The effect of acidification on epilithic algae in the Loch Ard catchment
A survey of epilithic filamentous algae was carried out at 15 sites on 10 streams with a range of mean pH from 4.37 to 6.67 in the Loch Ard area of the Trossachs, between 1986 and 1988. Monitoring of physical and chemical parameters was carried out in parallel. Photosynthetically Available Radiation (PAR) was measured using electronic integrators developed during the course of the study. Samples of epilithic algae were taken from natural and artificial substrates( microscope slides) to determine the relative contribution of different species to the community structure. Taxa could not be identified to species level in most cases, and are described by genus and cell diameter. Forty-nine taxa were distinguished in the filamentous algal communities found. Relative abundance of taxa was estimated. The mean value of abundance was calculated for use in subsequent statistical analyses. As an alternative, presence-absence(p/a) on each occasion was scored,a nd a mean value (pooled p/a) similarly calculated. Samples were taken also to determine algal biomass, as acetone or methanol extractable pigments (chlorophyll a and carotenoid) and ash-free dry weight (AFDW). The community structure at the different sites was investigated using statistical ecology computer programs. Using Inverse Association Analysis and Canonical Correspondence Analysis, distinct species assemblages were found in sites with mean pH values towards the extremes of the range encountered. Using Normal Association Analysis, Cluster Analysis and Canonical Correspondence Analysis, it was shown that sites are separable on the basis of the species they contain into groups related by mean pH. Using the results of Canonical Correspondence Analyses carried out by the program CANOCO (Ter Braak, 1988) on one half of the data set, field pH may be inferred from the species data, showing the indicator value of the community structure. In all analyses it was found that the pooled p/a data gave essentially the same result as the relative abundance data. The effects of changes in the pH and Aluminium content of water on the growth rate of selected species of green algae was investigated using a laboratory-scale recirculating miniature artificial stream apparatus which allowed six variations to be tested at one time. Algae were grown attached to microscope slides. Growth rates were measured by counting cells in individual filaments over several successive time intervals. Mixed cultures of up to eight species were employed. Species characteristic of the lower-pH streams such as Hormidium subtile, Geminella 8μm. Stigeoclonium 5μm. and some species of Mougeotia have a pH optimum for growth rate between pH 4.5 and 5.5. Species characteristic of circumneutral streams, Draparnaldia sp. Stigeoclonium 8μm. and Oedogonaum species, have a pH optimum between pH 5.0 and 6.0 or above. Hormidium subtile and Geminella 8μm. can grow at a reduced rate in a monomeric Aluminium concentration of 200 μg litre to the power -1 in which the majority of species tested are rapidly killed. Evidence was found for ecotypes with respect to pH in the genus Stigeoclonium. Biomass in natural waters was positively correlated with pH in contrast to some previous reports. In mixed cultures in the laboratory, the maximum biomass developed was in the range pH 4.5 to 5.5. around the pH optimum of the species present. At higher pH values (5.5 to 6.0) diatoms were predominant giving a brown-coloured periphyton layer which is less visually obtrusive than the bright green growth of the filamentous chlorophytes. Therefore anecdotal reports of an increased biomass upon acidification may reflect only a shift to more visible species. Inhibition of diatom growth by Germanium addition provided no evidence in favour of competition between diatoms and chlorophytes. Differences in community structure and changes in biomass with pH in laboratory culture cannot be ascribed to changes in invertebrate grazing or heterotrophic microbial activity. It is concluded therefore that - differences between species in tolerance towards pH or associated water chemistry variables are sufficient to explain differences in algal community structure in the field.