Background Recent studies show that coastal regions suffering from oxygen depletion have increased exponentially since the 1960s, with serious consequences for ecosystem functioning. These ‘dead zones’ exist in approximately 400 aquatic systems worldwide, affecting a total area of more than 245 000 km2, including one in the Baltic Sea. Because hypoxia is a direct consequence of nutrient pollution and eutrophication it is essential to reduce the amount of nutrient loads to marine environments. Upstream nutrient reductions, however, are difficult to achieve and it takes a long time before effects in marine environments are observed. In addition, global warming is expected to increase hypoxia. This implies that mitigation measures that lead to direct improvements are required. Objectives The main objective of the WEBAP project was to demonstrate a cost-effective wave-powered device, the ‘Wave Energized Baltic Aeration Pump’ (WEBAP), which mitigates the problem of oxygen depletion (hypoxia) in coastal zones and open seas. The main aims of the project were to demonstrate the technical and ecological feasibilities of the aeration pump, to prepare for a full-scale implementation of the aeration pump, to demonstrate its transferability as a mitigation measure to other hypoxic marine environments, and to disseminate project results in Europe and other regions. Results The WEBAP project demonstrated a cost-effective wave-powered device, the ‘Wave Energized Baltic Aeration Pump’ (WEBAP), to help mitigate the problem of oxygen depletion (hypoxia), particularly in coastal zones. The project developed a pilot wave-powered pumping device, along with a smaller electricity-powered unit for reference and verifications. The device was used to pump oxygen-rich surface water down into oxygen-poor, hypoxic, bottom layers in coastal areas near Stockholm and Simrishamn in Sweden. Further, the project elaborated methods enabling the monitoring and evaluation of these underwater interventions. The equipment units proved feasible, functional and effective at delivering fresh water down to hypoxic layers. However, the functionality depended on weather conditions (high seas reduced efficiency), which need to be continuously monitored to guarantee efficient operation. Over a period of more than 12 months, the pumping devices worked very efficiently in technical terms and, with low energy consumption, moved oxygen-rich surface water down to oxygen-poor deeper sea levels. The pumping activity was accompanied by active sampling of marine and biological data around the test areas. Subsequent monitoring and evaluation of the marine environment, water, biota and bottom sediments indicated that the enhanced water streams did not disturb the salinity stratification, which is important for all marine life. The main environmental impact derived from increased oxygen content in the bottom water layers, resulting in increased biological activity, and a slight reduction in harmful phosphorus concentrations, which potentially reduces the occurrence of algae blooms. Increasing the water’s oxygen content decreases the phosphorus leakage from bottom sediment, and will eventually enhance phosphorus binding as more aerobic conditions occur. The pumping activity was sensitive enough not disturb the bottom sediments, which might be, in the worst case, a huge phosphorus source that could enhance algae blooms and eutrophication. The monitoring and evaluation did not reveal any adverse impacts on bottom-dwelling flora and fauna or increased toxic contents. The project substantially improved the knowledge about the current situation in the Baltic Sea regarding hypoxia and phosphorous dynamics. Combatting oxygen depletion utilising the WEBAP method could be an effective and efficient alternative (or complement) to other methods, like diesel/electricity-powered water injection or chemical treatment, and a complement to the efforts to reduce the run-off of fertiliser and wastewater effluents. The method can be scaled up at relatively low costs, with an evaluation predicting that a 10-fold increase in terms of water volumes, compared with the WEBAP pilot, could have a real impact on hypoxia and thus improve the conditions on the sea bottom in coastal zones. Within Europe there are a large number of areas that could benefit from the method. Its implementation supports a number of EU policy objectives, for example, the Water Framework Directive (2000/60/EC), the Bathing Water Directive (2006/7/EC) and Integrated Coastal Zone Management. By using freely-available renewable energy to mitigate the harmful impacts of anthropogenic activities on water bodies, the approach was found to be cost-effective in the long-term. The project team calculated only modest potential climate change mitigation benefits could derive from this technology. Improvements to conditions for the biota at the sea bottom might have a positive impact on fish stocks and therefore fisheries, and the general improvement of water quality, especially the absence of algae blooms, could enhance tourism. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).