Article

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

ABSTRACT

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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    • "The subthermocline waters are rich in nutrients and contribute to the flux of nutrients to the nearshore, thereby helping to fuel nearshore kelp forest ecosystems and communities; however, the subthermocline waters are also low in DO and pH (Leichter et al., 1996; Booth et al., 2012; Washburn and McPhee-Shaw, 2013). While it is well-documented that seasonal upwelling contributes to nearshore DO variability and the potential development of coastal hypoxia, as well as pH variability and ocean acidification processes, this research suggests that the internal tide interacting with the canyon rim may contribute to rapid fluctuations in DO/pH in the nearshore with extremely fast onset times of these low DO/pH waters relative to those expected under upwelling alone (Chan et al., 2008; Booth et al., 2012; Walter et al., 2014b). The internal boluses also may contribute to the cross-shelf transport (Walter et al., 2014b) of larvae by delivering these organisms from the ambient coastal shelf waters adjacent to the canyon region to their natal nearshore habitats located in shallower subtidal and intertidal environments (Pineda, 1994), a process that may be important for connectivity and recruitment. "
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    ABSTRACT: This study utilizes more than a year of observations made in shallow waters (~30 m) at the head of the Monterey Submarine Canyon to assess variability in the physical environment and internal bore field. The interaction of the internal tide with the canyon rim results in a semidiurnal tidal period pumping of cold-water masses (subthermocline waters) onto the adjacent shelf (i.e., internal bores). These internal bores are shown to be significantly coherent with the local sea surface height with minimal spatial variability when comparing two sites near the canyon head region. During the summer months, and periods of strong regional wind-driven upwelling and shoaling of the offshore thermocline, the canyon rim sites display elevated semidiurnal temperature variance. This semidiurnal variability reaches its annual minimum during the winter months when the regional upwelling favorable winds subside and the offshore thermocline deepens. Additionally, the observed internal bores show a distinct asymmetry between the leading (gradual cooling with velocities directed onto the shelf) and trailing edges (sharp warming with velocities directed into the canyon). It appears that the semidiurnal internal tide at the canyon head is a first-order control on the delivery of subthermocline waters to the nearshore coastal environment at this location.
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    • "The occurrence of episodes of anoxia in nearshore waters in the northeastern Pacific has been observed and marked by large-scale mortalities of animals that lack the ability to move away from anoxic regions (Grantham et al. 2004, Chan et al. 2008). Animals vary greatly in their reliance on aerobically powered activity, their abilities to extract oxygen from seawater and transport it to respiring tissues, and their capacities for sustaining metabolism using anaerobic pathways of ATP generation when hypoxia occurs, either from low DO (environmental hypoxia) or elevated metabolic rates (physiological hypoxia; see supplemental text 2 for definitions of oxygen parameters; Hochachka and Somero 2002). "
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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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