Baltic Sea Dead Zones

• The Baltic Sea is the youngest sea in the world and a popular travel destination
• It serves as a habitat for fish and plants
• The Sea suffers from oxygen depletion leading to the creation of dead zones
• Uli Kunz, a marine biologist and research diver, is investigating these dead zones.

Oxygen Dependency of Marine Life

• Divers rely on oxygen tanks underwater to breathe
• Marine animals extract oxygen from the water using their gills.
• Oxygen deficiency in the water leads to uninhabitable conditions for fish and other marine life
• Oxygen depletion leads to essentially empty waters, devoid of life.

Extent and Monitoring of Oxygen-Poor Zones

• Researchers at the Helmholtz Centre for Ocean Research are using an advanced underwater robot to determine the number and size of oxygen-poor zones in the Baltic Sea
• The autonomous rover weighs 1.2 tons and is the size of a small car
• It is equipped with sensors and measuring technology, and can remain underwater for up to a year,
• The rover provides extensive data, including oxygen levels, revealing that dead zones are expanding
• Initially, the issue was confined to regions hundreds of meters deep
• In recent years, oxygen deficiency has spread to more biodiverse coastal regions.

Factors Contributing to Oxygen Deficiency

• The Baltic Sea's low salinity plays a role in oxygen depletion
• Over 200 rivers pour freshwater into the Baltic Sea
• Saltwater enters the sea from the North Sea
• The water's salt content drops to only 0.3% in the northern reaches of the Baltic Sea
• North Sea water contains over ten times more salt
• The interplay between saltwater and freshwater contributes to the formation of oxygen-poor zones, explained through a simple physics experiment
• Freshwater is lighter than saltwater, creating a layer on top of the saltier seawater, which prevents mixing
• Oxygen in the deeper layers gets used up and is not replenished unless fresh, oxygen-rich water enters from the North Sea.
• Specific wind conditions involving easterly winds followed by westerly winds are necessary for saltwater infusion from the North Sea
• Until the late 1980s, those conditions occurred every one to two years
• The life-giving saltwater infusion now occurs less frequently, sometimes with gaps of over ten years
• The reasons for this shift are under investigation, with climate change suspected as a contributing factor.

Eutrophication

• Another cause for oxygen decline is eutrophication, or the over-fertilization of the seas
• Fertilizers used by farmers near the Baltic Sea enrich the soil with nutrients like phosphorus and nitrogen
• Excess nutrients are washed into the Baltic Sea via rivers, promoting the growth of blue-green algae
• Extensive algal blooms, create algal carpets visible from space
• As the algae die, they sink to the bottom and decompose, consuming oxygen and depleting the supply for fish.
• Stricter fertilizer regulations, limiting the amount of fertilizer use, aim to mitigate this issue and reduce nutrient runoff into the Baltic Sea.