Suspended Particulate Organic Carbon Research Articles

Mussel Culture Activities Facilitate the Export and Burial of Particulate Organic Carbon

The recent expansion of shellfish mariculture could significantly impact the ocean carbon cycle and its associated biogeochemical processes. To understand the source and fate of particulate organic carbon (POC), a summer cruise was conducted from September 8 to 10, 2022, at a mussel farm on Gouqi Island and its adjacent areas located in the East China Sea. Parameters included in situ temperature and salinity, contents of dissolved oxygen (DO), suspended particulate matter (SPM), POC, and chlorophyll a (Chl a), as well as the stable carbon isotopic composition (δ13C) of organic matter in particle and sediment samples, which were analyzed to facilitate a comparative assessment of the areas inside and outside the mussel farm. The POM was much fresher (POC/Chl a < 150) inside the farm with little impact from sediment resuspension (lower SPM content, 11.6 ± 6.6 mg/L), while a significant influence of sediment resuspension was found outside the farm (SPM > 20 mg/L, POC/Chl a > 150). A two end-member mixing model showed that 82.0 ± 6.0% of POC originated from marine algae within the farm, much higher than that outside the farming area (66.1 ± 7.8%). Moreover, elevated DO saturation but relatively low Chl a concentration within the farm suggested continuous algae consumption following potential high productivity. The averaged δ13C values were similar among suspended POC, sinking POC, and sedimentary organic carbon within the farm, implying the fast export and burial of POC. This is likely due to the filter-feeding habits of mussels, who ingest fresh POC and then pack it as fecal pellets that rapidly settle into the sediment. This study sheds light on the distribution and sources of POM inside and outside the mussel farm on Gouqi Island, enhancing our understanding of the marine carbon cycle on shellfish farms and providing insights into the underlying biogeochemical processes.

Contribution of aged organic carbon to suspended particulate organic carbon in the western equatorial Indian Ocean

We investigated dual carbon isotopes within the vertical water column at sites 67-1 and 67-2 of the western equatorial Indian Ocean to determine the source and age of particulate organic carbon (POC) and thus evaluated the contributions of modern and fossil (aged) POC. The concentration of POC ranged from 7 to 47.3 μgC L−1, δ13CPOC values ranged from –31.8 to –24.4‰, and Δ14CPOC values ranged from –548 to –111‰. Higher values of δ13CPOC and Δ14CPOC near the surface indicated an influence of autochthonous POC, whereas decreasing trends toward the bottom suggested a contribution of aged OC sources to the total POC pool. The contribution of fossil POC was lower near the surface, accounting for only 12% and 6% of the total POC at sites 67-1 and 67-2, respectively; however, in the deeper layers below 1,000 m, the contribution of fossil POC increased to 52% and 44% of the total POC at the two sites. Mechanisms for the increased contributions of fossil OC within deeper POC include the inflow of aged OC from sediments resuspended near slopes, the adsorption of old dissolved organic carbon in deep water masses, and the impact of aged OC that may originate from hydrothermal sources. This study highlights the importance of aged OC in the carbon cycle of the equatorial Indian Ocean.

Hydrology drives export and composition of carbon in a pristine tropical river

ABSTRACTThe Ruki is a pristine blackwater tributary in the Congo Basin draining tropical lowland forest. Daily discharge and fortnightly concentrations, isotopic ratios, and molecular composition of carbon and organic matter were measured for 1 yr (2019–2020). Like the Congo River, discharge peaked from November–January, with a smaller secondary peak in June. Dissolved organic carbon (DOC), inorganic carbon (DIC), carbon dioxide (pCO2), and methane (pCH4) concentrations were high (21.3 ± 4.8 mg C L−1, 5.8 ± 0.9 mg C L−1, 6370 ± 1740 ppm, and 250 ± 100 ppm, respectively) and positively correlated with discharge, indicating transport limitation. Total suspended solids and particulate organic carbon (POC) concentrations were generally low (3.68 ± 1.61 mg L−1, 0.88 ± 0.33 mg C L−1, respectively) and varied inversely with discharge, indicating source limitation. The Ruki exported a total of 3.25 Tg C yr−1, of which DOC, DIC, and POC comprised 76%, 20%, and 3%, respectively. This DOC flux represents ~ 20% of the annual Congo Basin flux from about 5% of its area, highlighting the high yield. Isotopic ratios of DOC and POC indicate modern C3 forest vegetation as a source, except for a few older samples potentially indicating peat inputs. The bulk molecular composition of dissolved organic matter was seasonally consistent; however, a more oxidized and aromatic assemblage occurred at high discharge, corresponding with forest vegetation, while a more aliphatic, nitrogen‐, and sulfur‐enriched assemblage was found during low discharge, corresponding with soil‐derived organic matter. Overall, these results underscore how hydrology controls C concentrations in the Ruki River and how this blackwater river contributes disproportionately to C export per unit area within the Congo Basin hydrosystem.

