Distribution of coliforms in the Seine River and estuary (France) studied by rapid enzymatic methods and plate counts

Fecal contamination in the Seine River and its estuary was studied for the first time by rapid enzymatic methods in parallel with traditional enumerations on selective culture media. The study consisted of four sampling campaigns focused on a 450-km stretch including the Parisian area and presenting highly variable levels of fecal pollution. Enzymatic assays (based on the activity of the β-D-galactosidase and β-D-glucuronidase enzymes in total [TC] and fecal [FC] coliforms, respectively) were in good agreement with classical plate counts of TC and FC. Both methods reflected the strong impact of the wastewater discharge from the Parisian area and of the presence of a maximum turbidity zone (at the mouth of the estuary) on the abundance of fecal bacteria in the river. Downstream from the Parisian outfalls, enzymatic measurements probably detected enzymatically active but nonculturable bacteria disregarded by plate counts. Enzymatic measurements in the Seine River downstream from the Parisian area were used to estimate net disappearance rates of coliforms in the river, which were close to total mortality rates measured with a method based on bacterial DNA labeling with tritiated thymidine. The part of total mortality due to grazing by protozoa was also investigated by the thymidine method. Grazing activity was responsible for 47% to 99% of the mortality of coliforms in the river. Attachment of coliforms to suspended matter (SM) was another factor which could be important in controlling the dynamics of coliforms in the Seine River and particularly in its estuary. Results of filtration and decantation experiments suggested that a significant part of the coliforms can be linked to SM depending on the nature and concentration of the SM.