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Abstract This contribution provides an introduction to a sequence of five papers (CCE I- CCE V) that describe the impact of the Warm Anomaly of 2014–15 and El Niño 2015–16 on the pelagic food web of the southern California Current Ecosystem. These contributions analyze the influence of these two warm water perturbations on satellite-based measures of ocean fronts, export efficiency out of the euphotic zone, copepod egg production, mesozooplankton community structure, and a synthesis of primary production, mesozooplankton grazing, and gravitational fluxes of organic carbon. [ABSTRACT FROM AUTHOR]

Estimates of dissolved organic carbon (DOC) release by marine macrophyte communities (seagrass meadows and macroalgal beds) based on in situ benthic chambers from published and unpublished are compiled in this study. The effect of temperature and light availability on DOC release by macrophyte communities was examined. Almost 85 % of the seagrass communities and all of macroalgal communities examined acted as net sources of DOC. Net DOC fluxes in seagrass communities increase positively with water temperature. In macroalgal communities net DOC fluxes under light exceeded those under dark condition, however, this trend was weaker in seagrass communities. Shading of a mixed seagrass meadow in The Philippines led to a significant reduction on the net DOC release when shading was maintained for 6 days compared to only 2 days of shading. Net DOC fluxes increased with increasing community respiration, but were independent of primary production or net community production. The estimated global net DOC flux, and hence export, from marine macrophytes is about 0.158 ± 0.055 Pg C yr-1 or 0.175 ± 0.056 Pg C yr-1 depending on the global extent of seagrass meadows considered.

Emissions of nitrogen (N) from anthropogenic sources enrich marine waters and promote planktonic growth. This newly synthesised organic carbon is eventually exported to benthic waters where aerobic respiration by heterotrophic bacteria results in the consumption of dissolved oxygen (DO). This pathway is typical of marine eutrophication. A model is proposed to mechanistically estimate the response of coastal marine ecosystems to N inputs. It addresses the biological processes of nutrient-limited primary production (PP), metazoan consumption, and bacterial degradation, in four distinct sinking routes from primary (cell aggregates) and secondary producers (faecal pellets, carcasses, and active vertical transport). Carbon export production ( P E ) and ecosystems eXposure Factors (XF), which represents a nitrogen-to-oxygen ‘conversion’ potential, were estimated at a spatial resolution of 66 large marine ecosystem (LME), five climate zones, and site-generic. The XFs obtained range from 0.45 (Central Arctic Ocean) to 15.9 kg O 2 kg N −1 (Baltic Sea). While LME resolution is recommended, aggregated P E or XF per climate zone can be adopted, but not global aggregation due to high variability. The XF is essential to estimate a marine eutrophication impacts indicator in Life Cycle Impact Assessment (LCIA) of anthropogenic-N emissions. Every relevant process was modelled and the uncertainty of the driving parameters considered low suggesting valid applicability in characterisation modelling in LCIA. [ABSTRACT FROM AUTHOR]

The Yellow River is the second largest river in China. Carbon transport by the Yellow River has significant influence on riverine carbon cycles in Asia. In order to monitor seasonal and spatial variations of carbon concentrations and to estimate carbon exports, water and suspended solids were sampled every 10 days at three representative stations (Qingtongxia, Tongguan, and Luokou) along the mainstream of the Yellow River. Results showed that riverine carbon was mainly in dissolved form, except during flood period and water and sediment regulation (WSR) scheme, when particulate organic carbon (POC) dominated. Concentration of dissolved inorganic carbon was mostly 5 to 10 times higher than that of dissolved organic carbon (DOC). DOC was mainly related to a natural process (leaching effect) in the upstream and anthropogenic activities in the midstream (domestic sewage and fertilizer application) and downstream (industrial wastewater). POC was connected with high suspended solids. Annually carbon delivered to the Bohai Sea was 1.34 × 1012 g/yr, accounting for 0.15% of the global total riverine carbon flux. Mean DOC exported accounted for 0.12% of the Asian rivers' DOC flux. WSR played an important role in the carbon transport, which accounted for 1/5 to 1/3 of the corresponding annual fluxes. [ABSTRACT FROM AUTHOR]

