by Tetjana Ross, Ana C. Franco, John A. Barth, Akash Sastri, Marie Robert, Debby Ianson, Charles Hannah, Francis Chan, Richard Feely, Richard Dewey, and Angelica Peña
Reprinted with permission: PICES North Pacific Marine Science Organization Newsletter Vol. 30, No.1 / Winter 2022 p.38
While hypoxic or low oxygen events are known to occur seasonally in coastal bottom waters along the west coast of North America, 2021 was more severely hypoxic. The onset of low oxygen water was the earliest in 35 years (NOAA, 2021), lasted longer (Lundeberg, 2021), was anomalously low (Figures 1 and 4), and covered a larger extent, reaching north into Canadian waters (Figure 5) where hypoxia is rare (Figure 6; Crawford and Peña, 2013).
The seasonal near-bottom hypoxia was observed as early as mid-May (Figure 1) on the Washington shelf. Low-oxygen water was observed near the seafloor over the Washington, USA, continental shelf for much of the summer 2021. Oxygen levels continued to decrease throughout the summer, reaching “severe” hypoxia (O₂< 0.5 ml/l)—that can harm important marine species like Dungeness crab (Keller et al, 2015)—at the end of July and even approached anoxia, zero oxygen, in late August/early September. This seasonal decrease in near-bottom dissolved oxygen is consistent with similar measurements off central Oregon (Adams et al., 2013), but these 2021 levels are lower than typical. The return of near-bottom dissolved oxygen to levels above hypoxia occurred in mid-late September (not shown) as early-season storms swept across the region.
Figure 1. Near-bottom dissolved oxygen measured at the mid shelf off Grays Harbor, Washington, USA, by an Ocean Observatories Initiative instrumented bottom platform in approximately 90 m of water. Black and red lines indicate the thresholds of 1.4 ml/l (equal to 62.2 µmol kg-1) for hypoxia and 0.5 ml/l (equal to 22.2 µmol kg-1) for severe hypoxia. Data available at https://oceanobservatories.org/.
The seasonal low-oxygen bottom waters in the northeast Pacific coastal areas are typically linked to upwelling, both driven by the sinking and decomposition of primary production fueled by upwelled nutrients (Connolly et al 2010), or sometimes upwelled low oxygen waters. The separation of the coastal hypoxic waters from deep hypoxic water in the offshore in a glider transect collected near the Ocean Observatories Initiative bottom platform (Figure 2) suggests that increased coastal productivity was responsible for the low oxygen on the Washington shelf in July 2021. An early onset of upwelling may be responsible for this productivity, though it could also be caused by an increase in stratification trapping phytoplankton nearer the surface, as they may be light limited earlier in the growing season due to deeper mixing (Thomson and Fine 2003).
Figure 2. Dissolved oxygen measured from an underwater vehicle glider operated by Oregon State University on a cross-shore transect off Grays Harbor, Washington, USA (plots available at http://nvs.nanoos.org and data available at the U.S. Integrated Ocean Observing System Glider Data Acquisition Center. Hypoxic water occupies the lower three-quarters of the water column near the mid-shelf mooring location (~80 m isobath) and stretches from the outer continental shelf, shoreward to at least the 50-m isobath.
Figure 3. Map of near bottom oxygen along Vancouver Island using cruise and glider data collected between Aug 25 – Oct 18 2021. The colour bar (in ml/l) is red for hypoxic waters (<1.4 ml/l, or approx. 62 µmol kg-1), orange for 1.4 ml/l, and green for > 1.4 ml/l.
Figure 4. Section of dissolved oxygen concentrations off the Vancouver Island coast (near Barkley sound or ~49oN). The thin white lines indicate isopycnals. All oxygen concentrations on the shelf are above 62 µmol kg-1 (approx 1.4 ml/l). Credit: NOAA Pacific Marine Environmental Laboratory/Richard Feely
Initially, the hypoxic area was south of the Canada-USA border, but in late summer it expanded north onto the Vancouver Island shelf (Figure 3). In May, oxygen concentrations were lower than the climatological value off Vancouver Island at the P01 (observations not shown), but they were not yet hypoxic. The NOAA West Coast Ocean Acidification Cruise, in late June/early July found hypoxic waters offshore of Washington and, but not off Vancouver Island (Figure 4).
Figure 5. Timeseries of oxygen sensor data from Ocean Network Canada’s Folger Deep mooring situated at 96 m depth near Barkley Sound on the west coast of Vancouver Island, British Columbia, Canada. The black line indicates hypoxia (oxygen below 1.4 ml/l).
Figure 6. Vertical profiles of dissolved oxygen (µmol kg-1) collected at station P02 (Figure 5) during late summer expeditions (August and September) as part of Fisheries and Oceans Canada’s Line P monitoring program. Gray shading indicates one standard deviation from the climatological (2001-2021) summer mean. In 2021 (red thick dot-dash line), the bottom oxygen was the lowest observed in two decades and one of only two recent years where bottom waters reached hypoxic levels at this location.
Oxygen data from a sensor near the bottom (96 metres) at Ocean Network Canada’s Folger Deep mooring (Figure 5; black star in Figure 3) had some data loss in 2021, but suggests that hypoxic bottom water was present close to shore on the Vancouver Island starting in mid-July. By late summer, Fisheries and Oceans Canada’s routine summer surveys observed hypoxic bottom waters over much of the Vancouver Island shelf (Figure 3), including a Line P station (P02) where the bottom oxygen was lower than ever observed before (Figure 6; see also Crawford and Peña, 2013).
By mid-October, the bottom waters on the Vancouver Island shelf had returned to normal (oxic) oxygen levels, as observed by an ocean glider (diamond symbols in Figure 3).
This article was reprinted with permission from PICES North Pacific Marine Science Organization Newsletter Vol. 30, No.1 / Winter 2022 p.38
References
Adams, K.A., J.A. Barth, and F. Chan. 2013. Temporal variability of near-bottom dissolved oxygen during upwelling off central Oregon. Journal of Geophysical Research 118:4,839–4,854, https://doi.org/10.1002/jgrc.20361.
Connolly, T. P., Hickey, B. M., Geier, S. L., & Cochlan, W. P. (2010). Processes influencing seasonal hypoxia in the northern California Current System. Journal of Geophysical Research: Oceans, 115(C3). https://doi.org/10.1029/2009JC005283.
Crawford, W. R., & Peña, M. A. (2013). Declining oxygen on the British Columbia continental shelf. Atmosphere-Ocean, 51(1), 88-103. https://doi.org/10.1080/07055900.2012.753028.
Keller, A.A., L. Ciannelli, W.W. Wakefield, V. Simon, J.A. Barth, and S.D. Pierce. 2015. Occurrence of demersal fishes in relation to near-bottom oxygen levels within the California Current Large Marine Ecosystem. Fisheries Oceanography 24:162–176, https://doi.org/10.1111/fog.12100.
Lundeberg, S. 2021; Sep 8. “Ocean hypoxia off Pacific Northwest coast more troubling than ever, experts say” Oregon State University Newroom, https://today.oregonstate.edu/news/ocean-hypoxia-pacific-northwest-coast-more-troubling-ever-experts-say.
NOAA Fisheries, 2021; July 23. “Low-Oxygen Waters Off Washington, Oregon Coasts Risk Becoming Large ‘Dead Zones’ ” NOAA Fishereis News, https://www.fisheries.noaa.gov/feature-story/low-oxygen-waters-washington-oregon-coasts-risk-becoming-large-dead-zones.
