When the oxygen content of bottom water gets low, eventually only bacteria are able to survive on the seabed. Here is the so-called dead layer, which consists of white sulfur bacteria. (Photo: Peter Bondo Christensen)

Published: 2014-04-01

Dead zones have increased by more than 10-fold in the last century

BNI researchers and their international colleagues report that the dead zones in the Baltic Sea have increased 10-fold over the last 115 years, growing from approximately 5 000 km² in 1900 to more than 60 000 km² in recent years. The study is published in the latest issue of PNAS, Proceedings of the National Academy of Sciences.

Largest dead zone in the world

The Baltic Sea has previously been identified as home to the world's largest anthropogenic "dead zone" (low oxygen zone), leading to death of organisms that live on the bottom.

BNI researchers Jacob Carstensen and Bo Gustafsson, together with Danish and Swedish colleagues, now show that the dead zones have increased 10-fold over the last 115 years. They have grown from approximately 5 000 km² in 1900 to more than 60 000 km² in recent years.

This increase shows large decadal fluctuations over the last century, reducing the potential fish yield and favoring noxious algal blooms.

The extent of dead zones can be seen in these figures, where areas in the Baltic Sea with low oxygen content are depicted in red and no oxygen areas in black. Fig. A shows the situation in 1906, fig. B in 1955 and fig. C the situation in 2012.

NEW METHOD TO ANALYZE HOW AND WHY DEAD ZONES DEVELOP

The research team has, as part of their research in the BONUS-project HYPER (Hypoxia mitigation for Baltic Sea ecosystem restoration) and the HELCOM-project TARGREV developed a new method to analyze how these large dead zones develop and what the underlying causes are.

By analyzing the different processes that affect oxygen concentrations in bottom waters, the team has reconstructed oxygen and stratification conditions, and has been able to separate the effects of climate, saltwater inflows and nutrients.

The study, "Deoxygenation of the Baltic Sea during the last century" shows that nutrient inputs are the primary cause of today’s severe hypoxia situation. In addition, there are indications that higher deep-water temperatures in recent years may have had an addition effect.

HUMAN DRIVERS BEHIND THE INCREASE IN DEAD ZONES

The study highlights that although climate warming plays an important role in increasing ocean deoxygenation, anthropogenic nutrient discharges are the primary driving factor creating widespread hypoxic conditions in the Baltic Sea.

These increased nutrient levels can be attributed to the use of fertilizers, large animal farms, the burning of fossil fuels, and effluents from municipal wastewater treatment plants.

NEED FOR IMMEDIATE NUTRIENT REDUCTIONS THROUGH IMPLEMENTING THE BSAP

To be able to halt the increase of dead zones nutrient reductions are needed and the researchers call for immediate implementation of the HELCOM Baltic Sea Action Plan (BSAP) and the national reductions that the Baltic Sea countries have agreed upon.

If actions are postponed further, the research team fears that the situation will continue to worsen, not least under future climate changes.

This could have significant and devastating effects on the Baltic Sea ecosystem at all trophic levels, influencing benthic communities, biodiversity and the ecosystem services provided.

READ MORE ABOUT THE STUDY

THE STUDY IN THE MEDIA

The study has been mentioned in a wide range of international media and below are some examples for our international audience. If you are interested in more information, please contact BNI's communication officer Marmar Nekoro.

Denmark:

Sweden:

Germany, the article was featured more than 120 times (!), including in:

Poland, e.g. Interia and RMF24

Finland: e.g. Vasabladet and Österbottens Tidning

Italy, e.g. Article in La Stampa and in freenewspos.com

USA, e.g. Science Daily and some examples from China, Australia, Austria, UKand Switzerland.

France - a feature in the June issue of the French science magazine La RecherchePDF (pdf, 1.6 MB)


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Web editor: Marmar Nekoro

Updated: 2014-06-10
Baltic Nest Institute Sweden
Baltic Sea Centre, Stockholm University
SE-106 91 Stockholm, Sweden, +46-8-16 37 18
Baltic Nest Institute Denmark
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Baltic Nest Institute Finland
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