PREDICTING LESSER SCAUP WETLAND USE DURING SPRING MIGRATION IN EASTERN SOUTH DAKOTA

The relative influence of physical, chemical, and biotic wetland characteristics on wetland use by spring migrating lesser scaup (Aythya affinis (Eyton); hereafter "scaup") is not well understood. We com- pared characteristics of used and unused wetlands in eastern South Dakota. Used wetlands were larger (>2 ha; P = 0.05), with higher amphipod densities (>500 individuals m -2 ; P = 0.01) and higher chlorophyll-a concentra- tions (>0.2; P < 0.05). These wetlands had lower ionic conductivity (1.8 mS; P = 0.02), lower nitrates (1.0 ppm; P = 0.01), lower submerged aquatic vegetation density (P < 0.01), and lower fine sediment proportions (≤150 µm grain size; P < 0.01). Wetland use was best described by amphipod and submerged aquatic vegetation densities. The predictive model explained 50.4% of the variation in scaup use in a reserve dataset. Thresholds of tolerance by amphipods in relation to wetland habitats in the upper Midwest should be investigated further as indicators of a broader range of water and habitat quality characteristics for scaup.