Response of Gammarus pulex and Baetis rhodani to springtime acid episodes in humic brooks

While chronic acidification of water bodies has been steadily decreasing, episodic acidification continues to affect stream biology by temporarily decreasing pH and mobilizing aluminum. These events are becoming more common as climate change renders more frequent and intense storms and flooding. Throughout Scandinavia, the effects of acidification have been mitigated by liming since the 1980s, but remediation efforts can now be reduced. While transient acidity may reduce fish populations, also other species in streams are affected. In this in-stream study, two macro-invertebrates (Gammarus pulex and Baetis rhodani), both known as salmonid prey organisms, were exposed to snowmelt in six humic brooks with a natural gradient of pH and inorganic monomeric Al (Ali). We hypothesize that acid toxicity thresholds can be defined using lethal (mortality) and sublethal (changes in body elemental content) metrics. Periodic observations were made of mortality and whole body concentrations of base cations (BC: Ca, Mg, Na and K) and metals (Al, Fe, Zn and Mn). Mortality increased dramatically at pH < 6.0 and Ali > 15 μg/L for G. pulex and at pH < 5.7 and Ali > 20 μg/L for B. rhodani. No accumulation of Al was found. The invertebrate body Na concentration decreased when pH dropped, suggesting that osmoregulation in both species was affected. In contrast to general BC pattern, Ca concentration in G. pulex and Mg concentration in B. rhodani increased when pH decreased. Although Ali strongly correlates to pH, the Al composition of soil and bedrock also influences Al availability, potentially contributing to toxic Ali episodes. The estimated values calculated in this study can be used to improve water quality criteria and as thresholds to adjust doses of lime compared to old recommendations in ongoing liming programs. Such adjustments may be critical since both Ali and pH levels have to be balanced to mitigate damage to recovering stream ecosystems.