Retention and depuration of E. coli by Dreissena polymorpha and its application in watershed management

Potential application of the zebra mussel (Dreissena polymorpha ) as an indicator of historical combined sewer overflow (CSO) events was examined. This unique use of the mussel may considerably reduce the temporal requirements of a riverine sampling schedule when attempting to locate the sources of E. coli to a watershed. It was observed in both laboratory and field experiments that the zebra mussel has the ability to rapidly uptake E. coli, such as during a CSO event, and slowly purge these bacteria from their tissue over the next several days. The river does not need to be sampled while the CSO event is occurring; rather the mussel can be sampled days after the event and still be found to contain an elevated E. coli concentration. This new method allows the examination of information that is typically lost or missed with conventional sampling strategies.;Mussels in a laboratory were exposed to a dilution of raw sewage and river water. The E. coli concentration in the exposure tank was maintained at approximately 1000 cfu/mL. The mussel typically concentrated these organisms to approximately 2000 cfu/mL, given 1.5 to 2 hours of exposure to an E. coli source. The mussel was shucked, homogenized and enumerated for E. coli throughout this exposure period. Mussels were removed from the E. coli source and placed in sterile aquarium water; where they were again enumerated for E. coli as a function of time. The rate order and rate constants were determined describing the uptake and depuration kinetics.;Caged mussels were placed in a river upstream and downstream of a suspected E. coli source in an attempt to use the mussel to detect the presence of this source. Mussels and river water were enumerated daily for 14 days. A small E. coli spike in the river was detected and magnified by an order of magnitude in the mussels. Thus, the zebra mussel provides a potentially valuable mechanism for detecting the presence of E. coli days after the occurrence of an overflow event. The strategic placement of caged mussels along a river and its tributaries could allow watershed managers to significantly reduce sampling frequencies normally required to identify critical locations (sources). This inherent "detection trait" of the mussel offers an opportunity to design a more cost-effective river monitoring strategy for bacteria contamination.