Determination of human impact and environmental factors that effect on the distribution of mollusks in Lunuwila Ela, Galle

Abstract

Molluscans are one of the most diverse and dominant animal groups among macroinvertebtares. However due to different reasons many species are under high rate of extinction. For conservation purposes, it is important to identify habitats specific molluscan species and the threats affecting their distribution. The current study was conducted to determine the different factors that affect the distribution of molluscans groups in selected area of Lunuwila Ela, Galle. The study was continued up to six months from January to June 2007. Both temporal and spatial data were collected to determine the following objectives. As the first objective the human impact on the distribution of mollusks was estimated. A constructed anicut across the stream was considered as the barrier for the distribution of molluscan fauna and the community structure of the both sides of the anicut was studied. The mean values of nine different water quality parameters were recorded. Among calculated mean values for two sites, the highest difference showed in salinity (19.0770 ppt in site B - 26.074 ppt in site A) and the lowest difference showed in Alkalinity (0.0065 mmolL"1 in site B - 0.0092 mmolL"1 in site A). Five parameters [Chemical Oxygen Demand (COD), Biological Oxygen Demand (BOD), Salinity (SAL), Hardness (HARD) and Total Dissolved Solids (TDS)] showed significant difference between two sides. Five molluscan genera (Faunus, Gangetia, Thiaria, Neritina, Ostria) and one unidentified genera were, recorded from both sides of the anicut. Among them five genera were identified from the upper side of the stream and three genera were identified from the lower side of the stream. Genus Faunas and Gangetia were common to both sides. The species abundance also showed a significant difference between two sides (P < 0.5). High abundance and low diversity values were recorded from the lower part of the stream. Jaccard coefficient (Sj) indicated low similarity between the two sites (P < 0.5). The results derived from the analysis of water quality and community parameters showed that human impact affects the distribution of molluscan group in this stream. Environmental parameters that affected the distribution of Faunas sp. (Mollusca: Gastropoda) were studied as the second objective. Four sites were selected along the Lunuwila Ela, which covered approximately 2 km in length. Nine water quality parameters and substrate type was considered for the analysis. Among nine water quality parameters evaluated, Five of them were BOD, SAL, Alkalinity (ALK), HARD, and TDS varied along the stream. Seven parameters [COD, BOD, Dissolved Oxygen (DO), SAL, Temperature (TEMP), HARD and abundance of Faunus sp.] showed significant difference among sites. According to regression analysis, none of the water quality parameter significantly correlated with the abundance of Faunas sp. Contents of the substrate were varied among four sites. The highest number of individuals of the Faunus sp. were accumulated on the substrate consisted with fine sand, clay and silt. Thus, it can be assumed the structure of the substrate type is one of the major factors that affect the distribution of Faunus sp. along this water body.