The marine research vessel Aranda at sea, January 2024. Photo: Ilkka Lastumäki / Finnish Environment Institute


The Baltic Sea is facing unprecedented environmental challenges, with the latest marine research indicating alarmingly high levels of phosphate, a situation that poses a threat to its ecological balance. The annual winter survey conducted by the research vessel Aranda has unveiled that the Baltic Sea, particularly the Gulf of Finland and the Archipelago Sea, is experiencing the highest phosphate levels recorded in the last three decades.

This surge in nutrient content, coupled with persistent oxygen deficiencies in certain areas, underscores the pressing need for intensified environmental management efforts.

In the Gulf of Finland, researchers have detected not only an increase in salinity at the surface but also phosphate concentrations reaching unprecedented levels across a broader region than previously observed. This rise in nutrient levels, attributed to the inflow of nutrient-rich water bodies, exacerbates the eutrophication problem but, paradoxically, slightly improves the oxygen conditions in deeper waters by encouraging water layer mixing.

The Archipelago Sea has not been spared, with nutrient levels soaring to new winter highs, likely influenced by the transference of nutrient-laden waters from the Gulf of Finland. The study also highlights deteriorating conditions in the Bothnian Sea, where oxygen levels in deeper parts are declining, and nutrient concentrations remain high, a trend that began in the early 21st century.

Conversely, the Kvarken and Bothnian Bay areas maintain normal nutrient and oxygen levels for the winter season. However, the main basin of the Baltic Sea continues to struggle with poor oxygen conditions, especially in areas where the water column is strongly stratified, leaving deep waters devoid of oxygen.

The report also addresses the impact of the December 2023 saltwater inflow through the Straits of Denmark, which, despite being moderate compared to previous events, promises some improvement to the deep-water oxygen conditions around the island of Bornholm. The full effects of this influx are anticipated to manifest in the northern parts of the main basin by late spring or early summer.

In addition to nutrient and oxygen assessments, the Aranda cruise extended its research to include the monitoring of harmful substances and underwater noise. Findings reveal an increase in low-frequency noise in the Gulf of Finland and the northern main basin, which could have detrimental effects on marine life, particularly fish and shellfish. The presence of hazardous chemicals, such as oil and fluorine compounds, was noted but remains below critical levels in most areas.

As the Aranda prepares for its next expedition in mid-April, focused on tracking the progression of phytoplankton spring blooms, the data collected serves as a crucial indicator of the Baltic Sea's health. The findings not only highlight the urgent need for effective nutrient management strategies but also underscore the importance of ongoing monitoring to safeguard this vital marine ecosystem.