Assessment of phosphorus loading and eutrophication in Rawal Lake

The phenomenon of water eutrophication has increased over the world. Eutrophication involves the excessive algal growth due to increased influx of nutrients, leading to reduced level of dissolved oxygen in water. A regular inflow of P is mainly responsible for eutrophication. The present study was designed to monitor the occurrence and assessment of eutrophication in Rawal lake, Islamabad. Three different types of parameters viz., dissolved phosphorus in lake water, microbial biomass and phosphorus fractions in sediments were analyzed for its assessment. Water samples were collected and analyzed for dissolved phosphorus on biweekly basis while sediments from lake were collected and analyzed for phosphorus fractions in October, 2007 and July, 2008. Rawal lake was categorized as oligotrophic in October 2007, moderately eutrophic in January, and eutrophic in June and July, 2008 with a maximum level of dissolved phosphorus of 0.063 mg L-1. The rainfall was recorded highest of 160 mm in July, 2008 depicting maximum inflow of P into the lake. Among the various P-fractions, acid-P and CaCl2-P were found to be the highest with 4.25 mg L-1 concentration, respectively, in July 2008. The water samples were filtered through 0.045 mu m filter paper and then assessed for microbial mass. Residues left on filter paper were tested for microbial biomass, which increased from 5.8-15 mu mg L-1, from October, 2007 to July, 2008. The results clearly showed that the phenomenon of eutrophication do exist in Rawal lake, both at the centre and bank. The possible sources of contamination seemed to be the adjoining human settlement and mushroom growth of poultry sheds in and around the catchment area of Rawal lake. It is necessary to check these sources of contamination to protect a valuable source of drinking water for Rawalpindi and Islamabad citizens.