Impact of cattle grazing and irrigation water influx to the nitrate and posphate levels of lagoon ecosysytem in Bundala National Park

Abstract

When intense grazing occurs at riparian areas, overland flow (runoff) from cattle-grazed pastures adds nutrients mainly nitrate and phosphate to water bodies. Runoff from agricultural land through irrigation systems is another major means of nutrient influx to water bodies. Both of these phenomena cause eutrophication, which was recognized as a pollution problem from mid-20th century. Studies have revealed that 54% of lakes and reservoirs in Asia are eutrophic and coastal and marine eutrophication is high in tropical countries. There is a growing concern that lagoon system of Bundala National Park is under threat of eutrophication. A study done by IWMI confirmed that nutrient levels in four major lagoons exceed minimum algal simulation and these high nutrient levels are due to accumulation of cattle and buffalo sewage over a long period of time. But a very important factor, the amount of nitrate and phosphate influx through drainage irrigation water has not been considered.

In this study, nitrate, phosphate and potassium in lagoon and inlet water were analysed over fifteen months. Cattles and buffalo encountered in the vicinity of lagoons were recorded. Based on standard discharge amounts of urine and dung by adult and cub per day the amount of nutrient accumulate per month was calculated. Nitrate concentrations of Malala and Embilikala lagoons showed a strong positive correlation with nitrate in inlet water (p<0.01 and p<0.05) respectively. The maximum, minimum and average nitrate levels in ppm were (8.20, 1.10, 3.96), (3.3, 0.34, 1.30), (2.32, 0.32, 1.34) and (3.3, 0.33, 1.3) in inlets of Malala, Embilikala and Bundala lagoons respectively The highest concentrations occur in water inlets which is about three times of mean lagoon nitrate concentration, which evidenced that the high nutrient influx to lagoons. The total number of cattle and buffalo did not correlate with nitrate concentration of each month in lagoon water.  The present results suggest that the local impact of sewage to chemistry of micro habitats should be studied in detail to understand the potential impact of nutrient.