Seasonal variations of the phytoplankton of the River Yeşilırmak were studied between June 1999 and May 2000. Similiar seasoanl variations were observed at stations 1, 2 and 3 whereas at station 4 seasonal variations of total organism numbers were completely different untill November then similiar developments to other stations were observed. Bacillariophyta dominated in plankton of the River Yeşilırmak. The number of species of other division were found to be less. Navicula cincta, N. cryptocephala and N. rhyncocephala significantly increased in St1., St2. and St3. in July, September, December and January. Algal members were rather low in February, March, April and no algae were found in January and March due to rainfall.

Seasonal variations of the phytoplankton of the River Yeşilırmak were studied between June 1999 and May 2000. Similiar seasoanl variations were observed at stations 1, 2 and 3 whereas at station 4 seasonal variations of total organism numbers were completely different untill November then similiar developments to other stations were observed. Bacillariophyta dominated in plankton of the River Yeşilırmak. The number of species of other division were found to be less. Navicula cincta, N. cryptocephala and N. rhyncocephala significantly increased in St1., St2. and St3. in July, September, December and January. Algal members were rather low in February, March, April and no algae were found in January and March due to rainfall.