Lake Gregory, alien flora and urban aqua-environments in a Misty city of Sri Lanka

Abstract

Lake Gregory in the uppermost hill capital of Sri Lanka is a man made water-body built to enhance the site esthetic beauty. Now it has to act as a waste sinker, having lost its ability to supply bathing/ recreational/agricultural water. A series of short studies on its floral composition and water quality parameters were done from 2007 to 2009 through direct visual observations and the APHA standard methods to quantify its current ecological conditions and future sustainability. Floral composition of the lake was found to dominate by Alien Plant Species (APS) demonstrating rapid spatial changes from time to time. Myriophyllum sp., Polygonium sp, Salvinia molesta, Eichhornia crassipes, Ceratophyllum demersum and Pistia stratiotes were the dominant APS of which E. crassipes was found to occupy nearly 1/3 of the lake. Others were in localized patches. There was a reduction of native water Lilly plant cover and amplification of extensive mat forming of APS Polygonium sp., Ludwigia sp. and Altermanthera philoxeroides in water edge areas. Wedelia trilobota, Ulex europaeus and Colocasia esculenta were found to form thickets in peripheral areas displacing the grass land gradually while modifying the structure and function of the lake. APS in water edge areas was found to facilitate sediment accumulation in greater amounts while converting lake into a semi-terrestrial area and choking in flow channels. Now its open water area estimated was <0.5 Km2. Lake water quality was found to deplete rapidly, where most parameters exceeded permitted levels i. e. pH 7.8, EC 103.5 mS/cm, ammonia 0.38 mg/l, nitrate 3.7 mg/l and phosphate 1.04 mg/l. The condition was attributed to dense APS patches. It is clear now that the lake is persisting in a meso to eutrophic condition. Comparably less similar conditions were recorded in down stream at Meepilinama (Nanu Oya; BOD 8.14 mg/l, pH 7.10, DO 7.00 mg/l, EC 99.72 mS/cm, ammonia 0.14 mg/l, nitrate 2.34 mg/l and phosphate 1.54mg/l) in spite of receiving high quality water from a natural forest stretch. It appears that water quality depletion is so severe at the lake that it enables its down stream to self purification at this extension. Heavy use of agrochemicals and fertilizers in riparian areas and high silation were found as main pollution causes. The additional contribution of APS colonies blocking the air-water interface resulted low oxygen levels (DO <5.2 mg/l). Dumping huge quantities of waste as well as outflows from neighboring hotels, houses, etc. were other known threats which contributed to make the lake sometimes unpleasant. Discarding unwanted vegetable parts into the lake areas was another big problem as it adds on to the organic pollution. Altered floral composition in Lake Gregory is mainly due to heavy loading of nutrients and sedimentation from crop/tea farming and ecotourism based activities. Therefore, control/mitigation of such activities and removal of APS from time to time are highly recommended in addition to dragging as the lake is now rapidly loosing its ecological sustainability.