Aranda's monitoring cruise: Baltic Sea oxygen level normal, but nutrient conditions have deteriorated

Press release 2022-02-18 at 10:01
© Photo: Harri Kankaanpää

The research vessel Aranda has returned from its annual winter monitoring cruise. The cruise investigated the current physical and chemical state of Finnish sea areas and the levels of harmful substances in the sea. Phosphorus levels were found to have increased in Finnish sea areas.

The oxygen situation was good in December-January in the Archipelago Sea and the Bothnian Sea, but oxygen problems continue in the Northern Baltic Proper and in the near-bottom water of the western Gulf of Finland. Phosphorus levels were high in all of Finland's sea areas. The observed concentrations in the Bothnian Bay were the highest ever recorded, and exceptionally high in the Bothnian Sea. Concentrations were also high in the Gulf of Finland, in some places almost reaching the highest levels ever recorded this time of year.

The oxygen conditions in the Gulf of Finland were poor, as usual, and phosphorus levels remain high

The near-bottom water of the Gulf of Finland contained less oxygen than a year ago. In the western part of the Gulf of Finland the water remains oxygen depleted, and toxic hydrogen sulphide appears locally. Phosphorus concentrations in the surface water of the Gulf of Finland were high for the time of year.

It appears that the phosphate content in the central part of the Gulf of Finland shifted to a permanently higher level as of 2005. The near-bottom water contained more phosphate than a year ago. Especially in the western part of the Gulf of Finland, phosphate concentrations were high in the near-bottom water.

“The nutrient situation of the Gulf of Finland is probably more affected by the exchange of water coming from the Baltic Proper than by changes in the land-based nutrient discharges. In addition, in the shallow Gulf of Finland, the bottom water often mixes with upper layers of water, which makes the nutrients available to blue-green algae in the summer”, says cruise leader Harri Kankaanpää of the Finnish Environment institute.

The oxygen conditions are good in the Archipelago Sea, but phosphorus concentrations were significantly higher than in the previous winter. The highest phosphorus concentrations were observed in the southern part of the Archipelago Sea, where more nutrient-rich water flows in from the Baltic Proper.

In the Bothnian Sea and especially in its north-western part the oxygen situation is clearly better than last year. However, phosphorus levels have increased.

The oxygen conditions in the Bothnian Bay have remained good. Concentrations of dissolved nutrients have increased in the Bothnian Bay in recent years. Although nitrogen and phosphorus concentrations are low compared with other sea areas, record-high concentrations were recorded this winter in some parts of the region.

Oil concentration in the Baltic Sea has decreased in recent decades

Observed oil concentrations in the surface water in the Baltic Sea were low, and at levels that are typical for the winter. The oil concentration in the Baltic Sea has decreased in recent decades. The most likely reasons are stricter legislation, international agreements, the effectiveness of Finland's aerial surveillance of its sea areas, and a general change in attitude. Monitoring oil concentrations in the Baltic Sea is part of the Finnish monitoring programme for marine management.

SYKE and the Finnish Meteorological Institute jointly monitor the state of the open sea

Water and sediment samples were taken on the cruise of the research vessel Aranda 17–29 January 2022, recording temperatures, salinity, oxygen levels, nutrients, and harmful substances such as pharmaceuticals. The sea areas that were monitored included the Gulf of Finland, the Archipelago Sea, the Northern Baltic Proper, the Bothnian Sea and the Bothnian Bay. The ice cover extent and ice properties were also surveyed in the Bothnian Bay.

During the cruise, a sediment trap was moored close to the bottom in the eastern part of the Gulf of Finland. It will collect microplastics and other materials sedimenting through the water column. The results will provide information on the amount and quality of the microplastics and harmful substances that sink toward the bottom.

SYKE and the Finnish Meteorological Institute (FMI) jointly monitor the state of the open sea. The monitoring is part of the monitoring programme of the Baltic Marine Environment Protection Commission (HELCOM) and the Finnish marine strategy. SYKE is responsible for chemical and biological monitoring and the monitoring of underwater noise, and FMI is responsible for physical monitoring. Measurements taken in January during a monitoring cruise by the Swedish Meteorological and Hydrological Institute (SMHI) in the central and southern part of the Baltic Sea were also used in the evaluation of state of the Baltic Sea.

Further information

  • The state of the Gulf of Finland, harmful chemicals, underwater noise: Leading Researcher, cruise leader Harri Kankaanpää, Finnish Environment institute SYKE, tel. +358 295 251 258,
  • State of the Gulf of Bothnia, the Archipelago Sea, and the Baltic Proper: Senior Researcher, cruise leader Pekka Kotilainen, Finnish Environment institute SYKE, tel. +358 295 251 317,
  • Internal nutrient flows of the Baltic Sea: Senior Scientist Jouni Lehtoranta, Finnish Environment institute SYKE, tel. +358 295 251 363,
  • Microplastic sedimentation: Research Professor Maiju Lehtiniemi, Finnish Environment institute SYKE, tel. +358 295 251 356,
  • Physical conditions in the Baltic Sea, climate change: Head of Unit, Laura Tuomi, Finnish Meteorological Institute, tel. +358 295 396 404,


Edited 21.2.2022, 9.40 am: replaced milligrams per litre with micrograms per litre.

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