Low Particulate Organic Carbon Research Articles

Distributions of nutrients, trace metals, phytoplankton composition, and elemental consumption in the Ross and Amundsen Seas

The Southern Ocean (SO) not only functions as an important carbon sink but also serves as a crucial source of intermediate and deep waters to lower latitudes. The Ross Sea and Amundsen Sea in the West Antarctic are highly productive regions of the Southern Ocean, critically vulnerable to climate change. Hence, comprehending the source-sink patterns of nutrients, particularly trace metals, in these regions is of paramount significance for understanding their ecological effect and the effect on elemental distribution in global ocean. Here, we reported the surface and vertical distributions of phytoplankton composition, Chlorophyll-a (Chl-a), particulate organic carbon (POC), macronutrients, and trace metals (TMs) in the Ross and Amundsen Seas. Deplete dissolved iron concentrations were observed in surface waters averaging 0.17 ± 0.09 nM. High POC and Chl-a were observed that corresponded to elevated Fe in shallow subsurface water (100–300 m), while low POC and Chl-a were found to associate with depleted iron (∼0.1 nM) throughout the upper water column down to 300 m. Sea-ice melting and islands were found to be possible external TMs sources in the Amundsen Sea offshore. Phytoplankton act as a sink for trace metals and regulate the elemental concentration and composition of water bodies, while also being influenced by environmental conditions such as temperature, salinity, and the mixing layer depth. Elemental consumptions in surface water were associated with diatom abundance in both Ross and Amundsen Seas. Based on elemental drawndown ratios in the surface mixed layer, the stoichiometries of the two major phytoplankton functional groups, diatoms and haptophytes, were estimated. Our observations offered insights into the relationships among iron supply, nutrients distribution patterns, and phytoplankton composition in the Southern Ocean.

Phytoplankton stoichiometry along the salinity gradient under limited nutrient and light supply.

Ongoing climate warming alters precipitation and water column stability, leading to salinity and nutrient supply changes in the euphotic zone of many coastal ecosystems and semi-enclosed seas. Changing salinity and nutrient conditions affect phytoplankton physiology by altering elemental ratios of carbon (C), nitrogen (N) and phosphorus (P). This study aimed to understand how salinity stress and resource acquisition affect phytoplankton stoichiometry. We incubated a phytoplankton polyculture composed of 10 species under different light, inorganic nutrient ratio and salinity levels. At the end of the incubation period, we measured particulate elemental composition (C, N and P), chlorophyll a and species abundances. The phytoplankton polyculture, dominated by Phaeodactylum tricornutum, accumulated more particulate organic carbon (POC) with increasing salinity. The low POC and low particulate C:N and C:P ratios toward 0psu suggest that the hypoosmotic conditions highly affected primary production. The relative abundance of different species varied with salinity, and some species grew faster under low nutrient supply. Still, the dominant diatom regulated the overall POC of the polyculture, following the classic concept of the foundation species.

Influence of Planting Techniques and Integrated Nutrient Management on Soil Health in Rice (Oryza sativa L.)

The study investigated the impact of different planting techniques and nitrogen management at Sardar Vallabhbhai Patel University of Agriculture & Technology, Meerut (UP) during the rainy (kharif) season of 2019-20 on soil health in rice. Soil samples were collected at two depths, air-dried, and analyzed for aggregate size distribution using wet sieving. Particulate organic carbon (POC) was determined through aggregate slaking, while total organic carbon (TOC) content was assessed using rapid titration. Microbial biomass carbon (MBC) was calculated by incubating soil samples, followed by fumigation-extraction methods. Results revealed that soil pH and electrical conductivity remained unaffected by planting techniques and fertility levels. Aggregate size distribution analysis revealed that the Furrow Irrigated Raised Bed (FIRB) treatment promoted larger macro-aggregates (>2mm) at varying soil depths, while Conventional Tillage (CT) encouraged smaller micro-aggregates. Reduced Tillage (RT) combined with Farmyard Manure (FYM) and chemical fertilizer increased POC and TOC content, emphasizing the importance of reduced soil disturbance and organic matter addition. Conversely, CT exhibited lower POC and TOC levels due to intensive soil disturbance. RT with FYM and chemical fertilizer significantly enhanced soil MBC, highlighting their role in improving soil health and fertility.

