Recolonization by the benthos of an acid stream following a 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 macroinvertebrates was rapid; after 3 days of flow, 665 larvae/m², 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/m² 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 pre-drought 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.