POC Flux Research Articles

Are all sediment traps created equal? An intercomparison study of carbon export methodologies at the PAP-SO site

Sinking particulate flux out of the upper ocean is a key observation of the ocean’s biological carbon cycle. Particle flux in the upper mesopelagic is often determined using sediment traps but there is no absolute standard for the measurement. Prior to this study, differing neutrally-buoyant sediment trap designs have not been deployed simultaneously, which precludes meaningful comparisons between flux data collected using these designs.The aim of the study was to compare a suite of modern methods for measuring sinking carbon flux out of the surface ocean. This study compared samples from two neutrally buoyant drifting sediment trap designs, and a surface tethered drifting sediment trap, which collected sinking particles alongside other methods for sampling particle properties, including in situ pumps and 234Th radionuclide measurements. Samples were collected at the Porcupine Abyssal Plain Sustained Observatory (PAP-SO) site in the Northeast Atlantic Ocean (49°N, 16.5°W).Neutrally-buoyant conical traps appeared to collect lower absolute fluxes than neutrally-buoyant, or surface-tethered cylindrical traps, but compositional ratios of sinking particles indicated collection of similar material when comparing the conical and cylindrical traps. In situ pump POC:234Th ratios generally agreed with trap ratios but conical trap samples were somewhat depleted in 234Th, which along with sinking particle size distribution data determined from gel traps, may imply under-sampling of small particles. Cylindrical trap POC fluxes were of similar magnitude to 234Th-derived POC fluxes while conical POC fluxes were lower. Further comparisons are needed to distinguish if differences in particle flux magnitude are due to conical versus cylindrical trap designs. Parallel analytical determinations, conducted by different laboratories, of replicate samples for elemental fluxes and gel trap particle size distributions were comparable. This study highlights that the magnitude of particle fluxes and size spectra may be more sensitive than the chemical composition of particle fluxes to the instrumentation used. Only two deployments were possible during this study so caution should be taken when applying these findings to other regions and export regimes. We recommend that multiple methodologies to measure carbon export should be employed in field studies, to better account for each method’s merits and uncertainties. These discrepancies need further study to allow carbon export fluxes to be compared with confidence across laboratory, region and time and to achieve an improved global understanding of processes driving and controlling carbon export.

Riverine export of water, sediment and carbon during flood events in the arid to semi‐arid Wuding River on the Chinese Loess Plateau

AbstractFloods have become increasingly important in fluvial export of water, sediment and carbon (C). Using high‐frequency sampling, the export of water, sediment and C was examined in the Wuding River catchment on the Chinese Loess Plateau. With groundwater as an important contributor to runoff all year round, floods were relatively less important in the export of water. However, large floods were disproportionately important in exporting sediment and inorganic C (DIC) and organic C (DOC and POC). The three largest floods in each year transported 53.6–97.3 and 41.4–77% of the annual sediment and C fluxes, respectively. An extreme flood in 2017 alone contributed 94.6 and 73.1% of the annual sediment and C fluxes, respectively, in just 7 days, which included 20.3, 92.1 and 35.7% of the annual DOC, POC and DIC fluxes, respectively. A stable carbon isotope (δ13C) analysis of POC indicated that modern soils and C3 plants were its primary source. Furthermore, floods greatly accelerated CO2 degassing due to elevated gas transfer velocity, although stream water CO2 partial pressure (pCO2) exhibited a decreasing trend with flow discharge. Although these results illustrated that increasing runoff diluted pCO2, the timing and magnitude of floods were found to be critical in determining the response of pCO2 to flow dynamics. Low‐magnitude floods in the early wet season increased pCO2 because of enhanced organic matter input, while subsequent large floods caused a lower pCO2 due to greatly reduced organic matter supply. Finally, continuous monitoring of a complete flood event showed that the CO2 efflux during the flood (2348 ± 664 mg C m–2 day–1) was three times that under low‐flow conditions (808 ± 98 mg C m–2 day–1). Our study suggests that infrequent, heavy storm events, which are predicted to increase under climate change, will greatly alter the transport regimes of sediment and C. © 2020 John Wiley & Sons, Ltd.

