Macro-invertebrate associations in sewage filter-beds and their relationship to operational practice.
The macro-invertebrate faunas of 67 filter-beds in 48 sewage works throughout the UK were surveyed in relation to the physico-chemical characteristics of the bed environment. Faunal samples were collected from the surface medium and from the bed effluent. 59 of the beds were sampled twice (autumn and spring); the others were sampled in the spring only. 87 species were found, of which 39 occurred in the surface samples. Of these latter species, 24 (9 oligochaetes, 8 insects, 4 copepods and 3 mites) were particularly successful, being widespread and often abundant. Lumbricillus rivalis, Psychoda alternata and Histiostoma feroniarum were almost ubiquitous. The filter-bed fauna was essentially hygropetric and microbivorous. All but one (the predatory mite Platyseius italicus) of the 24 particularly successful species grazed the biofilm. The successful species characteristically had prolonged or frequent periods of reproduction. Therefore, they could respond rapidly to changes in the bed environment and they were able to persist despite continuous wash-out from the bed. DECORANA ordination indicated that abiotic variables likely to affect faunal composition were principally: the organic loading the bed received, the amount of biofilm present, air temperature, the size of the bed medium, and bed age. All except air temperature are under operational control, so there is potential for the faunal community structure to be modified, if required, by manipulation of the bed environment. Classification by TWINSPAN identified Psychoda albipennis and Bryocamptus pygmaeus as indicators of beds that received a low organic loading, whereas abundant P. alternata (>1000 larvae litre-1 medium) indicated more heavily loaded beds. The results of the present study emphasize the prime importance of the physico-chemical environment in shaping faunal community structure in sewage filter-beds. The study illustrates how knowledge of the faunal community of sewage filter-beds and its relationship to the bed environment and to bed performance can potentially aid the management of such beds to improve purification efficiency or reduce fly nuisance.