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Drop in Oceanic Oxygen Levels Could Severely Affect Global Marine Nitrogen Cycle

A new study by researchers at the University of Bristol has revealed that the feedback mechanisms that were believed to keep the marine nitrogen cycle comparatively stable over geological time can break down when oxygen levels in the ocean drop considerably.

On August 11th, 2015, a NASA satellite captured this false-color image of a large bloom of cyanobacteria (Nodularia) swirling in the Baltic Sea. These cyanobacteria fix inorganic atmospheric Nitrogen (N2) into a form available to Life, a process fundamental for marine ecosystems. In the paper, researchers show that nitrogen fixation becomes even more important when the state of oxygenation of the ocean declines. Image Credit: NASA Earth Observatory/USGS.

The nitrogen cycle is vital to all life forms on Earth—nitrogen is a fundamental building block of DNA.

The marine nitrogen cycle is strongly regulated by biology, and small variations in the marine nitrogen cycle have huge implications on life. It is believed that the marine nitrogen cycle has stayed comparatively stable over geological time because of a range of diverse feedback mechanisms.

These feedback mechanisms are called “the nitrostat.” Yet, how the global marine nitrogen cycle and the related feedback mechanisms reacted to severe variations in marine oxygenation in the past is not properly understood.

The researchers used a data-constrained earth system model to demonstrate that under these deoxygenated conditions, nitrogen levels in the ocean could become exceedingly depleted as the total bioavailable nitrogen inventory breaks down relative to phosphorus.

Simultaneously, the ocean shifts from an oxic-nitrate ocean to an anoxic ammonium ocean. The significant reduction in the ocean bioavailable nitrogen inventory in reaction to the shift in marine oxygenation may signify a core biogeochemical vulnerability.

The study outcomes were reported recently in the journal Proceedings of the National Academy of Sciences of the United States of America.

Our results demonstrate that changing the amount of oxygen in the ocean can have disastrous effects on vital biogeochemical cycles such as the nitrogen cycle, which is essential for all forms of Life.

Dr David Naafs, Study Lead Author, School of Earth Sciences, University of Bristol

According to Dr Fanny Monteiro, co-author of the study from Bristol’s School of Geographical Sciences, “Our modelling results are in agreement with the sparsely available proxy data from the geological past.”

Our modelling results show the impact of changes in ocean oxygenation on the marine nitrogen cycle for places and time periods for which we do not (yet) have sufficient proxy data.

Ann Pearson, Study Co-author and Professor, Harvard University

The state and strength of the marine nitrogen cycle and biological pump in the ocean are extremely vulnerable to disruptions in the oceanic oxygen level.

At present, oxygen levels in the oceans are decreasing, and this is projected to decline further in the coming decades because of anthropogenic activities. The results show that the marine nitrogen cycle might be greatly disrupted in the future.


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