Material Fluxes Across Wetland Ecotones in Northern Landscapes.

Abstract

The spatial and temporal distribution of sediment, nutrients, and cations in a 2-ha wetland near Shawano, Wisconsin (USA) was used to evaluate the effect of wetland ecotones on water- and windborne fluxes at ecosystem and landscape scales. Within the wetland ecosystem the ecotones studied were beach ridges deposited by post-glacial White Clay Lake, and stream levees deposited by a second-order stream flowing through the wetland. Snow, soil, and surface water data collected from a 52-point sampling grid were geostatistically analyzed to derive contour plots of within-wetland concentrations. Wind-blown snow form the frozen lake surface accumulated in the wetland behind a low beach ridge to a depth of >65 cm, nearly 7 times the depth at wind-protected areas of the wetland. As a result of this snow accumulation, areas to the lee of the beach ridge annually received 10% more water and 1.5% more inorganic N from direct precipitation than did other areas of the watershed. The use of geostatistical plots to analyze wetland surface waters revealed that: (1) within-wetland spatial variability was high on every sampling date except early snowmelt; (2) on a given sampling date, concentration patterns differed for different elements, sometimes substantially; (3) with the exception of NO3 -N, the spatial patterns for a given material changed considerably over time; and (4) many of these spatial patterns were interpretable on the basis of observed water flow patterns and wetland ecology. In general, there was a decrease in the concentrations of inorganic solids and most ions, and an increase in Ca++ and chemical oxygen demand with distance from the stream. Overbank fluxes contributed P to the wetland during spring and fall floods, but diluted ambient P concentrations during snowmelt events. During snow-free periods there was a sharp gradient in NO3 -N concentrations from a streamside value of °5 mg/L to <0.4 mg/L in wetland areas >45 m from the stream. Measurement of the concentration gradient perpendicular to the stream indicated NO3 -N disappearance rates of 6.6 g/100 m of distance during the spring flood. Soil concentrations of mineral matter and P were highest in areas of the wetland closest to the stream; soil concentrations of ammonium and nitrate were spatially disaggregated, and related to levee elevation. At the watershed scale the wetland retained and/or denitrified 15.2% of the total solids, 13.7% of the N, and 14.2% of the P fluxes from the watershed, thus benefitting the water quality of White Clay Lake.