The influence of adjacent stream reaches on the local ecological status of Central European mountain streams

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Abstract
Monitoring results show that restoring river habitats at a site scale does not necessarily lead to an improvement of the biological state. It has been widely stated that neglecting processes acting at the catchment scale such as diffuse nutrient or sediment inputs and a low recolonization potential are possible reasons for the relatively small ecological effect of many restoration projects. However, information on the influence of intermediate spatial scales is missing. Therefore, the objective of the present study was to investigate the effect of adjacent up‐ and downstream reaches on a site's ecological quality, as determined by the invertebrate fauna.We used invertebrate samples to assess the local ecological quality, land‐use data to describe the land‐use pressure in the upstream catchment and hydromorphological data as a proxy for the ecological status of the adjacent up‐ and downstream reaches. In the stream type investigated, upstream morphology has a significant effect on local ecological status, even when local habitat quality at the sample sites and land‐use pressure in the upstream catchment are considered as co‐variables in multivariate statistical analysis. This upstream effect is due to a positive effect of near‐natural reaches and a negative effect of heavily degraded reaches. The results indicate that upstream morphology is of similar importance to local habitat quality and is especially high in stream reaches with a medium local habitat quality. Since we used the morphological state upstream as a proxy for the ecological quality, we were not able to distinguish abiotic and biotic effects. However, there is some evidence that the upstream influence is at least partly due to biotic effects. We argue that this biotic effect is somewhat different from what is usually meant by recolonization or dispersal and is possibly due to the continuous input of a larger number of individuals.We conclude that a third spatial scale—the adjacent upstream reaches several hundred metres to a few kilometres in length—should be considered in stream restoration and management besides local habitat quality and catchment scale processes. This upstream effect might be used to enhance urban streams by restoring upstream rural reaches where the potential for stream restoration is higher. Moreover, the results can be used as a first estimate of the maximum distance between stepping stones or near‐natural reaches to enhance the habitat connectivity. Copyright © 2009 John Wiley & Sons, Ltd.

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