Some Hydrographic aspects of Koggala Lagoon with preliminary results on distribution of the marine bivalve Saccostrea forskalli : pre-tsunami status

Selected hydrographic parameters of Koggala lagoon over a period of four months in 2002- 2003 are reported together with the preliminary results on the distribution of a marine bivalve within lagoon. Such data prior to the December 2004 tsunami may provide back- ground information for comparison of post-tsunami conditions. Moderate yet significant variation in most physico-chemical parameters was found among selected sites. Mean salinity decreased from 34.0 ppt at the mouth to 11.4 ppt at 2150 m into the lagoon indicating pronounced seawater influx and mixing with freshwater. Monthly inter-site variation in salinity was statistically significant (p<0.05). The marine bivalve Saccostrea forskalli (Chemnitz) was the dominant benthic mollusk with numbers (per m²) ranging from 111.5±9.7 at lagoon mouth to 23.5±2.1 at 2150m inner lagoon. Density distribution of S. forskalli was significantly correlated (p<0.05) with the overall variation in salinity (r=0.889). Salinity-dependent colonization success of S. forskalli demonstrates the dominance of marine conditions in Koggala lagoon.