Green algae in the phytoplankton of the danube (bulgarian sector) species composition, distribution, cell numbers and biomass

Abstract

Summary In the phytoplankton of River Danube (Bulgarian stretch) for period of eight years (1980–1988) 216 taxa of Euchlorophytina and 26 taxa of Zygnemophytina were found. The distribution of established taxa from the 2 subdivisions is: in summer − 142 and 8, in spring − 124 and 8, in autumn − 73 and 3, in winter − 63 and 0, respectively. The majority of the established taxa (55–76 per cent) are rarely distributed along the river flow. By their numbers and biomass the green algae (predominantly Chlorococcales) occupied an important place in the river phytoplankton and in some stations they dominate over diatoms. Average numbers of the planktonic green algae varied from 2.30 × 105 cells/l (in winter) to 37.70 × 105 cells/l (in summer) and average biomass from 0.05 mg/l in autumn and 0.06 mg/l in winter to 0.34 mg/l in summer (1987/88). For Zygnemophytina this average values are 0.01 × 105 cells/l (in spring, summer and autumn) and 0.003 mg/l. The number of taxa, cell numbers and biomass of the green algae decrease after big industrial and port towns and increase after shallows in river arms near islands and sand strips (in summer period).