Baltic Coast suffers from severe oxygen loss

The loss of oxygen in the Baltic Sea coastal areas has been unprecedented in the last century compre to the average rate of loss in the last 1,500 years, reports a study conducted in Finland and Germany.

The study report was published in the European Geosciences Union journal Biogeosciences, which said the Baltic Sea is home to some of the world’s largest dead zones, areas of oxygen-starved waters where most marine animals can’t survive, said a press release issued by the University of Turku.

According to the study findings, human-induced pollution, from fertilisers and sewage running off the countries surrounding the Baltic into the sea, is the main driver of recent oxygen loss in the region’s coastal waters.

The spread of low-oxygen areas can have dire consequences for the environment and for local populations as it can reduce fish yields and even lead to massive die-offs of marine animals.

“The Baltic was strongly impacted by human nutrient inputs in the 20th century and is still experiencing the legacy of those inputs today,” said Tom Jilbert, an assistant professor at the University of Helsinki and one of the study authors.

But despite recent measures to reduce the release of polluting nutrients, the researchers wrote in the study report that they found “no evidence of recovery” from oxygen depletion in the Archipelago Sea, a coastal area between the mainland Finland and Sweden that is part of the Baltic.

"A reason, they say, may be climate change. Since warm waters are less effective at holding oxygen, global warming is likely to exacerbate oxygen depletion," said Sami Jokinen, a researcher at the University of Turku and the lead author of the study.

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

To find out what fuelled oxygen loss in the past and what role the climate played, the team drilled and studied a 4m-long sediment core from the seafloor in the Archipelago Sea. This allowed them to see, for the first time, how oxygen levels changed in this area over the past 1,500 years. This period includes the Medieval Climate Anomaly, a time of warmer climate but low nutrient pollution from 900 to around 1350, as well as modern times.

"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 Jilbert.

The team found that oxygen levels were also low during the warmer medieval period, but they wrote in their report that the present rate of oxygen loss is “unprecedentedly severe”, showing how excess pollution and warmer temperatures can combine to make dead zones thrive.

They also found that this recent oxygen loss started in the early 1900s, decades before it had been previously thought and prior to regular water-quality monitoring.

"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. Landmass in the Baltic Sea area has been rising since the end of the Ice Age removed weighty ice sheets from the region, and this uplift makes some coastal areas more sensitive to oxygen loss. 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,” said Jokinen.

This nutrient loading has long-term effects, making it hard to stop the ongoing spread of dead zones. Rivers in the inhabited coastlines of the Baltic carry nutrients into the sea, triggering algae blooms. As the algae die, they sink to the seafloor and are decomposed by bacteria, which use up oxygen in this process.

“If human nutrient inputs are reduced, this might be expected to reduce the blooms and shrink the dead zone,” said Jilbert. But, in dead zones, the decaying algae release phosphorus more efficiently, which then flows back to surface waters where it leads to the growth of cyanobacteria (blue-green bacteria), which, in turn, capture nitrogen from 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, said Jilbert. – It is a self-sustaining vicious circle that can take decades to reverse,” Jokinen added.

“Nowadays, and likely in the future, oxygen loss in the Archipelago Sea is sustained by the continued leakage of nutrients from the agricultural land, the release of phosphorus from the sediments to the water column due to low oxygen levels, and by the ongoing global warming,” said Jokinen.

“The good news is that many countries in the Baltic catchment have taken significant steps towards nutrient loading reductions, said Jilbert, adding that in some coastal regions we are already seeing improvements.

Better under-standing of the balance between nutrient inputs and climate change will therefore help to guide management of the Baltic in the future.