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Aims: The Thau lagoon shellfish are regularly impacted by microbial pollution of faecal origin, which cause European health standards to be exceeded and closure of the shellfish harvest. The aims of this study were to investigate the impact of weather and hydrological conditions on Escherichia coli (E. coli) accumulation factor (AF) between water and oysters and to evaluate the relevance of the use of this ratio for the purpose of sanitary risk assessment. Methods and results: Water and oysters (Crassostrea gigas) were sampled simultaneously in situ during 18 months in periods of dry weather and after rainfall events. Shellfish sanitary thresholds were exceeded in both periods. The E. coli AFs measured after rainfall (median = 6) were lower than in dry weather (median = 32), suggesting different shellfish faecal contaminations were operating in this system process. Conclusion: The AFs we measured appeared to be relevant markers for generating sanitary risk assessments for Thau lagoon shellfish. Significance and impact of the study: The results of the study address the need to assess the relationship between water quality measures and shellfish flesh quality. This study will contribute to the elaboration of a management tool to guide local authorities in prioritizing the sources of pollution and in optimizing public investment in the watershed. IntroductionWith up to 13 000 tons of oysters (Crassostrea gigas) and 2500 tons of mussels (Mytilus galloprovincialis) marketed every year, the Thau lagoon is the main shellfish harvesting area in the Mediterranean. Filter-feeding bivalves, such as oysters, filter large volumes of water from their environment and are able to concentrate large numbers of particles, which include micro-organisms pathogenic to human consumers (Bernard 1989;Plusquellec et al. 1990). According to European sanitary rules, the Escherichia coli concentrations found in Thau shellfish correspond to a Class B area. As a result, shellfish must be depurated before they can be marketed. Because Thau lagoon shellfish regularly exceed current health standards, authorities periodically restrict the shellfish harvesting area and, in some cases, suspend production. Moreover, several enteric virus outbreaks due to Thau lagoon oysters occurred during winter gastroenteritis epidemics in the local population .Many studies have shown the impact of rainfall on microbial water quality in coastal and estuarine areas. Contamination due to rainfall most often results from urban wastewater discharges or from nonpoint pollution sources in the watershed (Lipp et al. 2001;Chigbu et al. 2005;Jeng et al. 2005;Coulliette and Noble 2008;Papastergiou et al. 2009;Chu et al. 2011;Conn et al. 2012). Microbial contamination can also occur in dry weather conditions, in which faecal material from seabirds resting on shellfish farming structures is the likely main source (Levesque et al. 2000;Wither et al. 2005;Ogburn and White 2009;Derolez et al. 2010).European regulations recommend monitoring coliform bacteria (e.g. E. coli...

Highlights► The variability of oyster production is mainly controlled by the hydrometeorology. ► Waste water treatment plant (WWTP) efficiency affect oyster production, without significantly improving the ecological status of the Thau lagoon. ► Scenario of oyster stocking density to its authorized maximum triggered an increase of oyster productions at the expenses of thinner oysters with a lower condition index. ► The Thau lagoon remains an exceptional site for oyster farming associated with "good" ecological status condition and where the carrying capacity was not limiting. ► Oyster production might be threatened by drastic measures to be undertaken on WWPT, particularly on phosphorus that seems to control primary production in the Thau ecosystem.