Environmental Quality Assessment of Selected Sites in Bolgoda North Lake Using Three Environmental Quality Determining Indices

Abstract

The Index of Biotic Integrity (IBI) based on fish has been identified as a reliable biomonitoring tool that can be used to assess the quality of aquatic ecosystems. This tool wasapplied to evaluate the habitat quality of three selected sites of the Bolgoda North Lake (6"40'-49' N: 70" 54'-58' E), a brackish water estuarine system located in the western province ofSri Lanka. In order to compare and contrast the accuracy of IBI based habitat qualityassessment, two other common indices, Water Quality Index (WQI) based on selectedenvironmental parameters and Shannon Weiner Diversity Index (SWDI) based onzooplankton and phytoplankton were determined for the three sites.

Three selected sites, namely, Thuduwa, Borupana and Attidiya were sampled for fish,selected environmental parameters and zooplankton and phytoplankton over a period of 6months from February to July 2012. Using 12 metrics that reflected taxonomic composition,habitat composition, tropic composition and fish health, 3 separate fish based Indices ofBiotic Integrity were calculated for these three sites.

The environmental quality of the three sites based on the calculated fish based Indices ofBiotic Integrity concluded that the condition at Thuduwa site was “good”; at Borupana“poor-very poor” and at Attidiya site “very poor”. The calculated WQI values for Thuduwasite qualify it as a site having “good” water quality, Borupana site having “medium” waterquality and Attidiya site having “bad” water quality. Results of both indices were generallysimilar for the three sites. The Shannon Wiener Diversity Index, on the other hand did notshow a significant difference between 3 sites, which could be clearly attributable to thehigh/low evenness values. However, differences shown in the occurrence of indicator speciesgenerally agreed with both IBI and WQI. For instance phytoplankton indicators of pollutionsuch as Microcystis sp, Pediastrum sp, Scenedesmus sp, Actinastrum sp were found in highnumbers at Attidiya, in moderate numbers at Borupana and only the latter two species in lownumbers at Thuduwa.

When all three indices were compared, IBI can be considered as the best index to describethe overall habitat quality, as it reflects the consequences of a long term habitat degradationdue to change in environmental parameters at sites shown by changes in fish assemblageswhereas WQI reflects the precise level of specific pollutants and SWDI reflects thezooplankton and phytoplankton assemblage at any given time (as they are short lived).