Limnology project at Mahaweli reservoirs:II A limnological study at Kotmale, Victoria and Randenigala rservoirs

Abstract

The objective of this study was to collect basic data to ascertain statusof Kotrnale, Victoria and Randenigala reservoirs and to understand the trophicevolution of the reservoirs. The transparency values changed with the reservoirs.The highest frequency percentage of secchi depth observed at eachreservoir are; Victoria 1.21--1.6Im, Randenigala 2.51-3.0Im and Kotmale2.10-2.30 indicatin.g highest value at Randenigala and the lowest value atVictoria. All three reservoirs are thermally stratified and the highest differencesin temperature gradients were observed from surface to about 25m depth.In the deeper layers the temperature gradient is distin.ctly low. The highestconductivity values were observed closer to the dams of the three reservoirsindicating accumulation of dissolved ions towards the dams. In all threereservoirs the pH gradient was higher at the surface compared to the valuesbellow 10m depths. The high dissolved Oxygen concentrations are closelylinked with the euphotic zones. Oxygen concentrations were markedly lowbelow the depth of 20m. However there is no completely deoxygenatedlayers at the bottom of the reservoirs.

Data on chemical substances also proves the development of a strongchemocline below 20m depth level. Water hardness values are high whenwater retention levels of the reservoirs are low. High levels of HydrogenSulfide concentrations have been observed at the bottom layers ofRandenigala reservoir from time to time. Sulfide concentration increasedwith the increasing depth of the reservoirs. The range of Nitriteconcentrations recorded for Victoria is 0.005-D.l ppm and for RandenigalaO.01-D.08 ppm. Ammonia was recorded only in deeper layersbelow 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 ofgreen and blue green algae.