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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]

Concentrations of bioavailable inorganic nitrogen (N) and phosphorus (P) are simultaneously depleted in the (sub)tropical North Atlantic Ocean, but it remains unclear if phytoplankton growth rates are N limited or N–P co‐limited. Here we present findings from three bottle‐scale experiments using a four‐by‐four matrix of low‐level N and P additions, conducted at one site in the subtropical North Atlantic Ocean. Phytoplankton responses were assessed both in terms of bulk chlorophyll a (Chl a) concentrations and intracellular Chl a of dominant Prochlorococcus and Synechococcus groups. Two matrix experiments suggested that N was independently limiting in situ growth, with no co‐limiting role for P, while the third showed co‐limitation by both N and P in this region. This switch from N limitation to N–P co‐limitation was attributed to an episodic wet deposition event that supplied N, thereby stimulating phytoplankton growth and consuming available P. Such rapid transitions in nutrient limitation in response to environmental forcing might be common in oceanic systems with multiple depleted nutrients. [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]

The eastern equatorial Pacific (EEP) is a source of atmospheric CO2 during the last deglaciation, but the associated oceanic dynamics in the broader low‐latitude Pacific is not fully understood. Here, we report 30,000‐year‐long surface and subsurface pCO2 records for the western equatorial Pacific (WEP), based on boron isotopes in two planktonic foraminiferal species from core MD10‐3340. Our results show that the WEP surface became a significant atmospheric CO2 sink despite that its subsurface waters were enriched by CO2 during the last deglaciation to early Holocene. Combined with EEP proxy data and model results, we suggest that a deglacial‐early Holocene zonal seesaw of sea‐air CO2 exchange across the equatorial Pacific led to a net CO2 outgassing much greater than the modern situation. This can be ascribed to strengthened Subtropical‐Tropical Circulation, resulting in stronger upper ocean stratification in the WEP concurrent with enhanced upwelling of CO2‐rich subsurface waters in the EEP. Plain Language Summary: The modern equatorial Pacific is one of the main sources of CO2 released from the ocean into the atmosphere, with the strongest outgassing in the central‐eastern part and nearly neutral state in the western part. However, according to our boron isotope data of planktonic foraminifera, the western equatorial Pacific was turned into a significant atmospheric CO2 sink rather than a source between 16,000 and 7,000 years ago. At the same time, the eastern equatorial Pacific released more CO2, resulting in a zonal seesaw of sea‐air CO2 exchange across the equatorial Pacific that generally accelerated the CO2 outgassing and favored global warming. We further argue that these deglacial‐early Holocene CO2 changes were ascribed to the strengthened Subtropical‐Tropical Circulation, in which the accelerated eastward Equatorial Undercurrent leads to stronger upper ocean stratification in the WEP, and also converges seawater CO2 into the upper thermocline and finally releases at the surface EEP. This finding helps us understand and quantitatively assess future global carbon budget, particularly how the upper ocean circulation modulates the sea‐air CO2 flux in a manner similar to the past. Key Points: Strengthened upper ocean stratification led to significant atmospheric CO2 sink in the western equatorial Pacific during ∼16–7 kaThe zonal seesaw of sea‐air CO2 exchange in the equatorial Pacific resulted in a net CO2 outgassing much greater than the modern situationThe sea‐air CO2 exchange across the equatorial Pacific was modulated by the Subtropical‐Tropical Circulation on orbital timescales [ABSTRACT FROM AUTHOR]

Mesoscale eddies are common in the subtropical Northwest Pacific, however, relatively little is known about their spatial variability and temporal evolution, and how these impact upper ocean biogeochemistry. Here we investigate these using observations of a cyclonic eddy carried out along four sequential transects. Consistent with previous observations of cyclonic eddies, the eddy core had doming isopycnals, bringing elevated nutrient waters nearer to the surface. However, we also found that the upper layer of the eddy above the nutricline had significantly lower phosphate concentrations within its core relative to its edge. We attributed this to elevated N2 fixation within the eddy core, which was likely driven by enhanced subsurface iron supply, ultimately resulting in increased phosphate consumption. Eddy‐enhanced N2 fixation was additionally supported by the elevation of nitrate + nitrite to phosphate ratios below the euphotic zone. Moreover, we observed that while the upward displacement of isopycnals within the eddy core led to an increase in phytoplankton biomass in the lower euphotic zone, there was no significant increase in total phytoplankton biomass across the entire euphotic zone. Cyclonic eddies in the subtropical North Pacific are projected to be becoming more frequent, implying that such dynamics could become increasingly important for regulating nutrient biogeochemistry and ultimately productivity of the region. Plain Language Summary: Cyclonic eddies are oceanographic features leading to upward transfer of deep‐water nutrients that support phytoplankton growth in the sunlit surface ocean. In the low‐nutrient Northwest Pacific, cyclonic eddies are considered to be an extremely important nutrient source in alleviating nutrient limitation; however, their biogeochemical effects are not well‐examined, in particular in the upper layers where nutrients are below the detection limit of conventional techniques. By examining a cyclonic eddy with four sequential transect observations, our results showed that the eddy core had higher nutrient concentrations in the lower euphotic zone compared to the eddy edge, consistent with previous studies; in contrast, concentrations of phosphate in the upper surface layer had significantly lower values within the core relative to those at the edge. Surface rate measurements suggested that this lower phosphate was potentially driven by elevated N2 fixation, likely due to enhanced iron supply from depth, thereby resulting in additional consumption of excess surface phosphate. However, this increased nutrient supply did not lead to the enhancement of total phytoplankton biomass across the entire euphotic zone. Key Points: Lower phosphate concentrations were observed above the nutricline within the eddy core in comparison to the edgeEnhanced N2 fixation within the eddy core is proposed to have driven increased phosphate consumptionNo substantial total phytoplankton biomass increase was found within the eddy core [ABSTRACT FROM AUTHOR]