Sinking Particulate Organic Carbon Research Articles

Modeling carbon dioxide removal via sinking of particulate organic carbon from macroalgae cultivation

Macroalgae cultivation is receiving growing attention as a potential carbon dioxide removal (CDR) strategy. Macroalgae biomass harvesting and/or intentional sinking have been the main focus of research efforts. A significant amount of biomass is naturally lost through erosion and breakage of cultivated or naturally growing seaweed, but the contribution of the resulting particulates to carbon sequestration is relatively unexplored. Here, we use a fully coupled kelp-biogeochemistry model forced by idealized parameters in a closed system to estimate the potential of macroalgal-derived particulate organic carbon (POC) sinking as a CDR pathway. Our model indicates that at a kelp density of 1.1 fronds m−3, macroalgal POC sinking can export 7.4 times more carbon to the deep sea (depths > 500m) and remove 5.2 times more carbon from the atmosphere (equivalent to an additional 336.0 gC m−2 yr−1) compared to the natural biological pump without kelp in our idealized closed system. The results suggest that CDR associated with POC sinking should be explored as a possible benefit of seaweed farming and point to the need for further study on organic carbon partitioning and its bioavailability to quantify the effectiveness and impacts of macroalgal cultivation as a CDR strategy.

Hydrostatic pressure impedes the degradation of sinking copepod carcasses and fecal pellets.

Fast-sinking zooplankton carcasses and fecal pellets appear to contribute significantly to the vertical transport of particulate organic carbon (POC), partly because of low temperature that decreases microbial degradation during the descent into the deep ocean. Increasing hydrostatic pressure could further reduce the degradation efficiency of sinking POC, but this effect remains unexplored. Here, the degradation of carcasses and fecal pellets of the abundant marine copepod Calanus finmarchicus was experimentally studied as a function of pressure (0.1-100MPa). Samples were either exposed to elevated pressure in short 1-day incubations or a gradual pressure increase, simulating continuous particle sinking during a 20-day incubation. Both experiments revealed gradual inhibition of microbial respiration in the pressure range of 20-100MPa, corresponding to 2-10-km depth. This suggests that hydrostatic pressure impedes carbon mineralization of fast-sinking carcasses and fecal pellets and enhances the deep-sea deposition rate of zooplankton-derived organic material.

Composition of the sinking particle flux in a hot spot of dinitrogen fixation revealed through polyacrylamide gel traps

Diazotrophs regulate marine productivity in the oligotrophic ocean by alleviating nitrogen limitation, contributing to particulate organic carbon (POC) export to the deep ocean. Yet, the characterization of particles composing the sinking POC flux has never been explored in such ecosystems. Moreover, the contribution of the direct gravitational export of diazotrophs to the overall flux is seldom assessed. Here we explore the composition of the sinking POC flux in a hot spot of N2 fixation (the western sub-tropical South Pacific) using polyacrylamide gel-filled traps deployed at two stations (S05M and S10M) and three depths (170 m, 270 m, 1000 m) during the TONGA expedition (November-December 2019). Image analyses of particles collected in the gels was used to classify them into 5 categories (fecal aggregates, phytodetrital aggregates, mixed aggregates, cylindrical fecal pellets, and zooplankton carcasses). Fecal aggregates were the most abundant at both stations and all depths and dominated the flux (average of 56 ± 28% of the POC flux), followed by zooplankton carcasses (24 ± 19%), cylindrical fecal pellets (15 ± 14%) and mixed aggregates (5 ± 4%), whereas phytodetrital aggregates contributed less (<1%). Since N isotope budgets show that export is mainly supported by diazotrophy at these stations, these results suggest that the diazotroph-derived N has been efficiently transferred to the foodweb up to zooplankton and fecal pellets before being exported, pleading for an indirect export of diazotrophy. However, random confocal microscopy examination performed on sinking particles revealed that diazotrophs were present in several categories of exported particles, suggesting that diazotrophs are also directly exported, with a potential contribution to overall POC fluxes increasing with depth. Our results provide the first characterization of particle categories composing the sinking flux and their contribution to the overall flux in a hot spot of N2 fixation.