The distribution of dissolved organic carbon (DOC) concentration across coastal waters was characterized based on the compilation of 3510 individual estimates of DOC in coastal waters worldwide. We estimated the DOC concentration in the coastal waters that directly exchange with open ocean waters in two different ways, as the DOC concentration at the edge of the shelf break and as the DOC concentration in coastal waters with salinity close to the average salinity in the open ocean. Using these estimates of DOC concentration in the coastal waters that directly exchange with open ocean waters, the mean DOC concentration in the open ocean and the estimated volume of water annually exchanged between coastal and open ocean, we estimated a median ± SE (and average ± SE) global DOC export from coastal to open ocean waters ranging from 4.4 ± 1.0 Pg C yr−1 to 27.0 ± 1.8 Pg C yr−1 (7.0 ± 5.8 Pg C yr−1 to 29.0 ± 8.0 Pg C yr−1) depending on the global hydrological exchange. These values correspond to a median and mean median (and average) range between 14.7 ± 3.3 to 90.0 ± 6.0 (23.3 ± 19.3 to 96.7 ± 26.7) Gg C yr−1 per km of shelf break, which is consistent with the range between 1.4 to 66.1 Gg C yr−1 per km of shelf break of available regional estimates of DOC export. The estimated global DOC export from coastal to open ocean waters is also consistent with independent estimates of the net metabolic balance of the coastal ocean. The DOC export from the coastal to the open ocean is likely to be a sizeable flux and is likely to be an important term in the carbon budget of the open ocean, potentially providing an important subsidy to support heterotrophic activity in the open ocean. [ABSTRACT FROM AUTHOR]

Recently, researchers have recognized the significant role of small mountainous river systems in the transport of carbon from terrestrial environments to the ocean, and the scale of such studies have ranged from channel bed units to continents. In temperate zones, these mountain river systems commonly drain catchments that are largely forested. However, the magnitude of carbon export from rivers draining old-growth redwood forests has not been evaluated to date. Old-growth redwood stands support some of the largest quantities of biomass in the world, up to 350 000 Mg of stem biomass km-2 and soil organic carbon can reach 46 800 Mg km-2. In north coastal California, suspended sediment samples were collected at three gaging stations for two to four years on streams draining old-growth redwood forests. Carbon content, determined through loss-on-ignition tests, was strongly correlated with turbidity, and continuous turbidity records from the gaging stations were used to estimate annual carbon exports of 1 · 6 to 4 · 2 Mg km-2 yr-1. These values, representing 13 to 33% of the suspended sediment load, are some of the highest percentages reported in the global literature. The fraction of organic carbon as part of the suspended sediment load decreased with discharge, but reached an asymptote of 5 to 10% at flows 10 to 20 times the mean annual flows. Although larger rivers in this region exhibit high sediment yields (up to 3600 Mg km-2 yr-1), mainly attributed to high rates of uplift, mass movement, and timber harvest, the small pristine streams in this study have sediment yields of only 8 to 100 Mg km-2 yr-1. Because the current extent of old-growth redwood stands is less than 5% of its pre-European-settlement distribution, the present organic carbon signature in suspended sediment loads in this region is likely different from that in the early 20th century. Published 2015. This article is a U.S. Government work and is in the public domain in the USA. [ABSTRACT FROM AUTHOR]

To quantify the balance between new production and vertical nitrogen export of sinking particles, we measured nitrate uptake, net nitrate drawdown, ΔO2/Ar-based net community production, sediment trap flux, and 234Th export at a coastal site near Palmer Station, Antarctica, during the phytoplankton growing season from October 2012 to March 2013. We also measured nitrate uptake and 234Th export throughout the northern western Antarctic Peninsula (WAP) region on a cruise in January 2013. We used a nonsteady state 234Th equation with temporally varying upwelling rates and an irradiance-based phytoplankton production model to correct our export and new production estimates in the complex coastal site near Palmer Station. Results unequivocally showed that nitrate uptake and net community production were significantly greater than the sinking particle export on region-wide spatial scales and season-long temporal scales. At our coastal site, new production (105 ± 17.4 mg N m−2 d−1, mean ± standard error) was 5.3 times greater than vertical nitrogen export (20.4 ± 2.4 mg N m−2 d−1). On the January cruise in the northern WAP, new production (47.9 ± 14.4 mg N m−2 d−1) was 2.4 times greater than export (19.9 ± 1.4 mg N m−2 d−1). Much of this imbalance can be attributed to diffusive losses of particulate nitrogen from the surface ocean due to diapycnal mixing, indicative of a 'leaky' WAP ecosystem. If these diffusive losses are common in other systems where new production exceeds export, it may be necessary to revise current estimates of the ocean's biological pump. [ABSTRACT FROM AUTHOR]

