THE OXYGEN MINIMUM ZONE (OMZ) IS THE ZONE WHERE OXYGEN SATURATION IN SEAWATER IN THE OCEAN IS AT ITS LOWEST. THIS ZONE OCCURS AT DEPTHS OF ABOUT 200 TO 1000 METERS, DEPENDING ON LOCAL CIRCUMSTANCES.

SURFACE OCEAN WATERS GENERALLY HAVE OXYGEN CONCENTRATIONS CLOSE TO EQUILIBRIUM WITH THE EARTH’S ATMOSPHERE. THIS WATER IS EXPOSED TO A RAIN OF ORGANIC MATTER FROM ABOVE, DECREASING SHARPLY ITS DOWNWARD FLUX WITH DEPTH. AEROBIC BACTERIA FEED FROM THIS ORGANIC MATTER, CONSUMING 80% TO 90% OF IT IN THE TOP 1000 METERS. OXYGEN IS USED AS PART OF THE BACTERIAL METABOLIC PROCESS LOWERING ITS CONCENTRATION WITHIN THE WATER. THEREFORE, THE CONCENTRATION OF OXYGEN IN DEEP WATER IS DEPENDENT ON THE AMOUNT OF OXYGEN IT HAD WHEN IT WAS AT THE SURFACE MINUS DEPLETION BY DEEP-SEA ORGANISMS.

THE DEEP OCEAN THUS HAS HIGHER OXYGEN BECAUSE RATES OF ITS CONSUMPTION ARE LOW COMPARED WITH THE SUPPLY OF COLD, OXYGEN-RICH DEEP WATERS FROM POLAR REGIONS. DEPTHS IN BETWEEN, HOWEVER, HAVE HIGHER RATES OF OXYGEN CONSUMPTION AND LOWER RATES OF SUPPLY OF OXYGEN-RICH WATERS.

THE LARGE-SCALE OCEAN CIRCULATION CONTROLS THE DISTRIBUTION OF THE OPEN-OCEAN OXYGEN MINIMUM ZONES. ESSENTIALLY, WATERS THAT ARE PART OF THE WIND-DRIVEN SUBTROPICAL GYRE CIRCULATIONS, ARE RAPIDLY EXCHANGED WITH THE SURFACE AND NEVER ACQUIRE A STRONG OXYGEN DEFICIT. HOWEVER, ALONG THE EQUATORIAL EDGE OF THE GYRES, ONE FINDS A STAGNANT POOL OF WATER THAT HAS NO DIRECT CONNECTION TO THE OCEAN SURFACE. AS A RESULT, THESE ZONES HAVE VERY LOW OXYGEN CONCENTRATIONS EVEN THOUGH IN REGIONS SUCH AS THE EASTERN TROPICAL OR THE NORTH PACIFIC, THERE MAY BE RELATIVELY LITTLE ORGANIC MATTER FALLING FROM THE SURFACE.

OXYGEN-POOR CONDITIONS HAVE FAR-REACHING IMPACTS ON ECOSYSTEMS BECAUSE IMPORTANT MOBILE MACROORGANISMS AVOID OR CANNOT SURVIVE IN HYPOXIC ZONES. CLIMATE MODELS PREDICT DECLINES IN OCEANIC DISSOLVED OXYGEN PRODUCED BY GLOBAL WARMING. REDUCED OXYGEN LEVELS MAY HAVE DRAMATIC CONSEQUENCES FOR ECOSYSTEMS AND COASTAL ECONOMIES.

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