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1. Depth Variance of Organic Matter Respiration Stoichiometry in the Subtropical North Atlantic and the Implications for the Global Oxygen Cycle

   https://doi.org/10.1029/2023GB007814  

   Gerace, Skylar D. ; Fagan, Adam J. ; Primeau, François W. ; Moreno, Allison R. ; Lethaby, Paul ; Johnson, Rodney J. ; Martiny, Adam C. ( November 2023 , Global Biogeochemical Cycles)

   Climate warming likely drives ocean deoxygenation, but models still cannot fully explain observed declines in oxygen. One unconstrained parameter is the oxygen demand per carbon respired for complete remineralization of organic matter (i.e., the total respiration quotient,r_Σ‐O2:C). Here, we tested ifr_Σ‐O2:Cdeclined with depth by quantifying suspended concentrations of particulate organic carbon (POC), particulate organic nitrogen (PON), particulate organic phosphorus (POP), particulate chemical oxygen demand (PCOD), and total oxygen demand (Σ‐O₂ = PCOD + 2PON) down to a depth of 1,000 m in the Sargasso Sea. The respiration quotient (r_‐O2:C = PCOD:POC) and total respiration quotient (r_Σ‐O2:C = Σ‐O₂:POC) declined with depth in the euphotic zone, but increased vertically in the disphotic zone. C:N andr_Σ‐O2:Nchanged with depth, but surface values were similar to values at 1,000 m. C:P, N:P, andr_Σ‐O2:Pmostly decreased with depth. We hypothesize thatr_Σ‐O2:Cis linked to multiple environmental factors that change with depth, such as phytoplankton community structure and the preferential production/removal of biomolecules. Using a global model, we show that the global distribution of dissolved oxygen is equally sensitive tor_‐O2:Cvarying between surface biomes versus vertically during remineralization. Additionally, adjusting the model'sr_‐O2:Cwith depth to match our observations resulted in less dissolved oxygen throughout the upper ocean. Most of this loss occurred in the tropical Pacific thermocline, where oxygen models underestimate deoxygenation the most. This study aims to improve our understanding of biological oxygen demand as warming‐induced deoxygenation continues.

    

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2. Latitudinal gradient in the respiration quotient and the implications for ocean oxygen availability

   https://doi.org/10.1073/pnas.2004986117  

   Moreno, Allison R. ; Garcia, Catherine A. ; Larkin, Alyse A. ; Lee, Jenna A. ; Wang, Wei-Lei ; Moore, J. Keith ; Primeau, Francois W. ; Martiny, Adam C. ( August 2020 , Proceedings of the National Academy of Sciences)

   Climate-driven depletion of ocean oxygen strongly impacts the global cycles of carbon and nutrients as well as the survival of many animal species. One of the main uncertainties in predicting changes to marine oxygen levels is the regulation of the biological respiration demand associated with the biological pump. Derived from the Redfield ratio, the molar ratio of oxygen to organic carbon consumed during respiration (i.e., the respiration quotient,$r_{-O2:C}$) is consistently assumed constant but rarely, if ever, measured. Using a prognostic Earth system model, we show that a 0.1 increase in the respiration quotient from 1.0 leads to a 2.3% decline in global oxygen, a large expansion of low-oxygen zones, additional water column denitrification of 38 Tg N/y, and the loss of fixed nitrogen and carbon production in the ocean. We then present direct chemical measurements of$r_{-O2:C}$using a Pacific Ocean meridional transect crossing all major surface biome types. The observed$r_{-O2:C}$has a positive correlation with temperature, and regional mean values differ significantly from Redfield proportions. Finally, an independent global inverse model analysis constrained with nutrients, oxygen, and carbon concentrations supports a positive temperature dependence of$r_{-O2:C}$in exported organic matter. We provide evidence against the common assumption of a static biological link between the respiration of organic carbon and the consumption of oxygen. Furthermore, the model simulations suggest that a changing respiration quotient will impact multiple biogeochemical cycles and that future warming can lead to more intense deoxygenation than previously anticipated.

