Hypoxia in the Baltic Sea

1. Hypoxia in the Baltic Sea Prepared by the HYPER project partnership http://hyper.dmu.dk/

2. What is hypoxia? Hypoxia or the lack of oxygen is a phenomenon that occurs in aquatic environments when oxygen dissolved in the water becomes reduced in concentration to a point detrimental to aquatic organisms at a concentration of less than 2 mg O2/l. Helle Munk Sørensen Eels desperately looking for oxygen Mariager Fjord, Denmark, 25 August 1997 How long can you hold your breath?

3. Spreading dead zones Hypoxia has been reported from more than 400 coastal systems and the number of reports of hypoxia are increasing Diaz and Rosenberg 2008

4. The Baltic Sea The Baltic Sea is a brackish sea located in Northern Europe. The Baltic Sea is about 1600 km long, a coastline of 8000 km, an average of 55 m deep, and the maximum depth is 459 m. About 85 million people live in the Baltic Sea drainage basin. Hypoxia in the Baltic Sea is the largest human caused hypoxic area (dead zone) in the world.

5. Changes in hypoxia area with time The amount of bottom covered by hypoxia averages 49,000 km2 over the last 50 years, an area larger than the size of Denmark (43,000 km2). Conley et al. 2009. Environmental Science & Technology43: 3412-3420.

6. How long has hypoxia been present in the Baltic Sea? We must examine geological data as measurements are not available.

7. Formation of the Baltic Sea Baltic Ice Lake Ancylus Lake Littorina Sea 12 000 years BP 10 500 years BP 8 000 years BP Melting of glaciers Rising sea levels Land rebound The formation of the modern Baltic Sea occurred only around 8,000 years before the present (BP).

8. Hypoxia Through Time Hypoxia occurred in the Baltic Sea as soon as salt water entered the Baltic and continued for nearly 4,000 years as shown by the occurrence of laminated sediments. Hypoxia was due to the higher salinity from a larger opening with the North Sea and warmer temperatures.

9. Hypoxia Through Time Hypoxia again occurred during the Medieval Warm Period as shown by the occurrence of laminated sediments. The cause(s) of hypoxia during this time are under intense scientific investigation and is believed to have occurred due to nutrient inputs from humans through forest clearance and development of agriculture.

10. Laminated sediments in the Gotland Basin Hypoxia was absent from the Baltic Sea until around 1900 when the entire Gotland Basin became hypoxic and began to deposit laminated sediments. It is still a dead zone even today. (Hille et al. 2006)

11. What are the causes of hypoxia in the Baltic Sea? Over-enrichment by the nutrients nitrogen and phosphorus stimulate the growth of algae which sink to the bottom and uses up the oxygen in bottom waters. Excess nutrient inputs come from a variety of sources including agriculture, human wastes and from the burning of fossil fuels. The area of bottom covered by hypoxia is partly controlled by physical factors especially by variations in the inflow of salt water.

12. The ”viscious circle” The size of spring bloom in the Baltic is controlled by nitrogen. The spring bloom sinks to the bottom, causing hypoxia releasing phosphours and stimulating the growth of cyanobacteria. Summer cyanobacteria blooms and nitrogen fixation Spring bloom kväve fosfor nitrogen phosphorus Phosphorus is bound to iron oxides under oxic conditions and then released under hypoxia.

13. Hypoxia = decomposition > renewal Saltwater inputs into the Baltic Sea creates density layers restricting circulation and enhancing hypoxia. North Sea Baltic Sea Blue color = saltier water (Major inflows)

14. What are the consequences of hypoxia in the Baltic Sea?

15. The lack of oxygen causes dead zones with all large animals and fish killed. Only microbes can survive. Dead bottoms covered by the bacteria Beggiotaia.

16. Massive amounts of phosphorus are released from sediments when bottom waters are hypoxic. Note: Colors are DIP Isopleths are oxygen concentrations Conley et al. 2002

17. What can be done to rid the Baltic Sea of hypoxia?

18. Engineering solutions have been suggested to reduce hypoxia in the Baltic Sea A recent study has shown that virtually all engineering projects to add oxygen to bottom waters or to change the circulation patterns in the Baltic Sea are unrealistic, not viable, and/or carry large environmental risks. For more details see: Conley et al. 2009. Environmental Science & Technology43:3407-3411

19. We must reduce nutrients in the Baltic Sea from all sources to improve condtions in the Baltic and its environmental health!

20. We are carrying out an EU BONUS funded project called HYPER (HYPoxia mitigation for Baltic Sea Ecosystem Restoration) to address the following research questions: What are the spatial differences in hypoxia and how does it effect biology and biogeochemistry? Can we separate the effects of humans from climate on the ocurrence of hypoxia? Are there regime shifts and thresholds with hypoxia?

21. HYPER Work Plan More details can be found at: http://hyper.dmu.dk/

22. Aarhus University Lund University Stockholm University Åbo Akademic Univ. Univ. of Helsinki Finnish Env. Inst. Zoological Inst., Russia University of Gdansk Leibniz Institute for Baltic Sea Research (IOW) Utrecht University Partners in HYPER