Knowledge about the annual and seasonal patterns of organic and inorganic carbon (C) exports from the major rivers of the world to the coastal ocean is essential for our understanding and potential management of the global C budget so as to limit anthropogenic modification of global climate. Unfortunately our predictive understanding of what controls the timing, magnitude, and quality of C export is still rudimentary. Here we use a process-based coupled hydrologic/ecosystem biogeochemistry model (the Dynamic Land Ecosystem Model) to examine how climate variability and extreme events, changing land use, and atmospheric chemistry have affected the annual and seasonal patterns of C exports from the Mississippi River basin to the Gulf of Mexico. Our process-based simulations estimate that the average annual exports of dissolved organic C (DOC), particulate organic C (POC), and dissolved inorganic C (DIC) in the 2000s were 2.6 ± 0.4 Tg C yr−1, 3.4 ± 0.3 Tg C yr−1, and 18.8 ± 3.4 Tg C yr−1, respectively. Although land use change was the most important agent of change in C export over the past century, climate variability and extreme events (such as flooding and drought) were primarily responsible for seasonal and interannual variations in C export from the basin. The maximum seasonal export of DIC occurred in summer while for DOC and POC the maximum occurred in winter. Relative to the 10 year average (2001-2010), our modeling analysis indicates that the years of maximal and minimal C export cooccurred with wet and dry years (2008: 32% above average and 2006: 32% below average). Given Intergovernmental Panel on Climate Change-predicted changes in climate variability and the severity of rain events and droughts of wet and dry years for the remainder of the 21st century, our modeling results suggest major changes in the riverine link between the terrestrial and oceanic realms, which are likely to have a major impact on C delivery to the coastal ocean. [ABSTRACT FROM AUTHOR]

Summertime carbon, nitrogen, and biogenic silica export was examined using 234Th:238U disequilibria combined with free floating sediment traps and fine scale water column sampling with in situ pumps (ISP) within the Eastern Tropical North Pacific and the Gulf of California. Fine scale ISP sampling provides evidence that in this system, particulate carbon (PC) and particulate nitrogen (PN) concentrations were more rapidly attenuated relative to 234Th activities in small particles compared to large particles, converging to 1-5 µmol dpm−1 by 100 m. Comparison of elemental particle composition, coupled with particle size distribution analysis, suggests that small particles are major contributors to particle flux. While absolute PC and PN export rates were dependent on the method used to obtain the element/234Th ratio, regional trends were consistent across measurement techniques. The highest C fixation rates were associated with diatom-dominated surface waters. Yet, the highest export efficiencies occurred in picoplankton-dominated surface waters, where relative concentrations of diazotrophs were also elevated. Our results add to the increasing body of literature that picoplankton- and diazotroph-dominated food webs in subtropical regions can be characterized by enhanced export efficiencies relative to food webs dominated by larger phytoplankton, e.g., diatoms, in low productivity pico/nanoplankton-dominated regions, where small particles are major contributors to particle export. Findings from this region are compared globally and provide insights into the efficiency of downward particle transport of carbon and associated nutrients in a warmer ocean where picoplankton and diazotrophs may dominate. Therefore, we argue the necessity of collecting multiple particle sizes used to convert 234Th fluxes into carbon or other elemental fluxes, including <50 µm, since they can play an important role in vertical fluxes, especially in oligotrophic environments. Our results further underscore the necessity of using multiple techniques to quantify particle flux given the uncertainties associated with each collection method. [ABSTRACT FROM AUTHOR]

