Particle Mixing by Freshwater Infaunal Bioirrigators: Midges (Chironomidae: Diptera) and Mayflies (Ephemeridae: Ephemeroptera)

Abstract

Mechanisms and rates of sediment mixing by the infaunal chironomids Coelotanypus sp. and Chironomus sp., and the mayfly nymph, Hexagenia spp., were quantitatively investigated in the laboratory using a multiple 137Cs tracer layer microcosm technique. These insect larvae construct and irrigate burrows up to 10 cm beneath the sediment-water interface. Obliteration of tracer layers by these larvae was attributed to mixing by both diffusive (random smearing)—and feeding (directed particle motion)—style processes. Biodiffusion coefficients were obtained from the rate of widening of the tracer layers and feeding rates were obtained from the rate of decrease in activity of the tracer layers. The biodiffusion coefficients and feeding rates normalized by experimental population densities were used to compare the results of these experiments with those obtained in similar experiments using other organisms. The results suggest that of the organisms studied here, only Chironomus sp. mix sediments by biodiffusion at rates comparable to conveyor-belt deposit feeding oligochaete worms on a per individual basis. Individual rates of non-local mixing by feeding for the chironomid and mayfly larvae in these experiments are similar to or less than those reported for oligochaetes in similar experiments. These results indicate that although chironomid larvae and mayfly nymphs are burrow irrigators, they may also deposit-feed or at least rework sediments in a non-local fashion. Some of the differences in reported individual biodiffusion coefficients and feeding rates may be related to organism size, because larger organisms exhibited greater biodiffusion coefficients and feeding rates per individual. The values for the biodiffusion coefficients on a per individual basis were multiplied by approximate in situ population densities to evaluate the relative importance of each of these species at natural densities. The results indicate that the sparse populations of bivalves, some chironomids ( Coelotanypus sp.), and mayflies cause a relatively small amount of mixing compared to the more abundant amphipods, tubificid oligochaetes, and Chironomus sp. Surprisingly, the bioirrigator Chironomus sp. appears to be as important in mixing sediment as depositfeeding conveyor-belt tubificid oligochaetes; however, whereas sediment mixing by tubificid oligochaetes is primarily the result of feeding, sediment mixing by Chironomous sp. appears to be largely the result of burrow construction.