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EGU 2011: Oxygen - how low can it go?

By Liz Kalaugher at the EGU General Assembly in Vienna

Oxygen minimum, or “dead”, zones are found below just two per cent of the surface of the world’s oceans but they’re responsible for roughly one-quarter to one-half of marine nitrogen removal. Once oxygen levels drop, standard lifeforms cannot survive and bacteria that use nitrogen rather than oxygen as fuel can take over.

It’s been hard to measure the precise threshold for this changeover, but now a new oxygen sensor that’s one hundred times more sensitive has revealed that it takes place at much lower oxygen levels - just 0.3 microM - than scientists believed.

Using the sensor, Tage Dalsgaard of Aarhus University Denmark and colleagues found oxygen concentrations of less than 0.01 microM (0.3 microgrammes per litre) over a distance of 2500 km along the coast of Chile and Peru. Previous best estimates had indicated levels of 1-2 microM per litre, Dalsgaard told a press conference at the EGU General Assembly. The team only found nitrogen-removing processes taking place when oxygen levels were less than 0.3 microM; these reactions occurred at a greater rate deeper into the dead zone.

Some of the most extensive oxygen minimum zones are found in the Eastern Tropical North Pacific, Eastern Tropical South Pacific and the Arabian Sea. The zones form when nutrient-rich waters from the depths rise to the surface and enable a bloom in plankton growth. Once the plankton reach the end of their lives, decomposition of their bodies as they sink to the depths consumes a large amount of oxygen. In a typical oxygen minimum zone in the open ocean, the top 50 m of water are oxygenated, the next 250 m contain little oxygen and levels of the gas rise again towards the seafloor.

Although the zones are a natural phenomenon, climate change is likely to reduce oxygen levels further. Indeed, Caroline Slomp of the University of Utrecht told reporters that low oxygen is the third major problem of climate change - runner up behind temperature rise and acidification. That’s because oxygen is less soluble in warmer water, and warmer surface waters don’t mix so well with those beneath. Increased levels of nitrogen entering coastal seas from activities such as fertiliser use are also creating dead zones close to shore.

Not only are these areas suffering stress because of low oxygen, explained Lisa Levin of Scripps Institution of Oceanography, but they’re increasingly likely to be exploited as fishing activities move outwards from continental shelves to continental slopes, oil and gas exploration continues and extraction of resources such as diamonds and phosphates begins in new sites. Levin is keen that we understand more about the resilience of ecosystems in these areas before further exploitation occurs.

“Oxygen minimum zones play key roles in ocean biogeochemistry and are an important repository of microbial animal biodiversity,” she said.

With this in mind, Levin and colleagues did field-work off the western coast of Goa. Their aim was to see how oxygen availability affects the recovery of sediment-dwelling organisms after disturbance. Introducing colonization trays containing soft sediment to the sea-floor at three different depths revealed that recolonization was strongly oxygen-dependent.

The tray on the seabed at 542 m, where oxygen levels were lowest, was not colonized at all. Levin says that this was no surprise as the background community did not contain any animals. At 800 m, where oxygen levels were ten times higher, only a few colonizers - mainly worm species - moved in. And on the seafloor at 1147 m, where oxygen levels were ten times higher again, there was much more extensive colonization. This time the incomers were mainly from one opportunistic polychaete worm species (Capitella) that is known as a pollution indicator.

Posted by Liz Kalaugher on April 7, 2011 1:42 PM | Permalink