Oregon Shelf Hypoxia Modeling

Andrey O. Koch; Yvette H. Spitz; Harold P. Batchelder

doi:10.1007/978-3-319-54571-4_9

Modeling Coastal Hypoxia pp 215-238 | Cite as

Oregon Shelf Hypoxia Modeling

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Andrey O. Koch
Yvette H. Spitz
Harold P. Batchelder

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First Online: 04 May 2017

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 variability

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Appendix A. Oxygen Formulation

The dissolved oxygen dynamics in our ecosystem model is governed by the following equation:

$$ \frac{{\partial O_{2} }}{\partial t} = V_{m} f\left( I \right)\left\{ {\frac{{NO_{3} }}{{K_{u} + NO_{3} }}e^{{ - \psi NH_{4} }} r_{{O_{2} :NO_{3} }} + \frac{{NH_{4} }}{{K_{u} + NH_{4} }}r_{{O_{2} :NH_{4} }} } \right\}P $$

$$ - 2\Omega NH_{4} -\Gamma r_{{O_{2} :NH_{4} }} Z - \varphi r_{{O_{2} :NH_{4} }} D + Q_{ge} (O_{2sat} - O_{2} ) $$

where P is phytoplankton, Z is zooplankton, D is detritus, V _m = 1.5 d⁻¹ is phytoplankton maximum uptake rate, f(I) is light limitation (see Spitz et al. 2005 for details), ψ = 1.46 (mmol N m⁻³)⁻¹ is NH ₄ inhibition parameter, Ω = 0.25 d⁻¹ is NH ₄ oxidation coefficient, Γ = 0.1 d⁻¹ is zooplankton specific excretion and mortality rate, φ = 0.1 d⁻¹ is detritus decomposition rate, $ r_{{O_{2} :NO_{3} }} $ and $ r_{{O_{2} :NH_{4} }} $ are oxygen-nitrate and oxygen-ammonium conversion parameters, respectively; $ Q_{ge} = \frac{{K\nu_{{O_{2} }} }}{\Delta zn} $, where Δzn is the height of the top cell and $ K\nu_{{O_{2} }} = 0.31u^{2} \sqrt {660/S_{c} } $ is the gas transfer coefficient with u being the average wind speed and S _c being Schmidt number calculated after Keeling et al. (1998): S _c = 1638.0 − 81.83T + 1.483T ² − 0.008004T ³; O _2sat = e ^A1000/22.9316 is the saturation concentration of oxygen with A = 2.00907 + 3.22014TS + 4.05010TS ² + 4.94457TS ³ − 0.256847TS ⁴ + 3.88767TS ⁵ + S(−0.00624523 − 0.00737614TS − 0.0103410TS ² − 0.00817083TS ³) − 4.88682·10⁻⁷ S ², where TS = ln((298.15 − T)/T _k), T is in Celsius, T _k in Kelvin (after Garcia and Gordon 1992).

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Authors and Affiliations

Andrey O. Koch
- 1
Email author
Yvette H. Spitz
- 2
Harold P. Batchelder
- 2
- 3

1.Department of Marine ScienceUniversity of Southern MississippiHattiesburgUSA
2.College of Earth, Ocean, and Atmospheric SciencesOregon State UniversityCorvallisUSA
3.North Pacific Marine Science OrganizationSidneyCanada

Oregon Shelf Hypoxia Modeling

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