Nutrient dynamics in relation to geomorphology of riverine wetlands

Variation in water depth and soil properties associated with geomorphic str uctures can affect riverine wetland nutrient dynamics by altering biogeoche mical processes. We examined the seasonal influence of soils and geomorphol ogy on nutrient forms and concentrations in riverine wetlands in northeaste rn Minnesota (silty soils) and northwestern Wisconsin (clayey soils). Soil, water, and plant biogeochemistry were contrasted between and within the we tlands according to geomorphic features (riverbed, levee, and backwater zon es). There were few inter-wetland differences, and most were the result of differences in river water chemistry and levee elevation between the two si tes. Levees were hot spots of NO3-N, with spring porewater NO3-N concentrat ions (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 c oncentration in soil, vegetation, and summer surface water. Sedimentation r ates 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 summ er surface water concentrations of NO3-N (approximate to4 mug L-1, NH4-N (a pproximate to6 mug L-L), total P (TP) (similar or equal to 80 mug L-1), tot al 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 levee s, but the different water chemistry of riverbed vs. backwater zones indica ted minimal water exchange between them. This hydrologic zonation of riveri ne wetlands by geomorphic structures was the major source of intra-wetland variability.