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.

Suspended particulate organic carbon and its carbon isotopic composition in the surface water around the Antarctic Peninsula during summer 2017–2018

AbstractThe concentration of suspended particulate organic carbon (POC) and its carbon isotopic composition (δ13CPOC) were analysed in this study with the aim of exploring the sources and factors influencing levels of POC in the surface water around the Antarctic Peninsula. The scanning electron microscopy results suggest that diatom particles formed the main component of suspended particulate matter, indicating that POC was mainly from in situ primary production. The high concentrations of chlorophyll a and POC in sea water mainly occurred in nearshore and sea-ice edge regions, which might be controlled by nutrient and reactive iron inputs stemming from sea-ice melting. The δ13CPOC in the study area is significantly lower than that in low-latitude waters, with a range of -31.8‰ to -22.8‰ (mean -28.9‰), which was controlled by the high CO2 concentration in the Southern Ocean and might be influenced by phytoplankton growth rates and assemblages. This study helps us to understand material cycling in the Antarctic region under the conditions of global climate change.

Radiocarbon Constraints on Carbon Release From the Antarctic Ice Sheet Into the Amundsen Sea Embayment

AbstractThe Amundsen Sea Embayment in West Antarctica is experiencing rapid ice mass loss, resulting in biogeochemical changes via altered nutrient and organic matter supply. However, organic carbon released from melting ice has not yet been accurately quantified. In this paper, we have integrated new dissolved organic carbon (DOC) data obtained close to the melting Dotson Ice Shelf (DIS) with published radiocarbon (Δ14C) data on sinking and suspended particulate organic carbon (POC), sedimentary OC, DOC and dissolved inorganic carbon to quantify the effect of ice melt to the carbon cycle. Elevated DOC concentrations in deep water near the DIS indicate the transport of carbon sources from the ice shelf to the water column at a rate of 4.6 ± 2.0 × 1010 g C yr−1. Furthermore, Δ14C‐DOC measurements suggest there is a possible dark chemoautotrophic production under the influence of meltwater input. The vertical profile of Δ14C in the sedimentary OC from the Sea Ice Zone and the edge of the DIS demonstrates the presence of aged organic carbon sources during warm episodes at ∼11.5 and 15.9 ka BP. Our study indicates that deep water is not only affected by OC discharge from meltwater but also by biological processes due to altered nutrient inputs. Limited data hampers a precise assessment of the influence of meltwater on the carbon cycle. Further sampling in front of the DIS will be beneficial to enhance our understanding of the role of Antarctic Ice Sheet melting in the downstream ecosystem.

Contrasts in dissolved, particulate, and sedimentary organic carbon from the Kolyma River to the East Siberian Shelf

Abstract. Arctic rivers will be increasingly affected by the hydrological and biogeochemical consequences of thawing permafrost. During transport, permafrost-derived organic carbon (OC) can either accumulate in floodplain and shelf sediments or be degraded into greenhouse gases prior to final burial. Thus, the net impact of permafrost OC on climate will ultimately depend on the interplay of complex processes that occur along the source-to-sink system. Here, we focus on the Kolyma River, the largest watershed completely underlain by continuous permafrost, and marine sediments of the East Siberian Sea, as a transect to investigate the fate of permafrost OC along the land–ocean continuum. Three pools of riverine OC were investigated for the Kolyma main stem and five of its tributaries: dissolved OC (DOC), suspended particulate OC (POC), and riverbed sediment OC (SOC). They were compared with earlier findings in marine sediments. Carbon isotopes (δ13C, Δ14C), lignin phenol, and lipid biomarker proxies show a contrasting composition and degradation state of these different carbon pools. Dual C isotope source apportionment calculations imply that old permafrost-OC is mostly associated with sediments (SOC; contribution of 68±10 %), and less dominant in POC (38±8 %), whereas autochthonous primary production contributes around 44±10 % to POC in the main stem and up to 79±11 % in tributaries. Biomarker degradation indices suggest that Kolyma DOC might be relatively degraded, regardless of its generally young age shown by previous studies. In contrast, SOC shows the lowest Δ14C value (oldest OC), yet relatively fresh compositional signatures. Furthermore, decreasing mineral surface area-normalised OC- and biomarker loadings suggest that SOC might be reactive along the land–ocean continuum and almost all parameters were subjected to rapid change when moving from freshwater to the marine environment. This suggests that sedimentary dynamics play a crucial role when targeting permafrost-derived OC in aquatic systems and support earlier studies highlighting the fact that the land–ocean transition zone is an efficient reactor and a dynamic environment. The prevailing inconsistencies between freshwater and marine research (i.e. targeting predominantly DOC and SOC respectively) need to be better aligned in order to determine to what degree thawed permafrost OC may be destined for long-term burial, thereby attenuating further global warming.

