Shocking oxygen loss in the Baltic Sea as dead zones expand

A new study published by the European Geosciences Union has found a shocking amount of oxygen loss in the Baltic Sea. While the region is home to the world’s largest oxygen-deprived areas of water, known as dead zones, the loss of oxygen in Baltic coastal areas has been unprecedented over the last 1,500 years.

According to an international team of researchers, the main driver of oxygen loss in the Baltic Sea is human-induced pollution from fertilizers and sewage runoff.

The expansion of the so-called dead zones can have major consequences for the environment. In addition, the reduction of local fish populations is worsened.

Study co-author Tom Jilbert is an assistant professor at the University of Helsinki in Finland.

“The Baltic was strongly impacted by human nutrient inputs in the 20th century and is still experiencing the legacy of those inputs today,” said Professor Jilbert.

Despite recent efforts to reduce the release of polluting nutrients, the researchers said that they found “no evidence of recovery” in oxygen across the Archipelago Sea, which is a Baltic area between Finland and Sweden.

Study lead author Sami Jokinen is a researcher at the University of Turku.

According to Jokinen, warm waters are less effective at holding oxygen, which means that “global warming is likely to exacerbate oxygen depletion.”

“Climate change was not the main cause of the current dead zone, but it is an important factor delaying the recovery,” said Jokinen.

For their investigation, the team drilled and studied a sediment core from the floor of the Archipelago Sea with a length of four meters. The scientists observed how oxygen levels have changed in this area over the past 1,500 years.

“The interesting finding from our study is that, in the coastal areas, oxygen loss in the modern period really stands out, due to the strong signal of recent human nutrient inputs,” said Professor Jilbert.

The study revealed that oxygen levels were low during the medieval period as well, but the experts pointed out that oxygen loss has now become “unprecedentedly severe.”

According to the study, the oxygen loss began in the early 1900s. This dates back decades further than what was previously realized.

“This is surprising because the 1950s is often regarded as the period of rising oxygen depletion in the Baltic Sea, which has been linked to the substantial increase in human-induced nutrient loading around that time,” explained Jokinen.

“On top of this, we found evidence of marked human-induced nutrient loading already at the turn of the 20th century, which likely stimulated oxygen depletion in coastal areas.”

Nutrient loading has long-term impacts, and makes it difficult to stop the ongoing spread of dead zones.

“The good news is that many countries in the Baltic catchment have taken significant steps towards nutrient loading reductions,” said Professor Jilbert.

“In some coastal regions we are already seeing improvements. Better understanding of the balance between nutrient inputs and climate change will therefore help to guide management of the Baltic in the future.”

The research is published in the journal Biogeosciences.

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By Chrissy Sexton, Earth.com Staff Writer

Image Credit: Kari Mattila, The Archipelago Research Institute