Long-term trends in pH, aluminium and dissolved organic carbon in Scottish fresh waters; implications for brown trout (Salmo trutta) survival

Over the past 30 years upland streams and lochs in Scotland have shown significant signs of recovery from acidification, particularly in terms of declining concentrations of non-marine sulphate (nm-SO4). Long-term index sites in central and southwest Scotland reveal a significant decline in the concentration of biologically important components, notably acidity (H+) and labile (toxic) forms of aluminium (Al-L), whilst dissolved organic carbon (DOC) and complexed forms of aluminium (AL-NL) have increased significantly. Although these improvements should increase the probability of brown trout survival, and have resulted in increased acid neutralising capacity (ANC) in fishless streams to values close to current critical limits, there is still a relatively poor correlation between ANC and current fish status. Site to site variability appears to be linked to the relative contribution of (H+), Al-L and DOC within the critical ANC (ANCCRIT) range. It is proposed that ANCCRIT should cover a range of ANC values as a function of Al-L concentration. Based on field studies an empirical range of critical ANC values are suggested which better predict the presence of a sustainable brown trout population.