Seasonal and Spatial Variation of Dissolved Oxygen and Nutrients in Padaviya Reservoir, Sri Lanka

Lakes, reservoirs, rivers, and aquifers are important freshwater sources for basic human needs such as drinking, sanitation, and agriculture. The anthropogenic influences on the natural environment, especially on freshwater resources, have increased dramatically during the last few decades. Eutrophication and pollution are major threats to many of these water bodies. There are thousands of man-made reservoirs, which are centuries old in Sri Lanka, and only a handful of them have been extensively studied and monitored. This study investigates the spatial and seasonal variations of water quality in Padaviya Reservoir by studying the vertical distribution of physical parameters and inorganic nitrogen species: ammonia, nitrite and nitrate, reactive phosphate, and dissolved oxygen. Padaviya Reservoir, which is an ancient man-made irrigation reservoir, has never been studied in detail to assess its water quality. Sharp chemical gradients for ammonia, nitrite, nitrate, reactive phosphate, and dissolved oxygen were observed between surface and bottom waters of the reservoir, suggesting that it does not overturn completely. The temperature difference is between the surface and bottom waters of about 2°C, which is not large enough to cause thermal stratification. The most probable reason for the stratification is extensive photosynthesis at surface waters with subsequent decomposition of the organic material at the bottom.