Abstract
Urban rivers are socioecological systems, and restored habitats may be attractive to both sensitive species and recreationists. Understanding the potential conflicts between ecological and recreational values is a critical issue for the development of a sustainable river-management plan. Habitat models are very promising tools for the ecological evaluation of river restoration projects that are already concluded, ongoing, or even to be planned. With our paper, we make a first attempt at integrating recreational user pressure into habitat modeling. The objective of this study was to analyze whether human impact is likely to hinder the re-establishment of a target species despite the successful restoration of physical habitat structures in the case of the restoration of the Isar River in Munich (Germany) and the target fish species Chondostroma nasus L. Our analysis combined high-resolution 2D hydrodynamic modeling with mapping of recreational pressure and used an expert-based procedure for modeling habitat suitability. The results are twofold: (1) the restored river contains suitable physical habitats for population conservation but has low suitability for recruitment; (2) densely used areas match highly suitable habitats for C. nasus. In the future, the integrated modeling procedure presented here may allow ecological refuge for sensitive target species to be included in the design of restoration and may help in the development of visitor-management plans to safeguard biodiversity and recreational ecosystem services.
Highlights
Centuries of human activities have directly or indirectly degraded, damaged, transformed, or entirely destroyed aquatic ecosystems [1], threatening worldwide riverine ecosystem services [2], e.g., water security for fauna, flora, and humans [3]
In the case of the common nase (Chondrostoma nasus L.) in the restored section of the Isar River in the center of Munich, (i) whether hydro-morphological urban river restoration succeeded in providing physical habitats for different life-cycle stages of the target species, (ii) how urban recreational pressure is distributed with regard to fish habitats and how it modifies the availability of highly suitable habitats, and (iii) whether urban recreational pressure is likely to explain the absence of C. nasus from the studied restored river section
Our study has demonstrated the existence of habitats with suitable flow velocities, depths, and substrata for all lifecycle stages of C. nasus historically observed in this Isar River section, and it suggests that the reduction of adequate spawning and juvenile habitats by physical destruction and by user pressure is responsible for the absence of recruitment
Summary
Centuries of human activities have directly or indirectly degraded, damaged, transformed, or entirely destroyed aquatic ecosystems [1], threatening worldwide riverine ecosystem services [2], e.g., water security for fauna, flora, and humans [3]. River restoration is a term applied to a wide range of activities concerned with “repairing waterways that can no longer perform essential ecological and social functions” [5]. Project surveys in the United States, Japan, Australia, Germany, and France have shown that improving instream physical habitats through hydro-morphological changes is one of the most common goals of restoration [6,7,8,9,10,11]. Despite significant hydro-morphological changes, indicator species may not recover [12,13,14,15], which suggests that stressors other than hydro-morphological degradations still affect the biota in restored river sections [10,12]
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