Spatiotemporal variability of organic carbon in streams and rivers of the Northern Hemisphere cryosphere

The earth's cryosphere is the outpost of climatic warming, which leads to rapid changes of organic carbon (OC) transport from terrestrial to aquatic ecosystems. OC in the cryosphere rivers plays vital roles in the carbon cycle and river ecosystem health. Yet, we still lack a comprehensive assessment of the spatiotemporal patterns of riverine OC across the Northern Hemisphere cryosphere. Here, we compiled OC concentration, radiocarbon (14C), and the specific ultraviolet absorbance (SUVA) of dissolved OC (DOC) at 254 nm data from 1007 unique sites, extracted from 138 published literature between 1972 and 2022. Overall, the average DOC and particulate OC (POC) concentrations are 6.34 and 2.61 mg C L−1, respectively, with the average age of DOC and POC being ~1100 and ~4300 years BP, respectively, indicating the release of aged carbon pools. Seasonal variations in DOC and POC concentrations, Δ14C-DOC and SUVA254 were observed, with distinct spatial variations closely linked to specific watershed characteristics. We found permafrost-impacted watersheds displayed significantly higher DOC concentrations, younger OC ages but lower POC concentrations compared to glacier-impacted watersheds. Meanwhile, in boreal forest watersheds, DOC is the most concentrated and youngest in varied ecoregions. Additionally, in permafrost regions characterized by higher permafrost extent, ground ice content, or lowlands with thick overburden cover, riverine DOC is more concentrated and aromatic. We estimated that specific OC fluxes in glacier rivers are higher than that in permafrost rivers (4.77 and 1.86 g C m−2 yr−1, respectively). Our results highlight the complex and variable spatiotemporal patterns of riverine OC in the northern cryosphere, which are essential for assessing the impact of OC on the global carbon cycle and climate warming.

Particulate organic carbon in the deep-water region of the Gulf of Mexico

Ocean eddies play a major role in lateral and vertical mixing processes of particulate organic carbon (POC), as well as in the transport of heat, salinity, and biogeochemical tracers. In the open waters of the Gulf of Mexico (GoM), however, there are limited observations to characterize how these mesoscale structures affect the spatial distribution of POC in the upper water column, which is important for organic matter cycling and export. We present the distribution patterns of POC relative to mesoscale features throughout the water column in the deep-water region of the GoM during three oceanographic cruises held during the summer months of 2015, 2016, and 2017. Samples were collected under well-stratified upper ocean conditions, which allowed us to assess the spatial and temporal distribution of POC as a function of non-steric sea surface height, density, apparent oxygen utilization, and chlorophyll fluorescence. We further explored the variability of integrated surface layer POC concentrations at stations located within the cores and the edges of cyclonic and anticyclonic eddies, and those collected outside these structures. Although our results indicate mesoscale eddies modulate several important physical and biogeochemical variables and POC concentrations in the upper ocean, these features do not fully explain the spatial distribution of POC concentrations throughout the deep-water region of the GoM. Relatively lower POC concentrations were observed in the border of the cyclonic and the center of the anticyclonic eddies, in contrast to the relatively higher POC concentrations at the center of the cyclonic and the border of anticyclonic eddies. We observed high variability in POC concentration variability outside mesoscale structures, which may be attributed to other processes such as upwelling over the shelves, and the contribution by rivers during the summer especially in the northern and southern GoM.