Benthic megafauna assemblage change over three decades in the abyss: Variations from species to functional groups

Megafaunal seafloor assemblages on the Monterey Fan in the NE Pacific (Station M, 4000 m depth) were studied between 2006–2018 using remotely operated vehicles (ROVs) as part of a continuing time-series study that began in 1989. Since 2006 we identified nearly 120,000 individual animals representing over 142 morphospecies, and observed continuous changes in the megafaunal assemblage. This study, which tracked variation in observed morphospecies over a 13-year period, is one of the most detailed long-term records of megafaunal change for abyssal depths. Our investigation shows that new variations continued to emerge, reinforcing the concept that the deep-sea is dynamic over short time scales, rather than static over long periods. Some species were uncommon, but later observed in high numbers, then decreased to very low or undetectable levels (e.g. Elpidia sp. A), while others (e.g. Psychropotes longicauda) exhibited a relatively persistent presence with less fluctuation in abundance. Decreasing total echinoderm density from 2013–2018 did not correspond with the continued occurrence of large episodic POC flux events between 2016–2018. This may be attributed to the quality of food supply arriving at the seafloor and the varied ability of organisms to utilize it. Long-term tracking (30 years) of 10 specific epibenthic echinoderm species originally quantified from camera-sled images shows a pattern of assemblage structure, perhaps returning toward the composition observed in the 1990s and early 2000s. Many questions remain as to how this abyssal site and others will change with continued, and potentially increasing, anthropogenic change in the upper ocean. For example, the marine heat anomaly known as the ‘Warm Blob’ may have influenced major ecological processes at the abyssal seafloor in terms of morphospecies and functional group composition due to changes in POC flux. The degree of dynamism continues to indicate that ad hoc or short-term investigations provide a limited perspective for assessing community structure in conservation or resource exploitation impact assessment studies in the deep sea.

Seafloor biodiversity of Canada's three oceans: Patterns, hotspots and potential drivers

AbstractAimWe examined the relationships between bathymetry, latitude and energy and the diversity of marine benthic invertebrates across wide environmental ranges of Canada's three oceans.LocationCanadian Pacific, Arctic and Atlantic Oceans from the intertidal zone to upper bathyal depths, encompassing 13 marine ecoregions.MethodsWe compiled 35 benthic datasets that encompass 3,337 taxa (70% identified to species and 21% to genus) from 13,172 samples spanning 6,117 sites. Partitioning the analyses by different gear types, ecoregions or sites, we used Hill numbers to examine spatial patterns in α‐diversity. We used resampling and extrapolation to standardized sampling effort and examined the effects of depth, latitude, chemical energy (export particulate organic carbon [POC] flux), thermal energy (bottom temperature) and seasonality of primary production on the benthic biodiversity.ResultsThe Canadian Arctic harboured the highest benthic diversity (e.g. epifauna and common and dominant infauna species), whereas the lowest diversity was found in the Atlantic. The Puget Trough (Pacific), Beaufort Sea, Arctic Archipelago, Hudson Bay, Northern Labrador and Southern Grand Bank (Atlantic) were the “hotspots" of diversity among the ecoregions. The infauna and epifauna both exhibited hump‐shaped diversity–depth relationships, with peak diversity near shelf breaks; latitude (positively) predicted infaunal diversity, albeit weakly. Food supply, as inferred from primary production and depth, was more important than thermal energy in controlling diversity patterns. Limitations with respect to calculating POC flux in coastal (e.g. terrestrial runoff) and ice‐covered regions or biological interactions may explain the negative POC flux–infaunal diversity relationship.Main ConclusionsWe show previously unreported diversity hotspots in the Canadian Arctic and in other ecoregions. Our analyses reveal potential controlling mechanisms of large‐scale benthic biodiversity patterns in Canada's three oceans, which are inconsistent with the prevailing view of seafloor energy–diversity relationships. These results provide insightful information for conservation that can help to implement further MPA networks.

Changes in riverine organic carbon input to the ocean from mainland China over the past 60 years

Compared to rivers in Europe and North America, Chinese rivers that discharge into oceans have different organic carbon (OC) transport characteristics. Out of the top 25 largest rivers worldwide, three (Changjiang, Huanghe, and Zhujiang rivers) are located in China, along with numerous small rivers. Thus, synthesized estimates of total riverine OC flux from Chinese rivers into marginal seas are critical but remain deficient. In this study, we developed relationships between riverine OC (dissolved OC, or DOC, and particulate OC, or POC) and basin characteristic variables (basin population density, precipitation, and riverine suspended sediment concentration) to estimate annual riverine DOC and POC fluxes during 1953–2016. The results showed that rivers in mainland China transported 9.63 Tg C of OC to the marginal seas in 2008, with 4.61 Tg C of DOC and 5.02 Tg C of POC. Of this transported OC, 14.28% DOC and 17.49% POC were transported by small southeastern rivers, whose drainage areas covered only 6.68% of the total. Because of intensifying human activities, DOC export increased but POC export decreased during 1953–2016. Additionally, basin population growth and reservoir water capacity were the major factors for increasing DOC flux and decreasing POC flux, respectively. Overall, the DOC/POC ratio increased for OC transport in Chinese rivers. Therefore, this study is important for understanding human-induced impacts on environmental change and the carbon cycle in marginal seas.

