Dissolved oxygen level in world oceans declining

A new analysis of decades of data on oceans across the globe has revealed that the amount of dissolved oxygen contained in the water – an important measure of ocean health – has been declining for more than 20 years.

Researchers at Georgia Institute of Technology looked at a historic dataset of ocean information stretching back more than 50 years and searched for long term trends and patterns. They found that oxygen levels started dropping in the 1980s as ocean temperatures began to climb.

“The oxygen in oceans has dynamic properties, and its concentration can change with natural climate variability,” said Taka Ito, an associate professor in Georgia Tech’s School of Earth and Atmospheric Sciences who led the research. “The important aspect of our result is that the rate of global oxygen loss appears to be exceeding the level of nature’s random variability.”

The study, which was published April in Geophysical Research Letters, was sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration. The team included researchers from the National Center for Atmospheric Research, the University of Washington-Seattle, and Hokkaido University in Japan.

Falling oxygen levels in water have the potential to impact the habitat of marine organisms worldwide and in recent years led to more frequent “hypoxic events” that killed or displaced populations of fish, crabs and many other organisms.

Researchers have for years anticipated that rising water temperatures would affect the amount of oxygen in the oceans, since warmer water is capable of holding less dissolved gas than colder water. But the data showed that ocean oxygen was falling more rapidly than the corresponding rise in water temperature.

“The trend of oxygen falling is about two to three times faster than what we predicted from the decrease of solubility associated with the ocean warming,” Ito said. “This is most likely due to the changes in ocean circulation and mixing associated with the heating of the near-surface waters and melting of polar ice.”

The majority of the oxygen in the ocean is absorbed from the atmosphere at the surface or created by photosynthesizing phytoplankton. Ocean currents then mix that more highly oxygenated water with subsurface water. But rising ocean water temperatures near the surface have made it more buoyant and harder for the warmer surface waters to mix downward with the cooler subsurface waters. Melting polar ice has added more freshwater to the ocean surface – another factor that hampers the natural mixing and leads to increased ocean stratification.

“After the mid-2000s, this trend became apparent, consistent and statistically significant — beyond the envelope of year-to-year fluctuations,” Ito said. “The trends are particularly strong in the tropics, eastern margins of each basin and the subpolar North Pacific.”

In an earlier study, Ito and other researchers explored why oxygen depletion was more pronounced in tropical waters in the Pacific Ocean. They found that air pollution drifting from East Asia out over the world’s largest ocean contributed to oxygen levels falling in tropical waters thousands of miles away.

Once ocean currents carried the iron and nitrogen pollution to the tropics, photosynthesizing phytoplankton went into overdrive consuming the excess nutrients. But rather than increasing oxygen, the net result of the chain reaction was the depletion oxygen in subsurface water.

That, too, is likely a contributing factor in waters across the globe, Ito said.

— source rh.gatech.edu

Extreme low-oxygen Eddies in the Atlantic produce Greenhouse Gases

Oxygen in the seawater is not only vital to most marine organisms, its concentrations also affect the chemistry of the ocean and that of the atmosphere above. In oceanic regions with very little oxygen, for example, large amounts of the potent greenhouse gas nitrous oxide, also called laughing gas, are produced via biogeochemical processes and can then be released to the atmosphere.

— source geomar.de

Acidified ocean water widespread along North American West Coast

A three-year survey of the California Current System along the West Coast of the United States found persistent, highly acidified water throughout this ecologically critical nearshore habitat, with “hotspots” of pH measurements as low as any oceanic surface waters in the world. The researchers say that conditions will continue to worsen because the atmospheric carbon dioxide primarily to blame for this increase in acidification has been rising substantially in recent years.

— source oregonstate.edu

Small Plastics Pose Big Problem

A decade or so ago, scientists first discovered that tiny pieces of plastic debris discarded by human civilization — some only a few thousandths of a millimeter in size — were finding their way into the oceans. But since then, it’s become increasingly apparent that microplastics, as the miniscule trash is called, represent a potentially huge threat to aquatic animals, according to an article in the July 11 edition of the journal Science.

The article, by marine scientists Kara Lavender Law of the Sea Education Association in Woods Hole, Mass. and Richard C. Thompson of the UK’s Plymouth University, notes that researchers increasingly are focusing upon the danger from microplastics, because their size makes it possible for a huge range of organisms — from large marine mammals, fish and birds to zooplankton — to ingest them. (Indeed, a 2012 study found that they pose a health threat to Baleen whales.)