Export of particulate organic carbon (POC) in the eddy region of the tropical northwest Pacific

We examined particulate organic carbon (POC) export using 238U–234Th disequilibrium in the tropical northwest Pacific Ocean, where numerous eddies are present. We obtained data from an anticyclonic eddy in 2019 and from both anticyclonic and cyclonic eddies in 2020. In 2019, excess 234Th and higher POC concentrations were observed in the upper 100 m layer inside the anticyclonic eddy compared with the outer area of the eddy (the reference site). We speculate that the peculiar feature of excess 234Th in the surface layer was caused by horizontal transport of POC into the eddy and consequent POC degradation and release of particulate 234Th to a dissolved form. However, in 2020, lower POC concentrations with 234Th deficiency were observed in both cyclonic and anticyclonic eddies relative to the reference site. In both years, POC export was lower in the cores of the anticyclonic and cyclonic eddies relative to the reference site. We propose that severe nutrient depletion in the upper 150 m layer hindered nutrient supply by vertical water movement in the eddies. Despite the low POC export at 100 m depth, POC export at 500 m depth was comparable to values observed at 500 m depth at Station Papa in the more productive northeastern Pacific region. Our results imply that POC export into the deep ocean interior in this region may not be as low as expected from the low primary productivity in the euphotic zone.

Variability of particulate organic carbon and assessment of satellite retrieval algorithms over the eastern Arabian Sea.

Particulate organic carbon (POC) and its variability were studied to assess the accuracy of ocean colour retrieval algorithms over the eastern Arabian Sea (EAS) as it controls the carbon sequestration, oxygen minimum zone and biogeochemical (C, N and P) cycles. The seasonality in the physical and biological processes strongly influenced the distribution of POC along the EAS. Higher POC and chlorophyll a (chl a) during the spring inter monsoon (SIM) in the north EAS were due to detrainment bloom. The lower POC:chl a ratios during the winter monsoon (WM) (299.8 ± 190.8) than the SIM (482.1 ± 438.3) were due to the influence of freshly derived organic matter with high nutrient levels. The moderate coefficient of regression values of POC with chl a (R2 = 0.49, N = 59) suggests the importance of dead organic materials in controlling the POC distribution in the EAS. Validation between satellite and in situ POC using the four ocean colour retrieval algorithms showed that the algorithm based on the ratio of remote sensing reflectance (Rrs) performed better (R2 = 0.6, N = 17). It also showed a linear trend of POC with absorption coefficients suggesting it as an optical proxy for the POC retrieval.

Modelling the effects of copepod diel vertical migration and community structure on ocean carbon flux using an agent-based model

Fecal pellets are a significant component of total particulate organic carbon (POC) flux in the ocean and a key component of the biological pump. The effect of copepod community structure, feeding behavior, and diel vertical migration (DVM) on fecal pellet carbon flux was investigated using an agent-based model, which was applied to two contrasting open ocean regions: an oligotrophic (Hawaii Ocean Time-series site ALOHA) and a mesotrophic (Japanese time-series site K2) system. Data from the VERtical Transport in the Global Ocean (VERTIGO) project was used to parameterize the model, and modelled fecal pellet fluxes were compared to field-derived neutrally buoyant sediment trap data to examine how community structure and DVM impact fecal pellet carbon flux in the water column. The model produces copepod fecal pellet fluxes that are consistent with sediment trap data from K2 and ALOHA and with other modeling studies. The vertical distribution of copepods matching nighttime field data resulted in a greater magnitude of fecal pellet flux at all model depths compared to daytime distributions. Communities dominated by omnivores resulted in higher fecal pellet carbon flux and lower POC attenuation than those dominated by carnivores or detritivores. Incorporating DVM into the simulations produced lower fecal pellet fluxes in some cases, while also marginally improving the agreement between the modeled and observed median fecal pellet fluxes. The relatively simplified model presented here shows that an agent-based model that uses trait-based relationships for physiological rates can be used to give insight into the variability of particle flux in the deep ocean.

