Limnology Project at Mahaweli Reservoirs:II A Limnological Study at Kotmale, Victoria and Randenigala Reservoirs

Abstract

The objective of this study was to collect basic data to ascertain status of Kotrnale, Victoria and Randenigala reservoirs and to understand the trophic evolution of the reservoirs. The transparency values changed with the reservoirs. The highest frequency percentage of secchi depth observed at each reservoir are; Victoria 1.21--1.6Im, Randenigala 2.51-3.0Im and Kotmale 2.10-2.30 indicatin.g highest value at Randenigala and the lowest value at Victoria. All three reservoirs are thermally stratified and the highest differences in temperature gradients were observed from surface to about 25m depth. In the deeper layers the temperature gradient is distin.ctly low. The highest conductivity values were observed closer to the dams of the three reservoirs indicating accumulation of dissolved ions towards the dams. In all three reservoirs the pH gradient was higher at the surface compared to the values bellow 10m depths. The high dissolved Oxygen concentrations are closely linked with the euphotic zones. Oxygen concentrations were markedly low below the depth of 20m. However there is no completely deoxygenated layers at the bottom of the reservoirs. Data on chemical substances also proves the development of a strong chemocline below 20m depth level. Water hardness values are high when water retention levels of the reservoirs are low. High levels of Hydrogen Sulfide concentrations have been observed at the bottom layers of Randenigala reservoir from time to time. Sulfide concentration increased with the increasing depth of the reservoirs. The range of Nitrite concentrations recorded for Victoria is 0.005-D.l ppm and for Randenigala O.01-D.08 ppm. Ammonia was recorded only in deeper layers below the 45m level at Randenigala. The zooplankton community of the three reservoirs consists of Copepods, Rotifers and Cladocerans. The phytoplankton community of the three reservoirs consist mainly of green and blue green algae.