Cycling Rates of Particulate Organic Carbon Along the GEOTRACES Pacific Meridional Transect GP15

AbstractUnderstanding particle cycling processes in the ocean is critical for predicting the response of the biological carbon pump to external perturbations. Here, measurements of particulate organic carbon (POC) concentration in two size fractions (1–51 and >51 μm) from GEOTRACES Pacific meridional transect GP15 are combined with a POC cycling model to estimate rates of POC production, (dis)aggregation, sinking, remineralization, and vertical transport mediated by migrating zooplankton, in the euphotic zone (EZ) and upper mesopelagic zone (UMZ) of distinct environments. We find coherent variations in POC cycling parameters and fluxes throughout the transect. Thus, the settling speed of POC in the >51 μm fraction increased with depth in the UMZ, presumably due to higher particle densities at depth. The settling flux of total POC (>1 μm) out of the EZ was positively correlated with primary production integrated over the EZ; the highest export occurred in the subarctic gyre while the lowest occurred in the subtropical gyres. The ratio of POC settling flux to integrated primary production was low (<5%) along GP15, which suggests an efficient recycling of POC in the EZ in all trophic regimes. Specific rates of POC remineralization did not show clear variations with temperature or dissolved oxygen concentration, that is, POC recycling was apparently controlled by other factors such as microbial colonization and substrate lability. Particle cohesiveness, as approximated by the second‐order rate constant for particle aggregation, was negatively correlated with trophic regime: particles appeared more cohesive in low‐productivity regions than in high‐productivity regions.

Carbon and silica fluxes during a declining North Atlantic spring bloom as part of the EXPORTS program

The goal of NASA's EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) project is to develop a predictive understanding of the fate of global ocean primary productivity and export of carbon from the surface to the deep ocean. Thorium-234 (234Th, t1/2 = 24.1 d) was used to measure sinking particle export from an anticyclonic eddy during the EXPORTS North Atlantic cruise (May 2021) at the Porcupine Abyssal Plain. The four-week sampling period was broken into three time periods (“epochs”) where 800 234Th seawater samples were collected from over 50 CTD casts with high depth resolution over the upper 500 m. Size-fractioned particulate samples were collected to determine particulate organic carbon (POC) and biogenic silica (bSi) to 234Th ratios using in situ McLane pumps. A 234Th non-steady state model shows an eddy center epoch average progression of increasing 234Th export (∼2800 ± 300 (Epoch 1; standard deviation) to 4500 ± 700 (Epoch 3) dpm m−2 d−1) out of the top 110 m of the water column over the course of the cruise (29 d). This translates into an epoch average progression of ∼11 ± 1 to 14 ± 2 mmol C m−2 d−1 of sinking POC flux, and ∼ 3 ± 1 to 6 ± 1 mmol bSi m−2 d−1 of sinking bSi flux to deeper waters at 110 m. The overall efficiency of the biological carbon pump (amount of net primary production reaching 100 m below the euphotic zone) increases from ∼10% to ∼30% throughout the sampling period. The temporal trends discussed extensively in this paper show that POC and bSi export increase during diatom bloom evolution.

Export of Dissolved Organic Carbon (DOC) compared to the particulate and active fluxes near South Georgia, Southern Ocean

Quantifying the relative contributions of the export of particulate organic carbon (POC), dissolved organic carbon (DOC) and active fluxes by migrating organisms is essential to understand the functioning and vulnerability of the ocean's biological pump. However, these fluxes are rarely measured at the same time. Here we provide a first simultaneous comparison of these biological pump components in the region of South Georgia. We use a combination of in-situ data and an inverse model to calculate the DOC export and the suspended POC export and compare them to the sinking POC and active export. We find that, in this region, the DOC total export contributes about 6.6% (23.0–37.5 mg C m−2 day−1) to the total export flux, the active flux has no discernible contribution, and the sinking POC flux is dominant with a mean value of 409 mg C m−2 day−1. Diapycnal fluxes of DOC obtained from the cruise data constitute only a minor fraction (0.05–1.28 mg C m−2 day−1) of the total DOC export estimated by the inverse model and are exceeded on average by the diapycnal flux of suspended POC. Our results also indicate that the total export of DOC is driven by isopycnal transport. Future fieldwork in the region of South Georgia should focus on quantifying the isopycnal flux of DOC. Future measurement campaigns should also aim to simultaneously measure the particulate, dissolved and active components of the biological pump at contrasting locations and at different times to resolve the variability of their relative contribution.

