Gulf of Finland mends, Bothnian Sea gets sick

The state of the Gulf of Finland has improved in the past few years but the deteriorated state of the Bothnian Sea gives cause for concern, reports a Finnish Environment Institute (SYKE) assessment.

SYKE made an assessment of the state of the coastal waters of the Gulf of Finland and the open sea surrounding Finland from the southern Baltic Sea to the Bay of Bothnia, said a press release.

Towards the end of this summer, the oxygen conditions near the seabed of the Gulf of Finland were better than in the past few years. Entirely an oxic seabed and toxic hydrogen sulphide were observed only at the westernmost sampling point in the Gulf of Finland near the Estonian coast. As the seabed was oxic, the amount of phosphate phosphorus in the water above the seabed was also lower than in the past few years.

“The reason for the better state of the seabed in the open sea area in the Gulf of Finland can be found in the northern part of the Baltic Proper, where the halocline fell deeper last winter. This is still visible as lower salinity and weaker stratification are also seen in the Gulf of Finland. In the measurements conducted towards the end of the summer, increased phosphate concentrations in the central water column were observed only at some stations in the Gulf of Finland compared to the corresponding figures of the last 10 years,” said Research Professor Maiju Lehtiniemi.

The improved oxygen situation in the Gulf of Finland was also reflected in the state of the seabed and the benthic animal communities in the coastal areas and in the outer archipelago. The state of the surface sediment was interpreted to be toxic in almost two out of three of the examined samples. A year ago, the proportion was only just over one third. Benthic animals were observed at 90 per cent of the observation sites. Since 2001, the situation during the monitoring performed at the same observation sites has been better than this only once, says Senior Research Scientist Seppo Knuuttila, who led the monitoring cruise in the coastal waters.

The chlorophyll concentrations describing the amount of algae were close to the lowest values of the 2000s at the intensive monitoring station of Huovari in the eastern Gulf of Finland in July and August. Compared to the summer of 2018, which saw the worst algae situation of the past decade, the concentrations were lower by half on an average.

“Based on the observations collected in August, the oxygen condition in the Baltic Proper is still poor and the water is anoxic from 70 metres down to the seabed. The anoxic area extends from the southern Baltic Sea to the northern part of the Baltic Proper and up to the mouth of the Gulf of Finland to the east,” said Senior Researcher Pekka Kotilainen, the leader of Aranda’s long monitoring cruise.

The amount of oxygen in the basins of the Bothnian Sea was lower than that at the same time last year. In some places in the Bothnian Sea, the observed phosphate phosphorus concentrations in the entire water column were higher than the average long-term concentrations for summer. During the cruise, more algae blooms than usual were observed in the Bothnian Sea and Kvarken.

The most probable explanation for the increased phosphorus concentrations is the more phosphorous water flowing in from the Baltic Proper. The observations indicate changes in the state of the Bothnian Sea. The increasing algae blooms may further weaken the oxygen condition in the Bothnian Sea when the algal mass consumes oxygen in the water layers close to the seabed as it decomposes.