Large areas of open ocean starved of oxygen

Large regions of the open ocean are being starved of oxygen because of warmer sea temperatures according to studies showing that fish and other marine creatures are moving into narrower habitats to avoid suffocation.

Marine researchers said they have discovered growing areas of the ocean that suffer from hypoxia - oxygen depletion - which they believe is the result of warmer sea temperature caused by global warming.

Warmer sea temperatures increase stratification, where warm, stagnant bodies of surface water sit on top of cooler water and prevent the normal mixing that results from the vertical circulation currents of the ocean, said Professor Lisa Levin of the Scripps Institution of Oceanography in La Jolla, California.

Warmer water also holds less dissolved oxygen than cold water. This means that the stable regions of the deep ocean are also beginning to experience temperature rises that affect concentrations of dissolved oxygen, Professor Levin said.

“The water is getting warmer, and warm water holds substantially less oxygen than cold water - water at 2C holds 20 per cent more oxygen than 20C water....Off southern California over the past 22 years we’ve lost about 30 per cent of the oxygen at depths of around 200 to 300 metres,” Professor Levin said.

“It’s a big effect, but also the stratification of the ocean is reducing the amount of vertical mixing of the ocean which prevents surface layers which are rich in oxygen being carried to deeper layers of the ocean,” she said.

“We believe it is linked to atmospheric warming and ocean warming in as much as CO2 has come from fossil fuel burning, it is linked to humans,” she added.

At the same time, the oceans are becoming more acidic - with lower pH values - as they absorb more carbon dioxide from the atmosphere, she said.

“These low oxygen zones are also low pH [more acidic] zones because the same process that reduces oxygen, that is respiration, also produces carbon dioxide which reduces pH,” Professor Levin said.

“A key feature of these low oxygen, low pH zones is their narrow biodiversity and although these areas do occur naturally, they are expanding with low-oxygen areas moving up into shallow water,” she told the American Association for the Advancement of Science.

“The vertical distribution of deep-ocean species is shifted upwards. They have to move to avoid the low-oxygen waters. At the same time, the surface waters are getting warmer and more acidic and creating less suitable habitat from above,” she said.

“The appropriate habitat for many ocean animals is being compressed causing many animals to live in a smaller area at higher densities in waters that may be shallower than normal.

“This changes their interactions with other species, with predators and competitors and it also makes them much easier targets for fishermen,” Professor Levin told the meeting.

Deep-water temperature gauges off Spitsbergen in the Arctic and in the Southern Ocean near the Antarctic have recorded temperature increases of between 0.03C and 0.5C, and as much as 1C, which is highly significant for a a stable environment that does not change at all from one century to the next, she said.

“Those are significant numbers. The warming is more intense at the sea surface but it reaching the deep water,” Professor Levin said.

“Although we think of the deep sea as pristine, over the past 30 years since I’ve been studying the deep oceans they have become increasingly susceptible to human disturbance,” she said.