Evaluating the role of fluvial transfer of terrestrial organic carbon (OC) and subsequent burial in the global carbon cycle requires the sources and fluxes of fluvial OC to be assessed, which remains poorly constrained in the Huanghe (Yellow River). Here, we report the elemental, stable isotopic, and radiocarbon activity of particulate organic carbon (POC) sampled at the outlet of Huanghe in 2012-2013. We show that the Huanghe riverine POC can be explained by binary mixing of fossil (POCfossil) and non-fossil (POCnon-fossil) components, the former may reach ~40% of the total POC. The Huanghe POCnon-fossil is mostly sourced from C3 plants, with a mean residence time of c. 2200 years. The current human-controlled hydrological regime strongly influenced the POC sources, transport modes, and fluxes. In 2012-2013, the Huanghe delivered 0.73 Tg (1 Tg = 1012 g) of POC to the sea, and about 28% of the annual POC flux occurred within a short human induced flood event. Globally, the Huanghe should be one of the largest rivers in the transfer and re-burial of fossil OC. However, the fate of Huanghe fossil OC is still unconstrained and needs to be further investigated. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We studied biogenic rain to the ocean interior and sea floor in the Eastern Tropical South Pacific (ETSP), a region at the intersection of three oceanic regimes; coastal upwelling, equatorial divergence and the South Pacific oligotrophic gyre. Sediment cores from ocean depths >4000 m were collected, pore water was expressed using both whole core squeezing and rhizon techniques, and profiles of nitrate and dissolved Si were modeled to estimate remineralization fluxes. Nitrate modeling was interpreted as representative of C org remineralization assuming the oxic transformation of ammonium to nitrate. A broad range of TCO 2 fluxes were determined: 0.008–0.34 mmol C m −2 d −1 . The range in biogenic silica (bSi) remineralization flux was also large: 0.007–0.15 mmol Si m −2 d −1 . The pattern of TCO 2 flux showed higher particulate organic carbon (POC) inputs at sites closest to coastal and equatorial upwelling and lowest fluxes at the most oligotrophic site. Moored sediment traps, suspended at ~3700 m at 10°S, 100°W and 20°S, 100°W captured the annual pattern of mass and biogenic rain. The annual average mass flux was over five times greater at a 10°S site (30.8 mg m −2 d −1 ) compared to a 20°S site (5.5 mg m −2 d −1 ). The relative wt% of POC, PIC and bSi at these two stations were 4.9, 8.0, 15.5 and 6.6, 8.3, 2.7, respectively. The deep trap POC and bSi annual rain rates were within 0.5–4 times the benthic fluxes estimated from pore water models. The annual averaged surface ocean chlorophyll concentration estimated from satellites is a good predictor of POC rain to the ocean interior at the ETSP sites studied, as is 14 C primary production (PP). However, the POC rain into the deep ocean at ETSP sites per unit chlorophyll or per 14 C PP is significantly less than values obtained from the equatorial Pacific at 140°W or subtropical gyre station HOT. It appears that the ETSP, although underlain by an intense oxygen minimum zone, is inefficient at transferring C org production to deeply sinking POC rain. [ABSTRACT FROM AUTHOR]

Surface ocean productivity mediates the transfer of carbon to the deep ocean and in the process regulates atmospheric CO2 levels. A common axiom in oceanography is that large phytoplankton contribute disproportionally to the transfer of carbon to the deep ocean because of their greater ability to escape grazing pressure, build biomass, and sink. In the present study, we assessed the relationship of net community production to phytoplankton assemblages and plankton size distribution in the Sub-Antarctic Zone and northern reaches of the Polar Frontal Zone in the Australian sector of the Southern Ocean. We reanalyzed and synthesized previously published estimates of O2/Ar net community oxygen production (NCP) and triple-O2 isotopes gross primary oxygen production (GPP) along with microscopic and pigment analyses of the microbial community. Overall, we found that the axiom that large phytoplankton drive carbon export was not supported in this region. Mixed-layer-depth-integrated NCP was correlated to particulate organic carbon (POC) concentration in the mixed layer. While lower NCP/GPP and NCP/POC values were generally associated with communities dominated by smaller plankton size (as would be expected), these communities did not preclude high values for both properties. Vigorous NCP in some regions occurred in the virtual absence of large phytoplankton (and specifically diatoms) and in communities dominated by nanoplankton and picoplankton. We also observed a positive correlation between NCP and the proportion of the phytoplankton community grazed by microheterotrophs, supporting the mediating role of grazers in carbon export. The novel combination of techniques allowed us to determine how NCP relates to upper ocean ecosystem characteristics and may lead to improved models of carbon export. [ABSTRACT FROM AUTHOR]