Autonomous Multi-Trophic Observations of Productivity and Export at the Australian Southern Ocean Time Series (SOTS) Reveal Sequential Mechanisms of Physical-Biological Coupling

The timing of pelagic spring blooms has received attention to understand controls on open ocean productivity and its potential responses to climate change. Many studies have relied on surface chlorophyll (Chl) to define bloom initiation because of its availability from satellite observations, but this has limited utility because it ignores the full water column budget and because biomass represents only the small residual term in the balance between production and loss. Additional important measures include net community production (NCP) which determines maximal energy available to fuel phytoplankton and higher trophic level biomass accumulations, and particulate organic carbon export (POC flux) which determines the distribution of this energy across pelagic, mesopelagic and benthic communities. Here we present high temporal resolution records for the winter to spring transition (July-December 2012) obtained from moored sensors at SOTS in the Subantarctic Zone (SAZ) south of Australia. Measurements included physical drivers (temperature, salinity, surface mixed layer depth, currents, wind speeds, insolation, and air-sea heat fluxes) and biological responses (Chl from fluorescence and light attenuation, NCP from O2/N2 ratios and nutrient concentrations from an autonomous water sampler, POC flux from sediment traps, and zooplankton abundances from four-frequency acoustic backscatter profiles). These observations provide a phenology across the four trophic levels (NPZD) commonly used in ocean biogeochemical models. Chl column inventories began to increase in early winter while mixed layers were still deepening, and were accompanied by increases in net community production. Acoustic metrics for grazing pressure were very low at this time. In contrast, surface Chl did not increase until later when stratification developed. The levels of spring net community production were relatively high and balanced by sinking particle fluxes close to global median values, despite the relatively low surface biomass levels. Overall this phenology suggests that the extent of exchange with SAMW waters via deep mixing is a key driver of the seasonality of production, support of higher trophic levels, and the mediation of pelagic-benthic coupling, and occurs sequentially via trophodynamic (de-coupling of production and grazing) and physical (stratification) mechanisms.

Hydro-ecological controls on riverine organic carbon dynamics in the tropical monsoon region

Transport fluxes and properties of riverine organic carbon in the tropical monsoon region were the vital parameters in the global riverine organic carbon fluxes budget. The study focused on the riverine organic carbon in the Changhuajiang River (CHJR), locating at the mid-west of the Hainan Island, China. Dissolved organic carbon (DOC) concentrations in the CHJR ranged from 0.22 mg/L to 11.75 mg/L with an average of 1.75 mg/L, which was lower than the average of global rivers and had a significantly temporal and spatial variation. Output flux of riverine DOC was calculated as 0.55 t/km2/y, which could be revised up to 1.03 t/km2/y, considering that the riverine discharge before dam construction. A linear model of riverine DOC flux suitable in CHJR basin was established, which involved the factors, such as soil organic carbon, runoff depth and slope, etc. There was a large variation of POC concentrations in the CHJR where the average POC concentration in the dry season was 2.41 times of the wet season. Riverine POC flux in CHJR basin was calculated as 1.78 t/km2/y, higher than the average of global rivers and far lower than those in other domestic larger rivers. About 8.28 × 103 t POC were exported yearly in CHJR, of which, 7.15 × 103 t originated from terrestrial ecosystem and 1.13 × 103 t stemmed from aquatic ecosystem. Meanwhile, about 87.74% of terrestrial source happened in the wet season and 12.26% in the dry season. This research revealed that the riverine organic carbon mainly stemmed from the surface erosion processes in the drainage basin during the wet season.

