Diked coastal wetlands along the Great Lakes have stable water levels between management actions; however, internal nutrient processing can vary across a wetland and lead to net nutrient release or retention associated with shifts in hydrology via management. Determining the direction and rate of nutrient exchange at the sediment-water interface of a diked coastal wetland can indicate the potential for nutrient release upon a shift in management actions. Long established methods for measuring nutrient exchange at the sediment-water interface often involve the incubation of intact sediment cores, but isolating sediment from the surrounding ecosystem could lead to inaccurate estimates of nutrient exchange occurring in the field. We asked how nutrient exchange varies at the sediment-water interface across vegetation patches of a diked Lake Erie wetland? And, how in situ and ex situ methods of measuring nutrient exchange compare? Our objectives were to estimate ecosystem nutrient exchange rates across the wetland based on area weighted means from each vegetation patch and to better explain differences associated with methods. We measured sediment-water nutrient flux (mg/m2/d) using intact sediment core incubations (ex situ) and stacked-resin bag core deployments (in situ) across five distinct vegetation patches in a diked wetland on the southwest coast of Lake Erie. In intact sediment cores, soluble reactive phosphorus fluxes range from -10.6–11.1, nitrate fluxes range from -2.9–4.2, and ammonium fluxes range from -3.9–151.8 (negative fluxes indicate net transfer from surface waters to sediments). Nutrient exchange rates are generally higher in magnitude from intact core incubations than stacked-resin bags, which could be attributed to the discrepancy of the temporal scale captured by the two methods (3 vs 10 days, respectively) or by confounding variables occurring in situ which are excluded ex situ. Understanding how sediment-surface water nutrient exchange varies spatially and temporally can guide management decisions in Great Lakes coastal wetlands, but also must be measured at a scale relevant to land managers.
