Recolonization by the benthos of an acid stream following drought

This study investigates recolonization by benthic algae and macroinvertebrates following a drought and subsequent drying of an acid stream in southern England, normally characterised by permanent flows. Re-establishment of epilithic biofilm on the upper surfaces of mineral substrata was tracked in a shallow riffle between Sep. 1995 and Mar. 1996. Recolonization of these surfaces by macroinvertebrates was also followed, as was their benthic density. Post-drought abundance of biofilm and invertebrates was compared with that of the pre-drought year (Aug 1994-Mar 1995). We found that viable algal cells remained attached to the dry substratum despite 9 weeks of drying. On rewetting, these resistant benthic algal populations increased in biomass (evidenced by AFDM, chlorophyll density) and cell density, until a series of high flow events scoured the stones in winter 1995/1996. Spatial variation in the biofilm also existed: generally, algal biomass was higher on bedrock than on small or large stones. Recolonization of the previously dry stream bed by macro invertebrates was rapid; after 3 days of flow, 665 larvae/m(2), composed of a mean of 7.5 taxa, had been attained. The total numbers of invertebrates peaked 38 days after rewetting when a density of 2185 larvae/m(2) was found. This increase in abundance was mainly driven by the tanypod chironomid Zavrelimyia sp., which represented 56% of total numbers on day 38. Detrended correspondence analysis (DCA) revealed that, as colonization time progressed, the macroinvertebrate community became progressively more dissimilar to the predrought community, until high flows occurred in winter 1996 and community composition finally converged to that characteristic of the pre-drought community. Recovery following drought was rapid despite the depauperate and unproductive nature of this acid stream.