Assessing and predicting the changes for inorganic mercury and methylmercury concentrations in surface waters of a tidal estuary (Adour Estuary, SW France)

Total and dissolved concentrations of inorganic mercury (IHg) and methylmercury (MeHg) in water (Adour Estuary) were determined during three sampling campaigns and related to biogeochemical variables (nutrients, organic matter). Factors (sampling time, sample type) were included in analysis of covariance with effect separation. The urban estuary suffered historically from anthropogenic sources, however, decreased emissions have reduced Hg concentrations. Total IHg (0.51–3.42 ng L−1) and MeHg (25–81 pg L−1) concentrations are additively described by suspended particulate matter and particulate organic carbon. Higher total concentrations, carried by organic-rich particles, were found near specific discharge points (0.79–8.02 ng L−1 and 34–235 pg L−1 for IHg and MeHg, respectively). The associated high dissolved MeHg concentrations could not be explained only by biogeochemical variables. Better efficiency of the models is found for total than for dissolved concentrations. Models should be checked with other contaminants or with estuaries, suffering from downstream contamination.

Global Ocean Particulate Organic Phosphorus, Carbon, Oxygen for Respiration, and Nitrogen (GO-POPCORN)

Concentrations and elemental stoichiometry of suspended particulate organic carbon, nitrogen, phosphorus, and oxygen demand for respiration (C:N:P:−O2) play a vital role in characterizing and quantifying marine elemental cycles. Here, we present Version 2 of the Global Ocean Particulate Organic Phosphorus, Carbon, Oxygen for Respiration, and Nitrogen (GO-POPCORN) dataset. Version 1 is a previously published dataset of particulate organic matter from 70 different studies between 1971 and 2010, while Version 2 is comprised of data collected from recent cruises between 2011 and 2020. The combined GO-POPCORN dataset contains 2673 paired surface POC/N/P measurements from 70°S to 73°N across all major ocean basins at high spatial resolution. Version 2 also includes 965 measurements of oxygen demand for organic carbon respiration. This new dataset can help validate and calibrate the next generation of global ocean biogeochemical models with flexible elemental stoichiometry. We expect that incorporating variable C:N:P:-O2 into models will help improve our estimates of key ocean biogeochemical fluxes such as carbon export, nitrogen fixation, and organic matter remineralization.

Transport of particulate organic carbon in the lower Yellow River (Huanghe) as modulated by dam operation

The Yellow River has changed into a highly dam-controlled system over the recent decades. Based on gauging records of water-sediment discharge and particulate organic carbon (POC) datasets, this paper investigates the dam impacts on spatio-temporal variability of hydrology and POC transport. With a large part of suspended sediment and POC normally preserved within the Xiaolangdi (XLD) reservoir, erosion of riverbed downstream the reservoir has now become a primary source of sediment and POC to coastal ocean (172 Mt/yr for sediment and 0.56 Mt/yr for POC, respectively). Sediment accumulation and POC burial within the XLD reservoir are 246 Mt/yr and 0.60 Mt/yr, respectively, globally accounting for ∼10% and ∼1% of the retentions in reservoirs. Downstream the XLD reservoir, the averaged Δ 14 C values of POC was −451 ± 25‰, corresponding to a 14 C age of 4763 ± 366 yr BP, indicating that the POC in the lower reaches of the Yellow River was ∼2400 yr younger than that in the Loess Plateau region where most of the sediment and POC are sourced. Petrogenic POC fluxes upstream and downstream the XLD dam were 0.12 Mt/yr and 0.06 Mt/yr, respectively, clearly indicative of large-scale trapping of petrogenic POC by the XLD dam. At present, the XLD dam regulation intercepts this petrogenic POC from the Loess Plateau and enhances the downstream input of biospheric POC ( Fm = 0.85) to the coast. In contrast to the sediment and POC delivery of the rivers worldwide that are primarily dominated by tectonic, climatic and geographic settings, human activities, especially the dam regulation in the Yellow River, have significantly altered river hydrologic regime and overwhelmed the sediment and POC delivery within the river-coast continuum, which has profound impacts on carbon cycling in the coastal margin. Schematic diagram for POC and sediment budget within the Yellow River-Coast Continuum from 2002 to 2014 based on observed and reconstrcuted datasets. • The XLD reservoir acts as a sink of POC coupled with periodic rapid sediment accumulation. • Downstream riverbed erosion is a dominant source of POC to the coastal margin. • Dam regulation has substantially transformed the POC delivery during the WSRS.

