Baltic Sea hypoxia
Encyclopedia
Baltic Sea hypoxia refers to low levels of oxygen
in bottom waters, also known as hypoxia
, occurring regularly in the Baltic Sea
. 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
loading from human activities causing algal blooms. 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
.
Figure of hypoxic area from 2007. Red shows the areas that experience hypoxia (O2 < 2 ml/l) and black is the anoxic area with no oxygen left in the water. Figure from SMHI.
in coastal areas around the world is due to the excessive inputs of plant nutrients, such as nitrogen
and phosphorus
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
. 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
In the Baltic Sea, the input of salt water from the North Sea
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, lighter surface waters. Thus, large areas of hypoxia occur when more salt water comes into the Baltic Sea.
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 Littorina Sea starting around 8,000 calendar years BP. Hypoxia disappeared for a period of nearly 3,000 years appearing again just before the Medieval Warm Period around 400 calendar years AD and lasted for 800 years. 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 both climate and human impacts starting already several thousand years ago.
. In the Baltic Sea, low oxygen concentrations also reduce the ability of cod
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. 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 eutrophication
that helps to sustain itself.
White mats of Beggiatoa bacteria on the sediment surface that occur on the bottom when oxygen is no longer present in the water.
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in bottom waters, also known as hypoxia
Hypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
, occurring regularly in the Baltic Sea
Baltic Sea
The Baltic Sea is a brackish mediterranean sea located in Northern Europe, from 53°N to 66°N latitude and from 20°E to 26°E longitude. It is bounded by the Scandinavian Peninsula, the mainland of Europe, and the Danish islands. It drains into the Kattegat by way of the Øresund, the Great Belt and...
. 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
Nutrient
A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...
loading from human activities causing algal blooms. 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
Dead zone (ecology)
Dead zones are hypoxic areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. These occur near inhabited coastlines, where aquatic life is most concentrated...
.
Figure of hypoxic area from 2007. Red shows the areas that experience hypoxia (O2 < 2 ml/l) and black is the anoxic area with no oxygen left in the water. Figure from SMHI.
Causes of hypoxia
The rapid increase in hypoxiaHypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...
in coastal areas around the world is due to the excessive inputs of plant nutrients, such as nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
and phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
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
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...
. 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
In the Baltic Sea, the input of salt water from the North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
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, 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 sedimentsLamination (geology)
In geology, lamination is a small scale sequence of fine layers that occurs in sedimentary rocks. Lamination is normally smaller and less pronounced than bedding. Lamination is often regarded as every planar structure smaller than 1 cm, and bedding as every planar structure larger than that...
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 Littorina Sea starting around 8,000 calendar years BP. Hypoxia disappeared for a period of nearly 3,000 years appearing again just before the Medieval Warm Period around 400 calendar years AD and lasted for 800 years. 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 both climate and human impacts starting already several thousand years ago.
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 zonesDead zone (ecology)
Dead zones are hypoxic areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. These occur near inhabited coastlines, where aquatic life is most concentrated...
. In the Baltic Sea, low oxygen concentrations also reduce the ability of cod
Cod
Cod is the common name for genus Gadus, belonging to the family Gadidae, and is also used in the common name for various other fishes. Cod is a popular food with a mild flavor, low fat content and a dense, flaky white flesh. Cod livers are processed to make cod liver oil, an important source of...
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. 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 eutrophication
Eutrophication
Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...
that helps to sustain itself.
White mats of Beggiatoa bacteria on the sediment surface that occur on the bottom when oxygen is no longer present in the water.