Watershed sediment losses to lakes accelerating despite agricultural soil conservation efforts

Background/Question/Methods

Agricultural soil loss and deposition in aquatic ecosystems is a problem that impairs water quality worldwide and is costly to agriculture and food supplies. In the US, for example, billions of dollars have subsidized soil and water conservation practices in agricultural landscapes over the past decades. We used paleolimnological methods to reconstruct trends in sedimentation related to human-induced landscape change in 32 lakes in the intensively agricultural region of the Midwestern United States.  Sediment cores were collected from each lake and dated using the radio-isotope ²¹⁰Pb.  Bulk sediment was divided into erosional and in-lake produced sources based on the percentage loss on ignition.  Concurrent changes to land-use and agricultural practices were characterized based on agricultural census data and land-cover maps for the past 150 years.  

Results/Conclusions

Despite erosion control efforts, we found accelerating increases in sediment deposition from erosion; median erosion loss since 1800 has been 15.4 tons ha^-1. Sediment deposition from erosion increased >6-fold, from 149 g m^-2 yr^-1 in 1850 to 986 g m^-2 yr^-1 by 2010.  Average time to accumulate one mm of sediment decreased from 631 days before European settlement (ca. 1850) to 59 days mm^-1 at present. Up to 70% of the sediment deposited in lakes from this study was derived from erosional sources and most of this sediment was deposited in the last 50 years.  In addition to increasing erosional inputs, in-lake production, stimulated from nutrient enrichment, tripled since 1950. Increases in sedimentation were more strongly correlated to agricultural intensification post-1940 rather than initial land clearance or predominance of agricultural lands.   Despite the presence of conservation practices in all watersheds in this study, traditional soil conservation programs have not decelerated downstream losses. Despite large erosion control subsidies, erosion and declining water quality continue, thus new approaches are needed to mitigate erosion and water degradation.