The aim of this deliverable is to conceptually model and empirically test the response of biota to the effects of both hydromorphological pressures acting in concert with one another or with other types of pressures. Best use is made of existing large national monitoring datasets (Denmark, UK, Finland, France, Germany, Austria & WISER datasets), case studies and modeling to provide evidence of multiple stressors interacting to alter river biota (Biological Quality Elements: BQE).

In-stream river plants. The evidence from analyzing plant traits, from UK, German and Danish data is that macrophytes can indicate hydromorphological degradation. One can also gain insights into how different hydromorphology and other stressors are interacting. Channelised sites are not only physically altered but require ongoing maintenance in the form of vegetation management (riparian and in-stream) and dredging which impact on macrophyte traits. The observed interactions between eutrophication and different hydromorphological pressures are explored. There was also evidence that macrophytes may have a role in accumulating/retaining heavy metals in polluted rivers; a fact which requires consideration during the physical restoration of such systems.

Fish. The sensitivity of species with different physical habitat affinities is considered. The response of fish to over 100 years of hydromorphological degradation at three Austrian case study systems is described. Here the complex nature of hydromorphological impacts on hydromorphological processes is emphasised. In a second study the possibility of creating models which link pressure to processes to fish occurrence is explored. As a pressure may affect more than one hydromorphological process it is important to understand and clear define how pressures interact on physical processes at scales that fish respond to.

Invertebrates. Existing invertebrate metrics are problematic.  To explore alternatives we aimed to elucidate general patterns between the assessment of habitats/biotopes and the diversity of macroinvertebrates using a standardised biomonitoring sampling methodology. We use a high quality dataset that includes a pre-defined gradient in hydromorphological degradation. In addition to identity-based diversity metrics and traits were used. There was some evidence that traits held some potential but sampling methods also need to be revised.

Joint (BQE) Biota Analysis.Analysis of a very large European data set covering Finland, Denmark and France examined the relative sensitivity of macrophytes, diatoms and invertebrates to nutrient and hydromorphological stresses. Analysis at the community level suggested that nutrient impacts were more detectable than hydromorphological stressors. This finding is not surprising for Finland where hydromorphological degradation is rather low. Sites subject to multiple-stresses tended to have the most common species. There are differences in sensitivity between BQEs at community level; primary producers appear more sensitive to diffuse pollution than invertebrates. The challenges in using monitoring data to identify known causal interactions between biota and multiple stressors are discussed.

Weir removal. This intervention is now widely advocated across Europe as a means of improving ecological status. The benefits of weir removal are obvious, for example its removal facilitates upstream migration of anadromous fish; but in multi-stressor environments there may also be unforeseen disadvantages. The removal of a weir from a British weir is modeled and checked against empirical data. The results indicate that the weir creates conditions for denitrification and its removal increases the load of N exported downstream. The cumulative impact of widespread weir removal should be considered carefully in terms of the gross export of rivers to coastal areas where catchments suffer from eutrophication.

Stressor interactions.A clear conclusion from the evidence available across all biotic groups is that irrespective of whether or not hydromorphological stressors and other forms of stress interact synergistically or antagonistically to alter natural river biota assemblages,  each stressor on its own can and do have detrimental impacts. Therefore when remediating damaged sites it is important to understand that while it may be possible to improve system status by tackling one stressor good ecological status is unlikely to be achieved without tackling all significant stressors in systems subject to multiple stress.

Future Directions. Monitoring data are designed to detect change at individual sites through time and can miss some crucial hydromorphological impacts. Revision of some of the monitoring metholodologies can help, however adherence to monitoring data alone will not supply knowledge and system understanding. Common sense indicates an alternative approach for some WFD operational and investigative monitoring are well designed, replicated  field ‘experiments’ elucidate cause-effect relationships on case study systems and are used to complement monitoring data, see WP4 for examples.