Role of Tidal Wetland Stability in Lateral Fluxes of Particulate Organic Matter and Carbon

AbstractTidal wetland fluxes of particulate organic matter and carbon (POM, POC) are important terms in global budgets but remain poorly constrained. Given the link between sediment fluxes and wetland stability, POM and POC fluxes should also be related to stability. We measured POM and POC fluxes in eight microtidal salt marsh channels, with net POM fluxes ranging between −121 ± 33 (export) and 102 ± 28 (import) g OM·m−2·year−1 and net POC fluxes ranging between −52 ± 14 and 43 ± 12 g C·m−2·year−1. A regression employing two measures of stability, the unvegetated‐vegetated marsh ratio (UVVR) and elevation, explained >95% of the variation in net fluxes. The regression indicates that marshes with lower elevation and UVVR import POM and POC while higher elevation marshes with high UVVR export POM and POC. We applied these relationships to marsh units within Barnegat Bay, New Jersey, USA, finding a net POM import of 2,355 ± 1,570 Mg OM/year (15 ± 10 g OM·m−2·year−1) and a net POC import of 1,263 ± 632 Mg C/year (8 ± 4 g C·m−2·year−1). The magnitude of this import was similar to an estimate of POM and POC export due to edge erosion (−2,535 Mg OM/year and − 1,291 Mg C/year), suggesting that this system may be neutral from a POM and POC perspective. In terms of a net budget, a disintegrating wetland should release organic material, while a stable wetland should trap material. This study quantifies that concept and demonstrates a linkage between POM/POC flux and geomorphic stability.

Geomorphological controls on fluvial carbon storage in headwater peatlands

AbstractGeomorphological controls and catchment sediment characteristics control the formation of floodplains and affect their capacity to sequester carbon. Organic carbon stored in floodplains is typically a product of pedogenic development between periods of mineral sediment deposition. However, in organically‐dominated upland catchments with a high sediment load, eroded particulate organics may also be fluvially deposited with potential for storage and/or oxidation. Understanding the redistribution of terrestrial carbon laterally, beyond the bounds of river channels is important, especially in eroding peatland systems where fluvial particulate organic carbon exports are often assumed to be oxidised. Floodplains have the potential to be both carbon cycling hotspots and areas of sequestration. Understanding of the interaction of carbon cycling and the sediment cascade through floodplain systems is limited.This paper examines the formation of highly organic floodplains downstream of heavily eroded peatlands in the Peak District, UK. Reconstruction of the history of the floodplains suggests that they have formed in response to periods of erosion of organic soils upstream. We present a novel approach to calculating a carbon stock within a floodplain, using XRF and radiograph data recorded during Itrax core scanning of sediment cores. This carbon stock is extrapolated to the catchment scale, to assess the importance of these floodplains in the storage and cycling of organic carbon in this area. The carbon stock estimate for the floodplains across the contributing catchments is between 3482‐13460 tonnes, equating on an annualised basis to 0.8‐4.5% of the modern‐day POC flux. Radiocarbon analyses of bulk organic matter in floodplain sediments revealed that a substantial proportion of organic carbon was associated with re‐deposited peat and has been used as a tool for organic matter source determination. The average age of these samples (3010 years BP) is substantially older than Infrared Stimulated Luminesence dating which demonstrated that the floodplains formed between 430 and 1060 years ago. Our data suggest that floodplains are an integral part of eroding peatland systems, acting as both significant stores of aged and eroded organic carbon and as hotspots of carbon turnover. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Biofloat observations of a phytoplankton bloom and carbon export in the Drake Passage

Prolonged in-situ biogeochemical observations have historically been limited in the high nitrate, low chlorophyll (HNLC) regions of the Southern Ocean (SO). Recently, large biofloat missions, including the SO Carbon and Climate Observations and Modelling (SOCCOM) program, are filling the data void. However, the standard ten-day float profiling cycle often lacks the temporal resolution required to resolve mesoscale phytoplankton bloom or export processes. This is particularly important in regions of the SO dominated by high energy mesoscale features, like the Drake Passage. In this study, we observe a naturally occurring phytoplankton bloom and subsequent carbon export event in the Drake Passage using in-situ data from an Autonomous Profiling EXplorer (APEX) biofloat that profiled the water column every two days. The fast, two-day profiling cycle meant that the biofloat's trajectory was coherent with mesoscale processes. However, because of the quasi-Lagrangian nature of the biofloat, we could not control for both spatial and temporal changes simultaneously. Therefore we explore this quasi-Lagrangian dataset as both a spatial and temporal series to understand the mesoscale processes as they relate to the observed phytoplankton bloom and its subsequent export. Our evidence suggests that eddy-driven subduction and a change in the vertical structure were important in the time period leading up to enhanced carbon export. Recent studies have shown that eddy-driven subduction may be responsible for ~20% of the total biological carbon pump in the SO. We estimate that the POC flux out of the surface ocean was 250 ± 50 mg m−2 d−1 between the peak bloom through an observed export event. In addition, we estimate that ~18 ± 10% of the particulate organic carbon observed during the peak bloom was exported out of the surface ocean via subduction and sinking.

