Record hot water deep in Baltic Sea

Water temperatures in the Baltic Sea are increasing and the deep-water temperatures are record-high in some areas.

Phosphorus concentrations are rising in the central part of the Archipelago Sea and the Bothnian Sea, said the Finnish Environment Institute (SYKE) and the Finnish Meteorological Institute (FMI) in a joint press release on Thursday.

The observations were made during the annual winter monitoring cruise by research vessel Aranda in January which is part of the Monitoring Programme of the Finnish Marine Management Plan.

An increase of 2-3 degrees Celsius in the deep-water temperature has been observed in the Baltic Sea.

Record-high temperatures were measured in the Gulf of Finland, the Åland Sea, and the Gulf of Bothnia. In the Åland Sea Basin, which goes down to 285 metres (station F64), the recorded water temperature at 250 metres exceeded +7°C for the first time in the 57 years’ history of measurements.

“The high water-temperatures are probably related to the exceptionally warm winter of 2019-2020, movements of water masses in the Baltic Sea, and the effects of the climate change,” said Senior Scientist Pekka Alenius of the FMI.

Nutrient concentrations were high in the Kihti open sea area of the Archipelago Sea. The entire water column was evenly mixed down to the bottom and the oxygen situation was good. The high nutrient concentrations were mainly attributable to the mixing and inflow from the Northern Baltic Proper and the Gulf of Finland. The situation is also likely to be affected by the nutrient-rich water flowing from the inner Archipelago Sea.

“Phosphorus concentrations in the Bothnian Sea were high, as they have been in recent years and the change appears to be permanent. The oxygen situation in the deeps of the sea area has further deteriorated compared to that in the previous years. The blue-green algal blooms that have become increasingly frequent in the Bothnian Sea are related to the increase in phosphorus concentrations,” said SYKE Cruise Leader Pekka Kotilainen.

The oxygen situation in the Gulf of Finland has deteriorated from the previous winter, when the oxygen situation of the near-bottom water was exceptionally good due to effective mixing.

Now the deeps of the western Gulf of Finland were anoxic. There was also considerably less oxygen in the deepest parts of the central and eastern Gulf of Finland than that of a year ago. These situations with mixed and stratified water in the Gulf of Finland vary from year to year, depending on weather conditions.