Abstract
We conducted a field study including a series of cruises over an isolated offshore zebra mussel reef (7–11 m deep) in Western Lake Erie to examine the effect of zebra mussels ( Dreissena spp.) on the water column. The horizontal currents over the reef were found to be primarily due to the hydraulic flow and surface gravitational seiches. The turbulence generated by these currents was found to be too weak to fully mix the water column. Although seasonal stratification was not observed, solar heating during the day and intrusions of cold central basin water caused stable stratification of the water column 60% of the time. Results from the seston analysis taken at five depths showed a statistically significant mussel‐feeding signature in chlorophyll a and organic seston concentrations measured within 2 m above the reef. Estimates of clearance rates based on field data were consistent with rates measured in a flow chamber using water from the site, which indicated that zebra mussels could remove up to 40% of the total seston. The detection of a zebra mussel‐induced concentration boundary layer is due to: (1) reduced vertical mixing as a result of semidiurnal periodic stratification, (2) refiltration of bottom water in zebra mussel populations, and (3) in situ clearance rates that are lower than those observed in the laboratory. Thus, offshore zebra mussel colonies may have less of an effect on the water column than had been previously estimated by simple stirred reactor models, and the role of zebra mussels in the clarification of Lake Erie should be investigated further.