Virginia Institute of Marine Science
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.
GULF-OF-MEXICO; FINLAND BALTIC SEA; LONG-TERM CHANGES; BLACK-SEA; BENTHIC ECOSYSTEM; CHESAPEAKE BAY; GOTLAND BASIN; HYPOXIA; EUTROPHICATION; TRENDS
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This work is licensed under a Creative Commons Attribution 4.0 International License.
We thank Guillaume Hardy who imported oxygen data from the public databases into Matlab and wrote programs for statistical analysis and mapping. Benoit Roberge and Audrey Guerard digitized the oxygen time series from refereed journal publications. Christine Lemay helped us construct literature search equations in ASFA and SCOPUS. We are indebted to the thousands of scientists who made oxygen measurements and sent their data to public national and international data archive centers. We are also grateful to the two anonymous reviewers for making constructive suggestions that greatly improved the paper. This research was initiated as part of the activities of SCOR (Scientific Committee on Oceanic Research) Working Group 128 on "Natural and human-induced hypoxia and consequences for coastal areas", and was funded by the Climate Change Science Initiative of the Department of Fisheries and Oceans Canada.
Gilbert, D., Rabalais, N. N., Díaz, R. J., and Zhang, J.: Evidence for greater oxygen decline rates in the coastal ocean than in the open ocean, Biogeosciences, 7, 2283-2296, https://doi.org/10.5194/bg-7-2283-2010, 2010.