    

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3. Global Ocean Particulate Organic Phosphorus, Carbon, Oxygen for Respiration, and Nitrogen (GO-POPCORN)

   https://doi.org/10.1038/s41597-022-01809-1  

   Tanioka, Tatsuro ; Larkin, Alyse A. ; Moreno, Allison R. ; Brock, Melissa L. ; Fagan, Adam J. ; Garcia, Catherine A. ; Garcia, Nathan S. ; Gerace, Skylar D. ; Lee, Jenna A. ; Lomas, Michael W. ; et al ( December 2022 , Scientific Data)

   Abstract Concentrations and elemental stoichiometry of suspended particulate organic carbon, nitrogen, phosphorus, and oxygen demand for respiration (C:N:P:−O 2 ) play a vital role in characterizing and quantifying marine elemental cycles. Here, we present Version 2 of the Global Ocean Particulate Organic Phosphorus, Carbon, Oxygen for Respiration, and Nitrogen (GO-POPCORN) dataset. Version 1 is a previously published dataset of particulate organic matter from 70 different studies between 1971 and 2010, while Version 2 is comprised of data collected from recent cruises between 2011 and 2020. The combined GO-POPCORN dataset contains 2673 paired surface POC/N/P measurements from 70°S to 73°N across all major ocean basins at high spatial resolution. Version 2 also includes 965 measurements of oxygen demand for organic carbon respiration. This new dataset can help validate and calibrate the next generation of global ocean biogeochemical models with flexible elemental stoichiometry. We expect that incorporating variable C:N:P:-O 2 into models will help improve our estimates of key ocean biogeochemical fluxes such as carbon export, nitrogen fixation, and organic matter remineralization. 

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4. Diel Measurements of Oxygen‐ and Carbon‐Based Ocean Metabolism Across a Trophic Gradient in the North Pacific

   https://doi.org/10.1029/2019GB006518  

   Henderikx Freitas, Fernanda ; White, Angelicque E. ; Quay, Paul D. ( November 2020 , Global Biogeochemical Cycles)

   Daily in situ rates of gross production and community respiration estimated from high‐frequency diel cycles in oxygen (O₂) and optically derived particulate carbon from several platforms (both ship based and via profiling floats) were made across an ecological gradient in the North Pacific spanning the high‐nutrient/low‐chlorophyll sub‐Arctic to the oligotrophic subtropical gyre. Both oxygen and carbon‐based gross primary production and respiration rates indicated a ~3× increase between subtropical and subpolar stations. We consistently found that gross production and community respiration rates were in approximate balance at all stations across the full ecological gradient, implying that community respiration is fueled by recently produced organic matter and that recycling efficiency (~90%) is similar along the gradient. We determined that phytoplankton turnover time doubles (from 2 to 4 days) between subtropical and subpolar regimes, whereas biomass increases by approximately tenfold. We found a consistent photosynthetic quotient (1.4 ± 0.2 mol O₂mol C⁻¹), respiratory quotient (1.0 ± 0.2 mol O₂mol C⁻¹), and gross to net production ratio (2.0 ± 0.3) at all stations which underscores the similarity of fundamental ecological characteristics despite the transition f rom nutrient deplete to replete conditions. That the float‐ and ship‐based estimates of in situ production and respiration generally agreed well suggests that float‐based diel O₂and particulate organic carbon measurements have the potential to greatly expand our knowledge of spatial and temporal variability of productivity and respiration in the ocean.

    

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5. Modeling Spatiotemporal Patterns of Ecosystem Metabolism and Organic Carbon Dynamics Affecting Hypoxia on the Louisiana Continental Shelf

   https://doi.org/10.1029/2019JC015630  

   Jarvis, Brandon M. ; Lehrter, John C. ; Lowe, Lisa L. ; Hagy, James D. ; Wan, Yongshan ; Murrell, Michael C. ; Ko, Dong S. ; Penta, Bradley ; Gould, Jr., Richard W. ( April 2020 , Journal of Geophysical Research: Oceans)

   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.

    

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