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CHIRONOMID ANALYSIS OF LAKE SEDIMENTS FROM LAKE ST. CROIX, MINNESOTA TO RECONSTRUCT DISSOLVED OXYGEN FROM 1400 A.D. TO THE PRESENT
Caitlin Eyre Stewart, University of Massachusetts, Amherst
Lake St. Croix is a 37 km natural impoundment located at the southern most extent of the St. Croix River on the state borders of Wisconsin and Minnesota. Land use changes beginning at the time of European settlement have resulted in nutrient runoff into the river. Studies have shown that over time, Lake St. Croix has been adversely affected by nutrient inputs resulting in eutrophication and decreased dissolved oxygen (DO) levels.
Non-biting midges (Insecta: Diptera: Chironomidae) have proven to be excellent paleoecological indicators of water quality because during the larval stage, chironomids undergo ecdysis and molt their sclerotized chitinous head capsules that sink to the lake bottoms where they are preserved in the sediment. Subfossil remains retain diagnostic features that are essential for taxonomic identification. Short life cycles allow chironomids to respond quickly to aquatic perturbations, and many taxa are stenotopic, that is, they are capable of tolerating only a narrow range of aquatic environmental conditions. Species distributions are in near equilibrium with their aquatic environment.
The purpose of this project is to reconstruct dissolved oxygen from 1400 AD to the present using subfossil chironomid assemblages from two sediment cores that were extracted from Lake St. Croix in June, 2006. Magnetic susceptibility profiles were used to correlate these cores with Pb210-dated cores that were previously taken from the same locations. Sediment was processed and head capsules were mounted on slides and identified to the lowest taxonomic level possible. A transfer function relating chironomid species assemblages to dissolved oxygen conditions developed by Roberto Quinlan will be utilized in this study in order to reconstruct DO over the last 600 years. Results will aid in understanding if anthropogenic activities have influenced Lake St. Croix's water quality.