Abstract
Bottom hypoxia on the shelf in the Northeast Pacific is caused by different processes than coastal hypoxia related to riverine inputs. Hypoxia off the coast of Oregon is a naturally occurring process as opposed to the anthropogenically forced hypoxia found in many coastal environments (e.g., Gulf of Mexico shelf, Chesapeake Bay). Off Oregon, bottom hypoxia occurs in summers that have large upwelling-driven near-bottom transport of high nitrate, low dissolved oxygen (DO) waters onto the shelf. The combination of low DO and high nitrate provides initially low (but not hypoxic) DO conditions near the bottom, and nitrate fertilization of shelf surface waters, leading to substantial phytoplankton production. Some production is grazed, and some of it sinks to the bottom where it decomposes consuming oxygen, creating bottom hypoxia in some years. Terrestrial runoff of nutrients into the system is small and not responsible for the development of bottom hypoxia. Similar processes contribute to natural hypoxia in other eastern boundary current upwelling regions, such as the Humboldt Current off Peru and the Benguela Current off Namibia and South Africa. We summarize the observational data on DO and illustrate the coupled biophysical modeling of hypoxia that has been done on the Oregon shelf. We compare hypoxia development in summer of 2002 and 2006, which differed in timing, spatial extent and intensity of hypoxia. Sensitivity analysis using various initial and boundary conditions for nitrate and dissolved oxygen reveals some of the essential conditions responsible for hypoxia development on the Oregon shelf.
Keywords
Oregon shelf Natural hypoxia Biophysical model of hypoxia Observation-model comparisons Interannual/seasonal variabilityReferences
- Bakun A (1990) Global climate change and intensification of coastal ocean upwelling. Science 247:198–201CrossRefPubMedGoogle Scholar
- Barth JA, Pierce SD, Castelao RM (2005) Time-dependent, wind-driven flow over a shallow midshelf submarine bank. J Geophys Res 110:C10S05. doi: 10.1029/2004JC002761
- Batchelder HP, Barth JA, Kosro PM, Strub PT, Brodeur RD, Peterson WT, Tynan CT, Ohman MD, Botsford LW, Powell TM, Schwing FB, Ainley DG, Mackas DL, Hickey BM, Ramp SR (2002) The GLOBEC Northeast Pacific California Current system program. Oceanography 15(2):36–47Google Scholar
- Castelao RM, Barth JA (2005) Coastal ocean response to summer upwelling favorable winds in a region of alongshore bottom topography variations off Oregon. J Geophys Res 110:C10S04. doi: 10.1029/2004JC002409
- Chan R, Barth JA, Lubchenco J, Kirincich A, Weeks H, Peterson WT, Menge. BA (2008) Emergence of anoxia in the California Current large marine ecosystem. Science 319:920. doi: 10.1126/science.1149016 CrossRefPubMedGoogle Scholar
- Connolly TP, Hickey BM, Geier SL, Cochlan WP (2010) Processes influencing seasonal hypoxia in the northern California Current system. J Geophys Res 115:C03021. doi: 10.1029/2009JC005283 CrossRefPubMedPubMedCentralGoogle Scholar
- Crawford WR, Pena MA (2013) Declining oxygen on the British Columbia continental shelf. Atmos Ocean 51:88–103CrossRefGoogle Scholar
- Diaz RJ, Rosenberg R (2008) Spreading dead zones and consequences for marine ecosystems. Science 321:926. doi: 10.1126/science.1156401 CrossRefPubMedGoogle Scholar
- Escribano R, Schneider W (2007) The structure and functioning of the coastal upwelling system off south/central Chile. Prog Oceanogr 75:343–346CrossRefGoogle Scholar
- Garcia HE, Gordon LI (1992) Oxygen solubility in seawater: better fitting equations. Limnol Oceanogr 37(6):1307–1312CrossRefGoogle Scholar