Experimental analysis of natural organic matter controls on nitrogen reduction during bank storage

Particulate organic carbon (POC) significantly influences nitrogen processes in riparian zones. However, the role of different types of natural organic matter (e.g., plant leaves and mud deposits) in nitrate reduction during the hydraulically driven mixing between rivers and groundwater within bank storage (BS) is not well known. Here, we used laboratory columns filled with 30 cm of riparian soil and buried with POC of varying quantity and quality (leaf litter, mud deposit, a mixture of both and a sediment control) on the soil surface in the column to investigate the effects of POC addition on nitrate reduction in low-permeable media. Pore waters were collected and measured periodically to compare the physicochemical differences among these treatments over time during scenarios of downward and upward flow. The leaf litter treatment had a larger amount of POC and higher reactivity, driving pore water to be in suboxic conditions with lower Eh and pH values. Nitrate reduction occurred immediately with downward surface water, and NO3− was removed in the POC buried layer. Due to the low POC content and low reactivity of the carbon source in the mud deposit, denitrification primarily occurred in the deeper sediment during the downwelling stage, as well as when groundwater returned to the POC buried layer with longer travel times. Both POC quantity and POC quality had strong effects on nitrate reduction. Our results suggested that the leaf litter treatment was preferential for nitrate reduction over the mud deposit treatment, with a higher NO3− reduction rate and less NH4+ accumulation during the complete BS process.

Improving satellite retrieval of oceanic particulate organic carbon concentrations using machine learning methods

Particulate organic carbon (POC) plays vital roles in marine carbon cycle, serving as a part of “biological pump” moving carbon to the deep ocean. The blue-to-green band ratio algorithm is applied operationally to derive POC concentrations in global oceans; it, however, tends to underestimate high values in optically complex waters. With an attempt to develop accurate and robust oceanic POC models, this study aimed to explore machine learning methods in satellite retrieval of POC concentrations. Three machine learning methods, i.e. extreme gradient boosting (XGBoost), support vector machine (SVM) and artificial neural network (ANN), were tested, and the recursive feature elimination (RFE) method was employed to identify sensitive features. Matchups of global in situ POC measurements and Ocean Colour Climate Change Initiative (OC-CCI) products were used to train and evaluate POC models. Results showed that machine learning methods produced obvious better performance than the blue-to-green band ratio algorithm, and XGBoost was the most robust among the tested three machine learning methods. However, the blue-to-green band ratio algorithm still worked well for clear open ocean waters with low POC, and ANN was more effective for optically complex waters with extremely high POC. This study provided globally applicable methods for satellite retrieval of POC concentrations, which should be helpful for studying POC dynamics in global oceans as well as in productive marginal seas.

Sea ice increases benthic community heterogeneity in a seagrass landscape

Sea ice plays an important role in subpolar seagrass meadows. It protects meadows against wave action and extreme temperatures. On the other hand, sea ice destroys seagrass leaves and removes plots of sediments and organics debris, leaving long-lasting ice-made tidal pools of various shapes and sizes within the meadow. The present study aimed at investigating the effect of sea ice on benthic community structure and biogeochemical processes in a subpolar seagrass meadow. Vegetated areas (V), artificially-created (aTP), and natural (nTP) tidal pools were sampled from April to October 2018 in a seagrass meadow located at Manicouagan Peninsula (Québec; 49°5′36″N, 68°12′44″W). aTP and nTP showed similar sediment characteristics with coarser sediment and lower particulate organic carbon and total nitrogen content but also lower NOx and higher NH4+ and PO43− porewater concentrations as compared to V. Benthic macrofauna communities showed a strong seasonality with very reduced total density, biomass and species richness during wintertime (from December to April) relatively to summertime (from June to September). Benthic macrofauna communities were also more diversified and abundant in V than in aTP and nTP. Species assemblages in aTP and nTP represented a subset of species assemblages in V with any species found exclusively in tidal pools. However, total biomass was similar among treatments, suggesting that tidal pools sheltered larger individuals than vegetated areas. These results underline the importance of considering the spatial heterogeneity of seagrass meadows when assessing the functioning of these ecosystems.