Resource supply and organismal dominance are associated with high secondary production in temperate agricultural streams

Abstract Agricultural land‐use affects the environmental and biological characteristics of stream ecosystems through multiple pathways including nutrient and pesticide contamination, riparian clear‐cutting and hydromorphological degradation. These changes in the abiotic environment can have a direct effect on the productivity of macroinvertebrate communities through environmental filtering and via altered resource conditions encompassing a shift from allochthonous to autochthonous primary production and changes in elemental stoichiometry and food quality. Additionally, macroinvertebrate productivity can be affected indirectly via biological mechanisms, such as changes in species interactions, richness, competition, and predation. We studied the effects of agriculture on structural and functional descriptors of macroinvertebrate communities by assessing environmental characteristics and macroinvertebrate secondary production (MSP), biomass and density in two forested and two agricultural streams and investigated underlying biotic mechanisms. On average, MSP was 1.6–3.6, biomass 2.8–6.2 and density 5–13 times higher in agricultural than in forested streams. This pattern was associated with higher nutrient concentrations, standing crops of riparian herbaceous vegetation, suspended particulate organic carbon, quantity and quality of epilithic biofilms and chlorophyll‐a concentrations in seston and biofilm of the agricultural streams. Species richness and evenness were significantly lower in agricultural than in forested streams. A negative relationship between MSP and species richness and evenness indicated that density compensation and trait dominance were the prevalent mechanisms facilitating higher MSP in agricultural streams. Our findings suggest that the loss of riparian canopy and excess nutrient conditions are the major environmental drivers contributing to homogenization of ecological niches and dominance of highly productive non‐insect generalist species. This study highlights the importance of an ecosystem approach to understanding how complex aggregate stressors affect the regulation of consumer–resource interactions. There is an urgent need to preserve or restore natural riparian vegetation, fostering habitat and resource diversity and limiting nutrient contamination to stream ecosystems. Read the free Plain Language Summary for this article on the Journal blog.

Pelagic ecosystem dynamics between late autumn and the post spring bloom in a sub-Arctic fjord

Marine ecosystems, and particularly fjords, are experiencing an increasing level of human activity on a year-round basis, including the poorly studied winter period. To improve the knowledge base for environmentally sustainable management in all seasons, this study provides hydrographic and biological baseline data for the sub-Arctic fjord Kaldfjorden, Northern Norway (69.7° N, 18.7° E), between autumn 2017 and spring 2018. Field observations are integrated with results of a numerical ocean model simulation, illustrating how pelagic biomass, represented by chlorophyll a (Chl a), particulate organic carbon (POC), and zooplankton, is affected by stratification and circulation from October to May. We observed an unusually warm autumn that likely delayed the onset of cooling and may have supported the high abundances of holoplankton and meroplankton (5768 individuals m–3). With the onset of winter, the water column cooled and became vertically mixed, while suspended Chl a concentrations declined rapidly (< 0.12 mg Chl a m–3). In January and February, suspended POC concentrations and downward flux were elevated near the seafloor. The hydrodynamic model results indicate that the strongest currents at depth occurred in these months, potentially inducing resuspension events close to the seafloor. In spring (April), peak abundances of suspended biomass were observed (6.9–7.2 mg Chl a m–3 at 5–15 m; 9952 zooplankton ind. m–3 at 0–100 m), and field observations and model results suggest that zooplankton of Atlantic origin were probably advected into Kaldfjorden. During all investigated seasons, the model simulation suggests a complex circulation pattern, even in such a small fjord, which can have implications for environmental management of the fjord. We conclude that the pelagic system in Kaldfjorden changes continually from autumn to spring and that winter must be seen as a dynamic period, not a season where the fjord ecosystem is ‘at rest’.

