The influence of pH and aluminium on the g rowth of filamentous algae in artificial streams

Community structure of freshwater filamentous benthic algae can be statistically related to acidification, with taxa ranked in terms of the mode of their abundance with respect to pH. However, pH is closely correlated with other variables, and statistical procedures relating species abundance to environmental variables cannot discriminate between closely correlated variables as potential causes of the species distribution. To investigate the independent influences of pH, aluminium and silicon levels, we set up two series of experiments using filamentous algal isolates in recirculating artificial streams. The isolates belonged to the genera Draparnaldia, Geminella, Hormidium, Mougeotia, Oedogonium and Stigeoclonium. Growth rate was calculated from rate of increase of cell numbers in algal filaments attached to glass slides, and the accumulation of total biomass was also monitored. In experiment 1, pH was adjusted to values between 4 and 6, with other conditions fixed. Growth rates of all taxa changed with pH and all had a maximum within the range tested. Final biomass was lower at the extremes of pH. In experiment 2, pH was set to 5.5 and levels of dissolved aluminium and silica varied with reference to a control. In the enhanced Al treatment, the effect of 7.4 μmol/l of total aluminium was to cause a rapid decline in growth rate in most species, and a lower final biomass. These results are discussed in relation to field observations of the distribution of the genera tested.