Productivity controls macrofauna diversity in the deep northern Gulf of Mexico

The α-diversity in two large-scale, systematic macrobenthos surveys (1983–1985 and 2000–2001) in the deep northern Gulf of Mexico (GoM) has been compared under strong vertical and horizontal productivity gradients to examine underlying factors that control the distribution of deep-sea diversity. We calculated the effective numbers of equally abundant species (Hill numbers) from 100 randomly selected individuals to examine spatial patterns of species richness and evenness. Macrofauna α-diversity was a negative parabolic function of depth; the minima occurred at high export POC flux on the upper continental slope and on the abyssal plain with extremely low input of POC; the maximum occurred at intermediate levels of macrofauna biomass and input of POC. The overall parabolic pattern of α-diversity was stable over the 20-year period and not statistically different between meso-scale depressions (e.g., basins and canyons) and their adjacent slope habitats; however, the diversity was significantly higher in the NE than the NW slope habitats. The observed Mid-Depth-Maximum (MDM) in α-diversity, we suggest, was likely a result of dynamic equilibrium between productivity and competitive exclusion. The higher diversity and productivity in the NE than the NW GoM at similar depth ranges suggests that, for the first time, the productivity-diversity relationship was observed independent of other depth-related factors. The findings provide a foundation for predicting the possible responses of diversity to climate change and anthropogenic impacts in the deep sea.

Impact of global major reservoirs on carbon cycle changes by using an advanced eco-hydrologic and biogeochemical coupling model

Recent research suggests that reservoirs might be a potentially important source of greenhouse gases to the atmosphere that is often unaccounted for in carbon cycle. The authors developed an advanced model coupling eco-hydrology with biogeochemical cycle (NICE-BGC). The model incorporates complex coupling of hydrologic-carbon cycle in terrestrial-aquatic linkages. In this study, NICE-BGC was further extended to apply to river basins to include the effect of 82 major reservoirs created by dams on hydrologic and biogeochemical cycles. The model result showed the differences in horizontal carbon transport (DOC flux = −66.4 TgC/yr, POC flux = −11.8 TgC/yr, and DIC flux = −43.5 TgC/yr) and vertical fluxes (CO2 evasion = 76.6 TgC/yr, and carbon burial = 45.0 TgC/yr) in presence of the reservoirs. The model also calculated CO2 evasion and carbon burial in global 82 reservoirs were 66.5 ± 35.9 TgC/yr and 54.7 ± 29.1 TgC/yr. Further, the result showed that the land carbon sink decreases more with the presence of reservoirs (−0.97 ± 0.61 PgC/yr) than without the presence of reservoirs (−1.05 ± 0.62 PgC/yr). These results are great improvements from previous research based on only field measurement because field measurements only accounted for emissions directly from the reservoirs and didn’t evaluate total carbon cycle. The model calculates the emission from the river downstream from the reservoirs in addition to the emission directly from the reservoirs. Therefore, the total simulated CO2 evasion increased not only in rivers with reservoirs but also in rivers downstream from reservoirs, which implies the need to include field observations of emissions from rivers downstream from reservoirs to quantify the total emissions due to reservoirs in the future. This finding provides valuable insights for re-evaluation of carbon cycle change of the biosphere in reservoirs.

Carbon export through zooplankton active flux in the Canary Current

Abstract Zooplankton vertical distribution, migrant biomass, and respiratory flux were studied in two transects performed in the Canary Current at 21°N and 26°N from the eu- and mesotrophic upwelling zone off Northwest Africa to the oligotrophic central gyre waters. Migrant biomass was estimated by sampling during day and night using a Longhurst-Hardy Plankton Recorder (LHPR) net in the 0–900 m depth water column. The electron transfer system (ETS) enzymatic activity was measured as a proxy for zooplankton respiration in the 0–900 m depth water column. Respiratory flux was obtained from the migrant biomass and the daytime respiration in the 200–800 m layer, assumed as the residence depth of migrant zooplankton. The southern transect at 21°N showed higher primary production, zooplankton biomass, migrant biomass, and lower passive flux. Zooplankton respiratory flux varied from 2.8 to 88.3% of the POC flux, being lower in the northern (5.3% ±2.7) than in the southern transect (46.1% ±32.9). Migrant biomass was significantly correlated to primary production (r2 = 0.570, p

