Evidence for greater oxygen decline rates in the coastal ocean than in the open ocean
- 1Institut Maurice-Lamontagne, Pêches et Océans Canada, 850 Route de la mer, Mont-Joli, Québec, G5H 3Z4, Canada
- 2Louisiana Universities Marine Consortium, 8124 Highway 56, Chauvin, LA 70344, USA
- 3Virginia Institute of Marine Science, College of William and Mary, Gloucester Point, VA 23062, USA
- 4State Key Laboratory of Estuarine and Coastal Research, East China Normal University, 3663 Zhongshan Road North, Putuo District, Shanghai, 200062, China
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 calculations reveal that median oxygen decline rates are more severe in a 30 km band near the coast than in the open ocean (>100 km from the coast). Percentages of oxygen time series with negative oxygen trends are also greater in the coastal ocean than in the open ocean. Finally, a significant difference between median published oxygen trends and median trends calculated from raw oxygen data suggests the existence of a publication bias in favor of negative trends in the open ocean.