Upwelling-driven nearshore hypoxia signals ecosystem and oceanographic changes in the northeast Pacific

Abstract

Seasonal development of dissolved-oxygen deficits (hypoxia) represents an acute system-level perturbation to ecological dynamics and fishery sustainability in coastal ecosystems around the globe1,2,3. Whereas anthropogenic nutrient loading has increased the frequency and severity of hypoxia in estuaries and semi-enclosed seas3,4, the occurrence of hypoxia in open-coast upwelling systems reflects ocean conditions that control the delivery of oxygen-poor and nutrient-rich deep water onto continental shelves1. Upwelling systems support a large proportion of the world's fisheries5, therefore understanding the links between changes in ocean climate, upwelling-driven hypoxia and ecological perturbations is critical. Here we report on the unprecedented development of severe inner-shelf (<70 m) hypoxia and resultant mass die-offs of fish and invertebrates within the California Current System. In 2002, cross-shelf transects revealed the development of abnormally low dissolved-oxygen levels as a response to anomalously strong flow of subarctic water into the California Current System. Our findings highlight the sensitivity of inner-shelf ecosystems to variation in ocean conditions, and the potential impacts of climate change on marine communities.

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Figure 1: Location of the 2002 hypoxic zone and hydrographic transects off Oregon.
Figure 2: Timeline of observations, both continuous (ac) and discrete (1–6).
Figure 3: Impact of hypoxia on rockfish communities.
Figure 4: Dissolved-oxygen profiles.

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Acknowledgements

We thank A. Kirincich for ADCP analyses, S. Oda and A. Walters for assistance and A. Pazar for crab fishery data. The Partnership for Interdisciplinary Studies of Coastal Oceans, funded by the David and Lucile Packard Foundation, and the US GLOBEC program, jointly funded by NSF and NOAA, both contributed to this work. J.A.B. acknowledges additional funding from NSF. ODFW ROV survey work was funded in part by the Oregon Department of Land Conservation and Development through NOAA.

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Correspondence to Francis Chan.

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Supplementary information

Supplementary Table S1

Average, minimum and maximum DO and chl-a concentrations along the SH hydroline. (DOC 24 kb)

Supplementary Figure S1

Dissolved oxygen source properties and depletion patterns across the shelf. (DOC 1209 kb)

Supplementary Figure S2

Composite cross-shelf profiles of temperature, salinity, density, nitrate+nitrite, chlorophyll-a, and dissolved oxygen, SH line, 11-12 August, 2002. (DOC 78 kb)

Supplementary Figure S3

Chl-a concentration (mg l-1) at 5 meters depth over Heceta Bank from 9-11 August 2002. (DOC 214 kb)

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Grantham, B., Chan, F., Nielsen, K. et al. Upwelling-driven nearshore hypoxia signals ecosystem and oceanographic changes in the northeast Pacific. Nature 429, 749–754 (2004). https://doi.org/10.1038/nature02605

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