Emergence of Anoxia in the California Current Large Marine Ecosystem

Department of Zoology, Oregon State University, Corvallis, OR 97331, USA.
Science (Impact Factor: 33.61). 03/2008; 319(5865):920. DOI: 10.1126/science.1149016
Source: PubMed


Eastern boundary current systems are among the world's most productive large marine ecosystems. Because upwelling currents
transport nutrient-rich but oxygen-depleted water onto shallow seas, large expanses of productive continental shelves can
be vulnerable to the risk of extreme low-oxygen events. Here, we report the novel rise of water-column shelf anoxia in the
northern California Current system, a large marine ecosystem with no previous record of such extreme oxygen deficits. The
expansion of anoxia highlights the potential for rapid and discontinuous ecosystem change in productive coastal systems that
sustain a major portion of the world's fisheries.

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    • "Such expansions have already been observed in tropical and subtropical regions (Stramma et al., 2008, 2010), across the northern North Pacific (Watanabe et al., 2003; Whitney et al., 2007; Crawford and Peña, 2013), and in the southern California Current Ecosytem (CCE) (Bograd et al., 2008; McClatchie et al., 2010) where deoxygenation has been linked to intensification of the California Undercurrent (Bograd et al., 2014). Deleterious effects of ocean deoxygenation have been observed for shallow-dwelling and coastal organisms exposed to extreme hypoxic events such as the summertime " dead zone " appearance in the Gulf of Mexico (Rabalais et al., 2002), entrapment of epipelagic fishes in California harbors (Stauffer et al., 2012), and strong upwelling events along the Oregon coast (Grantham et al., 2004; Chan et al., 2008). The mesopelagic zone is one of the largest ecosystems on earth, and the resident fauna is both diverse and abundant (Robison, 2009). "
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    ABSTRACT: Climate change-induced ocean deoxygenation is expected to exacerbate hypoxic conditions in mesopelagic waters off the coast of southern California, with potentially deleterious effects for the resident fauna. In order to understand the possible impacts that the oxygen minimum zone expansion will have on these animals, we investigated the response of the depth of the deep scattering layer (i.e., upper and lower boundaries) to natural variations in midwater oxygen concentrations, light levels, and temperature over time and space in the southern California Current Ecosystem. We found that the depth of the lower boundary of the deep scattering layer (DSL) is most strongly correlated with dissolved oxygen concentration, and irradiance and oxygen concentration are the key variables determining the upper boundary. Based on our correlations and published estimates of annual rates of change to irradiance level and hypoxic boundary, we estimated the corresponding annual rate of change of DSL depths. If past trends continue, the upper boundary is expected to shoal at a faster rate than the lower boundary, effectively widening the DSL under climate change scenarios. These results have important implications for the future of pelagic ecosystems, as a change to the distribution of mesopelagic animals could affect pelagic food webs as well as biogeochemical cycles.
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    • "This may be due in part to decreasing oxygen concentrations with time in the oceanic water masses that feed into the California Current system (Whitney et al., 2007) and affect oxygen concentrations on the continental shelf (Grantham et al., 2004; Connolly et al., 2010). As the upwelled water often originates from depths of 100 m or more, it is usually low in dissolved oxygen, and may be nearly hypoxic (2 mg/L O 2 ; 62 mM); however the occurrence of severe hypoxia requires onshelf respiration to drive O 2 to levels well below source water concentrations (Grantham et al., 2004; Hales et al., 2006; Chan et al., 2008; Connolly et al., 2010). "
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    ABSTRACT: Benthic chamber incubations were performed in the mid-shelf region on the Oregon shelf in June and August 2009 to measure fluxes of oxygen, nutrients, and iron and their effect on water column chemistry. Chamber oxygen and nitrate fluxes were into the sediments while silicate, iron and ammonium fluxes were out of the sediments. Benthic fluxes were similar between the two months, except that dissolved iron fluxes were higher at some sites in August. Bottom waters were consistently hypoxic (43-64 μM O2) and had ammonium concentrations from 0-2.6 μM in the mid-shelf region. Given measured ammonium fluxes (0.2 to 1.4 mmol m-2 d-1), we used a simple stoichiometric model for a 10 m bottom boundary layer to calculate that benthic fluxes only contributed ∼16-41% of the bottom water ammonia. Benthic oxygen fluxes (-4.3 to -12.5 mmol O2 m-2 d-1) were responsible for ∼38-51% of oxygen drawdown in the benthic boundary layer. In both cases, the remainder may be attributed to water column respiration. Benthic iron and nitrate fluxes have opposite effects on productivity. Iron fluxes (0-71 μmol m-2 d-1, average: 5 μmol m-2 d-1) increased bottom water concentrations while nitrate was lost (-1.2 to -2.9 mmol m-2 d-1 NO3-) due to denitrification. By supplying iron and consuming nitrogen, benthic diagenetic processes reinforce an iron-replete, nitrate-limited coastal ecosystem.
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    • "Sea level rise associated with climate change will also increase the intrusion of oceanic water into estuaries, particularly, if there are also declines in freshwater inflow. It has been suggested that there has been an increase in the occurrence of severe hypoxic condition on the Oregon shelf (Chan et al. 2008; Pierce et al. 2012). If future studies demonstrate anthropogenic-related increases in the amount of coastal upwelling, the occurrence of hypoxia on the inner shelf or increases in intrusion of oceanic water into the estuaries , then those exceedances identified as associated with ocean input may have some component related to anthropogenic activities, which will greatly complicate decisions with regard to compliance monitoring for water quality criteria. "
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    ABSTRACT: Wind-driven coastal upwelling along the Pacific Northwest Coast of the USA results in oceanic water that may be periodically entrained into adjacent estuaries and which possesses high nutrients and low dissolved oxygen (DO). Measurement of water quality indicators during these upwelling water entrainment events would represent extreme values for water quality thresholds derived from typical estuarine conditions. Tools are therefore needed to distinguish upwelled waters from other causes of exceedances of water quality thresholds within estuaries of the region. We present an example application of logistic regression models to predict the probability of exceedance of a water quality threshold, using DO data from the Yaquina estuary, Oregon, USA. Models including water temperature and salinity correctly classified exceedances of DO about 80 % of the time. Inclusion of in situ fluorescence in the logistic regression model for DO improved the model performance and reduced the rate of false positives.
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