FILAMENTOUS ALGAL COMMUNITIES RELATED TO PH IN STREAMS IN THE TROSSACHS, SCOTLAND

1. A survey of acidified streams in the Loch Ard area of The Trossachs , Scotland, was carried out between 1986 and 1988, to determine the in fluence of physical and chemical factors on the distribution of benthi c algae. Samples were taken on thirty-five occasions from fifteen site s on ten streams. Forty-nine operational taxa of filamentous algae wer e distinguished. Relative abundance of taxa in samples was scored on a seven-point scale. Estimates of standing crop were made by pigment ex traction and ash-free dry weight (AFDW) determination from biomass dev eloped on artificial substrata. 2. No evidence was found for seasonali ty in standing crop, nor for an increase in standing crop with a decre ase in pH. Seasonality in relative abundance was evident for few taxa. Species richness and diversity were highly correlated with pH and cor related chemical variables. 3. Canonical correspondence analysis (CCA) carried out using the CANOCO program ordinates species and sites and relates them directly to environmental variables. The most important v ariable was found to be pH (or an acidification-related variable that is highly correlated with pH), with percentage forest cover and total oxidized nitrogen being of secondary importance. The effect of subdivi ding the data set, using data based on presence-absence as well as est imated relative abundance, was tested. A comparison of the effects on the distribution of site and species values on the pH vector showed th at the results are robust, confirming the value of this semi-quantitat ive sampling method for use with a difficult algal group. It was possi ble to infer pH from algal community structure using CANOCO. 4. The pH ranking of species derived from CCA revealed that larger cell diamete r taxa do not predominate at low pH, implying that reduction of invert ebrate grazing was not an important determinant of algal community com position.