EFFECTS OF RIVER DISCHARGES ON SEASONAL VARIATIONS OF WATER QUALITY AND PRIMARY PRODUCTIVITY WITH  REFERENCE TO NILWALA AND WALAWE RIVER MOUTH

River discharge is one of the important factors that transport nutrients and sediments from  lands and, it plays a major role to change the water quality of the Ocean. However, only very few findings are available on the effects of discharges in Sri Lankan rivers. Therefore, this study was carried out to determine the effect of the river discharges on nearshore water quality and primary production of Nilwala and Walawe rivers in Southern Province of Sri Lanka. Sampling was carried out on a monthly basis from May 2016 to April 2017 considering four monsoon periods. Eleven sampling points were selected within the study area for both sites. River discharges were calculated by the mid section method. Temperature, pH, Salinity, Electrical Conductivity (EC), and Dissolved oxygen (DO) were measured onsite whereas Total Suspended Solids (TSS), Chlorophyll-a, nitrate, phosphate and Colored Dissolved Organic Matter (CDOM) were measured in the laboratory. Phytoplankton and zooplankton samples were filtered through plankton nets with mesh diameters of 10 µm and 80 µm respectively. They were identified to the nearest taxonomic group using standard identification keys. Further, composition, density and diversity  parameters were determined. The water temperature at Nilwala and Walawe river sites  varied between 26 °C - 30 °C whereas pH varied from 7.8 to 8.78 during the study period.  Temperature (r = 0.517; p< 0.01) and TSS (r = 0.710; p< 0.01) showed a significant positive  correlation with the Nilwala river discharge. Whereas, phosphate (r = - 0.318; p< 0.05) and Chlorophyll-a concentration (r = - 0.861; p< 0.01) showed a significant negative correlation with Nilwala river discharge. Salinity values of the Nilwala and Walawe sites varied between 4.3 ppt to 33.05 ppt and 0.46 ppt to 33.6 ppt respectively. Salinity and TSS showed  a significant negative correlation (r = -0.612; p