Functional processes in a Swiss Midland river in relation to land use changes: leaf decomposit
One of the most important anthropogenic factors influencing freshwater systems globally is land use. Aquatic ecosystems are greatly influenced by the riparian zone (acting as a buffer between stream and watershed) but also the surrounding landscape. Several studies showed a direct linkage between surrounding land use and instream invertebrate and fish diversity. Land used for agriculture or urbanization can thus influence functional and structural processes in streams. To determine instream ecological conditions, leaf litter decomposition rates can be used. Only a small fraction of the organic carbon production of terrestrial plants is removed by herbivores or sequestered. This makes decomposition of plant litter one of the most important ecosystem processes in the biosphere. Changes in leaf litter decomposition rates can therefore reveal the impact of land use changes on streams.
Decomposition of allochthonous leaf litter was assessed in the Gürbe river located south of Bern, Switzerland using the mesh bag method. This stream is influenced by agriculture and urbanization throughout its catchment area. Instream decomposition at these agricultural and urban influenced sites was determined by use of leaf packs in relation to relatively pristine sites in the same channel. Decomposition values observed indicate an acceleration in decomposition at anthropogenic influenced sites. Microbes probably contribute most to the decomposition process in this catchment. Coarse mesh bags (macroinvertebrate, physical and microbial decomposition) only showed 25% higher decomposition compared to microbial restricted fine mesh bag decomposition. Previous studies found similar results.
Macroinvertebrate assemblages in the leaf packs differed as well along this gradient from pristine to land-use change influenced sites. Higher numbers of macroinvertebrates, as well as the more pollution tolerant macroinvertebrate species, were found at the influenced sites. Pristine sites showed the lowest number of macroinvertebrates but had higher diversity indices, indicating less single species domination at these sites. Chironomidae (Diptera) were dominant in the leaf packs, taking advantage of the trapped particles as well as the biofilm on the leaves. Plecoptera numbers were locally also very high. This particularly holds for the pristine sites.
Decomposition in coarse mesh bags was higher compared to fine mesh bag decomposition throughout the catchment. However, shredder densities were low at all sites. This could indicate the importance of physical factors contributing to decomposition, next to dominance of microbes in the decomposition process in this stream. Fungal biomass on leaves did not show a pattern related to land use changes, and results were very variable within and among sites. Previous studies indicate that increased nitrate concentrations stimulate microbial growth and increase in fungal diversity on leaves, resulting in higher decomposition rates. Possibly due to the presence of Chironomidae (which feed on microbial biofilm on leaves) in the leaf packs, higher fungal biomass was not found on leaves in agricultural/urban influenced sites. These results suggest that decomposition is higher at agricultural and urban influenced sites because of higher numbers of macroinvertebrates and not due to microbial decomposition.