The effect of shredder species richness on the production and character of fine organic particles in aquatic habitats

Background/Question/Methods   The species richness and identity of shredding invertebrates can affect the rate of decomposition in aquatic systems; however, there are knowledge gaps in understanding how shredder richness and identity may affect the stoichiometry and size distribution of the fine organic particles that they produce. Fine organic particles are consumed by filtering and collecting invertebrates and act as a subsidy linking upstream and downstream habitats. Changes to the rate of particle production as well as the stoichiometry and size distribution of particles could affect consumers that subsist on fine organic matter. The effect of shredder species richness and identity on particle characteristics was tested in mesocsoms using three dominant shredders found in the slow moving streams characteristic of the Upper Peninsula of Michigan: Limnephilis sp, Caecidotea sp.,and Hyallela azteca. Treatments were composed of all species combinations (n=7+1 control, r=4) with biomass between treatments held constant. Shredders were kept in flow through mesocosms stocked with .3g speckled alder leaves, Alnus incana, at the beginning of the experiment and particles were collected every 4 days for 24 days. Particles collected from each treatment each day were analyzed for total mass, particle size distribution, and C/N ratio.

Results/Conclusions   Larger particles tended to be more nutrient rich across treatments. In the mixed species treatments particle production was greater than expected from additive models parameterized with single species treatment data. The observed particle size distribution in mixed species treatments also deviated from expected; although the direction and magnitude of deviations varied between treatments. The results suggest that shredder species richness and identity not only can positively affect total organic matter processing; but that species richness and identity can drive fine scale differences in the stoichiometry and particle size distribution of the fine particles produced by decomposition in aquatic systems. Fine scale differences, while often overlooked in leaf mass loss experiments, could potentially have important ecological effects. Changes to fine organic particle size distribution could impact competitive interactions between filtering invertebrates that niche differentiate based on particle size, and changes to particle stoichiometry could affect total nutrient export from upstream habitat and the secondary production of filtering and collecting invertebrates.