Extreme rainstorms drive exceptional organic carbon export from forested humid-tropical rivers in Puerto Rico

Extreme rainfall events in the humid-tropical Luquillo Mountains, Puerto Rico export the bulk of suspended sediment and particulate organic carbon. Using 25 years of river carbon and suspended sediment data, which targeted hurricanes and other large rainstorms, we estimated biogenic particulate organic carbon yields of 65 ± 16 tC km−2 yr−1 for the Icacos and 17.7 ± 5.1 tC km−2 yr−1 for the Mameyes rivers. These granitic and volcaniclastic catchments function as substantial atmospheric carbon-dioxide sinks, largely through export of river biogenic particulate organic carbon during extreme rainstorms. Compared to other regions, these high biogenic particulate organic carbon yields are accompanied by lower suspended sediment yields. Accordingly, particulate organic carbon export from these catchments is underpredicted by previous yield relationships, which are derived mainly from catchments with easily erodible sedimentary rocks. Therefore, rivers that drain petrogenic-carbon-poor bedrock require separate accounting to estimate their contributions to the geological carbon cycle.

Long‐term suspended sediment and particulate organic carbon yields from the Reynolds Creek Experimental Watershed and Critical Zone Observatory

AbstractLong‐term (>20 y) suspended sediment (SS) and particulate organic carbon (POC) records are relatively rare and yet are necessary for understanding linkages between climate, erosion and carbon export. We estimated long‐term (>23 y) SS and POC yields from four nested catchments that ranged from <1 to 54 km2 in area across the Reynolds Creek Experimental Watershed and Critical Zone Observatory (RCEW‐CZO) in southwestern Idaho, USA. We found strong relationships between log10SS and log10POC (R2 = 0.38–0.86) that varied across catchments but remained robust across years, one dry and one of the wettest water years on record. Mean annual SS yields varied from 18 to 89 g SS m−2 y−1 and POC from 0.6 to 11.0 g C m−2 y−1 across the four catchments. Water yield explained much of the temporal variation (72%–85%) in SS and POC yields except in a small, snow‐dominated headwater catchment where it explained 15%–51%. The largest five water years accounted for 69%–84% of the total SS and POC yields in catchments with 24 y records. All catchments had positive slopes (>0) for SS and POC concentration‐discharge (C‐Q) relationships, with large catchments exhibiting greater slopes (0.66–0.97) than smaller ones (0.14–0.16). In addition, most catchments were dominated (80%) by clockwise hysteretic curves. Lack of seasonal exhaustion in the SS‐POC relationships, positive C‐Q and clockwise relations indicated that these systems were transport‐rather than supply limited, and that sediment and POC appeared to be sourced from channel/bank erosion and remobilization. POC yields represent 1%–10% of mean water year net ecosystem exchange depending on elevation; lower elevation catchments may shift from being carbon sinks to sources after accounting for fluvial POC export associated with changes in climate.

Correlation between the carbon isotopic composition of planktonic foraminifera-bound organic matter and surface water pCO2 across the equatorial Pacific

