Habitat variation at different scales and biotic linkages in lotic systems: consequences for monitorization

Abstract

The Water Framework Directive (WFD) requires the hydromorphological assessment of water bodies, thereby acknowledging the importance of these features in supporting biological quality elements and providing a more complete ecological characterization of surface water bodies. Using a dataset covering mainland Portugal (about 300 sites spread along the different river types) and based on the River Habitat Survey (RHS) field methodology, our aim was to test the spatial variation and the relative role of an array of multi-scale habitat descriptors, in order to: (a) to analyse their geographical variation; (b) to identify the principal variables that express human disturbance; and (c) to assess how three different aquatic communities (invertebrates, fish, and macrophytes) were related to those environmental descriptors. We found that hydromorphological variables described by RHS varied significantly over large geographical scales and were more strongly associated with the principal catchments rather than river type (derived from climatic, geological, and typological factors). RHS-derived descriptors were of greater importance in assessing disturbance and were closely related to land use and vegetation on the banks and along the river corridor, despite the considerable geographical variation. Habitat variables were more clearly associated with macrophytes and fish than with the benthic macroinvertebrates, a facet of sampling design, since the scale of biological survey for the former two biological groups approaches the scale used to characterize the surrounding environment. An array of environmental variables, ranging from instream features to bankside and river corridor land use features, was associated with each community, making it difficult to discern any common underlying pattern. Based upon our findings, we propose that hydraulic variables should be included in hydromorphological assessment to improve both the performance of physical indicators and the correspondence with the demands of the WFD.