Modeling Spatiotemporal Patterns of Ecosystem Metabolism and Organic Carbon Dynamics Affecting Hypoxia on the Louisiana Continental Shelf. Issue 4 (18th April 2020)

• Record Type: Journal Article
• Authors: Jarvis, Brandon M.; Lehrter, John C.; Lowe, Lisa L.; Hagy, James D.; Wan, Yongshan; Murrell, Michael C.; Ko, Dong S.; Penta, Bradley; Gould, Richard W.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 4(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-04-18
• Subjects: Louisiana Continental Shelf -- Hypoxia -- Primary Production -- Respiration -- Carbon; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC015630
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: The hypoxic zone on the Louisiana Continental Shelf (LCS) forms each summer due to nutrient‐enhanced primary production and seasonal stratification associated with freshwater discharges from the Mississippi/Atchafalaya River Basin (MARB). Recent field studies have identified highly productive shallow nearshore waters as an important component of shelf‐wide carbon production contributing to hypoxia formation. This study applied a three‐dimensional hydrodynamic‐biogeochemical model named CGEM (Coastal Generalized Ecosystem Model) to quantify the spatial and temporal patterns of hypoxia, carbon production, respiration, and transport between nearshore and middle shelf regions where hypoxia is most prevalent. We first demonstrate that our simulations reproduced spatial and temporal patterns of carbon production, respiration, and bottom‐water oxygen gradients compared to field observations. We used multiyear simulations to quantify transport of particulate organic carbon (POC) from nearshore areas where riverine organic matter and phytoplankton carbon production are greatest. The spatial displacement of carbon production and respiration in our simulations was created by westward and offshore POC flux via phytoplankton carbon flux in the surface layer and POC flux in the bottom layer, supporting heterotrophic respiration on the middle shelf where hypoxia is frequently observed. These results support existing studies suggesting the importance of offshore carbon flux to hypoxia formation, particularly on the west shelf where hypoxic conditions are most variable. Plain Language Summary: Formation of hypoxia, or low dissolved oxygen, is a seasonal occurrence on the Louisiana Continental Shelf associated with stratification of the water column and excess nutrient loads delivered via the Mississippi and Atchafalaya River systems. Frequently referred to as "dead zones, " bottom‐water hypoxia results in stress or death of aquatic organisms, especially those that cannot move to areas with more oxygen. To study the sources and distribution of organic matter that supports oxygen consumption, we applied a three‐dimensional hydrodynamic‐biogeochemical model named CGEM (Coastal Generalized Ecosystem Model). CGEM simulations between 2003 and 2007 successfully simulated spatial and temporal patterns of hypoxia and important biological processes that control its formation. Our simulations revealed that highly productive nearshore waters serve as a source of organic matter that supports oxygen consumption offshore. We identified seasonal bottom‐layer currents that transport organic matter offshore and interannual variations in river discharge that influence biological production and bottom‐layer oxygen consumption offshore. The ecological processes described in this study increase our understanding of how nearshore processes affect the development and maintenance of offshore hypoxia. Key Points: Simulation models describe ecological processes controlling spatial patterns in production and respiration and effect on hypoxia Offshore organic matter flux from highly productive nearshore areas supports increased respiration offshore Offshore carbon flux is an important driver of oxygen demand and resulting hypoxia at intermediate depths where hypoxia commonly occurs