Decoupling of ΔO<sub>2</sub>∕Ar and particulate organic carbon dynamics in nearshore surface ocean waters

Abstract. We report results from two Lagrangian drifter surveys off the Oregon coast, using continuous shipboard sensors to estimate mixed-layer gross primary productivity (GPP), community respiration (CR), and net community production (NCP) from variations in biological oxygen saturation (ΔO2∕Ar) and optically derived particulate organic carbon (POC). At the first drifter survey, conducted in a nearshore upwelling zone during the development of a microplankton bloom, net changes in ΔO2∕Ar and [POC] were significantly decoupled. Differences in GPP and NCP derived from ΔO2∕Ar (NCPO2/Ar) and POC (NCPPOC) time series suggest the presence of large POC losses from the mixed layer. At this site, we utilized the discrepancy between NCPO2/Ar and NCPPOC, and additional constraints derived from surface water excess nitrous oxide (N2O), to evaluate POC loss through particle export, DOC production, and vertical mixing fluxes. At the second drifter survey, conducted in lower-productivity, density-stratified offshore waters, we also observed offsets between ΔO2∕Ar and POC-derived GPP and CR rates. At this site, however, net [POC] and ΔO2∕Ar changes yielded closer agreement in NCP estimates, suggesting a tighter relationship between production and community respiration, as well as lower POC loss rates. These results provide insight into the possibilities and limitations of estimating productivity from continuous underway POC and ΔO2∕Ar data in contrasting oceanic waters. Our observations support the use of diel POC measurements to estimate NCP in lower-productivity waters with limited vertical carbon export and the potential utility of coupled O2 and optical measurements to estimate the fate of POC in high-productivity regions with significant POC export.

Carbon sequestration and mineralization in soil aggregates under long-term conservation tillage in the North China Plain

Understanding the process of soil organic carbon (SOC) sequestration and mineralization in aggregates is pertinent to mitigate climate change and minimize risks of soil degradation. Thus, soil samples were obtained after a 10-year field experiment to identify the influences of tillage on aggregate-associated SOC sequestration and mineralization in the North China Plain (NCP). Four tillage practices investigated were as follows: no-till with straw retention (NTS, conservation tillage), rotary tillage with straw incorporation (RTS), conventional tillage with straw incorporation (CTS), and conventional tillage with straw removal (CT). Significantly negative correlations were observed between SOC concentration and potentially mineralized carbon in aggregates under different treatments for the 0–10 cm soil layer. The large macro-aggregates (>2 mm) with the highest proportion of size distribution represented the major pool of SOC stock (47.3–51.2%) and mineralization amount (38.2–43.6%) in the 0–30 cm layer, followed by that in the small macro-aggregates (0.25–2 mm), regardless of tillage practices. However, the mineralization quotient (mineralization per unit SOC concentration) of macro-aggregates (>0.25 mm) was lower than that for the other size classes. The NTS enhanced the macro-aggregate formation in the 0–20 cm layer and associated SOC concentration in the 0–10 cm layer. Furthermore, NTS decreased total potential mineralization in the 0–30 cm layer compared with the other tillage practices, attributed to decrease in the large macro-aggregates (30.0–51.4%) with low particulate organic carbon (POC) concentration. The NTS with low straw inputs had higher incremental efficiency with straw incorporation than that in the RTS and CTS by 45.0% and 13.5%, respectively (P < 0.05). Overall, the higher proportion of macro-aggregates recorded under NTS decreased carbon mineralization, and consequently, increased incremental efficiency with straw incorporation, and improved SOC sequestration in the surface soil layer in the NCP.

