Widespread loss of ocean oxygen to become noticeable in 2030s

(April, 2016)  A reduction in the amount of oxygen dissolved in the oceans due to climate change is already discernible in some parts of the world and should be evident across large regions of the oceans between 2030 and 2040, according to a new study.

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Upwelling intensity modulates nitrous oxide concentrations over the western Indian shelf


"Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continental shelf of India in 2012 revealed high concentrations of 45 ± 32 nM (off Kochi) and 73 ± 63 nM (off Mangalore) during the summer monsoon (SM). N2O concentrations increased nonlinearly during the peak of the SM upwelling, when low O2 (<25 µM) conditions prevailed in the water column. Off Kochi, N2O levels fell gradually from the fall intermonsoon (20 ± 8 nM) to the winter monsoon (8.8 ± 2 nM) and remained low (9.2 ± 5.2 nM) through the spring intermonsoon season. The N2O supersaturation off Kochi (574 ± 720%) was presumably due to its high yield during sediment denitrification, whereas the higher N2O supersaturation observed off Mangalore (1046 ± 885%) was due to its production during denitrification in both the anoxic water column and the underlying sediments. Such distinctive biogeochemical behavior between these two shelf segments is at first augmented by the natural origin of intense upwelling at Mangalore relative to Kochi wherein suboxic to anoxic oxygen minimum zone waters spread from offshore to the shelf of Mangalore, over which the runoff and terrestrial nutrients supply acts in unison.[...]"


Deep sea life faces dark future due to warming and food shortage

New study reveals negative impact of climate change, human activity, acidification and deoxygenation on ocean and its creatures


Major impacts of climate change on deep-sea benthic ecosystems


"The deep sea encompasses the largest ecosystems on Earth. Although poorly known, deep seafloor ecosystems provide services that are vitally important to the entire ocean and biosphere. Rising atmospheric greenhouse gases are bringing about significant changes in the environmental properties of the ocean realm in terms of water column oxygenation, temperature, pH and food supply, with concomitant impacts on deep-sea ecosystems. [...]" 


First evidence of denitrification vis-à-vis monsoon in the Arabian Sea since Late Miocene


"In the Arabian Sea, South Asian monsoon (SAM)-induced high surface water productivity coupled with poor ventilation of intermediate water results in strong denitrification within the oxygen minimum zone (OMZ). Despite the significance of denitrification in the Arabian Sea, we have no long-term record of its evolution spanning the past several million years. Here, we present the first record of denitrification evolution since Late Miocene (~10.2 Ma) in the Eastern Arabian Sea, where the SAM generates moderate surface water productivity, based on the samples retrieved during the International Ocean Discovery Program (IODP) Expedition 355. [...]"


Fish under threat from ocean oxygen depletion, finds study

Oxygen levels in oceans have fallen 2% in 50 years due to climate change, affecting marine habitat and large fish such as tuna and sharks


Oceans on the edge of anoxia


For the past several hundred million years, oxygen concentrations in Earth's atmosphere have been comparatively high (1, 2). Yet, the oceans seem never to have been far from anoxia (oxygen depletion) and have occasionally suffered major oceanic anoxic events (OAEs), recognized in the rock record through accumulations of dark, organic-rich shales (3). OAEs seem to be promoted by warm climates, and some have been associated with major environmental crises and global-scale disturbances in the carbon cycle. New insights into the causes of OAEs are now emerging (4, 5). Furthermore, ocean oxygen concentrations are declining in the modern ocean (6). A full-scale OAE would take thousands of years to develop, but some of today's processes are reminiscent of those thought to have promoted OAEs in the distant past." 


Bottom trawling and oxygen minimum zone influences on continental slope benthic community structure off Vancouver Island (NE Pacific)


"Understanding responses of benthic ecosystems to cumulative impacts of natural stressors, long-term ocean change and increasing resource exploitation is an emerging area of interest for marine ecologists and environmental managers. Few, if any, studies have quantitatively addressed cumulative effects in the deep sea. We report here on a study from the continental slope off Vancouver Island (Canada) in the northeast Pacific Ocean, where the Oxygen Minimum Zone impinges on seabed habitats that are subjected to widespread bottom trawling, primarily by the fishery for thornyhead (Sebastolobus ssp.). We examined how the benthic megafauna in this area was influenced by varying levels of dissolved oxygen and trawling activity, along a depth gradient that was also likely to shape community composition. [...] "


Decline in global oceanic oxygen content during the past five decades


"Ocean models predict a decline in the dissolved oxygen inventory of the global ocean of one to seven per cent by the year 2100, caused by a combination of a warming-induced decline in oxygen solubility and reduced ventilation of the deep ocean. It is thought that such a decline in the oceanic oxygen content could affect ocean nutrient cycles and the marine habitat, with potentially detrimental consequences for fisheries and coastal economies. [...]"

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Scientists have just detected a major change to the Earth’s oceans linked to a warming climate

"A large research synthesis, published in one of the world’s most influential scientific journals, has detected a decline in the amount of dissolved oxygen in oceans around the world — a long-predicted result of climate change that could have severe consequences for marine organisms if it continues."



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