An Ecological Study of the Enchytraeid Worm Population of Sewage Bacteria Beds: Synthesis of Field and Laboratory Data

Abstract

The Huddersfield bacteria beds are unusual in harboring a persistent population of the enchytraeid worm Enchytraeus albidus, while the more common Lumbricillus (=Pachydrilus) lineatus is scarce. This resulted from the inability of L. lineatus to tolerate the chemical wastes in the Huddersfield sewage; egg development was totally inhibited and immature and adult stages were also affected. E. albidus was much more tolerant at all stages of its life cycle. Nevertheless, its population was much smaller than that of L. lineatus in beds treating domestic sewage. Three bed types, each offering different environments, due to treating sewages of varying toxicity and at different rates, were studied. Allowing for differences in population age, it is probable that population size was detd. mainly by the avg. toxicity of the sewage. The least favorable bed did not contain any enchytraeids. This was not entirely due to the toxicity of the sewage but also to the secondary development of septic conditions in the absence of an adequate scouring fauna. It is improbable that L. lineatus was able to breed in these beds. Comparison of its incidence with rainfall suggested that it was washed into the beds from natural habitats by flood water. The vertical distribution of E. albidus was abnormal, showing a maximum concn. below 12 in.; usually enchytraeids live nearer the surface. This was attributed to the higher initial toxicity of the sewage, which is gradually reduced by passage through the bed. E. albidus showed a marked seasonal cycle with a small summer and higher winter population. The winter max. was attributed to its habit of breeding most actively during this season and also to smaller losses from washing out, A rapid collapse occurred in the spring due indirectly to the destruction of the fungus Oospora (Fungi Imperf ecti) by the larvae of the rapidly growing population of flies, Psychoda alternata. This fungus forms the main food of the worm, and when it is converted to humus by the larvae and washed out of the bed the worms pass out with it. Recovery of the worm is delayed until the autumn when breeding becomes active again. The winter fluctuations in the worm population were also related to periodic off-loading of solids due to Psychoda larvae. The bacteria bed community is discussed. Compared with natural habitats it is simple and factors influencing population cycles more easily analyzed. At Huddersfield there is a seasonal fluctuation of Oospora and P. alternata due to differential reaction to temp. The fungus builds up during the winter when the fly is depressed and is depleted in the spring, by the larvae. The fly population in turn is reduced during late summer due to food shortage. Thus, both climatic and biological factors determine the cycle. E. albidus plays a minor role in this relationship.