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Are the oceans turning into deserts? Rising temperature, increasing surface stratification, and decreasing vertical inputs of nutrients are expected to cause an expansion of warm, nutrient deplete ecosystems. Such an expansion is predicted to negatively affect a trio of key ocean biogeochemical features: phytoplankton biomass, primary productivity, and carbon export. However, phytoplankton communities are complex adaptive systems with immense diversity that could render them at least partially resilient to global changes. This can be illustrated by the biology of the Prochlorococcus "collective." Adaptations to counter stress, use of alternative nutrient sources, and frugal resource allocation can allow Prochlorococcus to buffer climate‐driven changes in nutrient availability. In contrast, cell physiology is more sensitive to temperature changes. Here, we argue that biogeochemical models need to consider the adaptive potential of diverse phytoplankton communities. However, a full understanding of phytoplankton resilience to future ocean changes is hampered by a lack of global biogeographic observations to test theories. We propose that the resilience may in fact be greater in oligotrophic waters than currently considered with implications for future predictions of phytoplankton biomass, primary productivity, and carbon export. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Marine phytoplankton stoichiometry links nutrient supply to marine carbon export. Deviations of phytoplankton stoichiometry from Redfield proportions (106C : 1P) could therefore have a significant impact on carbon cycling, and understanding which environmental factors drive these deviations may reveal new mechanisms regulating the carbon cycle. To explore the links between environmental conditions, stoichiometry, and carbon cycling, we compared four different models of phytoplankton C: P: a fixed Redfield model, a model with C: P given as a function of surface phosphorus concentration (P), a model with C: P given as a function of temperature, and a new multi-environmental model that predicts C: P as a function of light, temperature, and P. These stoichiometric models were embedded into a fivebox ocean circulation model, which resolves the three major ocean biomes (high-latitude, subtropical gyres, and tropical upwelling regions). Contrary to the expectation of a monotonic relationship between surface nutrient drawdown and carbon export, we found that lateral nutrient transport from lower C: P tropical waters to high C: P subtropical waters could cause carbon export to decrease with increased tropical nutrient utilization. It has been hypothesized that a positive feedback between temperature and pCO2,atm will play an important role in anthropogenic climate change, with changes in the biological pump playing at most a secondary role. Here we show that environmentally driven shifts in stoichiometry make the biological pump more influential, and may reverse the expected positive relationship between temperature and pCO2,atm. In the temperature-only model, changes in tropical temperature have more impact on the Δ pCO2,atm (~41 ppm) compared to subtropical temperature changes (~4.5 ppm). Our multi-environmental model predicted a decline in pCO2,atm of ~46 ppm when temperature spanned a change of 10 °C. Thus, we find that variation in marine phytoplankton stoichiometry and its environmental controlling factors can lead to nonlinear controls on pCO2,atm, suggesting the need for further studies of ocean C: P and the impact on ocean carbon cycling. [ABSTRACT FROM AUTHOR]

Marine phytoplankton stoichiometry links nutrient supply to marine carbon export. Deviations of phytoplankton stoichiometry from Redfield proportions (106C : 1P) could therefore have a significant impact on carbon cycling, and understanding which environmental factors drive these deviations may reveal new mechanisms that regulate the carbon cycle. To explore the links between environmental conditions, stoichiometry, and carbon cycling, we compared four different models for variations in phytoplankton C : P: a fixed Redfield model, a model with C : P given as a function of surface phosphorus concentration ([P]), a model with C : P given as a function of temperature, and a new multi-environmental model that predicts C : P as a function of light, temperature, and [P]. These stoichiometric models were embedded into a box model of the ocean circulation, which resolves the three major ocean biomes (high-latitude, subtropical gyres, and iron-limited tropical upwelling regions). Contrary to the expectation of a monotonic relationship between surface nutrient drawdown and carbon export, we found that lateral nutrient transport from lower C : P tropical waters to high C : P subtropical waters could cause carbon-export to decrease with increased tropical nutrient utilization. Temperature is thought to be one of the primary drivers of changes in atmospheric pCO2 (pCO2,atm) across glacial/interglacial periods, and it has been hypothesized that a positive feedback between temperature and pCO2,atm will play an important role in anthropogenic climate change, with changes in the biological pump playing at most a secondary role. Here we show that environmentally driven shifts in stoichiometry make the biological pump more influential, and may reverse the expected negative relationship between temperature and pCO2,atm. In the temperature-only model changes in tropical temperature have more impact on the Δ pCO2,atm (~ 41 ppm) compared to subtropical temperature (~ 4.5 ppm). Our multi-environmental model produced a decline in pCO2,atm of ~ 46 when temperature spanned a change of 10 °C. Thus, we find that variation in marine phytoplankton stoichiometry and its environmental controlling factor can lead to counterintuitive controls on pCO2,atm, suggesting the need for further studies of ocean C : P and the impact on ocean carbon cycling. [ABSTRACT FROM AUTHOR]