The effect of long-term nonpoint-source agricultural runoff on assimilative capacity of freshwater marshes: A paleoecological approach.

摘要

Wetlands may act as short-term sink for nutrients because they can remove these materials from runoff by microbial processing, sedimentation, or biomass production. Unfortunately, long-term (i.e., decades) records on the ability of wetlands to trap nutrients from runoff are limited. I quantified accumulation rates of selected nutrients in six sediment cores, dated with 210Pb and ancillary markers (137Cs, Ambrosia pollen), as a proxy for such long-term records in two marshes along the southwestern shore of Lake Erie, Ohio. Cores were sectioned in 1 cm slices and analyzed for grain-size distribution, total phosphorus (TP), bioavailable phosphorus (BAP), total nitrogen (TN), total carbon (TC), organic matter (OM), and selected metals. Both marshes have been managed similarly by the Winous Point Marsh Conservancy since 1850, except that the reference marsh was isolated from runoff from its watershed from 1978 to 1998. Both marshes have been protected by lakeward dikes since ca. 1920 and are situated between agricultural land to the north and Muddy Creek Bay to the south. Unsupported 210Pb activities decreased down-core and approached background at depths up to 30 cm. Dating uncertainty increased with sediment age and made 210Pb-dates older than 100 years unreliable, so that Ambrosia pollen was used to determine the age of older sediments. TP and BAP accumulation rates during the last decade more than tripled in the impacted marsh compared with rates prior to 1978. Such increases were not noted for TC and TN reflecting the importance of atmospheric emission rather than sediment storage for carbon and nitrogen. The ratios of TP to TC and TP to TN in the impacted marsh were higher during the most recent decade of deposition, whereas these ratios decreased in the reference marsh sediments since 1978. Detailed pollen analysis of cores from both marshes revealed the limited utility of Ambrosia as independent age marker in areas with historically abundant wet prairie. Pollen from local/near-shore taxa increased up-core reflecting the impact of marsh management aimed at promoting emergent vegetation. Continued accumulation of sedimentary phosphorus emphasizes the long-term utility of these marshes to mitigate the effects of nutrient-rich farm runoff on downstream systems.