Dr. Scott Petrie, Long Point Waterfowl Executive Director
Dr. Hugh Henry, University of Western Ontario
Dr. Michael Schummer, Long Point Waterfowl Scientist
Wetlands in the lower Great Lakes (LGL) region are altered or degraded by stressors which can reduce habitat quality by altering communities of submerged aquatic vegetation (SAV). Changes
in biomass and species composition of SAV can affect carrying capacity for staging waterfowl and other animals. Introduction and proliferation of zebra (Dreissena polymorpha) and quagga mussels (D. burgensis; hereafter Dreissenid mussels) has been associated with changes in the abundance, distribution, and community composition of SAV within the LGL. Filtering by Dreissenid mussels influences water clarity and quality, thereby influencing species-specific growth and altering SAV communities.
Because LPB is an important wetland habitat used by staging waterfowl and other fish and wildlife in the LGL, researchers have quantified the distribution, relative abundance and long-term temporal dynamics of the SAV community; Smith(1979) mapped the distribution of SAV in LPB during the 1960s and 1970s; Knapton and Petrie(1999) and Petrie and Knapton(1999) assessed the distribution and relative abundance of SAV and Dreissenid mussels in the mid-1990s and compared their findings with those of Smith (1979). However, no studies have determined seasonal dynamics of SAV at LPB between autumn and spring. Seasonal abundance is important in determining carrying capacity of LPB for waterfowl, and these data would be beneficial to conservation planners charged with determining habitat suitability and availability through the annual cycle of waterfowl.
With the exception of slender pondweed, the 5 most abundant SAV species in LPB have experienced declines in abundance since 1992. Although this response was predicted for charophyte species in LPB (muskgrass), it contradicted the predicted increase in angiosperm species abundance expected with more eutrophic conditions. Because nutrient enrichment and light availability are interrelated, it is possible that a synergistic effect could better explain SAV growth within LPB. Although angiosperm SAV species are known to proliferate in eutrophic water (Smith 1979), it is possible that there is a point at which there is not enough light available to stimulate growth, regardless of nutrient availability. Inputs of nitrogen and phosphorus from fertilizer applications can also influence declines of SAV and enhancement of phytoplankton growth. Increased abundance of suspended particles in the water column decrease light availability for SAV and reductions in growth are often observed. As such, the decline in filter-feeding Dreissenid mussels in LPB and an associated increase in phytoplankton growth may have reduced light availability for SAV below a threshold that retards growth of these plants. My results suggest that SAV abundance is related to Dreissenid mussel densities within LPB, with increased SAV and mussels in the 1990s. Further, SAV abundance decreased along with a 96% decrease in mussel density between the mid-1990s and 2009.
Food availability and, thus energetic carrying capacity in LPB currently meets and exceeds the requirements of migrating waterfowl in both the autumn and spring seasons. Considering all species of SAV, there is a 10-fold surplus of energy in LPB. However, previous studies suggest that some species of SAV are avoided relative to their abundance. When we excluded muskgrass (80+% of the SAV community by biomass) from our analysis, energy available and surplus were greatly reduced (Fig1). Habitat managers should assess the utility of muskgrass for the suite fish and wildlife that use LPB and, if justified, consider methods to reduce the abundance of muskgrass in LPB while concurrently increasing availability of other species of SAV to increase diversity and forage quality of the plant community for waterfowl.
Ducks Unlimited Canada
Long Point Waterfowl
Ontario Federation of Anglers and Hunters
Ontario Ministry of Natural Resources