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The bioavailable nitrogen fixed by diazotrophs is critical for sustaining productivity in the oligotrophic ocean. Despite this, understanding how diazotroph-derived nitrogen integrates into the nitrogen cycle within the euphotic zone remains unknown. Here, we investigated nitrogen fixation rates in the particulate and dissolved fractions within the euphotic zone of the North Pacific Subtropical Gyre. Our findings reveal the proportion of nitrogen fixation rates in the dissolved fraction increases with depth. Light manipulation experiments uncover that reduced light levels can stimulate the net release of diazotroph-derived nitrogen, aligning with our depth-related observations. Furthermore, we identify two distinct transfer pathways vertically associated with light-driven ecological niches. Specifically, the released diazotroph-derived nitrogen is transferred to non-diazotrophic plankton in the upper layers. Meanwhile, in the lower layers, it contributes to the nitrification process. Our results underscore the high bioavailability of diazotroph-derived nitrogen and its rapid integration into the nitrogen cycle through multiple pathways within the well-lit ocean. New study finds diazotroph-derived nitrogen is utilized by phytoplankton in upper euphotic zone, while below, it is used by nitrifiers. The newly identified pathways reveal the marine nitrogen cycle may be more interconnected than previously recognized. [ABSTRACT FROM AUTHOR]

Nitrification, the stepwise oxidation of ammonia to nitrate via nitrite, is a key process in the marine nitrogen cycle. Reported nitrite oxidation rates frequently exceed ammonia oxidation rates below the euphotic zone, raising the fundamental question of whether the two steps are balanced and if alternative sources contribute to nitrite production in the dark ocean. Here we present vertically resolved profiles of ammonia, urea, and nitrite oxidation rates and their kinetic traits in the oligotrophic Subtropical North Pacific. Our results show active urea‐derived nitrogen oxidation (urea‐N oxidation) in the presence of experimental ammonium amendment, suggesting direct urea utilization. The depth‐integrated rates of urea‐N oxidation and ammonia oxidation are comparable, demonstrating that urea‐N oxidation is a significant source of nitrite. The additional nitrite from urea‐N oxidation helps to balance the two steps of nitrification in our study region. Nitrifiers exhibit high affinity for their substrates, and the apparent half‐saturation constants for ammonia and nitrite oxidation decrease with depth. The apparent half‐saturation constant for urea‐N oxidation is higher than that for ammonia oxidation and shows no clear vertical trend. Such kinetic traits may account for the relatively higher urea concentration than ammonium concentration in the ocean's interior. Moreover, a compilation of our results and reported data shows a trend of increased urea‐N oxidation relative to ammonia oxidation from the eutrophic coastal zone to the oligotrophic open ocean. This trend reveals a substrate‐dependent biogeographic distribution of urea‐N oxidation across marine environments and provides new information on the balance and flux of the marine nitrification process. Key Points: Active urea‐derived nitrogen oxidation in the presence of added ammonium suggests direct urea utilization by marine ammonia oxidizersSubstrate affinity regulates the vertical distribution of ammonia, urea, and nitrite in the ocean's interiorNitrite production from urea is comparable to that from ammonia oxidation in the energy‐starved mesopelagic ocean [ABSTRACT FROM AUTHOR]

We examined the spatial variation in size‐fractionated (0.8–51 and > 51 μm) particulate cadmium (Cd) and phosphorus (P) based on a large dataset collected during a GEOTRACES Section cruise (GP09) in the North Pacific Subtropical Gyre (NPSG) to better understand the interrelationship between Cd and P biogeochemical cycles. Concentrations of particulate Cd (0.06–2.15 pmol L−1) and P (0.33–10.19 nmol L−1) showed an initial increase with depth followed by a decrease, and were among the lowest observed in the global ocean. Bulk particulate Cd : P ratios in the euphotic zone, indicative of phytoplankton Cd assimilation, showed strong geographic variability averaging 0.05 ± 0.02 within the NPSG interior vs. 0.14 ± 0.04 pmol nmol−1 at the southern boundary. Cadmium to P remineralization ratio in the mesopelagic zone had a roughly similar stoichiometry as what was produced in the euphotic zone, being ~ 0.05 ± 0.01 in the NPSG interior vs. 0.21 ± 0.04 pmol nmol−1 at the southern boundary. Cd–P decoupling was reflected in the elements' vertical distribution, showing unsynchronized changes through the water column, consistent with Cd–P differential remineralization resulting from multiple Cd and P pools. The Cd–P relationship also differed between small and large particles, suggesting differences in Cd assimilation among phytoplankton assemblages as well as particle dynamic processes. Our results highlight complex processes fractionating Cd from P in the oligotrophic ocean and complicate the use of Cd as a palaeo‐phosphate proxy. [ABSTRACT FROM AUTHOR]

