SULFUR CYCLING DYNAMICS AND HYDROLOGIC INTERACTIONS AMONG PRAIRIE WETLANDS: THE INFLUENCE OF LANDSCAPE POSITION AND HYDROPERIOD

Prairie wetlands are closed basin depressions with dynamic water regimes that fluctuate in relation to climate variability and position in the landscape, both of which also influence sulfur cycling and carbon sequestration. The goal of this study is to characterize the hydrologic interactions and seasonal aspects of sulfur cycling within and among several prairie wetlands located in eastern South Dakota. Surface waters and groundwater within two sub-catchments comprised of replicate seasonal and semi-permanent wetlands were sampled monthly from April through October. Standard hydrochemical measures were used to characterize surface and groundwater signatures. Stable sulfur and oxygen isotope values of dissolved sulfide and sulfate were used to document spatial and temporal changes in sulfur cycling. Stable isotopes of water were used to infer exchanges of water between wetlands and to quantify groundwater-surface water interactions.

Seasonal wetlands are positioned higher in the landscape compared to semi-permanent wetlands. As a result, hydroperiods were much more dynamic and seasonal wetlands generally dried by late summer. Specific conductance ranged from 437 to 1429 mS cm^-1 in seasonal wetlands and from 574 to 1730 mS cm^-1 in semi-permanent wetlands. Similar overlap was observed for dissolved sulfate concentrations (14.7-478.4 mg L^-1 and 42.8-552.9 mg L^-1 in seasonal and semi-permanent wetlands respectively). Oxygen isotope values of semi-permanent surface waters were several per mil enriched relative to seasonal wetlands while filling in spring. Surface water oxygen isotope values of semi-permanent wetlands more closely resembled groundwater signatures (>2 m). Temporal trends in wetland oxygen isotope values indicate strong enrichment, particularly in semi-permanent wetlands, and likely reflect evaporation and potentially stronger groundwater-surface water interactions. Additional data on the seasonality of sulfur cycling in prairie wetlands will be discussed. These data will be used to illustrate 1) the influence of seasonality on sulfur cycling in prairie wetlands, 2) the extent of hydrologic connectivity among adjacent wetlands, and 3) the importance of hydrology on nutrient subsidies available to wetland food webs.