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. Chlorophyta was the most dominant group followed by Cyanobacteria, Euglenophyta, Pyrrhophyta, Bacillariophyceae and Chrysophyceae. Dominant species were mainly the desmids; Cosmarium and Staurastrum. Chlorophyll a concentrations and phytoplankton 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 primary productivity and chlorophyll a occurred before the onset and immediately after the monsoon season in November-January. Total phytoplankton densities were highest in the top 5 m and decreased with depth. However, maximum primary productivity and chlorophyll a concentrations were found near the metalimnion (10 -15 m) which could be due to high autotrophic bacterial population at this depth. Occurrence of high phytoplankton densities and production during the pre and post monsoon period was probably due to the intrusion of the nutrient enriched water from the hypolimnion brought about by the cooler temperature and water turbulence during this period.