Description: The workshop aimed at bringing experts working in the field of offshore wind farms – benthos together for the first time in order to get an overview on the state of the art. This was achieved by an extended poster session. The second issue of WKEOMB was to identify knowledge gaps and evaluating monitoring strategies. This issue was evaluated by disentangling the cause-effect relationships affected by the pressures of the activities during the construction and operation phase of offshore wind farms. All cause-effect relationships were summarized in a schematic presentation. The identifi-cation and a comprehensive overview of cause-effect relationships is a prerequisite for an efficient, hypothesis driven approach towards the disentanglement of the vari-ous effects of offshore wind farms on the marine benthos as well as on the whole eco-system. Further, manifold cause-effect relationships were prioritized based on three main research themes, biological resources – biogeochemical reactions – biodiversity, disentangled by the participants as relevant. An important outcome of the workshop is that benthos receives by far too little atten-tion compared to other ecosystem components (e.g. seabirds, marine mammals), al-though it contributes to a great extent to marine ecosystem services and goods, e.g. biodiversity, long-term carbon storage and trophic supply for higher trophic-level species. A second main outcome of WKEOMB was that legal baseline monitoring merely allows for net-effect descriptions but not for identifying and understanding the underlying processes. Key processes should be, thus, identified and become sub-ject to hypotheses-based target monitoring and/or experimental studies.

Description: Offshore windfarms are expected to affect substantially the structure and functioning of marine ecosystems. Collision risks for migrating birds and noise impact on marine mammals and fish are issues of major public concern. Less charismatic organisms, however, from marine algae through to benthic invertebrates and demersal fish receive far less attention. We contend that the benthos deserves much greater attention owing to the numerous ecosystem goods and services, such as marine biodiversity and long‐term carbon storage and natural resources (e.g. for fish, birds, mammals, and finally humans), that are intimately linked to the benthic system. The installation and operation of extensive offshore windfarms in shallow shelf seas will initiate processes which are expected to affect benthic communities over various spatial and temporal scales. Extensive baseline monitoring programmes allow observations of structural changes to benthic communities, but this is a post‐hoc approach. To gain a mechanistic understanding of these processes that enables us to explain the observed changes, specific target monitoring and well‐designed experimental studies are required. In this conceptual talk we will discuss specific cause–effect relationships in the marine benthos arising from the anthropogenic activities associated with offshore windfarms. The identification of cause–effect relationships is the prerequisite for an efficient, hypothesis‐driven approach towards the disentanglement of the various effects of offshore windfarms on the marine benthos as well as on the whole ecosystem.

Description: In many European countries offshore windfarm projects are accompanied by obligatory environmental impact assessments, including baseline monitoring of the marine benthos and demersal fish. The effects of offshore windfarm developments on the benthic system are complex. However, legal baseline monitoring merely allows for net effect descriptions but not for identifying and understanding the underlying processes. Instead, key processes should be identified and become subject to hypotheses‐based target monitoring and/or experimental studies in order to make environmental impact assessments more efficient and reduce duplication internationally. We compiled an overview over the anthropogenic activities associated with the construction and operation of offshore windfarms and identified cause–effect relationship to facilitate the development of specific hypotheses. We expect offshore windfarming activities to modify the geomorphological and hydrodynamic environment at different temporal and spatial scales. The environmental effects will have consequences for the behaviour and physiology of benthic organisms, including demersal fish, restructuring natural local populations and communities. Major effects on biological production, biogeochemical processes, as well as on structure and function related to biodiversity, are expected from the massive colonization of the artificial underwater constructions by a specific hard‐bottom fauna which is naturally missing in soft sedimentary habitats. Understanding the mechanisms behind these changes is a priority for assessing and predicting the ecological implications for the benthic system. Such predictions may help to develop science‐based mitigation actions.