Researchers from Utrecht University show in a publication in Science Advances how algal blooms in the oceans can run wild. The climate scientists discovered that during periods of major climate change, phosphate was hardly buried in the seabed anymore. This fueled an enormous growth of algae in the ocean, making large parts of the ocean devoid of oxygen and unlivable. The scientists emphasize that something like this could happen again.
Millions of years ago, when there were even more greenhouse gases in the atmosphere than today, the oceans were warmer and more acidic. Large areas of the oceans were also devoid of oxygen: an explosive growth of algae, followed by large-scale decay, led to a massive loss of oxygen from the seawater. This made these parts of the ocean practically uninhabitable. Even today there are areas in the ocean where the oxygen content of the water drops so much that fish and many other marine life can no longer live there. These ‘dead zones’ are observed worldwide, for example in the Baltic Sea or in the Gulf of Mexico.
Nitrogen and phosphate in seawater give algae a growth boost. Spectacular, but sometimes harmful, algae growth is then the result, which can lead to the formation, or enlargement, of dead zones. Climate change and the warming of the oceans also play a role in this. The Utrecht researchers wanted to know what influence climate change has on the formation of dead zones. They studied various chemical and biological indicators from seabeds millions of years old, from periods when the concentration of greenhouse gases in the atmosphere was much higher than today.
The researchers were especially curious to find out if the availability of phosphate in these warmer oceans was different. They therefore looked at the reuse of phosphate, and it turned out that phosphate was reused much more efficiently in warm oceans. Much less phosphate than expected ended up in the ocean floor. So phosphate could return to the ocean water over and over to promote algae growth. This self-reinforcing cycle, in which phosphate became available again and again for algae growth, led to increased oxygen loss in the oceans and an increase in ocean dead zones.
What was the determining factor that halted the formation of the apatite minerals is not yet exactly clear. Despite this, the researchers’ findings have important implications for the impacts of current climate change. There is a risk that the current increase in CO2 concentrations in the atmosphere will accelerate the reuse of nutrients in the ocean, resulting in increased oxygen loss and other negative consequences for ocean life.
More information can be found in the publication ‘Enhanced phosphorus recycling during past oceanic anoxia amplified by low rates of apatite authigenesis‘ in Science Advances.
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