REGIONAL LAKE ECOSYSTEM RESPONSES TO CLIMATIC AND ANTHROPOGENIC CHANGE OVER THE PAST 2000 YEARS BASED ON BIOGENIC SILICA AND PHOSPHORUS FROM SEDIMENTS OF 18 LAKES IN SOUTHERN MINNESOTA

Euroamerican agricultural impacts on lake ecosystems in the Midwestern US are well documented but less known are lake ecosystem responses to climate change or the physical or biological factors mediating these responses. To better understand these responses we examined sediments deposited over the past 1000-2000 years from 18 small-medium sized lakes in southern Minnesota. Our proxies included biogenic silica (bSi), sediment-P, and sediment magnetics, and these were correlated with lake and catchment physical characteristics as well as historical vegetation. As expected, influx values for biogenic silica and total P were 8-14 times higher in modern sediments (1950-present) than historic (1800-1850) but the bSi:P ratio differed little (67 vs. 62). Across sites, historic bSi and P influxes were both most strongly correlated with catchment-basin ratio (r=0.50 and r=0.51 respectively). Within lakes, cores showed 2-3 fold variation in influx rates for bSi and P, and bSi and P were poorly correlated. There was no one response to on-set of the ‘Little Ice Age' although approximately 1/3 of lakes showed a peak bSi around 1200 cal Yr AD. Variation within and among lakes suggests the importance of local factors as mediators of lake response to climatic change.