Impact Type: Deposition of pollutant
Salmonid species are likely to be harmed by pH levels below 5.0, while levels below 4.0 are lethal (Alabaster & Lloyd 1980. In waters acidified by atmospheric pollution (i.e. rather than naturally organic acid waters) aluminium toxicity is the principal problem. Salmonids are most vulnerable at their early life stages in the nursery areas.
Arctic charr inhabit freshwater lochs and spawn on clean shallow gravels. They are more sensitive to the effects of acidification than salmon and trout (e.g. in L. Grannoch, Galloway) Arctic charr were eliminated but a remnant population of brown trout survived). In large lochs, acidification that may be lethal in tributaries is diluted and charr can also escape from polluted inflows. Medium and small loch populations however, can be subjected to prolonged acidification and reduced recruitment leading to small populations of irregular age structure or elimination.
The degree of loch acidification depends on deposition rates and the ANC (Harriman et. al. 1995) of the catchment, also loch morphometry and water retention time can be important. ANC may be reduced by afforestation (e.g. Rees & Ribbens 1995). Critical deposition loads can be calculated for individual sites.
Organically stained waters are believed to provide some protection from the deleterious effects of acidification. Recent reports on surface water chemistry in the UK indicate a reversal of acidification (e.g. Jenkins 1999).
|Habitat/ Ecosystem Type||Critical Load/ Level||Reliability||Indication of exceedance||Reference|
Calculated on a site basis.
|not relevant/applicable i.e. the approach is not relevant for this species or habitat||
The calculation is based on ANC and species presence on a site-by-site basis. The effect of exceedance is reduced recruitment leading to small populations of irregular age structure or elimination.