Growing Ocean Dead Zones Leave Fish Gasping

May 3, 2008 — -- "Dead zones" containing too little oxygen for fish to breathe are growing as global temperatures increase.

Warmer water dissolves less oxygen, so as temperatures rise, oxygen vanishes from oceans. Marine biologists are warning that if dead zones continue expanding, oceanic "deserts" could massively deplete marine life and fish stocks.

Previous studies have shown that surface layers of the ocean can be depleted of oxygen by pollution draining out from rivers, as in the Gulf of Mexico. However the new study finds depletion at intermediate ocean depths, between 300 and 700 metres. There has also been evidence of oxygen depletion closer to the sea bed in some regions, such as the Arabian Sea, but no one has looked before in detail at intermediate depths.

"From our observations we can only tell what happened in the past 50 years, but we need to find out what will happen in the future," says Lothar Stramma of the University of Kiel, Germany.

Oxygen Shortfall

Stramma's team used data from historical records of oceanic oxygen concentrations, collected mainly from research vessels. They combined it with recent data from buoys newly equipped to measure oxygen concentrations, as well as temperature and salinity. "We added our own data from recent cruises and floats where available to continue the older data set to the present," says Stramma.

The combined data set shows that, over the past 50 years, large volumes of ocean previously rich in oxygen have become "oxygen minimum zones" (OMZs) containing less than 120 micromoles of oxygen per kilogram of water. These are the concentrations at which fish, squid, crustaceans and other marine creatures begin to suffocate and die.

On average, the team calculate that oxygen dropped by between 0.90 and 0.34 micromoles per kilogram of ocean per year.

Worst affected of six areas sampled was a tropical region of the Atlantic Ocean to the west of Africa. Between 1960 and 2006, the layer with less than 90 micromoles of oxygen per kilogram of ocean grew dramatically – its vertical thickness increased by 85%, from 370 to 690 metres. The other region of particular concern was in the equatorial Pacific Ocean.