4.0 Limnology of Long Point Bay

4.3.2 Influence of zebra mussels on Lake Erie and Long Point Bay water quality

Figure 4.2 Secchi Disk Measurements (m) in Western Lake Erie for Three Pre-zebra Mussel Years (1984-1986) and Three Post-zebra Mussel Years (1990-1992).

Figure 4.3 Average April/May Weekly Total Diatom Frustules in Western Lake Erie for Eight Pre-zebra Mussel Years (1961-1986) and Three Post-zebra Mussel Years (1990-1992).

Figure 4.4 Mean Chlorophyll a concentrations in Western Lake Erie Between April and September, 1976-1991.

Zebra mussels have had a profound ecological influence on water quality and the aquatic food chain in the Great Lakes because they occur at substantially higher densities than native molluscs and they have prodigious filtering capacities (MacIsaac et al. 1992; Dermott et al. 1993). Abiotic as well as biotic particles are filtered indiscriminately from the water column and either rejected as pseudofeces, digested, or egested as feces (Holland 1993; Berg et al. 1996). Pseudofeces are particles that are taken into the mantle cavity, coated with mucus and ejected via the inhalant siphon or along the ventral mantle margin. In contrast, fecal pellets are made up of nondigestible remnants of absorbed materials and are expelled via the exhalent siphon (Berg et al. 1996). Pseudofecal and fecal particles are both aggregations of smaller particles which, being larger and more dense, are less likely to be suspended in the water column than the individual constituent particles. The result of this large scale removal of suspended particles from the water column has been a substantial increase in water transparency and a concomitant decline in all phytoplankton taxa and chlorophyll concentrations in Lake Erie (Figures 4.2, 4.3, 4.4)(Holland 1993; Leach 1993; Nicholls and Hopkins 1993).

Figure 4.2 Secchi Disk Measurements (m) in Western Lake Erie for Three Pre-zebra Mussel Years (1984-1986) and Three Post-zebra Mussel Years (1990-1992).

Figure 4.3 Average April/May Weekly Total Diatom Frustules in Western Lake Erie for Eight Pre-zebra Mussel Years (1961-1986) and Three Post-zebra Mussel Years (1990-1992).

Figure 4.4 Mean Chlorophyll a concentrations in Western Lake Erie Between April and September, 1976-1991.

Water temperature in the Inner Bay of Long Point between May and September ranges from 18°C to 26°C, well within the tolerance range of temperatures for reproduction in zebra mussels (Strayer 1991). Consequently, the Inner Bay, like the rest of Lake Erie, has supported high densities of zebra mussels since colonization (Knapton and Petrie 1998). Zebra mussels were monitored from 1991 to 1995 in the Inner Bay at Long Point (Knapton and Petrie 1998); they have had a strong influence on the water quality of Long Point Bay, as water transparencies were highest during 1992, the year that zebra mussel densities peaked on the Bay (Knapton and Petrie 1998). While phytoplankton levels were not monitored on Long Point Bay, the waters adjacent to Elgin and Dunnville experienced substantial declines in phytoplankton during 1988 and 1989, which coincides with the establishment of dense populations of zebra mussels in Lake Erie (Leach 1993). Zebra mussels can actually induce oligotrophication, as they incorporate nutrients into their bodies, as well as expel nutrients to the substrate (Stanczykowska 1984; Holland 1983). While the Bay was classified as eutrophic in 1978-1979 based on total phosphorus, nitrate-nitrogen, chlorophyll a, and secchi disc transparencies (Leach 1981), the filtering activities of zebra mussels are offsetting the effects of Big Creek nutrient and suspended solid deposition. Knapton and Petrie (1998), using Bunt et al's (1993) allometric equation for zebra mussel pumping rates, calculated that zebra mussels could filter the entire Inner Bay's water in 17 days. While water clarity is aesthetically pleasing (Holland 1993), and it may increase some forms of recreation on Long Point Bay, the ecological consequences for the food chain are unknown and quite possibly substantial.

In addition to their direct influence on water quality and the food web, mussels influence the cycling of contaminants that readily sorb to the suspended particles (Asmus and Asmus 1991; Geyer et al. 1982; Fisher et al. 1993; Leach 1993). Some of the more important contaminants that are cycled are polychlorinated biphenyls (PCBs) and polynuclear aromatic hydrocarbons (PAHs) (Fisher et al. 1993; deKock and Bowmer 1993; Swackhamer and Skoglund 1993). Zebra mussels can influence the cycling of contaminants by assimilating contaminants into their own tissue, and subsequently by providing a concentrated source (mussel tissue, pseudofeces, and feces) of contaminants to higher trophic levels (Bruner et al. 1994). While consumption of large quantities of contaminated zebra mussels can adversely influence reproduction in waterfowl (e.g. Tufted Duck Aythya fuligula, Scholten et al. 1989), little is known about the contamination level of Long Point's zebra mussels or if there is a present or potential threat to staging waterfowl.