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 sigma-O2:C). Here, we tested if r sigma-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 (sigma-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 sigma-O2:C = sigma-O2:POC) declined with depth in the euphotic zone, but increased vertically in the disphotic zone. C:N and r sigma-O2:N changed with depth, but surface values were similar to values at 1,000 m. C:P, N:P, and r sigma-O2:P mostly decreased with depth. We hypothesize that r sigma-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. Rising ocean temperatures are likely causing the observed decline of dissolved oxygen below the ocean surface. This continued oxygen loss threatens the survival of many marine animals. Currently, global models cannot fully explain the observed rate of oxygen loss with warming. One missing component could be variance in the respiration quotient, the ratio of oxygen consumed per organic carbon respired during microbial respiration. However, the respiration quotient has yet to be estimated at different depths by directly measuring the chemical composition of organic matter. Here, we used direct measurements to find that the respiration quotient varied with depth in the Atlantic Ocean. Therefore, the respiration quotient at the surface should not represent values at deeper depths. In addition, we used a global model to find that the respiration quotient mostly affects oxygen in the tropical Pacific Ocean, where unexplained oxygen loss is the highest. Thus, more extensive data on the respiration quotient may significantly improve global models. The respiration quotient of particulate organic matter varied with depthElemental ratios of particulate organic matter deviated from Redfield proportions at all depthsThe increase in the respiration quotient with depth may account for some previously unexplained oxygen loss