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

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 if r_Σ-O2:C declined 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 and r_Σ-O2:N changed with depth, but surface values were similar to values at 1,000 m. C:P, N:P, and r_Σ-O2:P mostly decreased with depth. We hypothesize that r_Σ-O2:C is 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 to r_-O2:C varying between surface biomes versus vertically during remineralization. Additionally, adjusting the model's r_-O2:C with 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.