Effects of within-lake gradients on the distribution of fossil chironomids from maar lakes in western Alaska: implications for environmental reconstructions

We examined fossil chironomids (Diptera: Chironomidae) in the surface sediments of four maar lakes in western Alaska to determine chironomid distribution patterns with respect to within-lake gradients of water depth, LOI (loss-on-ignition), and bottom-water temperature. Linear and non-linear regressions were undertaken to test whether the within-lake distributions of fossil chironomids were uniform. Additionally, water depths where abrupt changes or breakpoints in the assemblages occur were identified using piecewise regression. Direct gradient analysis was then used to examine variation in the assemblages explained by the environmental data. For the shallowest lake, chironomid abundances of individual taxa and inferred temperatures varied little within the lake. For the three deep lakes, seven of the sixteen commonest fossil taxa varied significantly with water depth, although some lake-specific patterns were evident. Water depth was generally identified as the principal environmental variable in explaining variation in the assemblages, although sediment organic matter content and bottom-water temperature were also important. Abrupt changes in assemblages occurred at different water depths in each lake, and at only one lake did the breakpoint occur within the range of water depths defining the thermocline. Chironomid-inferred temperature trends from the lakes also showed depth-related patterns: the warmest inferred temperatures were generally from both the shallowest and deepest water depths, whereas intermediate depths yielded temperature inferences about 0.5 to 1.0°C cooler than the average within-lake value. Nevertheless, we conclude that these patterns had only a slight impact on temperature reconstructions relative to the prediction error of the model. A greater understanding of taphonomic processes is needed to determine their influence on environmental reconstructions based on chironomids. Handling editor: J. Saros