6.0 Aquatic Macroinvertebrate Distribution and Abundance in Long Point Bay

6.2 Distribution and abundance of native aquatic macroinvertebrates in Long Point Bay

Table 6.1 Invertebrate Distribution and Composition in Long Point's Inner Bay, Shown as an Average Number per m2 at Each Sampling Station.

Table 6.2 Mean Numbers of Invertebrate Taxa in Samples Collected from Throughout the Nearshore Region of Lake Erie, Grouped by Type of Sediment and Water Depth.

Table 6.3 Density Of Macroinvertebrates (# per m2) Sampled at Selected Ponds on the Long Point National Wildlife Area.

Based on the LPWWRF 1992 survey, native aquatic macroinvertebrates are distributed somewhat unevenly throughout the Inner Bay (Table 5.1, see Figure 5.1 for the boundaries of sampling areas) and the densities are below those considered to be sufficient for successful growth and survival of ducklings in nearby Brant County (>3600 per mē)(Godin and Joyner 1981). The North Shore, and Big Creek areas in particular had low invertebrate densities, while the open water area had slightly higher densities (Table 6.1). Turkey Point, Crown Marsh and Thoroughfare Point had the highest invertebrate densities on the Inner Bay. This may be attributed to the fact that the distribution and abundance of aquatic macroinvertebrates is influenced both spatially and temporally by several abiotic and biotic factors. For instance, invertebrate community structure at any given time or location is influenced by the type of substrate, amount and species composition of aquatic vegetation, kinds and amount of organic matter available, and the quality (high phosphorus loadings increase the availability of phytoplankton to herbivorous invertebrates), depth, temperature and oxygen concentration of water (Krecker and Lancaster 1933; Dermott 1978; Barton 1988). As over 90% of the Inner Bay is covered by submerged aquatic plants, and the water characteristics and depth are fairly uniform, differences in invertebrate species composition and availability among areas is most likely a function of the availability of particular plant species and type of substrate. Due to a lack of thermal stratification and macrophyte photosynthesis, dissolved oxygen is probably not limiting to invertebrates on the Inner Bay in (Leach 1981).

Table 6.1 Invertebrate Distribution and Composition in Long Point's Inner Bay, Shown as an Average Number per m2 at Each Sampling Station, Summer 1992.

Table 6.2 Mean Numbers of Invertebrate Taxa in Samples Collected from Throughout the Nearshore Region of Lake Erie, Grouped by Type of Sediment and Water Depth. "Coarse" Includes Bedrock, Pebbles, and Gravel.

*ns means no sample taken                                                                                                            From Barton 1988

Wilcox and Knapton (1994) did not survey the invertebrates in the Outer Bay of Long Point. However, Barton (1988) did do a cursory survey at a few locations. He found that Gammarus fasciatus, Pseudochironomus sp, and Heterotrissocladius changi were largely restricted to Outer Long Point Bay, but he made no mention of biomass or species diversity. Gammarus fasciatus in particular was ubiquitous and randomly distributed within the Outer Bay, but was collected only rarely in the central basin of Lake Erie. While the relative availability of invertebrates in the Inner and Outer Bay have not been studied, invertebrates are much more abundant in vegetated areas than non-vegetated areas (Krull 1970). For example, Voigts (1976) found that maximum numbers of invertebrates occurred where beds of submerged aquatic plants were interspersed with stands of emergent vegetation, and Krieger (1992) determined that wetlands with ample submergent and floating-leaved vegetation possess a higher diversity and biomass of invertebrates than wetlands with limited aquatic plant availability. This can be attributed to the fact that aquatic plants are important as indirect and direct food sources; they also provide shelter from turbulence and predators, sites for oviposition, and sources of oxygen (Brooke 1984). Therefore, the reduced availability of aquatic vegetation and lower productivity of the Outer Bay probably predisposes it to having a lower native aquatic macroinvertebrate availability than does the Inner Bay (Leach 1981).

LPWWRF (unpublished data) sampled invertebrates on 6 ponds and creeks (Helmer's, Cedar Creek, Bouck's, Anderson, Duncan's and Long) on the Long Point spit during late summer, 1992. Surprisingly, no zebra mussels were recorded and the density and diversity of native invertebrates was low, relative even to the Inner Bay (Table 6.3). Amphipods (Gammaridae) and Diptera (Chironomidae) comprised about 80% of all organisms. While only a cursory investigation of Big Creek Marsh has been undertaken, benthic macroinvertebrates appear to be dominated by chironomid larvae and amphipods, while quantities of Physidae, Odonata, and Isopoda were also present from mid July to mid October (Wade 1979).

Table 6.3 Density Of Macroinvertebrates (# per m2) Sampled at Selected Ponds on the Long Point National Wildlife Area. (sites sampled on either 26 August or 4 September, 1992)