Nutrient loading and its consequences in a lake ecosystem

Cultural eutrophication has emerged as one of the most relevant causes of fresh water quality deterioration. Excessive enrichment of inland waters, coupled with dramatic urbanization, has led to formidable algal blooms in many parts of the world. In order to save inland waters from extenuation, tremendous investigative activities have been observed all over the world (Elser & Kimmel 1985; Hezlar et al. 1993; Nedoma et al. 1993; Raschke 1993; Zutshi & Khan 1992) but biological response to nutrient enrichment has merely been appreciated (Garg & Garg 1999, 2001). In the aforecited backdrop, a pilot study was undertaken to assess the potential fertility of lentic waters and to ellucidate the ecology of aquatic flora. In addition, studies were performed at microcosmal level and the results, so obtained, were applied at field level, to forsee the impact of nutrients on natural waters. On this basis, limits have been suggested for each nutrient, the excess of which may cause eutrophication or considerable loss of lake biota. Three major water resources of Bhopal, Upper lake (UL), Lower lake (LL) and Mansarovar reservoir (MR), have been selected for present investigation. Unfortunately, these beautiful water resorts are under great environmental stress owing to human influences, discharge of sewage, growth and deposition of organic matter, siltation and eventually eutrophication. Sampling stations were established near Vardaman park in Upper lake, opposite to fish aquarium in Lower lake and near Administrative Academy in Mansarovar reservoir. These transects were chosen after considering limnochemical representation, enutrint loading and input of sewage. Samples were drawn, from these lakes, on every 1st, 3rd, 5th, 10th, 15th and 30th day of the month and the parameters were simultaneously assayed for natural as well as microcosmal waters. To facilitate replicate tests and standardization, microcosmal experiments were conducted in the laboratory. Four cubical glass aquaria, with 50 litres capacity, were housed with 35 litres of water, sediment and phytoplankton penned-off from natural lake waters. Three of them were administered with 5.0 mg l-1, 10.0 mg l-1 & 15.0 mg l-1 of nutrients whereas the remaining one retained controlled condition. The experiments were conducted for 30 days and at a stretch only one nutrient was handled. Such experiments were performed in five replicates, one for each nutrient viz. calcium, magnesium, potassium, nitrogen and phosphorus. Thus, fifteen sets, five for each reservoir, with a total of sixty model ecosystems, were developed, over a spell of 15 months. Phytoplankton were identified following the manual of Ward & Whipple (1968). Compound Quotient, also regarded as Nygaard’s Coefficient, was computed for each lake as follows: Chlorococcales + Centrales + Cyanophytes + Euglenoids CQ = Desmids Limnochemical features were analyzed according to APHA (1989) and NEERI (1988). Water of all the three lakes were found to be hard with calcium contents ranging from 41.6 – 74.4 mg l-1 in Upper lake, 30.0 – 78.0 mg l-1 in Lower lake and 47.2 – 99.2 mg l-1 in Mansarovar reservoir. The latter held high concentration of cal-