Epiphytic bacteria downstream of sewage-works outfalls

Epiphytic microorganisms were investigated in two watercourses which were enriched by sewage-works effluent. These were a disused canal and a small stream in North-East England. Scanning electron microscopy, on the submerged macrophytes Callitriche sp. and Elodea canadensis, from the canal, demonstrated abundant epiphyton which was dominated by rod-shaped bacteria and pennate diatoms, especially Cocconeis sp. An in situ counting technique showed that the density of epiphytic bacteria was greater downstream of the sewage-works outfall on plants from both watercourses. The proportion of metabolically-active epiphytic bacteria, and cell length of epiphytic bacteria, were determined on Callitriche in the small stream, and were greater downstream of the outfall. Thus increase in (1) the population density of epiphytic bacteria, (2) the proportion of cells which were metabolically active, and (3) cell size, combined to increase the potential contribution of epiphytic bacteria to biopurification and downstream recovery. Epiphytic diatoms, in contrast to bacteria, apparently did not increase in abundance downstream of the outfalls.