Carbon Chemistry in the Mainstream of Kuroshio Current in Eastern Taiwan and Its Transport of Carbon into the East China Sea Shelf

Comprehensive carbon chemistry data were measured from the mainstream of Kuroshio, off eastern Taiwan, in May 2014. Results indicated that variations of pH@25 °C, POC, ΩCa, DIC, pCO2 and RF were closely related to the characteristics of various water types. Phytoplankton photosynthesis played important roles in DIC variation in Kuroshio Surface Water (KSW), whereas the DIC variation in Kuroshio Subsurface Water (KSSW) was probably influenced by the external transport of DIC-enriched water from the South China Sea. Vertical profiles of hydrological parameters and carbonate species indicated that the Kuroshio Current off eastern Taiwan could intrude into the ECS shelf as far as 27.9° E, 125.5° N in spring. What is more, the KSW, KSSW and Kuroshio Intermediate Water (KIW) could convey DIC into the East China Sea (ECS) with flux of 285, 305 and 112 Tg C/half year (1 Tg = 1012 g), respectively. The relevant flux of POC was 0.16, 2.93 and 0.04 Tg C/half year, respectively. Consequently, the intrusion of Kuroshio could probably exert a counteracting influence on the potential of CO2 uptake in the ECS, which needs further study.

POC export fluxes in the Arabian Sea and the Bay of Bengal: A simultaneous 234Th/238U and 210Po/210Pb study

Abstract Seasonal variation in POC export flux, from the upper ocean, was studied during post southwest monsoon (October–November 2013) and spring intermonsoon (May 2014) at selected locations in the central and southern Arabian Sea and during northeast monsoon (January 2014) and spring intermonsoon (March 2014) in the northern Bay of Bengal. POC fluxes estimated from two particle-reactive radionuclide flux proxies (234Th/238U and 210Po/210Pb) were compared. Spatial and temporal variations were observed in the POC/radionuclide ratios, net deficit flux of radionuclides and the estimated POC export fluxes in both the basins. During post southwest monsoon, the 234Th flux (1346 ± 101 dpm m− 2 d− 1), POC/234Th ratio (5.96 ± 0.29 μmol dpm− 1) and associated POC flux (8.0 ± 0.6 mmol m− 2 d− 1) were high in the southern Arabian Sea than those in the central Arabian Sea (450 ± 95 dpm m− 2 d− 1, 1.76 ± 0.07 μmol dpm− 1 and 0.8 ± 0.2 mmol m− 2 d− 1, respectively). However, during spring intermonsoon these trends were similar in both the central and southern Arabian Sea with mean 234Th fluxes, POC/234Th ratios and 234Th based POC export fluxes being 2762 ± 61 dpm m− 2 d− 1, 1.45 ± 0.05 μmol dpm− 1 and 4.0 ± 0.14 mmol m− 2 d− 1, respectively. On the other hand, the 210Po based POC export fluxes in the southern Arabian Sea (3.37 ± 0.32 and 6.73 ± 0.47 mmol m− 2 d− 1) were higher by a factor of 2–3 than those in the central Arabian Sea (2.08 ± 0.16 and 2.12 ± 0.28 mmol m− 2 d− 1) during post southwest monsoon and spring intermonsoon, respectively. In the northern Bay of Bengal, the 234Th based POC export fluxes were similar and low (0.6–1.2 mmol m− 2 d− 1) while the 210Po based POC export fluxes were much higher (3.75–8.48 mmol m− 2 d− 1) during both the northeast monsoon and spring intermonsoon seasons. One reason could be that more production and carbon export occurring in months prior to the period preceding sample collection due to weeks to several months difference in the mean-lives of the isotopes. The computed POC export fluxes and inferred efficiency of the biological carbon pump in the Arabian Sea and the northern Bay of Bengal are differentiated by this different span in half-lives of 234Th and 210Po (weeks to months, respectively). Also these values are distinct due to their biogeochemical behavior and preferred affinity for specific types of particles. This would require more joint application of each tracer in time and space for various biogeochemical regimes.

