Recent studies show that stoichiometric elemental ratios of marine ecosystems are not static at Redfield proportions but vary systematically between biomes. However, the wider Atlantic Ocean is under-sampled for particulate organic matter (POM) elemental composition, especially as it comes to phosphorus. Thus, it is uncertain how environmental variation in this region translates into shifts in C:N:P. To address this, we analyzed hydrography, genomics, and POM concentrations from 877 stations on the meridional transects AMT28 and C13.5, spanning the Atlantic Ocean. We observed nutrient-replete, high-latitude ecosystem C:N:P to be significantly lower than the oligotrophic gyres. Latitudinal and zonal differences in elemental stoichiometry were linked to overall nutrient supply as well as N vs. P limitation. C:P and N:P were generally higher in the P-stressed northern region compared to southern hemisphere regions. We also detected a zonal difference linked to a westward deepening nutricline and a shift from N to P limitation. We also evaluated possible seasonal changes in C:N:P across the basin and predicted these to be limited. Overall, this study confirms latitudinal shifts in surface ocean POM ratios but reveals previously unrecognized hemisphere and zonal gradients. This work demonstrates the importance of understanding how regional shifts in hydrography and type of nutrient stress shape the coupling between Atlantic Ocean nutrient and carbon cycles.

Climate warming is likely resulting in ocean deoxygenation, but models still cannot fully explain the observed decline in oxygen. One unconstrained parameter is the oxygen demand for respiring particulate organic carbon and nitrogen (i.e., the total respiration quotient, rΣ-O2:C). It is untested if rΣ-O2:C systematically declines with depth. Here, we tested for such depth variance by quantifying particulate organic carbon (POC), particulate organic nitrogen (PON), particulate organic phosphorus (POP), particulate chemical oxygen demand (PCOD, the oxygen demand for respiring POC), and total oxygen demand (-O2 = PCOD + 2PON) concentrations down to a depth of 1000 m in the Sargasso Sea. C:N and -O2:N changed with depth, but values at the surface were similar to those at 1000 m. C:P, N:P, and -O2:P exponentially decreased with depth. The respiration quotient (r-O2:C = PCOD:POC) and total respiration quotient (rΣ-O2:C = ‑O2:POC) were both higher below the euphotic zone. We hypothesize that rΣ-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 sensitive to changes in the PCOD surface production (PPCOD) and depth attenuation (bPCOD). These variables mostly affect oxygen in the tropical and North Pacific Ocean, where deoxygenation rates and model discrepancy are the highest. This study aims to improve our understanding of biological oxygen demand as warming-induced deoxygenation continues.