Declining Oxygen in the Northeast Pacific*

Abstract

Climate models predict a decrease in oceanic dissolved oxygen and a thickening of the oxygen minimum zone, associated with global warming. Comprehensive observational analyses of oxygen decline are challenging, given generally sparse historical data. The Newport hydrographic (NH) line off central Oregon is one of the few locations in the northeast Pacific with long oxygen records. Good quality data are available here primarily in two time blocks: 1960–71 and 1998–present. Standard sampling extends from midshelf (bottom depth of 58 m) to 157 km offshore (bottom depth of 2880 m). Shipboard measurements have been supplemented in recent years (2006–present) with data from autonomous underwater gliders. Oxygen declines significantly over this 50-yr period across the entire NH line. In addition to decrease in the vicinity of the oxygen minimum depth (~800 m), oxygen decreases across a range of density surfaces σθ = 26–27 within the thermocline, in the depth range 100–550 m. A core of decreasing oxygen (0.7 ± 0.2 μmol kg−1 yr−1 or 0.016 ± 0.005 ml l−1 yr−1) is also found over the upper slope at 150–200-m depths, within the region of average northward flow associated with the poleward undercurrent. During the summer upwelling season, the largest decline is observed near bottom on the shelf: the dissolved oxygen of upwelled water, already low, is further reduced by shelf processes, leading to near-bottom hypoxia (<60 μmol kg−1) on the Oregon shelf.

Ocean Research Enabled by Underwater Gliders

“…Deploying gear in the ocean involves some risk, as described for gliders by Brito et al (2014), whose view is based on many users new to gliders and is therefore rather bleak. My opinion is that more experienced glider operators have better performance, as must be the case for the glider programs that have been gathering data continuously for several years , Perry et al 2008, Todd et al 2011a, Pierce et al 2012, Pelland et al 2013, Mazzini et al 2014, Johnston & Rudnick 2015. The optimistic view is 10 August 2015 14:1 that ocean scientists willing to make an investment can approach the performance of experienced glider operators.…”

“…One application is to use gliders and satellite measurements to improve estimates of primary productivity ( Jacox et al 2015). Glider observations off Oregon have documented the declining levels of dissolved oxygen in coastal waters (Pierce et al 2012, Adams et al 2013). Continuing work is addressing the warm anomalies that have appeared throughout the eastern North Pacific during 2014-2015 (Figure 3).…”

Declining Abundance of Beaked Whales (Family Ziphiidae) in the California Current Large Marine Ecosystem

“…Mesopelagic fishes in the Southern California bight area have declined >60% in abundance since the 1980s in association with increasing deep-water hypoxia due to climate change [64]. Further projected expansion of oxygen minimum zones (OMZ) is expected to have significant ecosystem impacts [65]–[67]. Warming temperatures and OMZ expansion have facilitated invasion of the California Current by jumbo squid ( Dosidicus gigas ) since the late 1990s.…”

Looming hypoxia on outer shelves caused by reduced ventilation in the open oceans: Case study of the East China Sea

“…Recent studies based either on direct observations or modeling have demonstrated that subsurface water in the open ocean has decreased in DO content in recent decades, possibly owing to reduced seawater ventilation under global warming (Ono et al, 2001;Deutsch et al, 2006;Whitney et al, 2007;Pierce et al, 2012;Takatani et al, 2012). Obviously, an input of offshore seawater with a lower DO yet higher nutrients and AOU exacerbates coastal eutrophication and spreads the hypoxia.…”