Status of the Baltic Sea after a winter of climate change

Winter in Southern Finland was record-breakingly mild and snowless. During the winter, we witnessed in the worst possible way the challenges climate change can pose to waterway protection. In many rivers, record-breaking nutrient loads were measured. In southwestern Finland, for example, a river could carry more than half of its annual nutrient flow within the span of one week. Up until now, the greatest nutrient runoffs took place during the spring floods, but with the winters affected by climate change, the greatest runoffs are often measured in December, January, and February.

The results of the marine research vessel Aranda’s winter voyage indicate that the stormy and ice-free winter has efficiently mixed the oxygen-rich top water layer of the Gulf of Finland all the way down to the depths of the seabed. A good side in all this is that the oxygen status of the Gulf of Finland was exceptionally good, and there have been no indications of poisonous hydrogen sulphide. As a downside, the rich phosphorus reserves of the depths are now mixed with the entire mass of water, leading to a high concentration of phosphorus in the Baltic Sea’s surface waters when compared to the average of the past 20 years. At one measurement station, they even managed to reach the highest levels of phosphorus in measuring history. However, the levels of so-called calculated surplus phosphorus — i.e. phosphorus that remains in surface waters after a nitrogen-limited spring bloom, and directly feeds the blue-green algae blooms in the summer — were still below the levels of 2017 and 2019. This would indicate that blue-green algae blooms next summer will be slightly lower in volume than in previous years, of course depending on the winds and the temperature of the seawater.  A warm and windless summer favours the mass appearance of blue-green algae.

An unwelcome trend in the measurements of the past few years is the rise in the phosphorus concentration of the Bothnian Sea, clearly visible also in Aranda’s measurements from the winter of 2020. Concentrations of soluble phosphorus have been rising in the Bothnian Sea all through the 2010s, nearly doubling in the span of ten years, and increasing the likelihood of algae blooms also on the Bothnian Sea. Earlier, eutrophication was seen to be first and foremost a problem of the Gulf of Finland and the Archipelago Sea, but now we see an increase in eutrophication also in the more northern marine areas.

Marjukka Porvari
Director, Clean Baltic Sea projects, John Nurminen Foundation