Consultant's Report - Minnesota State Legislature

MINNESOTA DEPARTMENT OF NATURAL RESOURCES
Feasibility Study to Limit the Spread ofZebra Mussels from Ossawinnamakee Lal{e
that can become entrained in the inhalant current are sorted within the mussel. These non-edible
particles are then enveloped in mucous and are expelled as pseudofeces via the inhalant siphon
(Walz 1978, Ten Winkel and Davids 1982). Studies by MacIsaac and Rocha (1995) indicated
that zebra mussels increase feces and pseudofeces production when exposed to suspensions
containing 25 250 mg!l of clay particles. Griffiths (1993) that reported Secchi disk
transparency in Lake St. Clair increased from a range of 0.5 to 1.5 meters (1.6 to 4.9 feet) prior
to zebra mussel invasion to a range between 1.8 to 2.8 meters (5.9 to 9.2 feet) in 1990. Field
observations generally support the view that biodeposition of feces and pseudofeces associated
with zebra mussel filtering can improve water clarity (MacIsaac 1996).
Biotic Effects
European and North American freshwater ecosystems have experienced profound changes as a
direct and indirect result of zebra mussel invasion (MacIsaac 1996). The following short
discussions describe some of the observed ecological responses to the increased presence of
zebra mussels.
Phytoplankton
As filter feeders that primarily consume phytoplankton, it is not surprising that zebra mussels
have been implicated in the reduction of phytoplankton biomasses. Reeders et al. (1993)
reported a 46 percent decline in phytoplankton biovolume in a pond with zebra mussels, relative
to a reference pond lacking mussels. Chlorophyll-a concentrations in Lake Erie and Saginaw
Bay declined by approximately 60 percent between 1988 and 1991 subsequent to the
establishment of zebra mussels within the region (Leach 1993, Fahnenstiel et al. 1995). Within
Lake Erie, the depletion ofphytoplankton was noted to be most severe directly over zebra mussel
beds (MacIsaac et al. 1992).
Cyanobacteria
The data on the effect of zebra mussels on cyanobacteria (blue-green algae) are mixed.
However, Reeders et al. (1993)' reported that blue-green blooms ofAnabaena, Oscillatoria, and
Aphanizomenon developed in a reference pond but not in other ponds that contained zebra
mussels. In another study, Birger et al. (1978) found that zebra mussels do not feed well on
other blue-green species, and when exposed to Mycrocystis at 10-25 grams per liter, zebra
mussels experienced between 30 and 100 percent mortality. Heath et al. (1995) also observed a
negative response in zebra mussel filtering during cyanobacteria blooms.
Zooplankton
Zooplankton populations appear to be adversely impacted either indirectly (i.e., phytoplankton
food source is reduced due to presence ofzebra mussels) or directly (i.e., rotifers and other small
zooplankton are ingested by zebra mussels) (MacIsaac et al. 1991 and 1995). In Lake Erie,
rotifer abundance declined by 74 percent between 1988 and 1993, which coincides with the
period of time in which zebra mussels became established (Leach 1993). In other Lake Erie
IV-14 Review ofZebra Mussel (Dreissella polymorpha) Species