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.
Bibliographical noteFunding Information:
This study was supported by the Clean Water Fund, created by the Clean Water, Land and Legacy Amendment to Minnesota’s constitution. We thank the many members of the field crew who collected the field data upon which this analysis is based. LacCore was supported by the National Science Foundation (EAR-0949962). The field data modeled in this study are included as a supporting information. A partial data set is available in the EarthChem database: DOI: 10.1594/IEDA/100681. The full data set is available in the Data Repository for U of M (DRUM): DOI: 10.13020/D6130R; http://conservancy. umn.edu/handle/11299/185506.
- pore water