Turbidity Tolerances of Great Lakes Coastal Wetland Fishes

—Despite recent interest in assessing the condition of fish assemblages in Great Lakes coastal wetlands and a concern for increasing turbidity as a major stressor pathway influencing these ecosystems, there is little information on fish tolerance or intolerance to turbidity on which to base wetland assessment metrics. Existing studies have borrowed tolerance designations from the stream literature, but they have not confirmed that the designations apply to Great Lakes wetlands or that designations based on tolerance to degradation in general apply to turbidity in particular. We used a published graphical method to determine turbidity tolerances of Great Lakes fishes based on their pattern of occurrence and relative abundance across coastal wetlands spanning a turbidity gradient. Fish composition data were obtained from fyke-net and electrofishing surveys of 75 wetlands along the U.S. shoreline of the Laurentian Great Lakes, representing a turbidity range of approximately 0–110 nephelometric turbidity units (NTU). Turbidity levels of 10, 25, and 50 NTU (corresponding to the thresholds in use for state water quality criteria) were used to separate fish into tolerance classes. We found that the turbidity tolerances of many species in Great Lakes wetlands differed from the published tolerances to general degradation in streams. Also, the tolerance levels for many species were unclear owing to the species’ infrequent occurrence. Although many of the wetlands sampled had quite low turbidity, a large proportion of the fish species were tolerant or moderately tolerant to turbidity and very few were intolerant, suggesting that enumerating intolerant species may not be a useful metric or that the metric should be expanded to include moderately intolerant species. Our study lays the foundation for additional turbidity indicator development efforts for Great Lakes coastal wetlands. Coastal wetlands play an important role in supporting fishes in the Laurentian Great Lakes (Jude and Pappas 1992; Whillans 1992; Wei et al. 2004), and there is considerable interest in developing tools for assessing ecological conditions in these increasingly impacted and degraded ecosystems (e.g., Environment Canada and USEPA 2003; Lawson 2004). Decreasing water clarity, as measured by turbidity, is one major pathway via which anthropogenic nutrient and sediment loading affect fishes in coastal wetlands. An altered light regime has implications for visual foragers and predator–prey interactions as well as impacts on the reproductive and foraging habitat structure provided by submerged aquatic vegetation (Brazner and Beals 1997; Crosbie and Chow-Fraser 1999; Lougheed et al. 2001; Jude et al. 2005). Inclusion of metrics assessing fish responses to turbidity would therefore be a desirable part of broader fish condition assessments