Earth System Models generally predict increasing upper ocean stratification from 21st century planetary warming, which will cause a decrease in the vertical nutrient flux resulting in declining marine net primary productivity (NPP) and carbon export fluxes. Recent advances in quantifying marine ecosystem carbon to nutrient stoichiometry have identified large latitudinal and biome variability, with low-latitude oligotrophic systems harboring pico-sized phytoplankton exhibiting large phosphorus to carbon cellular plasticity. Climate forced changes in nutrient flux stoichiometry and phytoplankton community composition is thus likely to alter the ocean’s biogeochemical response and feedback with the carbon-climate system. We have added three pico-phytoplankton functional types within the Biogeochemical Elemental Cycling component of the Community Earth System Model while incorporating variable cellular phosphorus to carbon stoichiometry for all represented phytoplankton types. The model simulates Prochlorococcus and Synechococcus populations that dominate the productivity and sinking carbon export of the tropical and subtropical ocean, and pico-eukaryote populations that contribute significantly to productivity and export within the subtropical to mid-latitude transition zone, contributing a combined 50 – 70% of these fluxes. Pico-phytoplankton cellular stoichiometry and resulting sinking export patterns inversely track the distribution of surface phosphate, with the western subtropical regions of each basin supporting the most P-poor stoichiometries. Collectively, pico-phytoplankton contribute ~58% of global NPP and ~46% of global particulate organic carbon export below 100 meters. Subtropical gyre recirculation regions along the poleward flanks of surface western boundary currents are identified as regional hotspots of enhanced carbon export exhibiting C-rich/P-poor stoichiometry, preferentially inhabited by pico-eukaryotes and diatoms.

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.

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.

The ocean has many underwater light niches, but the selection pressure for chromatic acclimaters (generalists) compared to blue or green-specialists is not well understood. Here, we tested the hypothesis that changes in ocean spectra brought about by mixing on the order of days preferentially selects for generalists within a Synechococcus population. We investigated ocean conditions that led to high proportions of Synechococcus generalists versus specialists in a model ocean column, and compared simulations with in situ metagenomic and physical oceanographic data from major Bio-GO-SHIP cruises, supplemented with GEOTRACES and TARA Oceans, as well as the GOOS Argo Program and sea surface height from AVISO. We found that greater mixed layer depths selected for generalists in simulated Synechococcus populations, but explained only 14% of the partitioning between strategies in situ. Rather, variability due to upwelling and ocean fronts had larger effects, explaining ~40% of the partitioning between Synechococcus generalists and specialists in the ocean. Physical oceanographic drivers therefore offer a significant selection pressure on marine Synechococcus light-harvesting strategies. Our results motivate further study of the in situ light environments of upwelling zones and ocean fronts, which are currently understudied as potential light-driven niche habitats.