regularly at the end of May. They were intensified along the river course despite the lack of mesozooplankton. The dynamics of phytoplankton were reflected in the proportions of dissolved and particulate nutrients. The nutrient supply was always high enough to make limitations of algal growth improbable.