- Grantham BA, Chan F, Nielsen KJ, Fox DS, Barth JA, Huyer A, Lubchenco J, Menge BA (2004) Upwelling-driven nearshore hypoxia signals ecosystem and oceanographic changes in the Northeast Pacific. Nature 429:749–754CrossRefPubMedGoogle Scholar
- Hickey BM, Banas NS (2003) Oceanography of the US Pacific Northwest coastal ocean and estuaries with application to coastal ecology. Estuaries 26:1010–1031. doi: 10.1007/BF02803360 CrossRefGoogle Scholar
- Hodur RM (1997) The naval research laboratory’s coupled ocean/atmosphere mesoscale prediction system (COAMPS). Mon Weather Rev 125:1414–1430CrossRefGoogle Scholar
- Huyer A, Sobey EJC, Smith RL (1979) The spring transition in currents over the Oregon continental shelf. J Geophys Res 84:6995–7011CrossRefGoogle Scholar
- Kalnay E et al (1996) The NCEP/NCAR 40-year reanalysis project. Bull Am Meteorol Soc 77:437–471CrossRefGoogle Scholar
- Keeling RF, Manning AC, McEvoy EM, Shertz SR (1998) Methods for measuring changes in atmospheric O2 concentration and their application in Southern Hemisphere air. J Geophys Res 103:3381–3397CrossRefGoogle Scholar
- Koch AO, Kurapov AL, Allen JS (2010) Near-surface dynamics of a separated jet in the coastal transition zone off Oregon. J Geophys Res 115:C08020. doi: 10.1029/2009JC005704 Google Scholar
- Levitus S (1982) Climatological atlas of the world ocean. NOAA Professional Paper 13, US Government printing office, Washington DC, 173 ppGoogle Scholar
- Peterson JO, Morgan CA, Peterson WT, Di Lorenzo E (2013) Seasonal and interannual variation in the extent of hypoxia in the northern California Current from 1998-2012. Limnol Oceanogr 58:2279–2292CrossRefGoogle Scholar
- Shchepetkin AF, McWilliams JC (2003) A method for computing horizontal pressure-gradient force in an oceanic model with a nonaligned vertical coordinate. J Geophys Res 108(C3):3090. doi: 10.1029/2001JC001047 CrossRefGoogle Scholar
- Shchepetkin AF, McWilliams JC (2005) The regional ocean modeling system: a split–explicit, free–surface, topography–following coordinate oceanic model. Ocean Model 9:347–404. doi: 10.1016/j.ocemod.2004.08.002
- Shulman I, Kindle JC, deRada S, Anderson SC, Penta B, Martin PJ (2004) Development of a hierarchy of nested models to study the California Current system. Estuarine and coastal modeling. In: Malcolm L, Spaulding PE (eds) Proceedings of the 8th international conference on estuarine and coastal modeling. American Society of Civil Engineers, Reston, Va, pp 74–88Google Scholar
- Siedlecki SA, Banas NS, Davis KA, Giddings S, Hickey BM, MacCready P, Connolly T, Geier S (2015) Seasonal and interannual oxygen variability on the Washington and Oregon continental shelves. J Geophys Res Oceans 120. doi: 10.1002/2014JC010254
- Spitz YH, Allen JS, Gan J (2005) Modeling of ecosystem processes on the Oregon shelf during the 2001 summer upwelling. J Geophys Res 110:C10S17. doi: 10.1029/2005JC002870
- Strub PT, Batchelder HP, Weingartner TJ (2002) U.S. GLOBEC Northeast Pacific program: overview. Oceanography 15:30–35Google Scholar
- Ueno H, Yasuda O (2003) Intermediate water circulation in the North Pacific subarctic and northern subtropical regions. J Geophys Res 108(C11):3348. doi: 10.1029/2002JC001372
- Wetz JJ, Hill J, Corwith H, Wheeler PA (2004) Nutrient and extracted chlorophyll data from the GLOBEC long-term observation program, 1997–2004. Data report 193, COAS reference 2004-1 (Revised June 2005 [to include improved 2003 and 2004 data]). http://nepglobec.bco-dmo.org/reports/ccs_cruises/GLOBEC_nutchl_datareport_7_06_hpb.pdf
- Wheeler PA, Huyer A, Fleischbein J (2003) Cold halocline, increased nutrients and higher chlorophyll off Oregon in 2002. Geophys Res Lett 30. ISSN: 0094-8276. doi: 10.1029/2003GL017395