New analysis shows the coastal areas of the Baltic Sea have lost unprecedented amounts of oxygen during the 20th century.

The Baltic Sea hosts some of the largest dead zones, vast expanses of saltwater with little or no oxygen, in the world. Animal life cannot survive inside a dead zone.

The Baltic's massive dead zones are the result of decades of fertilizer and pollution runoff, fueling the growth of algal blooms. When the blooms die, microbial communities consume the decomposing algae, pulling large amounts of oxygen from the water during the process.

While human-caused nutrient inputs have decreased in recent years, the problem of hypoxia persists. Decaying algae do more than feed oxygen-eating microbes. The dying algae also releases phosphorous, fueling cyanobacteria blooms, which pulls nitrogen form the atmosphere.

"As a result, the total amount of nutrients -- phosphorus and nitrogen -- in the water remains high even after human inputs have been reduced," Tom Jilbert, an assistant professor at the University of Helsinki, said in a news release.

"It is a self-sustaining vicious circle that can take decades to reverse," said Sami Jokinen, a researcher at the University of Turku.

This negative feedback loop is exacerbated by global warming. Warmer water can't hold as much oxygen as colder water.

To place the region's oxygen losses in broader historical context, scientists drilled sediment cores from the bottom of the Archipelago Sea, a thin strip of the Baltic Sea lying between Finland and Sweden. The sediment cores helped scientists track the impacts of climate change on oxygen levels and nutrient pollution.

Their analysis -- published this week in the journal Biogeosciences -- showed oxygen levels were low between 900 to around 1350 AD, during a warm period known as the Medieval Climate Anomaly. However, modern coastal oxygen losses are much more severe.

"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," Jilbert said.

The research also showed modern oxygen losses picking up steam during the early 1900s.

To curb the problem of oxygen losses in the Baltic, scientists say more needs to be done to slow global warming and limit nutrient loading.

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

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