Bubbling trouble: Effects of supersaturated water on benthic macroinvertebrates

The saturation of total dissolved gases (TDG) in water remains around 100%. Certain circumstances can lead to TDG values exceeding 100%, resulting in TDG supersaturation (TDGS). TDGS above about 110% can be toxic to animals that rely on water for gas exchange. However, saturation beyond 200% can occur in freshwater downstream of dams and hydroelectric power plants. Despite its impact, TDGS is often overlooked as a hazard to aquatic life, particularly for benthic macroinvertebrates. This study aimed to examine the effects of TDGS on nine species of benthic macroinvertebrates. We used replicated tank studies to manipulate TDGS levels from 100% to 120% and investigated the overall survival and species-specific effects on survival and buoyancy. We also present a summary on the effects of TDGS on invertebrate species previously tested. The results indicate that seven of nine species exhibited increased buoyancy when exposed to TDGS, causing them to float on the water surface. Additionally, a Cox Proportional Hazards model revealed a significant effect of TDGS on the survival of the macroinvertebrates. The sensitivity towards TDGS varied greatly among species of benthic macroinvertebrates, and significant species-specific effects were only observed for Isoperla grammatica, Baetis rhodani and Asellus aquaticus. Among these, the two latter species showed clear dose-related effects caused by TDGS, enabling the assessment of LT50 (time required to kill half of the tested population). B. rhodani was most sensitive with a LT50 of 3.7 days at 119% TDGS. Both species had visible air bubbles under the exoskeleton. Our findings highlight that direct and indirect effects on benthic macroinvertebrates can occur even at low to moderate levels of gas supersaturation, likely causing reduced density, decreased species diversity and altered species composition. The emerging evidence strongly supports the implementation of regulations on TDGS in freshwaters.