SEDIMENT ACCUMULATION RATES IN MINNESOTA (USA) LAKES AS UNDERSTOOD THROUGH THE LENS OF LAND-USE HISTORY
Siliciclastic sediment accumulation rates (SAR) for individual lakes span a wide range, from ~10 to >2000 g m-2 yr-1 (across all time periods). Modern accumulation rates are greatest in lakes located within watersheds dominated by agricultural land uses (median 470 g m-2 yr-1, n=27) and lowest in northern forest lakes (median 77 g m-2 yr-1, n=54). Lakes in watersheds characterized by urban or mixed land uses in the central portion of the state show intermediate SAR (median 123 g m-2 yr-1, n=35). In all but 9 lakes, SAR has increased above natural background rates (pre-settlement). Median SAR has approximately doubled in the northern forested lakes but has not begun to return toward pre-settlement levels, perhaps due to legacy impacts from earlier logging. In the agricultural lakes, median SAR has increased more than fivefold, climbing sharply during initial land clearance and early farming but exhibiting a more muted rise after ~1940. The absence of any significant decline suggests that sediment-control benefits imparted by improved land management and soil conservation practices are: 1) overstated, 2) not yet observed in lake records due to considerable lag time; and/or 3) counterbalanced by increasing sediment fluxes from non-field sources.