Seasonality of downward carbon export in the Pacific Southern Ocean revealed by multi-year robotic observations

At high latitudes, the biological carbon pump, which exports organic matter from the surface ocean to the interior, has been attributed to the gravitational sinking of particulate organic carbon. Conspicuous deficits in ocean carbon budgets challenge this as a sole particle export pathway. Recent model estimates revealed that particle injection pumps have a comparable downward flux of particulate organic carbon to the biological gravitational pump, but with different seasonality. To date, logistical constraints have prevented concomitant and extensive observations of these mechanisms. Here, using year-round robotic observations and recent advances in bio-optical signal analysis, we concurrently investigated the functioning of two particle injection pumps, the mixed layer and eddy subduction pumps, and the gravitational pump in Southern Ocean waters. By comparing three annual cycles in contrasting physical and biogeochemical environments, we show how physical forcing, phytoplankton phenology and particle characteristics influence the magnitude and seasonality of these export pathways, with implications for carbon sequestration efficiency over the annual cycle.

The Outsized Role of Salps in Carbon Export in the Subarctic Northeast Pacific Ocean.

Periodic blooms of salps (pelagic tunicates) can result in high export of organic matter, leading to an "outsized" role in the ocean's biological carbon pump (BCP). However, due to their episodic and patchy nature, salp blooms often go undetected and are rarely included in measurements or models of the BCP. We quantified salp-mediated export processes in the northeast subarctic Pacific Ocean in summer of 2018 during a bloom of Salpa aspera. Salps migrated from 300 to 750m during the day into the upper 100m at night. Salp fecal pellet production comprised up to 82% of the particulate organic carbon (POC) produced as fecal pellets by the entire epipelagic zooplankton community. Rapid sinking velocities of salp pellets (400-1,200md-1) and low microbial respiration rates on pellets (<1% of pellet C respired day-1) led to high salp pellet POC export from the euphotic zone-up to 48% of total sinking POC across the 100m depth horizon. Salp active transport of carbon by diel vertical migration and carbon export from sinking salp carcasses was usually <10% of the total sinking POC flux. Salp-mediated export markedly increased BCP efficiency, increasing by 1.5-fold the proportion of net primary production exported as POC across the base of the euphotic zone and by 2.6-fold the proportion of this POC flux persisting 100m below the euphotic zone. Salps have unique and important effects on ocean biogeochemistry and, especially in low flux settings, can dramatically increase BCP efficiency and thus carbon sequestration.

What Controls the Large‐Scale Efficiency of Carbon Transfer Through the Ocean's Mesopelagic Zone? Insights From a New, Mechanistic Model (MSPACMAM)

AbstractA key challenge for current‐generation Earth system models (ESMs) is the simulation of the penetration of sinking particulate organic carbon (POC) into the ocean interior, which has implications for projections of future oceanic carbon sequestration in a warming climate. This paper presents a new, cost‐efficient, mechanistic 1D model that prognostically calculates POC fluxes by carrying four component particles in two different size classes. Gravitational settling and removal/transformation processes are represented explicitly through parameterizations that incorporate the effects of particle size and density, dissolved oxygen, calcite and aragonite saturation states, and seawater temperature, density, and viscosity. The model reproduces the observed POC flux attenuation at 22 locations in the North Atlantic and North Pacific. The model is applied over a global ocean domain with seawater properties prescribed from observation‐based climatologies in order to address an important scientific question: What controls the spatial pattern of mesopelagic POC transfer efficiency? The simulated vertical POC transfer is more efficient at high latitudes than at low latitudes with the exception of oxygen minimum zones, which is consistent with recent inverse modeling and neutrally buoyant sediment trap studies. Here, model experiments show that the relative abundance of large‐sized, rapidly sinking particles and the slower rate of remineralization at high latitudes compensate for the region's lack of calcium carbonate ballast and the cold‐water viscous resistance, leading to higher transfer efficiencies compared to low‐latitude regions. The model could be deployed in ESMs in order to diagnose the impacts of climate change on oceanic carbon sequestration and vice versa.

