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 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.

Oxygen in the deep Strait of Georgia, 1951–2009: The roles of mixing, deep‐water renewal, and remineralization of organic carbon

“…Falkowski et al (2011) noted that on the 26.5 kg m 23 isopycnal in the northeast Pacific Ocean, in addition to a general decline of about 0.67mmol L 21 since 1956, the concentration of oxygen had oscillated with a period of 18.6 yr, correlated with lunar precession, through a presumed mechanism of lunar influence on tide and consequently on ventilation of the isopycnal. In their global review, Gilbert et al (2010) pointed out that more of the ocean time series showed negative trends over 1976-2000 than over 1951-1975. This could imply that hypoxia is worsening, or, alternatively, that the current downward trend is part of a longer term cycle.…”

“…In the open ocean, progressive deoxygenation is caused by a combination of increasing surface temperature and stratification and declining ventilation of intermediate depth water (Whitney et al 2007;Falkowski et al 2011). The decline is occurring at a greater rate in coastal waters than in the open ocean (Gilbert et al 2010) because local effects augment the global change. Inlets with restricted exchange at the entrance have always been subject to hypoxia at depth (e.g., Saanich Inlet, Herlinveaux 1962), but now the concentration of oxygen is approaching thresholds of biological tolerance in other coastal waters.…”

“…Coastal waters can be susceptible to upwelling of hypoxic water from elsewhere, as in the California Current system off the west coast of North America (Grantham et al 2004;Bianucci and Denman 2012), or to a change in stratification due to winter warming of surface water, as has been observed in the Japan Sea (Kang et al 2004). In some coastal areas, such as the East China Sea (Chen et al 2007), South China Sea (Dai et al 2006), Gulf of Mexico (Rabalais et al 2001), and the lower Saint Lawrence estuary (Gilbert et al 2010), physical circulation has contributed the conditions necessary to support deoxygenation, while eutrophication caused by the discharge of municipal wastewater or agricultural runoff (Falkowski et al 2011) has provided a local stress that has pushed these areas into hypoxia.…”

Multivariate benthic ecosystem functioning in the Arctic – benthic fluxes explained by environmental parameters in the southeastern Beaufort Sea

“…Bottom water oxygen concentration of shelf waters changes on such timescales (Gilbert et al, , 2010, as does iron oxide and manganese oxide concentration (Gobeil et al, 2001;Katsev et al, 2006) in sediments. In open ocean regions, changes in the ocean's dissolved oxygen are likely related to physical processes and thermohaline circulation.…”

Multiple stressors of ocean ecosystems in the 21st century: projections with CMIP5 models

“…Reduced oceanic O 2 concentrations have been reported for coastal areas in response to human-induced eutrophication (Gilbert et al, 2010), and for the open ocean as a consequence of two major processes: a warming-induced reduction in solubility and increased stratification/reduced ventilation (Helm et al, 2011). This reduction in O 2 , referred to as ocean deoxygenation, between the 1970s and the 1990s amounts to a global average reduction of −0.93 mmol m −3 (Helm et al, 2011).…”