Global Warming

Ocean 'Dead Zones' Growing

The world’s oceans are plagued with the problem of “dead zones”, areas of high nutrients (such as nitrogen and phosphorus) in which plankton blooms cause a major reduction of oxygen levels in the water. Sea creatures need oxygen to breathe just as we do, and if oxygen levels fall low enough marine animals can suffocate. This commonly happens around coastlines where fertilisers are washed from fields into rivers and the sea, but also mid-ocean, where currents trap waters in gyres (large systems of rotating ocean currents).

To date most studies have shown that these dead zones have been growing with global warming. But a recent study published in Science by Curtis Deutsch and colleagues suggests that the ocean’s largest anoxic zone – where there has been a total depletion of oxygen – in the eastern tropical North Pacific, may in fact shrink due to weakening trade winds caused by global warming.

The trade winds drive water away from the coast, and the gap is filled by new cold and nutrient-rich waters that come up from the deep. These nutrients trigger algae and plankton blooms upon which larger animals feed, which builds up an accumulation of organic matter. As bacteria decompose this organic matter the oxygen in the water is depleted. This causes low oxygen areas, such as the oxygen minimum zones (OMZs) with very low oxygen content found at intermediate ocean depths.

Weaker trade winds would mean less upwelling of these deep nutrient-rich waters, and consequently less plankton and less oxygen depletion. Deutsch and colleagues affirm that although initial oxygen content will be lower due to higher temperatures, oxygen demand will decrease as trade winds do. So, the result would be that low oxygen areas in the tropical north Pacific would shrink.