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

• Although the acid load has decreased throughout Scandinavia, episodic acidification continues to affect stream biology by temporarily decreasing pH levels and mobilising aluminium. These events are becoming more common as climate change renders more frequent and intense storm floods. The transient acidity can reduce fish populations, but fish food resources can also be impaired because macro-invertebrates are affected. In this in-stream study, two salmonid prey organisms (Gammarus pulex and Baetis rhodani) were exposed to snowmelt in six humic brooks with a natural gradient of pH and inorganic monomeric Al (Ali). We hypothesise that there are thresholds for acid toxicity that can be defined by mortality and changes in body elemental content. Mortality was observed and the whole body content of base cations (BC, i.e., Ca, Mg, Na and K) and metals (Al, Fe, Zn and Mn) were determined. Both species began to die and the total bodily BC content decreased as the pH levels decreased and the Ali concentrations increased. In contrast to what has been observed for fish gills, no accumulation of Al was observed. The invertebrate body Na content decreased when the pH level decreased, which indicated that the osmoregulation in both species was affected. The Ca content in G. pulex and the Mg content in B. rhodani diverged from the general BC-pattern by increasing when the pH level decreased. The mortality increased drastically at pH <6.0 and Ali >15 μg/L for G. pulex and at pH <5.7 and Ali >20 μg/L for the somewhat less sensitive B. rhodani. Although Ali is tightly correlated to pH, the local Al availability in soil and bedrock also affect the Al release and toxic Ali episodes can consequently arise in some catchment areas. The estimated values can be used as water quality thresholds to adjust lime dose because both Ali and pH levels have to be balanced to prevent harm in recovering stream ecosystems.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Miljövetenskap (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Environmental Sciences (hsv//eng)

Nyckelord

Thresholds Ali & pH; Gammarus pulex; Baetis rhodani; ionoregulation; liming strategy

Applied Environmental Science

tillämpad miljövetenskap

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