A POST-SETTLEMENT TRANSITION TOWARDS PHYTOPLANKTON DOMINANCE IN SHALLOW WEST-CENTRAL MINNESOTA LAKES

Shallow lakes and wetlands are increasingly recognized for their important role as organic carbon sinks, yet geological reconstructions of the history of carbon burial and other dynamics in shallow lakes are relatively rare. Here we use geochemical proxies indicative of organic matter sources and primary productivity to explore the recent paleoecology (last few centuries) of numerous shallow lakes in west-central Minnesota, part of the Prairie Pothole Region. European settlement of the region (ca.1880) clearly marks the most significant event in these records, although in some cases site-specific management efforts in more recent decades are also notable. The majority of the lakes show post-settlement trends of decreasing carbon to nitrogen ratios (C/N) and increasing biogenic silica accumulation rates (BSi flux), suggesting a general shift towards increasing phytoplankton presence in these lakes. Many of the lakes also show a notable post-settlement change in N source (enriched bulk sediment δ¹⁵N) indicative of increased soil and/or livestock waste N. The post-settlement eutrophication of these lakes is consistent with observed increases in the number of shallow lakes in a turbid, phytoplankton-dominated regime throughout Minnesota. Recent work exploring a hypothesized link between lake regime (turbid to clear) and organic carbon burial suggests that the clear regime might increase burial efficiency. Without further management efforts, one consequence of the post-settlement transition in these lakes is a potential reduction in carbon burial.