A report issued in June by the Global Ocean Commission estimated that 10 million tons of plastic is dumped into the oceans each year. Some of the plastic is discarded into waterways and then is carried into the ocean, but it’s also lost or discarded at sea by ships, the article notes.

Larger plastic items degrade to form microplastic, but some of the particles also are being put directly into the sea, because bits of cosmetic beads and clothing fibers are small enough to pass through wastewater treatment systems.

Once in the oceans, the particles are transported far and wide in a complex pattern that is difficult to predict. However, scientists have found very high concentrations in the subtropical gyres — that is, areas where currents rotate rapidly — and in basins such as the Mediterranean.

Microplastics are themselves toxic, but they also soak up harmful chemicals that contaminate the ocean, such as DDT and PBDEs, so that they deliver a concentrated dose to the animals who ingest them. Marine scientists also worry that microplastics will end up in seafood-eating humans as well.

Microplastics are just one of the environmental woes afflicting the world’s oceans, and pushing them perilously close to ecological collapse, according to an article published last week in Foreign Policy, a political science journal.

Solving the problem is difficult because 65 percent of the oceans are outside the territorial waters of individual nations, and have become the equivalent of a chaotic, lawless “failed state” such as Somalia on land, the Foreign Policy article argued. [those countries are made to fail just for this purpose.]

— source news.discovery.com

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.

The paper, published Wednesday in the journal Nature by oceanographer Sunke Schmidtko and two colleagues from the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, found a decline of more than 2 percent in ocean oxygen content worldwide between 1960 and 2010. The loss, however, showed up in some ocean basins more than others. The largest overall volume of oxygen was lost in the largest ocean — the Pacific — but as a percentage, the decline was sharpest in the Arctic Ocean, a region facing Earth’s most stark climate change.

Ocean oxygen is vital to marine organisms, but also very delicate — unlike in the atmosphere, where gases mix together thoroughly, in the ocean that is far harder to accomplish. just 1 percent of all the Earth’s available oxygen mixes into the ocean; the vast majority remains in the air. Climate change models predict the oceans will lose oxygen because of several factors. Most obvious is simply that warmer water holds less dissolved gases, including oxygen.

But another factor is the growing stratification of ocean waters. Oxygen enters the ocean at its surface, from the atmosphere and from the photosynthetic activity of marine microorganisms. But as that upper layer warms up, the oxygen-rich waters are less likely to mix down into cooler layers of the ocean because the warm waters are less dense and do not sink as readily.

The resulting study attributes less than 15 percent of the total oxygen loss to sheer warmer temperatures, which create less solubility. The rest was attributed to other factors, such as a lack of mixing.

Because oxygen in the global ocean is not evenly distributed, the 2 percent overall decline means there is a much larger decline in some areas of the ocean than others.

Moreover, the ocean already contains so-called oxygen minimum zones, generally found in the middle depths. The great fear is that their expansion upward, into habitats where fish and other organism thrive, will reduce the available habitat for marine organisms.

In shallower waters, meanwhile, the development of ocean “hypoxic” areas, or so-called “dead zones,” may also be influenced in part by declining oxygen content overall.

On top of all of that, declining ocean oxygen can also worsen global warming in a feedback loop. In or near low oxygen areas of the oceans, microorganisms tend to produce nitrous oxide, a greenhouse gas, Gilbert writes. Thus the new study “implies that production rates and efflux to the atmosphere of nitrous oxide … will probably have increased.”

The new study underscores once again that some of the most profound consequences of climate change are occurring in the oceans, rather than on land. In recent years, incursions of warm ocean water have caused large die-offs of coral reefs, and in some cases, kelp forests as well. Meanwhile, warmer oceans have also begun to destabilize glaciers in Greenland and Antarctica, and as they melt, these glaciers freshen the ocean waters and potentially change the nature of their circulation.

When it comes to ocean deoxygenation, as climate change continues, this trend should also increase — studies suggest a loss of up to 7 percent of the ocean’s oxygen by 2100. At the end of the current paper, the researchers are blunt about the consequences of a continuing loss of oceanic oxygen.

