Aarhus University Seal / Aarhus Universitets segl

Eutrophication and oligotrophication in marine waters: Lessons learned from 4 decades of monitoring and assessment in Denmark

Activity: Talk or presentation typesLecture and oral contribution

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Jacob Carstensen - Lecturer

Nutrient inputs from land increased significantly up to the 1980s, when the deleterious effects of eutrophication were recognised and measures taken to reduce nutrient levels. It was anticipated that the nutrient reductions would lead to oligotrophication and return marine ecosystems to an earlier status. This fundamental tenet, assuming full reversibility of eutrophication, has been challenged recently. Although nutrient concentrations have declined significantly in response to reduced nutrient inputs from both point and diffuse sources, algal blooms are still frequent, benthic vegetation has not recovered and hypoxia is still widespread. This apparent lack of ecosystem predictability is likely caused by changes in other factors such as overfishing and climate change that shift the baselines, and by non-linear ecosystem responses potentially displaying hysteresis and regime shifts. As a consequence, phytoplankton biomass yield to nutrient levels have almost doubled over the last 2–3 decades, coastal waters are still turbid and do not allow for deeper colonisation of benthic vegetation, and nutrient leakage from the sediments remains high because of hypoxia. Hence, loss of important ecosystem services can maintain marine ecosystems in a bad state due to internal feed-back mechanisms, but marine ecosystems have also shown to recover fast once a healthy benthic community is established.
14 May 2012

Event (Conference)

TitleMarine Strategy 2012

ID: 45491019