Satellite Observations of the Diurnal Dynamics of Particulate Organic Carbon in Optically Complex Coastal Oceans: The Continental Shelf Seas of China

AbstractThe continental shelf seas of China (CSSC) broadly encompass the Bohai Sea, the Yellow Sea, and the East China Sea and exhibit highly variable optical properties. Accurate satellite estimates of particulate organic carbon (POC) remain challenging because optimal proxies for remotely sensed POC are largely obscure in these optically complex coastal waters. In this study, optical and biogeochemical data, including the particulate beam attenuation coefficient (cp), particulate backscattering coefficient (bbp), remote sensing reflectance (Rrs), POC, total suspended matter (TSM), and chlorophyll‐a (Chla), were collected over multiple seasons and years in the CSSC. We first classified the study area into three different water types with three different POC retrieval proxies: the TSM for high‐TSM waters, Chla for low‐TSM waters, and Rrs ratio (Rrs(490)/Rrs(555)) for moderate‐TSM waters. A composite POC algorithm using these three optimal proxies was then developed for Geostationary Ocean Color Imager (GOCI) satellite data (hereafter called the POC_CSSC algorithm). The validation results indicated that the accuracy of GOCI‐derived POC was greatly improved with a mean relative error of 32.08%. Application of the POC_CSSC algorithm to GOCI data over a tidally impacted estuary demonstrated the robustness of the algorithm and that tides play different roles in the broad CSSC. More specifically, tides have the strongest influence on nearshore estuarine waters, regulating the progression of high‐POC water masses from estuary to offshore environments, while offshore waters were the least influenced by tides with less variable, low POC concentrations.

Variation of carbon contents in eelgrass ( Zostera marina) sediments implied from depth profiles.

Seagrass meadows are able to store significant amounts of organic carbon in their underlying sediment, but global estimates are uncertain partly owing to spatio-temporal heterogeneity between and within areas and species. In order to provide robust estimates, there is a need to better understand the fate of, and mechanisms behind, organic carbon storage. In this observational study, we analyse a suite of biotic and abiotic parameters in sediment cores from 47 different eelgrass ( Zostera marina) beds spanning the distributional range of the Northern Hemisphere. Depth profiles of particulate organic carbon (POC) revealed three patterns of vertical distribution where POC either increased, decreased or showed no pattern with sediment depth. These categories exhibited distinct profiles of δ13C and C:N ratios, where high POC profiles had a proportionally larger storage of eelgrass-derived material whereas low POC profiles were dominated by phytoplanktonic and macroalgal material. However, high POC did not always translate into high carbon density. Nevertheless, this large-scale dataset provides evidence that the variability in organic matter source in response to natural and anthropogenic environmental changes affects the potential role of eelgrass beds as POC sinks, particularly where eelgrass decline is observed.

The export flux of particulate organic carbon derived from &amp;lt;sup&amp;gt;210&amp;lt;/sup&amp;gt;Po∕&amp;lt;sup&amp;gt;210&amp;lt;/sup&amp;gt;Pb disequilibria along the North Atlantic GEOTRACES GA01 transect: GEOVIDE cruise

Abstract. The disequilibrium between 210Po activity and 210Pb activity in seawater samples was determined along the GEOTRACES GA01 transect in the North Atlantic during the GEOVIDE cruise (May–June 2014). A steady-state model was used to quantify vertical export of particulate 210Po. Vertical advection was incorporated into one version of the model using time-averaged vertical velocity, which had substantial variance. This resulted in large uncertainties for the 210Po export flux in this model, suggesting that those calculations of 210Po export fluxes should be used with great care. Despite the large uncertainties, there is no question that the deficits of 210Po in the Iberian Basin and at the Greenland Shelf have been strongly affected by vertical advection. Using the export flux of 210Po and the particulate organic carbon (POC) to 210Po ratio of total (&gt; 1 µm) particles, we determined the POC export fluxes along the transect. Both the magnitude and efficiency of the estimated POC export flux from the surface ocean varied spatially within our study region. Export fluxes of POC ranged from negligible to 10 mmol C m−2 d−1, with enhanced POC export in the Labrador Sea. The cruise track was characterized by overall low POC export relative to net primary production (export efficiency &lt; 1 %–15 %), but relatively high export efficiencies were seen in the basins where diatoms dominated the phytoplankton community. The particularly low export efficiencies in the Iberian Basin, on the other hand, were explained by the dominance of smaller phytoplankton, such as cyanobacteria or coccolithophores. POC fluxes estimated from the 210Po∕210Pb and 234Th∕238U disequilibria agreed within a factor of 3 along the transect, with higher POC estimates generally derived from 234Th. The differences were attributed to integration timescales and the history of bloom events.