Low-intensity surface fire effects on carbon and nitrogen cycling in soil and soil solution of a Scots pine forest in central Germany

Dissolved and particulate organic carbon (DOC, POC) and nitrogen (DN, PN) are important constituents and indicators of the C and N dynamics in forested ecosystems, but little is known about fire effects on the fluxes of these elements. Biweekly fluxes at three different soil depths (organic layer O, mineral soil A, mineral soil B) were measured with zero-tension lysimeters before and after (prescribed) low-intensity surface fires in a Scots pine forest in central Germany. Measurements of soil organic C and total N concentrations, cold (soluble) and hot (labile) water-extractable DOC and DN, and soil respiration also were conducted for both pre- and post-burn bulk soils. Linear mixed-effect modelling (LMM) revealed that repeated low-intensity fire reduced DOC (−64%) and DN (−11%) fluxes in the organic layer, but increased soil CO2 fluxes (+7%). A nutrient flush from the charred material into the A horizon, as indicated by an enhanced solution pH (+11%) and electrical conductivity (+68%), may have stimulated microbial activity, leading to enhanced DOC (+47%) and DN (+202%) production and fluxes, respectively. The B horizon was unaffected by the fire treatment and retained DOC and DN. In contrast to DOC and DN fluxes, POC and PN fluxes were less affected by the fire treatment and decoupled from those of dissolved organic matter (DOM). Our findings indicate that low-intensity surface fires can significantly affect generally nutrient-poor soil systems by causing a short-term flush (“hot moment”) of DOM in the mineral A horizon (vertical “hot spot”) and by sorption in the mineral B horizon.

Scaled-dependence and seasonal variations of carbon cycle through development of an advanced eco-hydrologic and biogeochemical coupling model

Recent research has shown that inland water may play some role in carbon cycling, although the extent of its contribution has remained uncertain due to the limited amount of reliable data available. In this study, the author developed a new model coupling original eco-hydrology model and five biogeochemical cycle models (NICE-BGC), which incorporates complex coupling of hydrologic-carbon cycle in terrestrial-aquatic linkages and interplay between inorganic and organic carbon during the whole process of carbon cycling. This improved model was applied to Eurasian wetland by using three types of river network data to evaluate scaled-dependence of carbon cycle there. The model improved accuracy of low pH and alkalinity and DOC flux increase in the wetland in the northern part of study area with the finer river network data. The model showed that the difference in the carbon flux between different river network data becomes larger in downstream region, and that this difference is more predominant in the stream channel than in the hillslope, implying the importance of dry watercourses and intermittent rivers. The model was then extended to the global scale to evaluate seasonal variations of carbon cycle both in hillslope and river. The result extended from the previous studies to clarify that; (i) soil temperature has some effect on the carbon transport by biologic process responsible for carbon production in addition to clear relationship between runoff and carbon export, (ii) the high runoff during April to June and the large DOC and POC flux (about 458.38±474.41 TgC/season and 239.25±289.90 TgC/season) during January to March in hillslope, (iii) CO2 evasion becomes maximum about 294.66±93.80 TgC/season during January to June primarily affected by Amazon River, and (iv) sediment storage is larger and takes about 76.80±13.19 TgC/season during July to September particularly in Asian and North American rivers. This scaled-dependence and seasonal variations of carbon cycle helps to bridge the gap between carbon transport to the longitudinal direction and gas emission to the atmosphere in previous researches. This simulation system would also help for the further field observations, remotely-sensed imagery, and satellite datasets, and play important role in improvement in biogeochemical activity in spatio-temporal hot spots.

Flux and stable C and N isotope composition of sinking particles in the Ulleung Basin of the East/Japan Sea

Seasonal variability of sinking fluxes of total mass (TMF), particulate organic carbon and nitrogen (POC and PON) was examined using sinking particles collected from sediment traps during July 2011 to December 2011, and December 2012 to June 2013 at an offshore channel site; and from November 2013 to August 2014 at a nearshore slope site of the Ulleung Basin in the East/Japan Sea. δ13C and δ15N values of sinking particles were measured to elucidate the major export processes of POC and PON. Annual TMF (112–638gm−2yr−1) and fluxes of POC and PON (9.6–32.1gCm–2yr–1 and 1.2–4.5gNm−2yr−1, respectively) in the Ulleung Basin corresponded to the upper limit of values reported for other open seas and oceans in the world. No great seasonal variability in both quantitative (TMF, and fluxes and contents of POC and PON) and qualitative (C/N ratios, and δ13C and δ15N values) estimates of vertical fluxes was observed, reflecting a steady standing stock of chlorophyll a in the upper part of water column. Furthermore, high contents of POC and PON and nearly constant δ13C and δ15N values in sinking particles collected in the sediment traps, indicate that primary production in the euphotic zone may be a good predictor of TMF and export flux of organic matter. In this regard, our pilot study points out the importance of high annual primary production and low water temperature (<1°C) beneath the 200-m water depth, which would enable more sinking particles to be preserved during export process by limiting microbial decomposition activity in the water column, in determining the high annual flux of sinking particles in the Ulleung Basin (UB). A simple stable isotope mixing model of sinking particles indicates that despite a slight seasonal variation, the contribution of intact phytoplankton to sinking organic flux is significant to the POC and PON flux in the UB. Further continuous time series sediment trap experiments are proposed to estimate the contribution of allochthonus sources such as lateral advection through resuspended clay mineral, and aeolian and terrestrial inputs to the sedimentary flux.

