Preprints
https://doi.org/10.5194/bg-2022-3
https://doi.org/10.5194/bg-2022-3
 
11 Jan 2022
11 Jan 2022
Status: this preprint is currently under review for the journal BG.

Hydrodynamic and Biochemical Impacts on the Development of Hypoxia in the Louisiana–Texas Shelf Part I: Numerical Modeling and Hypoxia Mechanisms

Yanda Ou1 and Z. George Xue1,2,3 Yanda Ou and Z. George Xue
  • 1Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA, 70803, USA
  • 2Center for Computation and Technology, Louisiana State University, Baton Rouge, LA, 70803, USA
  • 3Coastal Studies Institute, Louisiana State University, Baton Rouge, LA, 70803, USA

Abstract. A three-dimensional coupled hydrodynamic–biogeochemical model with N, P, Si cycles and multiple phytoplankton and zooplankton functional groups was developed and applied to the Gulf of Mexico to study bottom dissolved oxygen dynamics. A 15-year hindcast was achieved covering the period of 2006–2020. Extensive model validation against in situ data demonstrates that the model is capable of reproducing vertical distributions of dissolved oxygen (DO), frequency distributions of hypoxia thickness, spatial distributions of bottom DO concentration and interannual variations of hypoxic area. The impacts of river plume and along-shore currents on bottom DO dynamics were examined based on multiyear bottom DO climatology, the corresponding long-term trends, and interannual variability. Model results suggest that mechanisms of bottom hypoxia developments are different between the west and east Louisiana–Texas Shelf waters. The mid-Atchafalaya nearshore (10–20 m) region firstly suffers from hypoxia in May, followed by the west-Mississippi nearshore region in June. Hypoxic waters expand in the following months and eventually merge in August. Sediment oxygen consumption (SOC) and water stratification (measured by potential energy anomaly, PEA) are two main factors modulating the variability of bottom DO concentration. Generalized Boosted Regression Models provide analysis of the relative importance of PEA and SOC. The analysis indicates that SOC is the main regulator in nearshore regions, and water stratification outcompetes the sedimentary biochemical processes in the offshore (20–50 m) regions. A strong quadratic relationship was found between hypoxic volume and hypoxic area, which suggests that the volume mostly results from the low DO in bottom water and can be potentially estimated based on the hypoxic area.

Yanda Ou and Z. George Xue

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on bg-2022-3', Anonymous Referee #1, 08 Mar 2022
  • RC2: 'Comment on bg-2022-3', Anonymous Referee #2, 16 Mar 2022

Yanda Ou and Z. George Xue

Yanda Ou and Z. George Xue

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Short summary
The Louisiana-Texas Shelf has been suffering recurring hypoxia (dissolved oxygen < 2 mg/L) over the past decades. In this study, we adapt and further developed a numerical ocean model to the Gulf of Mexico to simulate the physics and associated biogeochemical process to understand the mechanism of the hypoxia development. We performed a 15-year model simulation covering 2006–2020 and explore the controlling mechanism of hypoxia development in different parts of the shelf.
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