Relationships between land use and sediment accumulation in 116 Minnesota lakes, from pre-settlement to present

Background/Question/Methods

The native landscapes of Minnesota, USA have undergone enormous transformation since Euro-American settlement in the mid-19th Century.  Intensive logging has disturbed areas throughout the northern and eastern portions of the state while near-total conversion of tallgrass prairie into farmland, combined with substantial wetland drainage, has created human-dominated agroecosystems in the south and west.  These occurrences and subsequent land use practices have unquestionably altered the quantity and timing of sediment fluxes from the terrestrial landscape to aquatic environments, but to what extent?  Using paleolimnologic techniques, we explored the long-term historical relationships between land use and terrigenous (siliciclastic) sediment accumulation in 116 lakes spanning 3 ecoregions and multiple land use regimes, including intensive row-crop agriculture.  Estimates were constructed using sediment-core chronologies (based on ²¹⁰Pb, ¹³⁷Cs) and loss-on-ignition data (% siliciclastic material) for the past 150-200 years, with corrections for sediment focusing.  Because these lake sediment records provide an integrated signal of environmental change at the watershed scale, they may yield insights into the degree of aquatic impairment by excessive sedimentation and the effectiveness of conservation strategies in controlling sediment erosion. 

Results/Conclusions

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, SAR has increased more than fivefold.  Accumulation rates climbed sharply during initial land clearance and early farming, but increases after ~1940 appear to be somewhat diminished.  Nevertheless, median SAR has not exhibited any overall decline, suggesting that benefits from improved land management and soil conservation practices in this region are: 1) overstated; 2) not yet observed in lake records due to considerable lag time; or 3) counterbalanced by increasing sediment fluxes from non-field sources.