Slow-sinking particulate organic carbon and its attenuation in the mesopelagic water of the South China Sea

Coastal acidification has been widely investigated in terms of its rationale and ecological effects in the last decade. However, the driving mechanism for acidification in open seawater, especially in mesopelagic water, is still poorly understood. Here, the sinking velocity and flux attenuation of particulate organic carbon (POC) were examined based upon the radioactive 210Po-210Pb tracer to reveal the remineralization of POC in the mesopelagic zone in the northeastern South China Sea (SCS). Overall, the profiles of 210Po followed those of 210Pb, lending support to the particle sinking controlled top-down deficits of 210Po. Using an inverse model, the sinking velocity of particles, for the first time in the SCS, was estimated to vary from 3 to 34 m d-1 with the mean value of 15 ± 9 m d-1, indicating that the slow sinking particles largely contribute to the POC flux in the SCS. Beneath the euphotic zone, a consistent descending of the sinking speed implied continuous remineralization of sinking POC in the twilight zone. A preliminary estimate revealed that 1.9-5.4 mmol-C m-2 d-1 remineralized back to carbon dioxide within 100-500 m, representing about 70% of the exported autochthonous POC from the euphotic zone. In 100-1000 m, 2.4-6.6 mmol-C m-2 d-1 (i.e., 84%) remineralized. Thus, the upper twilight zone (i.e., 100-500 m) is the dominant layer of POC remineralization, and POC-induced acidification could be unneglectable there. These results provided insights into the POC-induced acidification mechanism in the mesopelagic water, especially in the upper mesopelagic layer.

Temporal and regional variabilities in the attenuation of sinking particulate organic carbon in the Kuroshio region

The biological pump that transports carbon from the surface ocean to the ocean interior is an important determinant of ocean carbon absorption from the atmosphere, and of biological activity beneath the euphotic zone. Various factors influence the magnitude and efficiency of this biological pump, such as primary production, particle size, and the composition of consumers of particulate organic carbon (POC). The complex factors influencing the biological pump induce high regional and temporal variability; however, the mechanisms of this variability are not understood and its causes remain unclear. The size of sinking particles has been hypothesized to affect the attenuation of POC flux via reduction of residence time within the water column. Here, we conducted sediment trap experiments in slope and subtropical gyre areas along the Kuroshio in the subtropical North Pacific. The ratio of POC flux at a depth 100 m below the base of the euphotic zone to that at the base of the euphotic zone (T100) varied from 0.307 to 0.646 and from 0.090 to 0.958 in the slope and subtropical gyre areas, respectively, and was positively correlated with variations in the volume-weighted mean particle diameter (Spearman rank order correlation coefficient: 0.700, p&amp;lt; 0.05, n = 9). However, the variation in T100 was not correlated with ambient water temperature (p &amp;gt; 0.05). The results support the hypothesis that attenuation of POC flux is influenced by the size of sinking particles in oligotrophic oceanic areas.

Carbon cycling in the East Sea (Japan Sea): A review

The East Sea (also known as the Japan Sea; hereafter, EJS) is a semi-enclosed marginal sea surrounded by the Korean Peninsula, Russia, and the Japanese Islands. The EJS is connected to the Pacific through shallow straits. Thus, the EJS has its own thermohaline circulation and the characteristic biogeochemistry. The deep overturning circulation plays a critical role in carbon cycling including absorption of atmospheric CO2 and its sequestration into the interior of the sea. The turnover time of the deep EJS (&amp;gt;1000 m) is ~ hundred years and probably varies depending on physical climate forcing. Thus, the effect of climate change on oceanic processes may be more easily detected in the EJS. In this paper, we summarize the current understanding of carbon cycling in the EJS. We focus especially on the Ulleung Basin in the southwestern EJS, from which more extensive data are available. Notable features of carbon cycling in the EJS include the following: primary productivity and the export/production ratio are higher than in the adjacent Pacific; the EJS is a net sink of atmospheric CO2 and anthropogenic CO2 content is ~1% of the dissolved inorganic carbon inventory; dissolved inorganic carbon in the sea interior is mostly supplied by organic matter decomposition rather than CaCO3 dissolution and thus, the deep waters are vulnerable to acidification; N:P molar ratio of the deep waters is ~13, lower than the Redfield ratio; concentration of dissolved organic carbon is significantly higher than in the oceans; and sediment resuspension and lateral transport is an important component of sinking particulate organic carbon (POC) flux. Another important feature is the temporal trends observed for the last few decades. For example, pH, calcium carbonate saturation status, and dissolved oxygen concentration in the sea interior have decreased, whereas dissolved inorganic carbon and likely, the inventory of anthropogenic CO2 have increased. These temporal trends have an implication on better understanding of the processes occurring more slowly in the oceans. Brief suggestions for future research that will improve our understanding of carbon cycling and its variability are provided at the end of the paper.

