INFLUENCE OF PURPLE LOOSESTRIFE INVASION ON POREWATER CHEMISTRY IN A MICHIGAN PEATLAND

Invasion of the exotic macrophyte, purple loosestrife (Lythrum salicaria), is of serious concern in Great Lakes basin wetland environments. Recent studies suggest that purple loosetrife colonization of wetlands may have a significant impact on sediment geochemistry. However, detailed studies of porewater chemistry, including redox sensitive species, nutrients and metals measured during several seasons at recently invaded sites are lacking. In this study, the porewater chemistry of Kleinstuck Marsh, a minerotrophic peatland in Kalamazoo, Michigan, is investigated at four sites, one on the east side of the marsh, and four on the west side of the marsh. The east site is vegetated primarily by purple loosestrife (~45 stems/m²). One west side site has a similar density of loosestrife (~45 stems/m²) together with 2 stems/m² of narrow-leaf cattails (Typha angustifolia) and several other types of plants (~80 stems/m²). 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). Sampling has been completed using porewater diffusion equilibrators to collect samples from the sediment surface to a depth of 50 cm, at ~1-2 cm intervals, in September and November 2006 and in March and June 2007. Pore waters have been analyzed for pH, DIC or alkalinity, dissolved Fe(II), Fe(III), sulfate, sulfide, ammonium and phosphate. Some trace elements were also analyzed. The pore waters are always redox stratified, but there are considerable differences between the cattail and loosestrife dominated sites during the different sampling events. The west marsh cattail site has a broad suboxic zone, with high accumulation of dissolved Fe(II), during September and June. In contrast, little Fe(II) is detected in the loosestrife site, which has high levels of dissolved sulfide. On the east side of the marsh, the suboxic zone is thin, with a broad sulfidic zone. The November and March samples display less variability between the purple loosestrife dominated and cattail dominated sites, and the profiles of redox-sensitive species are quite similar at all western marsh sites during these months.