Environmental factors affecting the presence of coliform bacteria in water and oysters (Crassostrea cucullata Born, 1778) in Negombo lagoon, Sri Lanka

The recent increase in sewage pollution in the Negombo Lagoon, Sri Lanka, has led to a growing interest in understanding its impact on the local aquatic ecosystem. Physicochemical and general microbiological parameters of the lagoon water (n = 84) were measured at seven sites with contrasted levels of fecal and organic pollution, and their correlation with the presence of total coliform bacteria (TC) was examined. A linear mixed-effect model revealed that heterotrophic bacterial concentrations and electrical conductivity significantly correlated with TC concentrations. Additionally, six individuals of Crassostrea cucullata oysters were sampled from five sites (n = 30) to assess their TC levels and compare their variation across sites. Significant differences in TC levels in oysters were observed across the study sites, with oysters from site S7 located in the Northern part of the lagoon being the most contaminated ones. Fecal indicator bacteria, Escherichia coli, were found to be present across all the studied sites except S5. Additionally, oysters from five sites tested positive for E. coli contamination. The smallest oysters were found at the site most contaminated by microbial load (S2), which may suggest that oysters had decreased filtering activity at the site in response to pollution. Overall, this is the first comprehensive study to provide comparative quantitative data on fecal contamination of oysters in the Negombo Lagoon and its surrounding water.