Carbon Isotopic Constraints on Basin‐Scale Vertical and Lateral Particulate Organic Carbon Dynamics in the Northern South China Sea

AbstractLateral particle transport in shelf/slope settings of marginal sea systems may potentially supply allochthonous particulate organic carbon (POC) to the ocean interior. The magnitude and prevalence of such processes has implications for our understanding of the functioning and efficiency of modern biological carbon pump processes, as well as for interpretation of deep ocean sediment records. Yet, ages and relative contribution of POC from different sources to deep ocean basins are much less understood. Here, based on radiocarbon (Fm, fraction modern), stable carbon (δ13Corg), and other geochemical analysis of particles intercepted by time‐series sediment traps deployed at three different depths (1,000, 2,150, and 3,200 m), we constrain the source of the sinking POC in the deep basin of northern South China Sea (SCS). The results show that the modern POC derived from surface ocean productivity, acting as the vertical vector, accounts for 87% ± 4% of sinking POC on average, while the lateral vector (aged POC hosted on resuspended sediment) comprises the remainder (13%). The contribution of laterally supplied POC increases with depth and is composed of aged POC derived from several sources. The majority stems from sediment resuspension of the northeastern SCS slope, that entrains aged marine POC (dominant), fossil OC, and soil OC (least significant). The diminished proportion of fossil OC in deep basin sinking POC relative to Taiwan‐proximal areas is likely due to its dispersion via resuspension–deposition loops and dilution by POC stemming from modern surface ocean productivity during the delivery.

Distributions and relationships of virio- and picoplankton in the epi-, meso- and bathypelagic zones of the Amundsen Sea, West Antarctica during the austral summer.

Virioplankton and picoplankton are the most abundant marine biological entities on earth and mediate biogeochemical cycles in the Southern Ocean. However, understanding of their distribution and relationships with environmental factors is lacking. Here, we report on their distribution and relationships with environmental factors at 48 stations from 112.5° to 150°W and 67° to 75.5°S in the Amundsen Sea of West Antarctica. The epipelagic stations were grouped into four clusters based on the virio- and picoplankton composition and abundance. Clusters three and four, which were associated with the ice-edge blooms in the coastal and Amundsen Sea Polynya (ASP) areas, had high abundances of autotrophic picoeukaryotes; this resulted in subsequent high abundances of heterotrophic prokaryotes and viruses. Cluster two stations were in open oceanic areas, where the abundances of autotrophic and heterotrophic picoplankton were low. Cluster one stations were located between the areas of blooms and the oceanic areas, which had a low abundance of heterotrophic prokaryotes and picoeukaryotes and a high abundance of virioplankton. The abundance of viruses was significantly correlated with the abundances of autotrophic picoeukaryotes and Chl-a concentration in oceanic areas, although this reflected a time-lag with autotrophic picoeukaryote and heterotrophic prokaryotes abundances in ice-edge bloom areas. The upwelling of Circumpolar Deep Water (CDW) might have induced the high abundance of autotrophic picoeukaryotes in the epipelagic zone, and the sinking particulate organic carbon (POC) might have induced the high abundance of heterotrophic prokaryotes and virioplankton in the meso- and bathypelagic zones. This study shows that the summer distribution of virio- and picoplankton in the Amundsen Sea of West Antarctica was mainly controlled by upwelling of the CDW and the timing of ice-edge blooms.

