THE SEASONAL INFLUENCE OF MACROPHYTES ON TRACE METAL PARTITIONING IN A MINEROTROPHIC FEN

Purple loosestrife (Lythrum salicaria) is an exotic macrophyte invading US wetlands at an estimated rate of 1000 km² per year (Thompson et al. 1987, USGS), reducing wetland biological diversity and native wildlife habitat. Although this is a significant problem, there has been very little study of the influence of this loosestrife invasion on wetland biogeochemistry. In this study, seasonal variations in wetland solid phase geochemistry are measured at several sites in a minerotrophic fen in Kalamazoo, MI to assess the influence of different macrophytes, including invading loosestrife, on trace metal partitioning in the subsurface. In July, four sites were chosen for study, only one with loosestrife, in a 1 m² area. In September, November, March and June, four sites, three within a 10-12 meter radius on the west side of the ~50 acre marsh preserve, and a fourth site on the east side of the marsh, were studied. The east side site has (~45 stems/m²) of purple loosestrife surrounding the sampling site. One west side site has a similar density of loosestrife (~45 stems/m²) together with 2 stems/m² of narrow-leaf cattails (Typha angustifolia). The second west side site has mostly cattails (~14 stems/m²) and the third site has a mixture of loosestrife and cattails (~28 and 8 stems/m², respectively). A Russian peat corer was used to extract 50 cm soil cores at each site. The cores were split into 5 cm increments and freeze-dried. An operationally-defined sequential extraction technique, modified from Tessier et al. (1979, Anal Chem) was used to extract trace metals (Fe, Mn, Ba, Ni, Zn, Pb, Cu, Co and Cr) associated with four fractions: exchangeable metals, carbonates, reducible (Fe and Mn oxides) and oxidizable (sulfides and organics) in each depth interval. In September, increased concentrations of Fe, Pb, Cu, Ni, Zn, Co and Cr were found in the oxidizable fraction of the peat at the loosestrife-dominated site, consistent with more reduced conditions indicated by pore water data from this site. Loosestrife-free sites had a higher proportion of metals near the peat surface associated with the reducible fraction, presumably bound to Fe and Mn oxides. Ongoing studies are being conducted to assess the influence of loosestrife and other macrophytes on trace metal partitioning in samples collected during the other four seasons.