Key role of bacteria in the short‐term cycling of carbon at the abyssal seafloor in a low particulate organic carbon flux region of the eastern Pacific Ocean

AbstractThe cycling of carbon (C) by benthic organisms is a key ecosystem function in the deep sea. Pulse‐chase experiments are designed to quantify this process, yet few studies have been carried out using abyssal (3500–6000 m) sediments and only a handful of studies have been undertaken in situ. We undertook eight in situ pulse‐chase experiments in three abyssal strata (4050–4200 m water depth) separated by tens to hundreds of kilometers in the eastern Clarion‐Clipperton Fracture Zone (CCFZ). These experiments demonstrated that benthic bacteria dominated the consumption of phytodetritus over short (~ 1.5 d) time scales, with metazoan macrofauna playing a minor role. These results contrast with the only other comparable in situ abyssal study, where macrofauna dominated phytodetritus assimilation over short (2.5 d) time scales in the eutrophic NE Atlantic. We also demonstrated that benthic bacteria were capable of converting dissolved inorganic C into biomass and showed that this process can occur at rates that are as high as the bacterial assimilation of algal‐derived organic C. This demonstrates the potential importance of inorganic C uptake to abyssal ecosystems in this region. It also alludes to the possibility that some of the C incorporation by bacteria in our algal‐addition studies may have resulted from the incorporation of labeled dissolved inorganic carbon initially respired by other unstudied organisms. Our findings reveal the key importance of benthic bacteria in the short‐term cycling of C in abyssal habitats in the eastern CCFZ and provide important information on benthic ecosystem functioning in an area targeted for commercial‐scale, deep‐sea mining activities.

Natural spatial variability of depositional conditions, biogeochemical processes and element fluxes in sediments of the eastern Clarion-Clipperton Zone, Pacific Ocean

The manganese nodule belt within the Clarion and Clipperton Fracture Zones (CCZ) in the abyssal NE Pacific Ocean is characterized by numerous seamounts, low organic matter (OM) depositional fluxes and meter-scale oxygen penetration depths (OPD) into the sediment. The region hosts contract areas for the exploration of polymetallic nodules and Areas of Particular Environmental Interest (APEI) as protected areas. In order to assess the impact of potential mining on these deep-sea sediments and ecosystems, a thorough determination of the natural spatial variability of depositional and geochemical conditions as well as biogeochemical processes and element fluxes in the different exploration areas is required.Here, we present a comparative study on (1) sedimentation rates and bioturbation depths, (2) redox zonation of the sediments and element fluxes as well as (3) rates and pathways of biogeochemical reactions at six sites in the eastern CCZ. The sites are located in four European contract areas and in the APEI3. Our results demonstrate that the natural spatial variability of depositional and (bio)geochemical conditions in this deep-sea sedimentary environment is much larger than previously thought. We found that the OPD varies between 1 and 4.5 m, while the sediments at two sites are oxic throughout the sampled interval (7.5 m depth). Below the OPD, manganese and nitrate reduction occur concurrently in the suboxic zone with pore-water Mn2+ concentrations of up to 25 µM. The thickness of the suboxic zone extends over depth intervals of less than 3 m to more than 8 m. Our data and the applied transport-reaction model suggest that the extension of the oxic and suboxic zones is ultimately determined by the (1) low flux of particulate organic carbon (POC) of 1–2 mg Corg m−2 d−1 to the seafloor, (2) low sedimentation rates between 0.2 and 1.15 cm kyr−1 and (3) oxidation of pore-water Mn2+ at depth. The diagenetic model reveals that aerobic respiration is the main biogeochemical process driving OM degradation. Due to very low POC fluxes of 1 mg Corg m−2 d−1 to the seafloor at the site investigated in the protected APEI3 area, respiration rates are twofold lower than at the other study sites. Thus, the APEI3 site does not represent the (bio)geochemical conditions that prevail in the other investigated sites located in the European contract areas. Lateral variations in surface water productivity are generally reflected in the POC fluxes to the seafloor across the various areas but deviate from this trend at two of the study sites. We suggest that the observed spatial variations in depositional and (bio)geochemical conditions result from differences in the degree of degradation of OM in the water column and heterogeneous sedimentation patterns caused by the interaction of bottom water currents with seafloor topography.