C, N, and P Mass Balances in the Bottom Seawater–Surface Sediment Interface in the Reducing Environment due to Anoxic Water of Gamak Bay, Korea

Current mass balances of C, N, and P were estimated using a model (Fluxin = Fluxout + ΔFlux) from Gamak Bay, Korea, in August 2017, where eutrophication and reducing conditions are prevalent. To examine the current fluxes of particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP), sinking and re-floating sediment traps were deployed, a sediment oxygen demand (SOD) chamber experiment and ex-situ nutrient incubation experiment were conducted, and Fick’s first law of diffusion was applied. The principal component analysis and cluster analysis were performed to identify the three groups of water masses based on the characteristics of the bay, including the effects of the reducing environment due to the anoxic water mass using 14 bottom water quality parameters. In the reducing environment (sampling point GA4), the SOD20 flux was 3047.2 mg O2/m2/d. Additionally, the net sinking POC flux was 861.0 mg C/m2/d, while 131.8% of the net sinking POC flux (1134.5 mg C/m2/d) was removed toward the overlying water. This indicates that the organic matter that had been deposited was decomposed as a flux of 273.6 mg C/m2/d. The net sinking PON flux was 187.9 mg N/m2/d, whereas 15.8% of the net sinking PON flux was eluted, and 84.2% remained in the surface sediments. The dissolved inorganic nitrogen (DIN) elution flux from the surface sediments consisted of NH4+ elution (33.7 mg N/m2/d) and NOx− elution (−4.1 mg N/m2/d) fluxes. Despite the net sinking POP flux being 26.0 mg P/m2/d, the 47.7 mg P/m2/d of DIP elution flux (179.5% of the net sinking POP flux) was eluted to the overlying water. Similar to C mass balance, the additional elution flux occurred. Therefore, severe eutrophication (16.5 of the Okaichi eutrophication index) with the lowest N:P ratio (2.6) in GA4 was noted. This indicates that not only the freshly exported organic matter to the surface sediments but also the biochemical processes under anoxic conditions played an essential role as a remarkable nutrient source–particularly P–for eutrophication in Gamak Bay, Korea.

Microbes contribute to setting the ocean carbon flux by altering the fate of sinking particulates

Sinking particulate organic carbon out of the surface ocean sequesters carbon on decadal to millennial timescales. Predicting the particulate carbon flux is therefore critical for understanding both global carbon cycling and the future climate. Microbes play a crucial role in particulate organic carbon degradation, but the impact of depth-dependent microbial dynamics on ocean-scale particulate carbon fluxes is poorly understood. Here we scale-up essential features of particle-associated microbial dynamics to understand the large-scale vertical carbon flux in the ocean. Our model provides mechanistic insight into the microbial contribution to the particulate organic carbon flux profile. We show that the enhanced transfer of carbon to depth can result from populations struggling to establish colonies on sinking particles due to diffusive nutrient loss, cell detachment, and mortality. These dynamics are controlled by the interaction between multiple biotic and abiotic factors. Accurately capturing particle-microbe interactions is essential for predicting variability in large-scale carbon cycling.

Sediment Trap Study Reveals Dominant Contribution of Metazoans and Dinoflagellates to Carbon Export and Dynamic Impacts of Microbes in a Subtropical Marginal Sea

AbstractFlux and efficiency of carbon export in the ocean has been widely studied using sediment traps, but due to limitations of traditional methods, differential contribution of different organisms is poorly understood. Here, we used DNA metabarcoding to document taxonomic composition for sediment traps deployed at a continental shelf (C6) and a continental slope (C9) site in the South China Sea. The results indicated that metazoans (mainly copepods) and dinoflagellates were the most dominant contributors overall, while dinoflagellates, diatoms, and haptophytes dominated the photosynthetic assemblage. For prokaryotes, Gammaproteobacteria dominated all samples. Furthermore, comparing the trap flora to overlying plankton revealed that metazoans and Polycystinea from Eukaryota and Gammaproteobacteria and Bacteroidetes from Prokaryota were enriched in the trap at both stations whereas haptophytes and Dinophyceae were consistently attenuated during sinking. In addition, Oceanospirillaceae and Rhodobacteraceae exhibited negative correlations with dinoflagellates, and based on overlying plankton metatranscriptomes, were most transcriptionally active lineages for metabolizing labile and semi‐labile organic carbon, potentially contributing to the Martin's decay of sinking particulate organic carbon. Using DNA and RNA sequencing technologies, this study provides novel insights into the lineage‐specific contribution and dynamic impacts of microbes to carbon export in a subtropical marginal sea.

