Given the varying tolerances that different plant species have for specific environmental variables, something that poses a threat or stress to a given species or group of species (i.e. eutrophication) will often act to enhance the survival of another, sometimes less-favored species. While aquatic macrophytes are fairly resilient to, and often benefit from, natural environmental change (e.g. fluctuating water levels), human activities (e.g. sedimentation, dredging, contaminants) can have substantial long term effects on the stability and community composition of aquatic macrophytes. The environmental variables that probably have the most influential effect on the distribution and abundance of submerged macrophytes in the Inner Bay are nutrient and contaminant concentrations, water levels (influence on light availability), turbidity, ice scouring, introduced exotics, interspecific competition and substrate and sediment composition (Knapton and Petrie 1999).
Fluctuating water levels are an essential component of wetland functioning. Periodic increases in water levels inhibit the formation of monoculture marshes by periodically killing dominant species such as Typha spp., thereby creating gaps for other species to colonize during low water periods (Keddy and Reznicek 1986; Klarer and Millie 1992). Low water periods facilitate wetland vegetative regrowth, because exposure of mudflat areas permits seeds to germinate (Klarer and Millie 1992). Therefore, fluctuating water levels result in increased aquatic plant diversity, and are an important regulating force determining macrophyte species composition (Prince and D'Itri 1985). High water also kills woody plants, thereby limiting the rate of marsh succession. Therefore, water level stabilization (see Chapter 2) would likely function to reduce marsh area, and the diversity of aquatic plants (Keddy and Reznicek 1986).
The primary sources of sedimentation and turbidity in Long Point Bay are via Big Creek Marsh and Lake Erie; sediment resuspension also occurs due to wave action (see Chapter 3). High turbidity results in decreased aquatic plant biomass and species richness. For instance, increased turbidity inhibits the presence of many submerged macrophytes including Elodea spp., Myriophyllum spp., and Potomogeton spp. (Robel 1961; Klarer and Millie 1992). Increased rates of sedimentation can also adversely affect aquatic plants. For instance, silt deposition from a single storm in the Chesapeake region resulted in a decline in the areas wild celery beds (Rybicki and Carter 1986).
Ice scouring can also have a significant impact on macrophyte communities, especially in areas close to shore (Crowder and Bristow 1986). Knapton and Petrie (1999) suggested that the complete disappearance of submerged macrophytes in some areas of the Bay, and the low species richness at others, is likely due to ice scouring in spring. However, as ice scouring does not alter growth conditions in the affected area, such losses of aquatic vegetation are probably short term.
Eurasian milfoil has been the most prominent flowering invader at Long Point, as it tends to form nearly monospecific communities, displacing native plant species. Its success at Long Point can probably be attributed to the fact that it grows well in eutrophic waters, and begins to grow early in the spring, thereby shading out and outcompeting many other species. This may be problematic as it results in the loss of other aquatic plants that are important foods for waterfowl. However, Eurasian milfoil generally only maintains its dominance for 10-20 years after introduction (Smith and Adams 1986). Eurasian milfoil may be experiencing this decline in dominance on the Inner Bay, as its percent frequency of occurrence declined substantially between 1992 and 1995 (Table 5.3).
Carp can also cause a threat to submerged aquatic plants, and they may be important in determining the aquatic macrophyte composition in Great Lakes coastal marshes. Carp can be extremely numerous in shallow water areas, and they consume fairly large amounts of submerged aquatic plant material (Powles et al. 1983) Even more importantly, they tend to uproot substantial quantities of vegetation and disturb sediments during feeding and spawning (Painter et al. 1988). Carp generally cause a reduction in vegetation biomass; the removal of carp and turtles from coastal portions of the Chesapeake Bay resulted in an increase in aquatic plant biomass (Carter and Rybicki 1985). Carp can have a particularly strong affect on Chara spp. (King and Hunt 1967), which is an important food source for waterfowl staging at Long Point. Muskrats also consume submergent vegetation, but probably not nearly to the extent that they eat emergent vegetation (Fuller et al. 1984). As the loss of staging habitats forces birds to congregate on remaining habitat, waterfowl consumption of aquatic macrophytes may also be adversely affecting preferred plant foods at Long Point (see Crowder and Bristow 1986).
Long Point Bay receives substantial quantities of nutrients from Big Creek and is somewhat eutrophic. Macrophyte production may initially increase in response to added nutrients, but increased levels of phytoplankton, epiphyton, and zooplankton will eventually result in the decline of submerged macrophytes through shading (Phillips et al. 1978). For example, decreasing species richness correlated with increasing eutrophication at Coote's Paradise, Burlington Bay (Crowder and Painter 1991). Therefore, any increase in the nutrient availability to Long Point Bay could precipitate a decline of preferred waterfowl plant foods. However, this is unlikely in the near future, given stricter legislation on phosphorus discharge to the Lake and the filtering affects of zebra mussels (See Chapter 4).
While there is some species specific variability, aquatic plants do not generally bioaccumulate metals and contaminants (see Crowder and Painter 1991). However, sodium chloride, primarily from road salts and sewage treatment plants, stresses certain aquatic plants and can result in a decline in species diversity (Bayley et al. 1978). Also, oil is toxic to aquatic plants, particularly wild celery (Burk 1977).
Construction of marinas has had a direct but localized effect on aquatic plant availability on the Inner Bay of Long Point, especially in the Coletta Bay and Turkey Point areas. Substantial increases in the number of boaters and other recreational users of Long Point Bay over the past few decades may also be influencing aquatic plant growth directly by damaging/disturbing plants and indirectly by increasing water turbidity.
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