Studies of phytoplankton in the streams of the river Merkys basin. – Botanica Lithuanica, 1: 35–47

The phytoplankton of the river Merkys basin (South East of Lithuania) was investigation in 1976–1994. Totally 148 algae species and 14 varieties were recorded in the investigated streams. The diatoms (Bacillariophycaea) and green algae (Chlorophyceae) dominated in the studied phytoplankton, respectively they comprised 52 % and 29 % of the recorded taxa. The number of true planttonic diatoms were rather small: most of the suspended algae were tychoplanktonic. The green algae were dominated by small planktonic Chlorococcales.

Variations in seasonal succesion were closely related to the stream order. In the first and second order stream diatoms were dominant during the year. In the third and fourth order streams a typical alteration of diatoms (spring, autumn) and green algae (summer) occerred.

Keywords: phytoplankton, species composition, seasonal succesion, streams.