— source washingtonpost.com by Chris Mooney

Ecotourism rise hits whales

Boat trips to watch whales and dolphins may increasingly be putting the survival of marine mammals at risk, conservationists have warned.

Research published this year shows that the jaunts can affect cetacean behaviour and stress levels in addition to causing deaths from collisions. But some animals are affected more than others and the long-term effects remain unclear, scientists at the International Marine Conservation Congress (IMCC) in Glasgow, UK, heard last week.

The number of people joining trips has expanded hugely since the 1990s, from 4 million in 31 countries in 1991 to 13 million in 119 countries in 2008, the most recent year for which full data are available. In 2008, the International Fund for Animal Welfare, an animal-protection charity in London, estimated the value of the industry at US$2.1 billion.

Although collisions with boats can hurt the animals, researchers are more concerned about effects such as animals failing to feed or using up energy swimming away from the vessels. These seemingly small events can add up, studies suggest.

Earlier this year, for example, marine biologist David Lusseau of the University of Aberdeen, UK, and his team showed that minke whales (Balaenoptera acutorostrata) in Faxaflói Bay in Iceland responded to whale-watching boats as they do to natural predators, upping their speed and respiring more heavily1. But whether this was a direct result of the boats is difficult to pin down: Lusseau, who was not at the meeting, says that soon-to-be-published research by his team shows that behavioural changes are probably not affecting actual numbers of the minke in Faxaflói Bay.

But Lusseau’s group has also shown that the bottlenose dolphins (Tursiops sp.) in Doubtful Sound, New Zealand, could be driven to extinction in decades2. The large number of dolphin-watching trips in the sound is driving the animals away from their preferred areas and forcing them to avoid boats instead of feeding. Dolphin numbers declined from 67 in 1997 to 56 in 2005, the team found.

Several delegates at the IMCC also described the effects on the roughly 70 endangered Irrawaddy dolphins (Orcaella brevirostris) living in the Mekong River between Cambodia and Laos, which are hounded by scores of tourist boats.

Determining which populations are most at risk could help to fix the problem, says Lusseau. He suggests plugging short-term observational data into longer-term population models to tease out whether behavioural changes are temporary or serious long-term threats. There are enough data on species types and locations to assess, at least roughly, where whale-watching should and should not be allowed, he says. But funding and political support are hampering the creation of detailed, localized plans. “There is a lot of lip service being paid to understanding the challenges tourism poses on wildlife, but in practice there is very little financial interest in finding this out,” he says.

Short-term fix

Guidelines such as specifying minimum distances between animals and boats, speed limits or no-go areas, can help. But codes vary widely: a 2004 study3 found that just 38% were binding; the rest were voluntary. They are also often inadequate. Even with guidelines in place, boats in the dolphin-watching haven of the Bocas del Toro region of Panama hit and killed at least 10 animals in a population of about 250 in 2012 and 2013, according to research presented to the International Whaling Commission in Cambridge, UK, this year.

Greg Kaufman, executive director of the Pacific Whale Foundation — an organization in Hawaii that runs whale-watching and research trips — holds up the Irrawaddy dolphin as a population desperately in need of protection. “They’re basically killing these animals one at a time,” he says.

But Brian Smith, a zoologist at the Wildlife Conservation Society in New York who has long studied the Irrawaddy group, says that although cetacean tourism is probably stressful for these animals, the main problem is entanglement in fishing nets. And the alternative to fishing for many people in the region is dolphin-watching.

Most of the speakers at the IMCC meeting agreed that more should be done to protect dolphins and whales from tourists. “Although whale-watching is not as bad as whaling,” says New, “it might be that last piece that pushes a species over.”

— source nature.com By Daniel Cressey

Warming temperatures could trigger starvation, extinctions in deep oceans

Researchers from 20 of the world’s leading oceanographic research centers today warned that the world’s largest habitat – the deep ocean floor – may face starvation and sweeping ecological change by the year 2100. Warming ocean temperatures, increased acidification and the spread of low-oxygen zones will drastically alter the biodiversity of the deep ocean floor from 200 to 6,000 meters below the surface. The impact of these ecosystems to society is just becoming appreciated, yet these environments and their role in the functioning of the planet may be altered by these sweeping impacts. Results of the study, which was supported by the Foundation Total and other organizations, were published this week in the journal Elementa.

— source phys.org