The Evolution of Sulfide in Shallow Aquatic Ecosystem Sediments: An Analysis of the Roles of Sulfate, Organic Carbon, and Iron and Feedback Constraints Using Structural Equation Modeling

The generation of elevated concentrations of sulfide in sediment pore waters that are toxic to rooted macrophytes is problematic in both marine and freshwaters. In marine waters, biogeochemical conditions that lead to toxic levels of sulfide generally relate to factors that affect oxygen dynamics or the sediment iron concentration. In freshwaters, increases in surface water sulfate have been implicated in decline of Zizania palustris (wild rice), which is important in wetlands across the Great Lakes region of North America. We developed a structural equation (SE) model to elucidate key variables that govern the evolution of sulfide in pore waters in shallow aquatic habitats that are potentially capable of supporting wild rice. The conceptual basis for the model is the hypothesis that dissimilatory sulfate reduction is limited by the availability of both sulfate and total organic carbon (TOC) in the sediment. The conceptual model also assumes that pore water sulfide concentrations are constrained by the availability of pore water iron and that sediment iron supports the supply of dissolved iron to the pore water. A key result from the SE model is that variations in three external variables (sulfate, sediment TOC, and sediment iron) contribute nearly equally to the observed variations in pore water sulfide. As a result, management efforts to mitigate against the toxic effects of pore water sulfide on macrophytes such as wild rice should approach defining a protective sulfate threshold as an exercise tailored to the geochemistry of each site that quantitatively considers the effects of ambient concentrations of sediment Fe and TOC. Plain Language Summary Aquatic plants, such as wild rice, white rice, and waterlilies that have roots in the saturated soils of wetlands are vulnerable to the buildup of toxic levels of hydrogen sulfide (also called sulfide). Anaerobic bacteria in the soil make the sulfide from sulfate that penetrates the soil from the overlying water. When sulfate in the waterbody is low, sulfide in the soil is low. But when sulfate is high, sulfide has been hard to predictsometimes low, sometimes high. The analysis of hundreds of wetland samples finds that sulfide can be predicted if two variables in addition to sulfate are considered: organic carbon in the soil, which is the food for the bacteria, and iron in the soil, which removes sulfide from solution. A model of the chemical reactions finds that the three variables, sulfate, organic carbon, and iron, are equally important in determining sulfide. The sensitivity of individual waterbodies to sulfate pollution effects on wild rice toxicity can thus be predicted from the analysis of the carbon and iron concentrations in the soil of a wetland.