The impact of environmental factors on nitrogen (N) and oxygen (O) isotope effects during algal nitrate assimilation causes uncertainty in the field application of sedimentary N isotope records and nitrate isotopes to understand the marine nitrogen cycle. Ocean acidification is predicted to change nitrogen cycling including nitrate assimilation, but how N and O isotope effects during algal nitrate assimilation vary in response to changes in seawater pH and partial pressure CO2 (pCO2) remains unknown. We measured N and O isotope effects during nitrate assimilation and physiological states of the marine diatom Thalassiosira weissflogii and Synechococcus under different pH (8.1 or 7.8) and pCO2 (400 or 800 μatm) conditions. Low pH and/or high pCO2 equally decreased N and O isotope effects during nitrate assimilation by diatoms possibly due to reducing cellular nitrate efflux/uptake ratio and decreased isotope effects for nitrate uptake, whereas they did not affect those by Synechococcus with low intracellular nitrate concentration and limited nitrate efflux. Our results provide compelling experimental evidence showing different changes in N and O isotope effects during nitrate assimilation by marine eukaryotic and prokaryotic phytoplankton at low pH and/or high pCO2. These findings suggest new insight into environmental controls on variability in the isotope effect during algal nitrate assimilation, and have implications for improving a predictive understanding of N and O isotope tools in acidified oceans. [ABSTRACT FROM AUTHOR]

Nutrient transfer into the sunlit surface ocean by cyclonic eddies is potentially crucial for sustaining primary productivity in the stratified subtropical gyres. However, the nature of productivity enhancements, including the flow of matter to higher trophic levels and its impact on carbon fluxes, remain poorly resolved. Here, we report a detailed assessment of the biogeochemical response to a cyclonic eddy in the subtropical Northwest Pacific via a combination of ship‐based and autonomous platforms. Primary production was enhanced twofold within the eddy core relative to reference sites outside, whereas phytoplankton biomass even decreased. Pico‐phytoplankton (< 2 μm) dominated (> 80%) both phytoplankton biomass and primary production inside and outside the eddy. The stimulated primary production in the eddy core was accompanied by an approximately twofold increase in mesozooplankton abundance, an approximately threefold increase in particle formation in the deep chlorophyll maximum layer, as well as significantly enhanced surface oceanic CO2 uptake and net community production. We suggest these observations carry important implications for understanding carbon export in the subtropical ocean and highlight the need to include such subtropical eddy features in ocean carbon budget analyses. [ABSTRACT FROM AUTHOR]

Rapid expansion of mariculture during past decades has raised substantial concerns about impacts on the coastal environment, notably eutrophication. This study focuses on one of the world's highest density mariculture sites, Sansha Bay, Fujian Province, China, featuring integrated multi-trophic aquaculture practices involving croaker, kelp and oyster, based on examination of nutrient distributions and releases. A two-endmembers-mixing model showed significant addition of dissolved inorganic nitrogen (DIN; 6.9 ± 4.1 μmol L-1) and phosphorus (DIP; 0.45 ± 0.29 μmol L-1) associated with mariculture activities in spring 2020. A mass balance model estimated an annual release of N and P from cage fish farming systems fed with mixed trash fish feed and formulated feed of (2.42 ± 0.15)×104 tons and (5.33 ± 0.37)×103 tons, respectively. Of the total feed input, 52.8 ± 4.7 % of DIN and 33.0 ± 3.7 % of DIP were released into seawater, values much higher than the riverine input and exchange with offshore coastal waters. A co-culture strategy involving kelp and oyster production in 2020 removed (1.08 ± 0.01)×103 tons of N and (1.56 ± 0.08) ×102 tons of P, respectively. Therefore, adjusting feed strategies and improving feed conversion rates could alleviate eutrophication caused by mariculture expansion in this ecosystem. [ABSTRACT FROM AUTHOR]

Rapid mariculture expansion has raised concerns about coastal eutrophication. This study assesses nutrient cycling in Sansha Bay, China, a eutrophic semi-enclosed bay with intensive mariculture. A two-endmember mixing model showed significant additions of dissolved inorganic nitrogen (DIN; 6.9 ± 4.1 μmol L -1 ) and phosphorus (DIP; 0.45 ± 0.29 μmol L -1 ) in May 2020, mainly from mariculture. Estimated N and P inputs from fish farming were 7789 ± 361 tons and 1497 ± 91 tons in spring, respectively, with N mainly in dissolved form and P in particulate form. And, trash fish feed caused higher nutrient release than formulated feed. Of the feed input, 52.8 ± 4.7 % of DIN and 33.0 ± 3.7 % of DIP were released into environment, exceeding riverine input and offshore exchanges. Co-culturing kelp and oysters removed 1079 ± 11 tons of N and 156 ± 8 tons of P. Therefore, adjusting feed types and planning co-cultivation strategies could alleviate eutrophication resulting from mariculture expansion., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Coastal oceans, traditionally seen as a conduit for transporting atmospheric carbon dioxide (CO 2 )-derived anthropogenic carbon (C ANT ) to open oceans, exhibit complex carbon exchanges at their interface. South China Sea (SCS) exemplifies this complexity, where interactions with the Pacific, particularly through Kuroshio intrusion, challenge the understanding of C ANT source and variability in a coastal ocean. Contrary to prevailing paradigm expectations, our high-resolution, long-term data reveal that C ANT in the SCS primarily originates from Pacific water injection across the Luzon Strait rather than atmospheric CO 2 invasion. Over the past two decades, the SCS has experienced increasing C ANT levels, with notable interannual fluctuations driven by El Niño and La Niña events influencing Kuroshio intrusion, generating anomalously high and low C ANT inventories, respectively. This highlights an overlooked C ANT transport pathway from open to coastal oceans, responsible for cumulative ocean acidification that has already affected coral reefs enriched in the SCS located west of the Coral Triangle.