For times prior to those represented by the air trapped in Antarctic ice core records, the concentration of CO2 in the atmosphere must be reconstructed using geochemical proxies. The δ13C of particulate organic carbon (POC) produced in ocean surface waters has previously been observed to covary with the concentration of CO2 in the water. Relative to bulk sedimentary organic carbon, the minute quantity of organic matter trapped within the shell walls of planktonic foraminifera, “foraminifera-bound organic matter” (FBOM), has the potential advantage of being protected from diagenetic alteration and contamination by allochthonous organic matter. Here, with new protocols and instrumentation, FBOM-δ13C is investigated as a potential proxy for the aqueous CO2 concentration ([CO2(aq)]) and thus the partial pressure of CO2 (pCO2) in past surface waters. We achieve a full method precision for FBOM-δ13C of 0.4‰ (1SD) on sample sizes of 20 nanomole C by adding a cryofocus step, a helium sheath flow to the oxidation and reduction reactors, and other modifications to an elemental analyzer and by introducing new approaches to reduce contamination during sample preparation. FBOM-δ13C was analyzed in core-top sediments from the eastern tropical Pacific that span the equatorial maximum in surface water [CO2(aq)]. FBOM-δ13C is lower than predicted for marine phytoplankton, consistent with a substantial lipid component in FBOM. Anticorrelation is observed between FBOM-δ13C and climatological [CO2(aq)]; the relationship is statistically significant (P <0.05) in mixed-species samples and in 4 out of 5 picked species. The slope of the FBOM-δ13C:[CO2(aq)] anticorrelation is equivalent to or greater than has been measured for the δ13C of suspended POC in the surface ocean. Based on the few species analyzed in this study, endosymbiont-bearing and -barren species do not clearly differ from each other either in terms of their average value of FBOM-δ13C or the strength of the FBOM-δ13C:[CO2(aq)] anticorrelation, despite the recognized importance of the photosynthetic endosymbionts as a source of organic carbon for the symbiotic species. Correcting for variations in the δ13C of CO2(aq) and phytoplankton growth rate did not improve the significance of the FBOM-δ13C:[CO2(aq)] anticorrelation. The findings support the possibility that FBOM-δ13C can be used as a paleoceanographic proxy for surface water [CO2(aq)] and thus atmospheric pCO2.

The Spatiotemporal Evolution of Storm Pulse Particulate Organic Carbon in a Low Gradient, Agriculturally Dominated Watershed

Streams and rivers integrate and transport particulate organic carbon (POC) from an array of aquatic and terrestrial sources. Storm events greatly accelerate the transport of POC. The sequences by which individual POC inputs are mobilized and transported are not well-documented but are predicted to be temporally transient and spatially dependent because of changes in forcing functions, such as precipitation, discharge, and watershed morphology. In this study, the 3rd−4th order agricultural stream network, Clear Creek in Iowa, U.S.A., was sampled at a nested series of stations through storm events to determine how suspended POC changes over time and with distance downstream. Carbon and nitrogen stable isotope ratios were used to identify changes in POC. A temporal sequence of inputs was identified: in-channel algal production prior to heavy precipitation, row crop surface soils mobilized during peak precipitation, and material associated with the peak hydrograph that is hypothesized to be an integrated product from upstream. Tile drains delivered relatively 13C- and 15N-depleted particulate organic carbon that is a small contribution to the total POC inventory in the return to baseflow. The storm POC signal evolved with passage downstream, the principal transformation being the diminution of the early flush surface soil peak in response to a loss of connectivity between the hillslope and channel. Bank erosion is hypothesized to become increasingly important as the signal propagates downstream. The longitudinal evolution of the POC signal has implications for C-budgets associated with soil erosion and for interpreting the organic geochemical sedimentary record.

Annual Net Community Production of Particulate and Dissolved Organic Carbon From a Decade of Biogeochemical Profiling Float Observations in the Northeast Pacific

AbstractCarbon export out of the surface ocean via the biological pump is a critical sink for atmospheric carbon dioxide. This process transports organic carbon to the deep ocean through sinking particulate organic carbon (POC) and the downward transport of suspended POC and dissolved organic carbon (DOC). Changes in the relative contribution of each pathway can significantly affect the magnitude and efficiency of carbon export to depth. Net community production (NCP), an analog of carbon export under steady state assumptions, is typically estimated using budgets of biologically important chemical tracers in the upper ocean constrained by ship‐board or autonomous platform observations. In this study, we use measurements from biogeochemical profiling floats, the Ocean Station Papa mooring, and recently developed algorithms for carbonate system parameters to constrain budgets for three tracers (nitrate, dissolved inorganic carbon, and total alkalinity) and estimate NCP in the Northeast Pacific from 2009 to 2017. Using our multiple‐tracer approach, and constraining end‐member nutrient ratios of the POC and DOC produced, we not only calculate regional NCP throughout the annual cycle and across multiple depth horizons, but also partition this quantity into particulate and dissolved portions. We also use a particle backscatter‐based approach to estimate POC attenuation with depth and present a new method to constrain particle export across deeper horizons and estimate in situ export efficiency. Our results agree well with previously published estimates of regional carbon export annually and suggest that the approaches presented here could be used to assess the magnitude and efficiency of carbon export in other regions of the world's oceans.

