Distribution of water chemistry, trace metals, and Chironomidae in coastal freshwater rock pools

Egan, Alex; Burge, David; Edlund, Mark; Lafrancois, Toben; Ferrington, Jr, Leonard

Coastal rock pools of Lake Superior in North America are a mosaic of biologically rich ecosystems that vary in their physical morphology and chemistry. Occurring in bedrock depressions between the forest edge and the lake, the aquatic communities in these pools are susceptible to impacts from both international shipping lane traffic and changing climate patterns. Individual pools are small, but the number of pools across the coastal landscape provide an expansive network of variable habitats, with >70,000 pools found in the 48 km of shoreline in the study area. In this study, we aimed to understand the relationships between aquatic invertebrates and abiotic conditions in coastal pools. The Dipteran family Chironomidae were studied because of their ubiquity and diversity in freshwater systems. We used nonmetric multidimensional scaling to describe how Chironomidae diversity and abundance were related to seasonality, habitat, and environmental factors (water chemistry and trace metals). We observed 2 distinct chironomid assemblages associated with habitat type based on chemical characteristics and physical location on the shoreline, including numerous rheophilic species near the lake, where consistent wave wash mimics lotic conditions, and species preferring lentic-mesotrophic conditions near the forest edge. Conductivity and pH were higher in pools near the lake, whereas temperature and dissolved O2 varied seasonally. Nutrients and metals differed between study sites and pools at each site. At 2 sites, naturally occurring Cu and Al were found at concentrations that exceeded estimated chronic toxicity for sensitive aquatic invertebrates. Pool separation of known tolerant or sensitive species indicated a potential response to elevated metal concentrations. Metals may be an important driver for sensitive species occupancy and survival in certain pools, raising more general concerns regarding community responses to natural conditions in pristine environments.