Acid deposition :: Macro invertebrates

Impact Type: Deposition of pollutant

Key Concerns:

Macroinvertebrates are vulnerable to the effects of acifi=dification of fresh waters. For example, Stoner et al. (1984) working in western Wales, found that where the pH was greater than 5.5 the macroinvertebrate community consisted of 60-70 taxa whereas below 5.5, only 23-37 taxa were present. The susceptibility of diferent freshwater macroinvertebrate grpoups to acidification varies widely. Macroinvertebrates may be affected by the physiological stresses arising from direct exposure to high concentrations of hydrogen ions or from indirect exposure to increased concentrations of toxic metal ions such as aluminium or copper. Other direct effects can result from the reduction of trace elements like calcium or from alterations in the food chain. Calcium is an important factor in contolling the distribution of molluscs and crustaceans which require calcium carbonate for their shells and exoskeletons.

Additional Comments:

Aquatic invertebrates constitute the main food supply for many species at higher trophic levels, so the effects of acidity should be considered on both groups. The impact of acid deposition depends largely on local and regional conditions but can range from minor to extremely harmful. In a survey of 1500 Norwegian lakes, snails and bivalves largely disappeared below pH 6.0 (Mason 1996) while work in Wales demonstrates that mayflies, caddis flies, beetles, molluscs and crustaceans are all scarce in streams below pH 5.7 (Reynolds and Ormerod 1993). Soulsby et al. (1996) showed that for a variety of streams draining the Cairngorm mountains in NE Scotland mayfly abundance reflected variations in acidity levels. The same authors noted an increase in abundance over the period 1988-1994 coincident with reductions in stream water non-marine sulphate concentrations. The response of aquatic macroinvertebrates to short-term events such as storms and snow melt is not well documented. Commonly, critical loads have not been set for freshwater macroinvertebrates but for higher fauna, e.g. salmonids. There is some evidence that a variable ANC, dependent on local site conditions would be more appropriate than a fixed value as the natural acidity of individual water bodies varies considerably.