Comparison of structural and functional stream assessment methods to detect changes in riparian vegetation and water pH

P>1. Streams draining conifer plantations are often assumed to suffer from reduced ecosystem function due to acidity and diminished basal food resources. Evidence exists for the deleterious impacts of conifer plantations on stream ecosystem function and also of the negative impacts of acidification. This study compared the performance of a measure of ecosystem function (leaf litter decomposition) with structural measures (shredder species richness; shredder abundance; ASPT (Average Score Per Taxon) and seven different acidity index scores) in determining the impacts of conifer plantations and acidification on streams. 2. The study was conducted in the Derbyshire Peak District (U.K.) from January to April 2003. Five pairs of sites with broadleaved versus coniferous riparian vegetation were selected for comparison. Three of the broadleaved sites and two of the coniferous sites were also acid (mean pH < 6.1). 3. Breakdown rates of alder and oak leaves as well as shredder species richness and abundance were determined from mesh bags. Benthic samples were also collected on the same sampling dates and these were used to calculate the various index scores for each site. Community composition was investigated using redundancy analysis. 4. Clear differences in mesh bags were observed between the two riparian vegetation types: all parameters measured were higher in conifer sites than broadleaved sites. Leaf litter breakdown also differed between pH classes: mass loss was higher in neutral than acid sites. Structural index scores were unable to differentiate between the riparian vegetation types and with the exception of the species level Acid Water Indicator Community score, no structural index was able to differentiate between the two acidity classes. 5. The results suggest that stream function (leaf litter breakdown) responds to differences in riparian land use and stream acidity more strongly overall than community structure (structural indices). This in turn suggests that stream structure and function may be only loosely coupled. Therefore, the use of water quality indices should ideally be combined with functional measures to obtain a complete assessment of stream ecosystems.