Mémoire
Résumé : The Negombo lagoon is one of the coastal lagoons in Sri Lanka that is exposed to sewage pollutiondue to dense human habitation around the lagoon. The purpose of this study was to assess theecological health status of the lagoon by using physicochemical and microbiological parameters asindicators of sewage pollution while investigating their impact on lagoon oysters Crassostreacucullata. Seven sinks of sewage effluents were identified after the survey of the localcommunities around the lagoon. The density of heterotrophic bacteria and salinity had a directeffect on the dissolved oxygen level of the Negombo lagoon. The pH had an indirect effect ondissolved oxygen as they affect the physiology of phytoplankton. Total coliform was found to bedifferent across the sampling areas. A further post hoc test revealed that the north of the lagoonhad the highest total coliform which declined towards the south. A temporal study of the totalcoliform across the different sampling weeks showed no significant difference. Assessment of theWater Quality Index of the sampling areas showed that the lagoon was frequently under threat ofsewage pollution. Also, oysters in the north of the lagoon were contaminated with Escherichia coli.Overall, the Negombo lagoon is generally threatened by sewage pollution except for the southernpart of the lagoon. Also, consuming C. cucullata from the north of the lagoon or utilizing it forrecreational swimming should be done with caution because of the potential risk of E. colicontamination.

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