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 μg L⁻¹ at Fond du Lac, 44 μg L⁻¹ at Pokegama) that were orders of magnitude higher than elsewhere in the wetlands. Summer denitrification potential was high in the levees (≈6 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 NO⁻ ₃-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⁻² 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 (≈4 μg L⁻¹), NH₄–N (≈6 μg L⁻¹), total P (TP) (≈80 μg L⁻¹), total suspended solids (TSS) (≈6 mg L⁻¹), and volatile suspended solids (VSS) (≈4 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.