Global warming intensifies eutrophication and oxygen depletion in marine environments; a Danish perspective

Climate change caused by anthropogenic emissions of greenhouse gases leads to ocean warming with strong effects on marine ecosystems. However, there has been limited attention on synergism between ocean warming and eutrophication. Danish authorities have in recent years focused on reducing nutrient inputs from land to alleviate marine eutrophication and prevent bottom water oxygen depletion. Water action plans have in recent decades reduced nitrogen and phosphorus loading to the ocean around Denmark by 45% and 80%, respectively. Nevertheless, the area of seabed with oxygen depletion has continued to increase dramatically in the last 15 years. The prerequisites for the action plans are apparently not valid in a warming ocean. Biogeochemical responses to increased temperatures are higher primary production and benthic decomposition, leading to more extensive oxygen depletion in stratified waters. It is important to comprehend, though, that internal loading derived from nutrient recycling via microbial remineralization of deposited algae is accelerated in a warmer ocean and propels the extent of oxygen depletion. Future action plans must therefore embrace effects of continued warming on nutrient recycling in the ocean by implementing compensatory reductions in external nitrogen loading. By failing to understand the true impact of ocean warming, there will be no control on eutrophication and bottom water oxygen depletion with long-lasting negative effects on marine ecosystems and human livelihood. The consequences of ocean warming as illustrated here for eutrophic Danish waters are probably valid also for other marine and estuarine environments throughout the World.