Variability in Water-Column Respiration and Its Dependence on Organic Carbon Sources in the Canary Current Upwelling Region

Plankton respiration (R) is a key factor governing the ocean carbon cycle. However, although the ocean supports respiratory activity throughout its entire volume, to our knowledge there are no studies that tackle both the spatial and temporal variability of respiration in the dark ocean and its dependence on organic carbon sources. Here, we have studied the variability of epipelagic and mesopelagic R via the enzymatic activity of the electron transport system (ETS) in microbial communities, along two zonal sections (21°N and 26°N) extending from the northwest African coastal upwelling to the open-ocean waters of the North Atlantic subtropical gyre, during the fall 2002 and the spring 2003. Overall, integrated R in epipelagic (Repi; 0-200 m) waters, was similar during the two periods, while integrated mesopelagic respiration (Rmeso; 200-1000 m) was >25% higher in the fall. The two seasons, however, exhibited contrasting zonal and meridional patterns of ETS distribution in the water column, largely influenced by upwelling effects and associated mesoscale variability. Multiple linear regression between average R and average concentrations of dissolved organic carbon (DOC) and slow-sinking (suspended) particulate organic carbon (POCsus) indicates that POCsus is the main contributor to Rmeso, supporting previous results in the same area. Rmeso exceeded satellite-derived net primary production (NPP) at all stations except at the most coastal ones, with the imbalance increasing offshore. Moreover, the export flux of sinking POC collected at 200 m with sediment traps, represented on average less than 6% of the NPP. All this indicates that Rmeso depends largely on small particles with low sinking rates, which would be laterally advected at mid water depths from the continental margin towards the open ocean, or transported by mesoscale features from the surface to the mesopelagic ocean, providing support to inferences from modeling studies in the region.

Sources of Particulate Organic Matter across Mangrove Forests and Adjacent Ecosystems in Different Geomorphic Settings

Mangrove forests are among the world’s most productive ecosystems and provide essential ecosystem services such as global climate regulation through the sequestration of carbon. A detailed understanding of the influence of drivers of ecosystem connectivity (in terms of exchange of suspended particulate organic matter), such as geomorphic setting and carbon stocks, among coastal ecosystems is important for being able to depict carbon dynamics. Here, we compared carbon stocks, CO2 fluxes at the sediment-air interface, concentrations of dissolved organic carbon and suspended particulate organic carbon across a mangrove-seagrass-tidal flat seascape. Using stable isotope signatures of carbon and nitrogen in combination with MixSIAR models, we evaluated the contribution of organic matter from different sources among the different seascape components. Generally, carbon concentration was higher as dissolved organic carbon than as suspended particulate matter. Geomorphic settings of the different locations reflected the contributions to particulate organic matter of the primary producers. For example, the biggest contributors in the riverine location were mangrove trees and terrestrial plants, while in fringing locations oceanic and macroalgal sources dominated. Anthropogenic induced changes at the coastal level (i.e. reduction of mangrove forests area) may affect carbon accumulation dynamics in adjacent coastal ecosystems.

From Land to the Ocean: The Interplay Between Allochthonous and Autochthonous Contribution to Particles in Nepheloid Layers of the Cariaco Basin, Venezuela

AbstractLithogenic sediment input to the Cariaco Basin on the eastern Venezuelan shelf is controlled by small mountainous rivers (SMRs). The Cariaco Basin is also an area of high phytoplankton productivity as a result of strong Trade Wind‐driven coastal upwelling. Characterizing the sources that supply particulate organic carbon (POC) to the deep Cariaco Basin is important for interpreting the paleoclimate record stored in its sediments. We measured suspended POC in the four main rivers draining into the Caraiaco Basin, the Tuy, Unare, Neveri, and Manzanares, between September 2008 and September 2009 and conducted basin‐wide oceanographic cruises in September 2008 (rainy season) and March 2009 (upwelling season). Riverine concentrations of dissolved organic carbon (DOC) and POC in the four rivers were comparable to observations made in similar tropical SMR systems (POC was between 0.3–2 mg C l−1; DOC was between 100–300 μM). Within the basin, the geochemical composition of surface particles and bottom nepheloid layers (BNLs) changed with season. During the rainy season, the isotopic composition of both surface particles and BNL was characteristic of continentally derived material (δ13Corg, approximately −30 to −26‰), while during upwelling, the composition shifted to values more typical of marine sources (δ13Corg, approximately −24 to −20‰). SMRs represent an important component of the global carbon budget, which are often overlooked in ocean carbon budgets and also in paleoclimate studies of coastal environments.