Variation in nutrient levels and other water quality parameters in minor irrigational tanks in the Kurunegala district, Sri Lanka, in relation to paddy cultivation: assessing suitability of tanks for aquatic life

Abstract

While providing water for irrigation, the reservoirs have been a source of fresh water for the rural communities, and, more recently, they have become the focus for expanding the inland fishery industry. This paper assessed nutrient enrichment in minor seasonal irrigation tanks in the Kurunegala District in relation to different stages in the paddy cultivation cycle. Findings of the study will be useful to determine their compatibility with safe levels for fisheries, among other uses. The study was extended to cover other selected parameters relating to water quality. Surface water from 50 minor tanks were sampled over pre-cultivation, cultivation and postcultivation stages of the paddy cultivation cycle in the Yala (n= 27) and Maha (n=23) seasons and analyzed for N03-N, P04-P, dissolved oxygen, turbidity, conductivity and pH, using standard methods and portable instruments. Results showed considerable spatial variation in nutrients between the 50 tanks and marked temporal variation through the three sampling stages. For N03-N, there was a shift in the proportion of tanks containing low and high nutrient levels through the pre-cultivation, cultivation, and post cultivation stages. The mean levels were 0.01-1 mg 1-1 during pre-cultivation and post-cultivation and 1.0-5.0 mg 1-1 during cultivation, with distinct peaks observed in the Yala and Maha seasons. The variation of P04-P was different, with the majority of the tanks having levels of 1-5 mg 1-1 P04-P during all sampling stages, and with no pronounced peaks. Nutrient levels indicate that a majority of the minor tanks become eutrophic during the two cultivation seasons. The other tested parameters also depicted spatial and temporal variation, and water quality indices revealed that the quality of water deteriorates during paddy cultivation. Based on these findings, particularly for nutrients, we conclude that many of the minor tanks become eutrophic during the cultivation season and may not offer optimal conditions for the intended uses, one of them being rearing of fish and shellfish. Mitigation measures must therefore be implemented to reap maximum benefits associated with these minor irrigation tanks.