Nutrient dynamics in relation to geomorphology of riverine wetlands

Variation in water depth and soil properties associated with geomorphic structures can affect riverine wetland nutrient dynamics by altering biogeochemical processes. We examined the seasonal influence of soils and geomorphology on nutrient forms and concentrations in riverine wetlands in northeastern Minnesota (silty soils) and northwestern Wisconsin (clayey soils). Soil, water, and plant biogeochemistry were contrasted between and within the wetlands according to geomorphic features (riverbed, levee, and backwater zones). There were few inter-wetland differences, and most were the result of differences in river water chemistry and levee elevation between the two sites. Levees were hot spots of NO3-N, with spring porewater NO3-N concentrations (340 mug L-1 at Fond du Lac, 44 mug L-1 at Pokegama) that were orders of magnitude higher than elsewhere in the wetlands. Summer denitrification potential was high in the levees (similar or equal to6 nmol N2O g(-1) h(-1)) and in organic backwater zones (8.3 nmol N2O g(-1) h(-1) at Fond du Lac, 4.8 nmol N2O g(-1) h(-1) at Pokegama), but denitrification was consistently NO3--limited throughout both wetlands. Riverbeds were zones of highest P concentration in soil, vegetation, and summer surface water. Sedimentation rates were higher in riverbeds (289 g m(-2) d(-1) at Fond du Lac 54 g m(-2) d(-1) at Pokegama) than in backwaters (80 g m(-1) d(-1) at Fond du Lac, 17 g m(-2) d(-1) at Pokegama). The two backwater zones had comparably low summer surface water concentrations of NO3-N (approximate to4 mug L-1, NH4-N (approximate to6 mug L-L), total P (TP) (similar or equal to 80 mug L-1), total suspended solids (TSS) (approximate to6 mg L-1), and volatile suspended solids (VSS) (approximate to4 mg L-1), This seasonal convergence of surface water chemistry implies that biotic processes common to the two backwater areas override their substrate differences. Backwaters were hydrologically connected to the river mainstem via openings in discontinuous natural levees, but the different water chemistry of riverbed vs. backwater zones indicated minimal water exchange between them. This hydrologic zonation of riverine wetlands by geomorphic structures was the major source of intra-wetland variability.