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Artificial oxygenation may improve the status of coastal waters

Press release 2012-08-22 at 12:00

Picture: Mixox pump 
(Vesi-Eko Oy) 

According to the results of the PROPPEN project, oxygenation pumping may be used to improve the oxygen conditions in the water layers near the bottom sediment, while also reducing the amount of nutrients released from the sediment, also referred to as "internal loading”. However, whether oxygenation is successful or not is dependent on the characteristics of the area in question and the technical oxygenation solutions. The dimensioning of pumping and the topography of the bottom, currents of water and vertical density of water greatly impact on the effects of pumping in the area.

This study also assessed the cost-efficiency and cost benefits of oxygenation and its ecological, technical and financial risks.Modelling was used to assess potential impacts on open sea areas. The joint Nordic project was coordinated by the Finnish Environment Institute SYKE, and its main financier is the Swedish Environmental Protection Agency.

Several test oxygenation phases were carried out in the project between 2009 and 2011 in two testing areas: at Sandöfjärden in the western Gulf of Finland in Finland and at Lännerstasundet in the inner Stockholm archipelago in Sweden. The chosen oxygenation method was oxygenation pumping, in which oxygen-rich surface water is pumped into the oxygen-depleted deep-water layers.

The water near the seabed at Lännerstasundet became oxygenated in a few weeks by means of pumping, while nutrient concentrations dropped significantly. The pumping reduced the density of water near the seabed, thus enabling higher-density water rich in oxygen to flow from nearby areas into the bottom of the pumping area.  

At Sandöfjärden, the pumping proved insufficient to keep the area oxygenated; water near the seabed ran out of oxygen in late July. Possible reasons for this include the warming of water near the seabed, partial flow of pumped water outside the oxygenation area and insufficient pumping capacity.

In neither case area did pumping cause significant amounts of nutrients to move into the surface water layers. However, water near the seabed did warm up in both case areas, which increases the consumption of oxygen while weakening water stratification. Due to the increase in the oxygen demand, the oxygen concentration below the thermocline decreased.

Modelling was used to estimate that large-scale oxygenation pumping could possibly be used to improve the oxygen concentration in deep-water areas on the open seas, provided that this water was pumped from a cold water layer at a depth of 30 to 50 metres. However, oxygen concentration would be reduced immediately below the halocline, which would increase the risk of oxygen-depletion and internal loading in areas affected by water having low oxygen concentration.

According to the project’s socio-economic analyses, oxygenation pumping is not a cost-efficient additional method for nutrient reduction at the current external-load removal efficiency in the coastal areas of the Gulf of Finland. The exceptions to this rule are specific semi-enclosed areas in which external local nutrient loads are insignificant or their removal very expensive. If oxygenation pumping were to be used to expedite sea recovery in accordance with the Baltic Sea Action Plan’s emission cuts, oxygenation pumping would produce positive net benefits in coastal waters – based on citizens’ expressed willingness to pay.

Our current knowledge of the ecological impacts of and socio-economic and technical preconditions for oxygenation in areas outside the archipelago is not adequate for us to recommend this method for extensive use.  Even when used in the archipelago, the method requires a background analysis concerning the suitability of the area in question for oxygenation. However, the results gained from the PROPPEN project give us every reason to continue conducting testing and research activities related to oxygenation in areas with various types of basins, flow conditions and density layers. In addition to the ecological impacts, the technical, political and financial issues related to oxygenation should be studied more extensively.

For more information

Heikki Pitkänen, Division Manager, SYKE
tel. +358 40 5823 182

Jouni Lehtoranta, Senior Research Scientist , SYKE
tel. +358 400 148 532

Erkki Saarijärvi, Managing Director, Water-Eco Oy
tel. +358 44 279 8603

Prof. Markku Ollikainen, University of Helsinki
tel. +358 50 344 4345

Juhani Anhava, Leading Researcher, Pöyry Finland Oy
tel. +358 40 7776 323

Aira Saloniemi, Chief Editor for Web Services
SYKE Communications, tel. +358 400 148 875

Maria Nurmi, Communications Intern, SYKE Communications, 
tel. +358 40 159 8852

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