Long-term phytoplankton changes in a tropical reservoir

Abstract

An understanding of the processes controlling primary production is necessary for effective conservation and management of lakes and reservoirs. Phytoplankton populations, limnological parameters and nutrient status have been monitored in the Pansoon Reservoir, Hulu Langat (Malaysia) — which has a mean depth of 30 m and a surface area of 130 km² — over a ten-year period. Recent data indicate a deterioration in water quality, with a dominance of chlorophytes and frequent occurrence of dinoflagellate blooms. Results from 1987–1990 are compared with data obtained during the initial stages of impoundment in order to determine the causal factors which triggered a rise in primary productivity. Nitrate-nitrogen and phosphate levels have increased ten- and four-fold, respectively, during the past ten years. The phytoplankton composition has shifted from an equal dominance of chlorophytes and cryptophytes in 1981 to a dominance by chlorophytes only. An increase in the supply of allochthonous nutrients to the lake is the probable explanation for the progressive rise in phytoplankton densities during recent years.

Two rivers draining into the reservoir are responsible for a large input of nutrients during the rainy season. Seasonal changes in phytoplankton populations result from competitive interactions as reflected in the dominance of dinoflagellates during the dry season when nutrient levels are low. The results of this study will contribute towards formulation of management strategies for Malaysian reservoirs.