Lagrangian Observation of 234Th and Its Application in Constraining the Sinking of Particulate Organic Carbon on the Slope of the Northeastern South China Sea

The 234Th–238U disequilibrium has been widely used to quantify the sinking flux of particulate organic carbon (POC) out of the upper ocean. However, the influence of the advection on the quantification is poorly understood due to the lack of in situ measured physical parameters. Here, a Lagrangian observation was deployed for 39 h to track the variability of 234Th along with the current on the slope of the northeastern South China Sea (SCS). Contrasting to the general ocean interior, 234Th showed deficits relative to 238U in the mesopelagic waters, indicating an enhanced removal of 234Th. Concurrently, elevated total particulate matter (TPM) and POC contents were observed in the mesopelagic waters, supporting the driving force of the cross-shelf dispersion of re-suspended shelf/slope sediments for the 234Th removal. The widely used 234Th-model (ignoring physical processes) produced a much lower sinking flux of POC than the sediment trap-derived POC flux during the same observation, indicating an unneglectable influence of advection and diffusion. By considering the main horizontal advection and vertical diffusion, the 234Th–238U method gave rise to comparable results to sediment trap. 234Th-derived POC fluxes showed an increased pattern from 300 to 1,000 m, consistent with the more abundant POC where. These results indicated that advection represents an unneglectable process during the quantification of the sinking flux of 234Th over the slope of the SCS.

Partitioning the Export of Distinct Biogenic Carbon Pools in the Northeast Pacific Ocean Using a Biogeochemical Profiling Float

AbstractWe leverage observations from chemical and bio‐optical sensors mounted on a biogeochemical profiling float in the Northeast Pacific Ocean to quantify the cycling and export potential of distinct biogenic carbon pools, including particulate inorganic carbon (PIC), particulate organic carbon (POC), and dissolved organic carbon (DOC). Year‐round observations reveal complex carbon cycle dynamics among these carbon pools. Net DOC production peaked during bloom initiation, about 3 months prior to the summer peak in POC production. We validate the float estimates of DOC cycling with seasonal accumulation and removal rates derived from ship‐board DOC observations over the same period. By combining chemical and bio‐optical tracers of POC cycling, we estimate the instantaneous POC sinking flux (). The cooccurrence of DOC consumption and POC production and sinking during fall and winter resolves the regional conundrum of a persistent particle sinking flux observed by sediment traps during a season that is known to be heterotrophic. PIC production is small, and uncertainties are large. By combining float‐based estimates of instantaneous net primary production (NPP) and , we quantify a real‐time carbon export ratio ([/NPP] × 100%) for the euphotic zone. Elevated export ratios during summer are associated with an increase in the fraction of particles larger than 100 μm in size. Elevated export ratios during winter are associated with the physical redistribution of particles through seasonal deep mixing. Our study demonstrates how the combined use of multiple sensors on biogeochemical profiling floats can provide more nuanced information about upper ocean carbon cycle dynamics.

Heterogeneous viral contribution to dissolved organic matter processing in a long-term macrocosm experiment.

Viruses saturate environments throughout the world and play key roles in microbial food webs, yet how viral activities affect dissolved organic matter (DOM) processing in natural environments remains elusive. We established a large-scale long-term macrocosm experiment to explore viral dynamics and their potential impacts on microbial mortality and DOM quantity and quality in starved and stratified ecosystems. High viral infection dynamics and the virus-induced cell lysis (6.23-64.68% d-1) was found in the starved seawater macrocosm, which contributed to a significant transformation of microbial biomass into DOM (0.72-5.32μgL-1d-1). In the stratified macrocosm, a substantial amount of viral lysate DOM (2.43-17.87μgL-1d-1) was released into the upper riverine water, and viral lysis and DOM release (0.35-5.75μgL-1d-1) were reduced in the mixed water layer between riverine water and seawater. Viral lysis was stimulated at the bottom of stratified macrocosm, potentially fueled by the sinking of particulate organic carbon. Significant positive and negative associations between lytic viral production and different fluorescent DOM components were found in the starved and stratified macrocosm, indicating the potentially complex viral impacts on the production and utilization of DOM. Results also revealed the significant viral contribution to pools of both relatively higher molecular weight labile DOM and lower molecular weight recalcitrant DOM. Our study suggests that viruses have heterogeneous impact on the cycling and fate of DOM in aquatic environments.