Long-term variation of mesopelagic biogenic flux in the central South China Sea: Impact of monsoonal seasonality and mesoscale eddy

The East Asian Monsoon and mesoscale eddies are known to regulate primary production in South China Sea (SCS), the largest tropical marginal sea; however, their contributions to the deep biogenic flux are yet to be quantified. Based on 7-year time series sediment trap observations at the depth of 1200m in the central SCS, we used the monthly average sinking biogenic fluxes to evaluate the impact of the monsoon and mesoscale cyclonic eddies on biogenic fluxes in combination with remote sensing physical parameters. The monthly average particulate organic carbon (POC) and opal fluxes, ranging from 3.0 to 5.2 and 14.8–34.9mgm−2d−1, respectively, were higher during the northeastern monsoon period. This corresponded to the deeper mixed layer depth and higher net primary production in this area, due to nutrient replenishment from the subsurface induced by monsoon transition and surface cooling. In contrast, lower POC and opal fluxes occurred during well-stratified inter-monsoon periods. In addition, CaCO3 flux (23.6–37.0mgm−2d−1) exhibited less seasonality and was assumed to originate from foraminifera. In terms of the long-term record, the combined effect of cyclonic eddies and mixing in the upper ocean could effectively regulate the temporal variation in the biogenic flux. In particular, the opal and POC fluxes in cyclonic eddies were 116% and 41% higher on average, respectively, than those during the non-cyclonic eddy period. Since the cyclonic eddies mainly occurred during the northeastern monsoon period, their contributions to biogenic flux via diatom blooms might overlap the regular winter flux peak, which could make the biological carbon pump more efficient at CO2 sequestration during this period thus amplifying the impact of seasonal transition.

Nestedness and species replacement along bathymetric gradients in the deep sea reflect productivity: a test with polychaete assemblages in the oligotrophic north‐west Gulf of Mexico

AbstractAimTo test the hypothesis that low productivity drives the nestedness component of β‐diversity in polychaetes at the deep‐sea floor.LocationWestern Gulf of Mexico.MethodsWe used A. Baselga's (2010, Global Ecology and Biogeography, 19, 134–143) metrics of species replacement and nestedness to assess their overall significance with differences in depth and particulate organic carbon (POC) flux to the seafloor. We used M. A. Rodríguez‐Gironés &amp; L. Santamaría's (2006, Journal of Biogeography, 33, 924–935) BINMATNEST to calculate the significance and direction of nestedness with depth and POC flux.ResultsNestedness was the most significant part of β‐diversity with depth and POC flux. The rank order of nestedness increased significantly with increasing depth, and decreased significantly with increasing POC flux. There was little endemism below 2000 m.Main conclusionThe polychaete fauna in the deepest western Gulf of Mexico is largely a nested subset of the shallower upper to mid‐bathyal fauna. While the causes of β‐diversity in the deep sea are undoubtedly multivariate, the relative importance of turnover and nestedness appears to be modulated by productivity in the form of POC flux to the seafloor. Under circumstances of extremely low POC flux and animal density in the deepest reaches of the Gulf, nestedness may be caused by some populations being maintained by immigration from larger populations upslope, or by biogeographic filtering for tolerance to abyssal conditions. Resources at great depths in regions of exceptionally low productivity may be too limited to permit adaptation resulting in endemism and continued downslope turnover. If productivity helps to explain the nestedness part of β‐diversity, this may lead to a more unified theory of deep‐sea biodiversity.