Physico-chemical properties and phytoplankton density was estimated in Tilyar Lake (28º 52’ 52.77” N and 76º 38’ 12.05” E.), Rohtak (Haryana) India. Different water quality parameters (pH, Temperature, DO, BOD, COD, Total Alkalinity, Total Hardness, Calcium Hardness, Magnesium Hardness, Acidity, CO2, TDS, TSS, Chloride, Total Kjeldal Nitrogen, Sulphate, phosphate, Net Primary Productivity and Phytoplankton Density) were estimated form January 2010 to December 2010 from three sampling stations sited in lake. The physico-chemical parameters have exhibited considerable variation during the study as pH (8.75-9.24), Temperature (13.50-32.67°C), DO (1.80-5.13), BOD (0.73-2.10), COD (18.47-62.67 mg/l), Total Alkalinity (85.33-106.67 mg/l), Total Hardness (84-104.67 mg/l), Calcium Harness (58.20-72.47 mg/l), Magnesium Hardness (5.69-8.94 mg/l), Acidity (1.80-6.67 mg/l), Free CO2 (0-3.93 mg/l), Chloride (10.37-20.87 mg/l), TDS (99.33-122.33 mg/l), TSS (28.67-60.67 mg/l), TKN (70.17-95 mg/l), Sulphate (0.95-1.55 mg/l), Phosphate (0.02-0.10 mg/l), Primary Productivity (2.13-3.70 mg/l) and Phytoplankton Density (36-68 mg/l). Due to low dissolved oxygen concentration and low primary productivity the ecological balance in the lake was observed very fragile. Statistically phytoplankton density was found significantly correlated with pH, magnesium hardness and total nitrogen (P= 0.05) and with total alkalinity, total hardness, calcium hardness, total dissolved solids and net primary productivity (P= 0.01).
