The Occurrence of Cyanobacteria in the Reservoirs of the Mahaweli River Basin in Sri Lanka
E.I.L. SILVA and M.J.S. WIJEYARATNE
About forty species of cyanobacreria belonging to twenty-four genera have been reported so far from Sri Lanka's reservoirs. Of these, except for Microcystis aeruginosa, the other species are either rare or occur only in small numbers. The composition of the phytoplankton in 21 reservoirs in the Mahaweli river basin were examined during February-March, 1994 to determine whether the cyanobacteria exhibit a particular pattern of distribution and abundance. Of the cyanobacteria found in the Mahaweli reservoirs, the genus Anabaena occurred in two reservoirs. It was dominant in the Ambewela reservoir (relative importance - 70%) while in the Kande-Ela reservoir, which is located in the immediate downstream of the Ambewela reservoir, its relative importance in the phytoplankton community was only 1%. The relative importance of the genus Microcystis ranged from 46% to 2% and was found in twenty water bodies. The genus Pseudanabaena was found in four reservoirs with a relative importance ranging from 4% to 46%. Coelosphaeriurn sp. was found in two water bodies but its relative importance varied from 7% to 1%. The genus Lynghia was found in one water body with a relative importance of 1%. In addition, several other genera of cyanobacteria reported by others were found in a few numbers. Of the five major genera of cyanobacteria, Microcystis had the highest distribution and relative abundance in the Mahaweli river basin. The lowest relative abundance and distribution were recorded for Planktolyngbia. The relative importance of cyanobacteria in the phytoplankton assemblage in most of the reservoirs were found to be high (>50%) or moderate (25-50%) when total phosphorus and NO3-N contents are relatively low. Further when the NO3-N content is very low, pH also appears to affect the relative importance of cyanobacteria.