Natural stressors in uncontaminated sediments of shallow freshwaters: the prevalence of sulfide, ammonia, and reduced iron.

Potentially toxic levels of 3 naturally occurring chemical stressors (dissolved sulfide, ammonia, and iron) can appear in freshwater sediments, although their roles in shaping ecosystem structure (i.e., plant and animal communities) and function (e.g., biologically mediated elemental cycles) have received little study. The present critical review discusses the prevalence and ecological effects of potentially toxic concentrations of sulfide, ammonia, and iron in uncontaminated freshwater sediments, including a review of the literature as well as a case study presenting previously unpublished data on sediment porewaters from a diverse set of shallow (<2 m) freshwater ecosystems in southwest Michigan, USA. Measured concentrations are compared with surface water quality criteria established by the US Environmental Protection Agency (USEPA) and with acute and chronic toxic thresholds in the published literature, where available. Based on USEPA criteria for aquatic life for these 3 stressors, the benthic environment of almost every freshwater ecosystem sampled was theoretically stressful to some component of aquatic life in some area or at some time (i.e., in at least 1 sample), and 54% of samples exceeded more than 1 criterion simultaneously. Organismal tolerances to chemical stressors vary, so the observed concentrations are likely shaping benthic animal communities and influencing rates of ecosystem processes. Consideration of the role of natural chemical stressors is important in shaping freshwater benthic environments and in developing bioassessments, restoration goals, and remediation plans. Environ Toxicol Chem 2015;34:467-479.