6.0 Aquatic Macroinvertebrate Distribution and Abundance in Long Point Bay

6.5 Importance of macroinvertebrates to waterfowl staging on Long Point Bay

Table 6.8 Aggregate Percent Dry Mass and Percent Occurrence of Invertebrates Consumed by 12 Species of Waterfowl (579 Birds) Collected During Fall on the Inner Bay of Long Point.

Despite popular belief, Long Point's importance to waterfowl is primarily as a staging habitat during migration, rather than as a breeding area. While the ultimate reason for Long Point's lack of use by breeding waterfowl is unknown, low macroinvertebrate densities relative to inland wetlands may be at least partially responsible. All areas of Long Point's Inner Bay, as well as 6 creeks and ponds surveyed on the spit, had invertebrate densities below the 3600 per mē considered optimal for duckling growth and survival (Godin and Joyner 1981), and plant foods alone do not meet the dietary requirements of young waterfowl (Sugden 1973). The protein content of invertebrates is much higher than that of plants, and pre-laying and laying female ducks switch to a diet that is dominated by invertebrates just prior to and during egg laying (Krapu 1974; Swanson et al. 1974; Hohman 1985). Therefore, ducks select habitats that support high concentrations of aquatic insects during breeding (Joyner 1980; Murkin et al. 1982; Murkin and Kadlec 1986; Krapu and Reinecke 1992).

All of the dietary samples presented in Table 6.8 were collected from hunter-shot birds at the Long Point Waterfowler's Association check station (LPWWRF 1991-1995). The benefit of collecting dietary samples in this manner is that it does not require any additional birds to be killed. The drawback is that not removing dietary samples immediately after collection predisposes those samples to post mortem digestion. This results in the loss of soft bodied invertebrates, as they tend to be digested much more quickly, resulting in a sample that is biased towards less readily digested plant matter. Also, the gizzard contents of several birds were incorporated into the sample, causing a further bias towards plant matter and quite possibly, zebra mussels. The other limitation of using hunter- shot birds is that they do not provide any information about what birds are eating during the spring.

Recognizing the aforementioned limitations, fall staging waterfowl at Long Point consumed extremely low quantities of native invertebrates (Table 6.8). Although Wilcox and Knapton's (1994) sampling indicates a density of up to 2500 invertebrates per mē in certain areas (Table 6.1), these data were collected in mid summer when many invertebrate species reach annual population peaks (Krieger and Klarer 1991). This is supported by the fact that lower temperatures and ice scouring virtually eliminate invertebrates from the 0-2 m zone along the north shore of Lake Erie during spring and fall (Barton and Hynes 1978). This is particularly true of Amphipoda, Trichoptera and Chironamidae, all of which are readily consumed by waterfowl when available. Therefore, the availability of invertebrates to staging waterfowl at Long Point is generally lower than Table 6.1 would suggest.

Table 6.8 Aggregate Percent Dry Mass and Percent Occurrence of Invertebrates Consumed by 12 Species of Waterfowl (579 Birds) Collected During Fall on the Inner Bay of Long Point.

Non-breeding waterfowl will switch to a diet dominated by animal matter when submerged macrophyte availability drops below a critical threshold. For instance, Canvasbacks wintering in North Carolina and Chesapeake Bay have responded to declines in plant food availability by consuming large quantities of molluscs (Perry et al. 1981). That 3 of 6 diving duck species, and all 6 dabbling duck species sampled, virtually ignored zebra mussels, suggests that the availability of aquatic macrophyte food sources is not limiting at Long Point, and that certain species are simply taking advantage of this readily available food source. As the diet of most diving ducks generally includes a high percentage of animal material, including molluscs (Bellrose 1976; Wormington and Leach 1992), those species that have switched to zebra mussels may well have been consuming less readily available native invertebrates before the zebra mussel invasion. Zebra mussels may be particularly important for nutrient reserve accumulation in waterfowl during the spring, as zebra mussels attain their maximum body fat and body mass at this time and this mass subsequently declines to minimum levels in late summer/fall (60% weight loss)(Nalepa et al. 1993).

However, zebra mussels are filter feeders and tend to accumulate contaminants more readily than native Great Lakes bivalves (Brieger and Hunter 1993). These contaminants are subsequently passed up the food chain to the waterfowl that consume them. Lesser Scaup, Greater Scaup and Buffleheads collected on western Lake Erie exhibited elevated tissue concentrations for a broad number of contaminants compared to other species that consume aquatic plants (Mazak et al. 1997). This may be problematic, as the transfer of contaminants from zebra mussels to waterfowl has been shown to adversely affect reproductive output (Scholten 1989; de Kock and Bowmer 1993). Therefore, while large numbers of scaup spp. and Buffleheads have responded to the ready availability of zebra mussels in the lower Great Lakes, the consumption of these introduced mussels may be indirectly influencing their survival and/or reproductive output, and by extension, their continental populations.