Teasing out the Effects of Natural Stressors at Chemically Contaminated Sites.

This article focuses on identifying the primary stressors responsible for ecological impairments in an aquatic ecosystem affected by groundwater contaminant upwellings near an industrial settling basin in Pere Marquette Lake, Michigan, USA. Using a four-phase weight-of-evidence (WOE) approach, including in situ Toxicity Identification and Evaluation (iTIE) testing and limnocorral experiments with the amphipod Hyalella azteca, the study found that diurnal fluctuations in dissolved oxygen (DO), particularly low nighttime DO levels, were the main cause of impaired benthic macroinvertebrate survival rather than elevated concentrations of contaminants of concern (COCs) such as ammonia, chloride, pH, and total dissolved solids. Although ammonia levels in sediment porewater were elevated, they were diluted near the sediment–water interface to nontoxic levels. These findings emphasize the importance of distinguishing between natural and anthropogenic stressors to inform effective aquatic ecosystem management and restoration.