Despite efforts to reduce nutrient inputs into the Baltic, a new study has found climate change induced warming has continued to perpetuate a severe lack of oxygen and the expansion of oxygen minimum zones by stimulating bacterial growth. With intensifying warming and stratification, hypoxia is occurring more frequently, undermining ongoing restoration efforts and preventing ecosystems from recovering.
Due to enhancements with wastewater treatment technology, the coastal Baltic region has made significant strides in minimising nutrient inputs into the Sea, reducing loads by 18-20%. However, despite this progress nutrient levels still remain excessive, and eutrophication remains prevalent. Additionally, phosphorus previously bound in the seabed is being re-released into the water column due to the increased occurrence of oxygen minima, a process that also releases ammonium.
Baltic ecosystems are some of the most altered marine ecosystems globally, and the GEOMAR Helmholtz Center for Ocean Research Kiel used long-term measurements to investigate how environmental conditions changed between 1991 – 2019. The study focused on bacterial biomass production (BBP) in the southwestern Baltic Sea, using BBP to describe the growth of bacteria and other microorganisms that break down organic nutrients.
They discovered that in the wake of the Spring phytoplankton bloom, BBP increases significantly in the Summer, causing a spike in oxygen consumption levels that are particularly pronounced in the deeper layers. Meanwhile, warmer surface waters act as a lid preventing mixing between different water layers, and thus the transport of fresh oxygen into deeper layers.
As temperatures have increased over the decades due to climate change, the stratification of the water column has begun to extend into the Autumn months. In extreme years, heat waves have penetrated into the deeper layers, further exacerbating oxygen depletion. While there are no effective solutions available the situation is predicted to continue worsening, according to Dr Hepach, a leading researcher on the project.
A large area of the Southern Baltic recently suffered from methane leaks in the wake of the Nord Stream pipeline destruction, with methane levels 1,000 times higher than normal following the pipeline explosion in 2022. Researchers from VOTO and the University of Gothenburg have used their measurements to create a robust model of how the methane was dispersed in the water. They reported the methane spread across 14% of the Baltic, and was later transported to 23 marine protected areas via ocean currents.