The effect of large woody debris on stream community structure across an enrichment gradient

Background/Question/Methods

The natural physical and biological states of rivers have been severely altered by long-term exploitation and modification. As a consequence river restoration is critical to help mitigate impacts on biodiversity-ecosystem functioning.

River habitat enhancement is often used to reinstate ecosystem properties yet many restoration studies focus on target species (e.g. brown trout) or assemblages (e.g. macroinvertebrates). Knowledge of the potential effects at the more complex organisational levels that bind these components together has yet to be applied (i.e. food webs, communities, ecosystems). To test the effectiveness of large woody debris (LWD) as a restoration tool, five calcareous streams were sampled before and after at control, reference and treatment sites.

Previous studies have indicated that ecological response is constrained in systems with high nutrient concentrations. In this study therefore, macroinvertebrates have been sampled from 19 base-rich streams with naturally occurring LWD and a range of nutrient levels to test for a threshold whereby habitat supersedes water quality as the primary ecological determinant. 

Results/Conclusions

Preliminary results indicate that total invertebrate abundance and species richness are significantly higher in the presence of woody debris, and that this relationship is not affected by nutrient concentrations. Fish response is less clear, however, the inclusion of data relating to other groups enabled assessment of biomass stocks, food webs, and body-size distributions. The effects of the abundance and biomass of fish, macroinvertebrates and diatoms on structural properties of ecosystems will be presented.