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Many estuaries experience eutrophication, deoxygenation and warming, with potential impacts on greenhouse gas emissions. However, the response of N₂O production to these changes is poorly constrained. Here we applied nitrogen isotope tracer incubations to measure N₂O production under experimentally manipulated changes in oxygen and temperature in the Chesapeake Bay—the largest estuary in the United States. N₂O production more than doubled from nitrification and increased exponentially from denitrification when O₂was decreased from >20 to <5 micromolar. Raising temperature from 15° to 35°C increased N₂O production 2- to 10-fold. Developing a biogeochemical model by incorporating these responses, N₂O emissions from the Chesapeake Bay were estimated to decrease from 157 to 140 Mg N year⁻¹from 1986 to 2016 and further to 124 Mg N year⁻¹in 2050. Although deoxygenation and warming stimulate N₂O production, the modeled decrease in N₂O emissions, attributed to decreased nutrient inputs, indicates the importance of nutrient management in curbing greenhouse gas emissions, potentially mitigating climate change. 

Abstract. 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 undersampled for particulate organic matter (POM) elemental composition, especially when it comes to phosphorus (i.e., POP). Thus, it is uncertain how environmental variation in this region translates into shifts in the C:N:P ratio. 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 that in the oligotrophic gyres. Latitudinal and zonal differences in elemental stoichiometry were linked to overall nutrient supply as well as N vs. P stress. 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 stress. 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. 

Abstract Concentrations and elemental stoichiometry of suspended particulate organic carbon, nitrogen, phosphorus, and oxygen demand for respiration (C:N:P:−O 2 ) play a vital role in characterizing and quantifying marine elemental cycles. Here, we present Version 2 of the Global Ocean Particulate Organic Phosphorus, Carbon, Oxygen for Respiration, and Nitrogen (GO-POPCORN) dataset. Version 1 is a previously published dataset of particulate organic matter from 70 different studies between 1971 and 2010, while Version 2 is comprised of data collected from recent cruises between 2011 and 2020. The combined GO-POPCORN dataset contains 2673 paired surface POC/N/P measurements from 70°S to 73°N across all major ocean basins at high spatial resolution. Version 2 also includes 965 measurements of oxygen demand for organic carbon respiration. This new dataset can help validate and calibrate the next generation of global ocean biogeochemical models with flexible elemental stoichiometry. We expect that incorporating variable C:N:P:-O 2 into models will help improve our estimates of key ocean biogeochemical fluxes such as carbon export, nitrogen fixation, and organic matter remineralization. 

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 A key uncertainty for predicting future ocean oxygen levels is the response and feedback of organic matter respiration demand. One poorly constrained component of the respiration demand is the oxygen‐to‐carbon remineralization ratio—the respiration quotient. Currently, multiple biological hypotheses can explain variation in the respiration quotient of organic matter produced in the surface ocean. To test these hypotheses, we directly quantified the particulate respiration quotient in 715 samples along a meridional section of the Atlantic Ocean and compared to previous Pacific Ocean observations. We demonstrate significant regional shifts in the respiration quotient and a two‐basin average of 1.16. Possible diel oscillations were also observed in the respiration quotient. Basin and regional variation in the respiration quotient were positively linked to temperature, N versus P stress, and plankton size structure. These observations suggest a complex regulation of the respiration quotient with important implications for the regional coupling of carbon and oxygen cycling. 

Abstract Detailed descriptions of microbial communities have lagged far behind physical and chemical measurements in the marine environment. Here, we present 971 globally distributed surface ocean metagenomes collected at high spatio-temporal resolution. Our low-cost metagenomic sequencing protocol produced 3.65 terabases of data, where the median number of base pairs per sample was 3.41 billion. The median distance between sampling stations was 26 km. The metagenomic libraries described here were collected as a part of a biological initiative for the Global Ocean Ship-based Hydrographic Investigations Program, or “Bio-GO-SHIP.” One of the primary aims of GO-SHIP is to produce high spatial and vertical resolution measurements of key state variables to directly quantify climate change impacts on ocean environments. By similarly collecting marine metagenomes at high spatiotemporal resolution, we expect that this dataset will help answer questions about the link between microbial communities and biogeochemical fluxes in a changing ocean. 

Abstract Environmentally driven variability in the elemental stoichiometry of ocean plankton plays a key role in ocean biogeochemical processes. Recent studies have identified clear regional variability in C:N:P, but less is known about the environmental regulation of diel variability in plankton elemental stoichiometry. Here, we quantified the amplitude of the diel variability in C:N of surface ocean particles (<30 μm,C:N_amp) across large latitudinal gradients in the Indian and Atlantic Oceans. We commonly observed diel oscillations in C:N and biome‐specific variability inC:N_amp. Temperature emerged as the strongest predictor ofC:N_amp, relative to the supply of nitrate. We propose thatC:N_ampis positively related to photosynthesis and respiration and thus phytoplankton growth rates. We find that independent growth rate proxies and an ecosystem model support this hypothesis. In addition, the temperature sensitivity ofC:N_amphas aQ₁₀of 1.78 corroborating studies of phytoplankton growth rates. Surface communities across the Indian Ocean transect had a very small dependency on nitrate, whereas recycled nitrogen sources were by far the most preferred and the ratio of recycled‐N:nitrate utilization increased with increasingC:N_amp. To predict future changes inC:N_amp, we combined our statistical model with data from the fifth Coupled Model Intercomparison Project for the years 1990 and 2090. The results suggest that future rising temperatures will yield increasedC:N_amp. Collectively, our results imply that rising surface ocean temperatures lead to elevated phytoplankton growth rates supported by recycled nutrients. 

Abstract A past global synthesis of marine particulate organic matter (POM) suggested latitudinal variation in the ratio of surface carbon (C): nitrogen (N): phosphorus (P). However, this synthesis relied on compiled datasets that may have biased the observed pattern. To demonstrate latitudinal shifts in surface C:N:P, we combined hydrographic and POM observations from 28°N to 69°S in the eastern Pacific Ocean (GO‐SHIP line P18). Both POM concentrations and ratios displayed distinct biome‐associated changes. Surface POM concentrations were relatively low in the North Pacific subtropical gyre, increased through the Equatorial Pacific, were lowest in the South Pacific subtropical gyre, and increased through the Southern Ocean. Stoichiometric elemental ratios were systematically above Redfield proportions in warmer regions. However, C:P and N:P gradually decreased across the Southern Ocean despite an abundance of macro‐nutrients. Here, a size‐fraction analysis of POM linked increases in the proportion of large plankton to declining ratios. Subsurface N* values support the hypothesis that accumulated remineralization products of low C:P and N:P exported POM helps maintain the Redfield Ratio of deep nutrients. We finally evaluated stoichiometric models against observations to assess predictive accuracy. We attributed the failure of all models to their inability to capture shifts in the specific nature of nutrient limitation. Our results point to more complex linkages between multinutrient limitation and cellular resource allocation than currently parameterized in models. These results suggest a greater importance of understanding the interaction between the type of nutrient limitation and plankton diversity for predicting the global variation in surface C:N:P.