Influence of stream flow on epilithic diatom communities and its consequence for water quality monitoring

Abstract

Epilithic diatom communities and relevant physico-chemical variables were studied between 3 May and 16 July 1993 in Harwood Beck, an oligotrophic stream in Upper Teesdale, and the River Browney, a lowland eutrophic river. The effect of stormflow on diatom-based water quality indices was assessed. In both rivers epilithic chlorophyll ḁ (µg cm(^-2)) was highly variable and very low after mid- June. In Harwood Beck there was a significant difference in epihthic chlorophyll a between cobbles and boulders, with the greatest difference after stormflow. Epilithic chlorophyll a in the River Browney was initially high, but declined markedly after stormflow. River Browney chlorophyll a was inversely correlated with current speed on cobbles and total inorganic nitrogen on cobbles and boulders. Uptake of inorganic nitrogen may be directly related to epilithic diatom biomass. A sharp rise in the percentage of Cocconeis placentula between mid-June and mid-July suggests that grazing may be an important factor limiting algal biomass. The possible roles of algal grazers, Cladophora glomerata and silicate concentration, in progressively limiting diatom biomass during the summer are discussed. Percentage composition changes of taxa after stormflow are generally related to taxa morphology and mode of adherance to the substrate. There were no significant differences in the percentages of the five most common taxa between cobbles and boulders. Percentage changes in taxa were not significantly correlated with nutrients, with the exception of Navicula gregaria and N. lanceolata (in the River Browney) which were inversely correlated with the concentration of total inorganic nitrogen. There appears to be a succession of dominant species, possibly related to the influence of flow and nutrient concentrations, which tended to increase during the study period. Diatom water quality indices remained relatively stable over the 10 week period. Effectively equal water quality classifications were derived from the Specific Pollution Index (SPI) and Generic Diatom Index (GDI). The zoning system proposed by Round (1993) may be broadly useful, but requires careful interpretation.