North Sea needs more protection from nutrient inputs
Reductions in the nutrients nitrogen and phosphorus flowing from rivers into the North Sea have had clear benefits on marine health in coastal waters, according to a recent study. However, the reductions are less effective in improving the condition of deeper offshore waters. Tougher measures to manage nutrient loads and prevent eutrophication are recommended.
Reductions in nutrient levels in recent years are the result of the PARCOM recommendation of the OSPAR Commission1, signed in 1988. Together with the European Commission, OSPAR links fifteen governments in Western Europe to protect and conserve the marine environment of the North-East Atlantic. PARCOM is not legally binding, but functions as a guideline in designing measures to reduce nutrient inputs. In 1997, OSPAR developed the Common Procedure (OSPAR CP) for the Identification of the Eutrophication Status of Maritime Areas. A set of Ecological Quality Objectives (EcoQO) were established as criteria to assess eutrophication status.
PARCOM calls for a 50 per cent reduction in dissolved inorganic nitrogen and phosphorus flowing into the sea by 2010 compared with 1985 levels. Nutrients released into rivers and estuaries by human activities, such as agriculture, are the main cause of eutrophication. Eutrophication causes accelerated growth of algae and other plant life, affecting the balance of organisms and water quality.
The researchers, working under the EU ECOOP project2, modelled the effects of reduced nutrient flows during 1985-2006 on five marine zones of the North Sea near the Netherlands and Germany. They used river and nutrient load data from countries surrounding the North Sea in the 3D physical-chemical-biological model to identify the long-term effects on marine health. Influencing factors taken into account included water temperature, salinity, climate, heat fluxes, wind stress, atmospheric pressure fields, tides, radiation and light.
Nutrient levels in the sea dropped in line with the 50 per cent reduction measures. The results indicated that the three study areas closer to major estuaries rapidly responded to the measures. For example, average winter concentrations in these areas were reduced by 50 per cent for nitrogen and 35 per cent for phosphorus, stabilising after 2-3 years. The greatest reductions in nutrient concentrations are along the Dutch coast. In contrast, offshore areas were affected by reductions to a lesser extent with only a very weak increase in oxygen levels. Decreased oxygen levels are a symptom of eutrophication. However, the complex interplay of tides, currents and winds influence oxygen levels, and this interplay is likely to have prevented the effects of reduced nutrients reaching offshore areas. While there were some limits to the model, the results broadly corresponded with direct measurements observed in previous research.
The OSPAR CP and EcoQOs have obvious strengths due to their clear goals and simplicity, and the nutrient reductions achieved as the result of PARCOM have led to some important improvements. However, the researchers believe that a 50 per cent reduction is not sufficient for the North Sea. Further reductions are needed to prevent eutrophication in the long term, especially under a changing climate. Climate change may cause increased nutrient inputs, for example.
The researchers call for an international nutrient monitoring and modelling programme, with a common system of measuring and classifying. Harmonising the eutrophication strategies of OSPAR, the EU's Water Framework Directive3 and the European Marine Strategy4 is one of the greatest challenges facing the North Sea, they suggest.
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