Evidence for greater oxygen decline rates in the coastal ocean than in the open ocean

Abstract

Abstract. In the global ocean, the number of reported hypoxic sites (oxygen <30% saturation) is on the rise both near the coast and in the open ocean. But unfortunately, most of the papers on hypoxia only present oxygen data from one or two years, so that we often lack a long-term perspective on whether oxygen levels at these locations are decreasing, steady or increasing. Consequently, we cannot rule out the possibility that many of the newly reported hypoxic areas were hypoxic in the past, and that the increasing number of hypoxic areas partly reflects increased research and monitoring efforts. Here we address this shortcoming by computing oxygen concentration trends in the global ocean from published time series and from time series that we calculated using a global oxygen database. Our results show that oxygen concentrations are generally decreasing in the ocean. They also reveal a greater percentage of negative trends for published oxygen time series than for oxygen time series computed from the global oxygen database, particularly in the open ocean. Finally, the oxygen decline rates computed from the global oxygen database are more severe in a 30 km band near the coast than in the open ocean, probably in response to human-caused eutrophication.