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Massive Fish Deaths Result From An Ocean Short Of Oxygen: Dead Zones Expanding
Every summer, a few places in the north-east Pacific now see vast numbers of dead marine animals including fish, shrimp or molluscs being washed up on the beach by the waves. It is a financial catastrophe for the local fishing industry and an ecological one for the sea. These mass mortalities are caused by the animals suffocating because their water contains too little oxygen, or even none at all.
Millions of dead sardines on Redondo Beach, California: the fish suffocated in oxygen-depleted water.
Although this phenomenon is still localised, that could change in the future, as revealed by a new publication in “Nature Geoscience”. The two authors, Samuel Jaccard from the Geological Institute at ETH Zurich and Eric Galbraith from McGill University, Canada, have evaluated oxygen data from seafloor sediment cores. Based on this data, they have reconstructed how the oxygen content in the oceans throughout the world has changed in the past 20,000 years, focusing particularly on the Pacific and Indian Oceans.
When temperature rises, oxygen vanishes
Their analyses show that the marked average global temperature rise of around two degrees centigrade occurring between the peak and the end of the last ice age, i.e. 20,000 to 10,000 years ago, had a massive effect on the oxygen content of seawater. The oxygen content of many oceans fell dramatically as a result of the temperature increase, and oxygen-depleted marine zones expanded more than during the cold period.
Jaccard stresses that, “We are now seeing a comparable warming, but it is taking place over a mere 100 years.” The global average temperature has increased by almost one degree Celsius since the Industrial Revolution. Models are predicting a worldwide average warming of at least two degrees by 2100. Although their analyses do not permit any conclusions by analogy for its future development, Jaccard and Galbraith believe that the current rapid climate warming could lead to a similar phenomenon.
Expansion of the dead zones
At present around 15 percent of the oceans are oxygen-depleted or anoxic “dead zones”. They are expanding in the north Pacific especially. Jaccard fears that these dead zones will expand further in the wake of climate change. The ETH Zurich researcher says, “Our assessment confirms studies that describe an enlargement of the oxygen-depleted sea areas, and to this extent the fears that rising temperatures remove oxygen from the oceans are justified.”
These dead zones are particularly critical because they are situated in areas of the ocean that are home to the largest quantity of life. This means a shrinkage of habitat especially for the large fish of the open ocean. Entire food chains are threatened with deprivation. Biodiversity in the oceans will probably decrease, which will also pose serious problems for the fishing industry.
Poor mixing
The oxygen in seawater originates mainly from gas exchange between the sea surface and the atmosphere. Algae and other green life-forms also produce the vital gas. However, oxygen is consumed when dead organic material sinks down towards the sea bed and is degraded in the process by various organisms, mostly microbes. Warmer temperatures in the uppermost layers of water disturb this delicate equilibrium, because for physical reasons warm water is able to absorb less oxygen than colder water.
Furthermore, more stable layers form when seawater warms up. Warm, lower-density water lies on top of cold, denser water. These layers scarcely intermix, reducing the efficiency of the gas exchange with the atmosphere and between the various bodies of water.
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Story by Peter Rüegg
ETH Life
Citation: Jaccard SL & Galbraith ED. Large climate-driven changes of oceanic oxygen concentrations during the last deglaciation. Nature Geoscience (2011). Published online 18 December 2011. DOI: 10.1038/ngeo1352
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fear-mongering with junk science 101