I documented zebra mussel (Dreissena polymorpha) population dynamics in a Saginaw Bay, Lake Huron, coastal wetland and investigated the potential effects this filterer might have on marsh zooplankton and macroinvertebrate communities. This research was conducted during 1993, 1994, and 1995. Clusters of mussels over-wintered in the outer area of the emergent marsh and were the source of recruitment for the marsh population. Recruitment was primarily onto stems of the dominant emergent plant, three-square bulrush (Scirpus americanus). Peak larval production and settlement occurred between 300 and 350 m from shore in July and August, respectively. Disappearance of mussels from stems decreased abundance by up to 80% in this area by the end of September in 1994. Lack of recruitment closer to shore and high loss of mussels in the middle section of the marsh was probably due to very little water movement in the center and nearshore areas.
The ability of this population to impact the wetland invertebrate community was examined both experimentally and by comparing the pre-existing community to that present after the invasion. Zooplankton remains, particularly rotifer loricas, were observed in the feces of wetland mussels. Predation by mussels on rotifers and small Bosmina longirostris was experimentally confirmed for high mussel filtration capacities. Mussel filtration capacities more representative of the marsh population did not significantly reduce the abundance of any zooplankton. A laboratory experiment was designed to examine the potential for mussel competition with marsh Cladocera. After being cultured for 14 days in the presence of a range of mussel filtration capacities, Ceriodaphnia dubia abundances were significantly lower at higher filtration capacities. Negative effects of competition on C. Dubia became apparent at mussel filtration capacities higher than those calculated for the marsh mussel population. A comparison of the Cladocera community before (1990) and after (1994) zebra mussels colonized the marsh showed no evidence of changes attributable to zebra mussels.
The wetland macroinvertebrate community showed more evidence of changes potentially related to zebra mussels than did the wetland zooplankton. The abundances of larval Tanytarsini increased on plant stems (p=0.03) between 1990 and 1994 while at the same time decreasing in density in the sediment (p=0.08), perhaps indicating a habitat shift. Abundances of tubificid oligochaetes were significantly higher (p=0.04) in marsh sediments in 1994. No other groups had significant changes in abundance, nor was there a change in the number of taxa present between 1990 and 1994. These results indicate that zebra mussels are not having a major impact on coastal wetland invertebrates.
NOTE:
For work on other macroinvertebrates in Saginaw Bay, see the page on The Invertebrates of a Great Lakes Coastal Marsh.
Some photographs, other information on Dr. Brady can be found at the HSCW Speed Record page.