Trend analysis of Escherichia coli levels in French shellfish harvesting areas

Following the regulation (EC) n°8542004, the French microbiological monitoring program of bivalve molluscan shellfish harvesting areas (REMI) evaluates and monitors Escherichia coli levels in shellfish from 362 sampling... [ view full abstract ]

Following the regulation (EC) n°8542004, the French microbiological monitoring program of bivalve molluscan shellfish harvesting areas (REMI) evaluates and monitors Escherichia coli levels in shellfish from 362 sampling sites. Since 1989, this network has collected more than 120 000 data. Trends analysis of such a dataset could be a tool to evaluate changes in fecal microbial input from the watershed, in order to assess the efficiency of fecal pollution management policies or to identify priorities in pollution reduction action programs.

The trend analysis method needs to be adapted to E. coli concentration data, which do not follow a normal distribution, present strong seasonal patterns. The seasonal Mann-Kendall test (Gilbert, R.O. 1987. Statistical Methods for Environmental Pollution Monitoring) was tested for each sampling station with 10 years of monthly E. coli data, with a 6 months seasonal cycle. Additionally, data were plotted to show inter-annual and seasonal variability over the 10 years period of each site.

This method revealed gradual and continuous changes in E.coli levels, either upward or downward, over a ten years period. Upward trends could be linked to continuous increase of streamflow as urbanization increases over the watershed. The principal limit of this monotonic method is its inability to detect an abrupt shift of E.coli level and to detect trends over a short period. The approach needs to be completed by other statistical tests allowing the detection of “steps” or non-continuous changes.