Baltic Sea hypoxia refers to low levels of

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

in bottom waters, also known as

hypoxia Hypoxia means a lower than normal level of oxygen, and may refer to: Reduced or insufficient oxygen * Hypoxia (environmental), abnormally low oxygen content of the specific environment * Hypoxia (medical), abnormally low level of oxygen in the tis ...

, occurring regularly in the

Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and from ...

. the total area of bottom covered with hypoxic waters with oxygen concentrations less than 2 mg/L in the Baltic Sea has averaged 49,000 km² over the last 40 years. The ultimate cause of hypoxia is excess

nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excret ...

loading from human activities causing

algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompas ...

s. The blooms sink to the bottom and use oxygen to decompose at a rate faster than it can be added back into the system through the physical processes of mixing. The lack of oxygen ( anoxia) kills bottom-living organisms and creates dead zones.

Causes

The rapid increase in

in coastal areas around the world is due to the excessive inputs of plant nutrients, such as

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

and

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

by human activities. The sources of these nutrients include agriculture, sewage, and atmospheric deposition of nitrogen containing compounds from the burning of fossil fuels. The nutrients stimulate the growth of algae causing problems with

eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...

. The algae sink to the bottom and use the oxygen when they decompose. If mixing of the bottom waters is slow, such that oxygen stocks are not renewed, hypoxia can occur.

Description

the total area of bottom covered with hypoxic waters with oxygen concentrations less than 2 mg/L in the Baltic Sea has averaged 49,000 km² over the last 40 years. In the Baltic Sea, the input of salt water from the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...

through the Danish Straits is important in determining the area of hypoxia each year. Denser, saltier water comes into the Baltic Sea and flows along the bottom. Although large salt water inputs help to renew the bottom waters and increase oxygen concentrations, the new oxygen added with the salt water inflow is rapidly used to decompose organic matter that is in the sediments. The denser salt water also reduces mixing of oxygen poor bottom waters with more

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

, lighter surface waters. Thus, large areas of hypoxia occur when more salt water comes into the Baltic Sea.

Geological perspective

Geological archives in sediments, primarily the appearance of laminated sediments that occur only when hypoxic conditions are present, are used to determine the historical time frame of oxygen conditions. Hypoxic conditions were common during the development of the early Baltic Sea called the

Mastogloia Sea The Mastogloia Sea is one of the prehistoric stages of the Baltic Sea in its development after the last ice age. This took place 8000 years ago following the Ancylus Lake stage and preceding the Littorina Sea stage. Overview Towards its demise, ...

and

Littorina Sea Littorina Sea (also Litorina Sea) is a geological brackish water stage of the Baltic Sea, which existed around 7500–4000 BP and followed the Mastogloia Sea, a transitional stage of the Ancylus Lake. This stage and form of the body of wat ...

starting around 8,000 calendar years

Before Present Before Present (BP) years, or "years before present", is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Becaus ...

until 4,000 BP. Hypoxia disappeared for a period of nearly 2,000 years, appearing a second time just before the Medieval Warm Period around 1 AD until 1200 AD. The Baltic Sea became hypoxic again around 1900 AD and has remained hypoxic for the last 100 years. The causes of the various periods of hypoxia are being scientifically debated, but it is believed to result from high surface salinity, climate and human impacts.

Impacts

The deficiency of oxygen in bottom waters changes the types of organisms that live on the bottom. The species change from long-living, deep-burrowing, slow-growing animals to species that live on the sediment surface. They are small and fast-growing, and can tolerate low concentrations of oxygen. When oxygen concentrations are low enough only bacteria and fungi can survive. These areas are called dead zones. In the Baltic Sea, low oxygen concentrations also reduce the ability of

cod Cod is the common name for the demersal fish genus '' Gadus'', belonging to the family Gadidae. Cod is also used as part of the common name for a number of other fish species, and one species that belongs to genus ''Gadus'' is commonly not call ...

to spawn in bottom waters. Cod spawning requires both high salinity and high oxygen concentrations for cod fry to develop, conditions that are rare in the Baltic Sea today. The lack of oxygen also increases the release of phosphorus from bottom sediments. Excess phosphorus in surface waters and the lack of nitrogen stimulates the growth of

cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...

. When the cyanobacteria die and sink to the bottom they consume oxygen leading to further hypoxia and more phosphorus is released from bottom sediments. This process creates a vicious circle of

that helps to sustain itself.

Solutions

The countries surrounding the Baltic Sea have established the

HELCOM The Baltic Marine Environment Protection Commission (Helsinki Commission, HELCOM) is an intergovernmental organization governing the Convention on the Protection of the Marine Environment of the Baltic Sea Area (Helsinki Convention). A regional ...

Baltic Marine Environment Protection Commission to protect and improve the environmental health of the Baltic Sea. In 2007, the Member States accepted the Baltic Sea Action Plan to reduce nutrients. Because the public and media have been frustrated by the lack of progress in improving the environmental status of the Baltic Sea, there have been calls for large-scale engineering solutions to add oxygen back into bottom waters and bring life back to the dead zones. An international committee evaluated different ideasConley, D.J., E. Bonsdorff, J. Carstensen, G. Destouni, B.G. Gustafsson, L.-A. Hansson, N.N. Rabalais, M. Voss, L. Zillén. 2009. ''Viewpoint:'' Tackling hypoxia in the Baltic Sea: Is engineering a solution? ''Environ. Sci. Tech.'' 43: 3407-3411. and came to the conclusion that large-scale engineering approaches are not able to add oxygen to the extremely large dead zones in the Baltic Sea without completely changing the Baltic Sea ecosystem. The best long-term solution is to implement policies and measures to reduce the load of nutrients to the Baltic Sea.

Causes

Description

Geological perspective

Impacts

Solutions

References

External links