Vertical and seasonal distribution of phytoplankton in a tropical reservoir, Malaysia

Phytoplankton of Kenyir reservoir (04 degrees 47' N to 5 degrees 15' N and 102 degrees 32' E to 102 degrees 55' E) was studied during 1992. Chlorophyt a was the most dominant group followed by Cyanobacteria, Euglenophyta, Pyrr hophyta, Bacillariophyceae and Chrysophyceae. Dominant species were mainly the desmids; Cosmarium and Staurastrum. Chlorophyll a concentrations and ph ytoplankton productivity ranged from <1.1 mg/m(3) to 38.5 mg/m(3) and from < 2.8 - 54.4 mgC/m(3)/hr respectively. Maximum phytoplankton density, net p rimary productivity and chlorophyll a occurred before the onset and immedia tely after the monsoon season in November-January. Total phytoplankton dens ities were highest in the top 5 m and decreased with depth. However, maximu m primary productivity and chlorophyll a concentrations were found near the metalimnion (10 -15 m) which could be due to high autotrophic bacterial po pulation at this depth. Occurrence of high phytoplankton densities and prod uction during the pre and post monsoon period was probably due to the intru sion of the nutrient enriched water from the hypolimnion brought about by t he cooler temperature and water turbulence during this period.