Variability in runoff fluxes of dissolved and particulate carbon and nitrogen from two watersheds of different tree species during intense storm events

Abstract. Heavy storm events may increase the amount of organic matter in runoff from forested watersheds as well as the relation of dissolved to particulate organic matter. This study evaluated the effects of monsoon storm events on the runoff fluxes and on the composition of dissolved (&lt; 0.45 µm) and particulate (0.7 µm to 1 mm) organic carbon and nitrogen (DOC, DON, POC, PON) in a mixed coniferous/deciduous (mixed watershed) and a deciduous forested watershed (deciduous watershed) in South Korea. During storm events, DOC concentrations in runoff increased with discharge, while DON concentrations remained almost constant. DOC, DON and NO3–N fluxes in runoff increased linearly with discharge pointing to changing flow paths from deeper to upper soil layers at high discharge, whereas nonlinear responses of POC and PON fluxes were observed likely due to the origin of particulate matter from the erosion of mineral soil along the stream benches. The integrated C and N fluxes in runoff over the 2-month study period were in the order of DOC &gt; POC and NO3–N &gt; DON &gt; PON. The integrated DOC fluxes in runoff during the study period were much larger at the deciduous watershed (16 kg C ha−1) than at the mixed watershed (7 kg C ha−1), while the integrated NO3–N fluxes were higher at the mixed watershed (5.2 kg N ha−1) than at the deciduous watershed (2.9 kg N ha−1). The latter suggests a larger N uptake by deciduous trees. Integrated fluxes of POC and PON were similar at both watersheds. The composition of organic matter in soils and runoff indicates that the contribution of near-surface flow to runoff was larger at the deciduous than at the mixed watershed. Our results demonstrate different responses of particulate and dissolved C and N in runoff to storm events as a combined effect of tree species composition and watershed specific flow paths.

210Po and 210Pb disequilibrium at the PN section in the East China Sea

Lead-210 and 210Po have been widely used as tracers for quantifying particulate scavenging in the upper layer of the oceanic water column. In this study, we investigated the 210Po/210Pb disequilibrium in the water column of the PN section in the East China Sea (ECS) during autumn 2013. In most of the water column, a deficiency of 210Po was observed with respect to its parent nuclide 210Pb (i.e., a 210Po/210Pb activity ratio < 1.0). The (210Po/210Pb)dissolved, (210Po/210Pb)particulate and (210Po/210Pb)total activity ratios ranged from 0.29 to 0.71 (average: 0.53 ± 0.13, n = 27), 0.31 to 1.42 (average: 0.70 ± 0.27, n = 27) and 0.22 to 0.62 (average: 0.50 ± 0.12, n = 27), respectively. The distribution coefficients (Kd) of 210Po and 210Pb were 12.1× 104 ml g−1 and 8.8× 104 ml g−1, with an average (210Po/210Pb) total activity ratio of (0.50 ± 0.12, n = 27). However, over the continental shelf, planktonic detritus and fecal pellets appear to be the main carriers for 210Po, which preferentially scavenges 210Po and produces a lower (210Po/210Pb) total activity ratio (0.49 ± 0.12, n = 22) with a Kd for 210Po of 13.8× 104 ml g−1 in the water column. The variations in the fractionation factor (1.48 ± 0.66) of 210Po/210Pb reveal distinct differences between the distribution and scavenging of 210Po and 210Pb by particulate matter in different marine environments: in the estuarine zone (a high turbidity area), terrigenous suspended particulate matter scavenges 210Pb from the water column, while in areas dominated by biogenic particular matter, 210Po is preferentially scavenged from the water column. Using the 210Po/210Pb disequilibrium in the water column, we estimated the removal fluxes of POC from the upper waters downward to be 25.0 mg C m−2 d−1, comparable to those in other marginal seas. Moreover, a decreasing trend of POC removal fluxes was observed with increasing distance offshore.