What drives invertebrate communities in a chalk stream : from trophic relationships to allometric scaling

Abstract

Despite a slow start freshwater meiofauna research is now gathering pace. Evidence is accumulating which indicates the importance of their inclusion in lotic metazoan studies. Here I contribute towards this research effort by conducting an investigation of meiofauna and macrofauna from a chalk stream. I sampled meiofauna for a 19 month period, and macrofauna for a 12 month period between April 2004 and October 2005 from the subsurface, macrophyte stands and gravel beds. The chalk stream community was highly diverse with 57 taxa identified from the subsurface and 186 from the benthos. Meiofauna outnumbered macrofauna in all habitats in terms of density. Both meio- and macroinvertebrates preferred macrophyte stands over gravel beds as habitat, indicated by higher densities, biomass and species richness. Speciesabundance relationships and density-size spectra indicated the invertebrate assemblages of the benthos to be stable over the period of the study as patterns varied little between sampling months and habitats. Production and standing biomass were dominated by the macroinvertebrates which suggests meiofauna had a limited role within functioning of the stream. However, gut content data indicated meiofauna may play an important trophic role, linking basal resources and top consumers. Combined gut content and stable isotope analysis suggested a strong pattern of generalist feeding throughout the whole spectrum of body size in the community, rejecting the concept of functional feeding groups. Predominance of generalist feeding also suggested a large number of weak interactions in food webs. While higher species richness lower in food webs indicated greater functional redundancy of lower trophic levels. Density-body size distributions were shallow with a biased distribution of energy towards larger size classes. Moreover, testing of production, standing biomass and PIB body size allometry was inconclusive with regards to theoretical predictions. The interrelationship ofbiodiversity, stability, and trophic dynamics, with body size determine the structure and dynamics of the chalk stream community, not metabolism.