The researchers led by Lothar Stramma from the Leibniz Institute of Marine Sciences in Kiel, Germany, discovered oxygen levels in tropical oceans at depths of 985 to 2,300 feet have declined during the past 50 years. The ecological impact of that decline and the growth of such areas, the scientists said, could have substantial biological and economic consequences.

The study involved an analysis of an historical database of ocean oxygen measurements.

"We found the largest reduction … in the tropical northeast Atlantic, whereas the changes in the eastern Indian Ocean were much less pronounced," said Stramma. "Whether or not these observed changes in oxygen can be attributed to global warming alone is still unresolved. The reduction in oxygen may also be caused by natural processes on shorter time scales."

The study that included U.S. National Oceanic and Atmospheric Administration scientist Gregory Johnson, Janet Sprintall of the Scripps Institution of Oceanography in San Diego and Volker Mohrholz from the Institute for Baltic Sea Research in Warnemunde, Germany, appears in the journal Science.