Selective removal of nitrogen (N) and phosphorus (P) from the marine dissolved organic matter (DOM) pool has been reported in several regional studies. Because DOM is an important advective/mixing pathway of carbon (C) export from the ocean surface layer and its non-Redfieldian stoichiometry would affect estimates of marine export production per unit N and P, we investigated the stoichiometry of marine DOM and its remineralization globally using a compiled DOM data set. Marine DOM is enriched in C and N compared to Redfield stoichiometry, averaging 317:39:1 and 810:48:1 for C:N:P within the degradable and total bulk pools, respectively. Dissolved organic phosphorus (DOP) is found to be preferentially remineralized about twice as rapidly with respect to the enriched C:N stoichiometry of marine DOM. Biogeochemical simulations with the Biogeochemical Elemental Cycling model using Redfield and variable DOM stoichiometry corroborate the need for non-Redfield dynamics to match the observed DOM stoichiometry. From our model simulations, preferential DOP remineralization is found to increase the strength of the biological pump by ~9% versus the case of Redfield DOM cycling. Global net primary productivity increases ~10% including an increase in marine nitrogen fixation of ~26% when preferential DOP remineralization and direct utilization of DOP by phytoplankton are included. The largest increases in marine nitrogen fixation, net primary productivity, and carbon export are observed within the western subtropical gyres, suggesting the lateral transfer of P in the form of DOP from the productive eastern and poleward gyre margins may be important for sustaining these processes downstream in the subtropical gyres. [ABSTRACT FROM AUTHOR]

The flux of organic carbon from the surface ocean to mesopelagic depths is a key component of the global carbon cycle and is ultimately derived from primary production (PP) by phytoplankton. Only a small fraction of organic carbon produced by PP is exported from the upper ocean, referred to as the export efficiency (herein e-ratio). Limited observations of the e-ratio are available, and there is thus considerable interest in using remotely sensed parameters such as sea surface temperature to extrapolate local estimates to global annual export flux. Currently, there are large discrepancies between export estimates derived in this way; one possible explanation is spatial or temporal sampling bias in the observations. Here we examine global patterns in the spatial and seasonal variability in e-ratio and the subsequent effect on export estimates using a high-resolution global biogeochemical model. The model used here represents export as separate slow- and fast-sinking detrital material whose remineralization is respectively temperature dependent and a function of ballasting minerals. We find that both temperature and the fraction of export carried by slow-sinking particles are factors in determining e-ratio, suggesting that current empirical algorithms for e-ratio that only consider temperature are overly simple. We quantify the temporal lag between PP and export, which is greatest in regions of strong variability in PP where seasonal decoupling can result in large e-ratio variability. Extrapolating global export estimates from instantaneous measurements of e-ratio is strongly affected by seasonal variability and can result in errors in estimated export of up to ±60%. [ABSTRACT FROM AUTHOR]

The advantages of remote sensing using Unmanned Aerial Vehicles (UAVs) are a high spatial resolution of images, temporal flexibility and narrow-band spectral data from different wavelengths domains. This enables the detection of spatio-temporal dynamics of environmental variables, like plant-related carbon dynamics in agricultural landscapes. In this paper, we quantify spatial patterns of fresh phytomass and related carbon (C) export using imagery captured by a 12-band multispectral camera mounted on the fixed wing UAV Carolo P360. The study was performed in 2014 at the experimental area CarboZALF-D in NE Germany. From radiometrically corrected and calibrated images of lucerne (Medicago sativa), the performance of four commonly used vegetation indices (VIs) was tested using band combinations of six near-infrared bands. The highest correlation between ground-based measurements of fresh phytomass of lucerne and VIs was obtained for the Enhanced Vegetation Index (EVI) using near-infrared band b899. The resulting map was transformed into dry phytomass and finally upscaled to total C export by harvest. The observed spatial variability at field- and plot-scale could be attributed to small-scale soil heterogeneity in part.

Assessing the sources and quantifying the contributions of particulate organic matter (POM) in estuaries is a challenge. Here we apply source-specific biomarkers to assess POM sources in an estuary receiving suspended material from freshwater wetlands, fringe mangroves and coastal environments. A three end-member mixing model, including terrestrial, estuarine and marine end-member contributions was developed and successfully validated to assess general OM dynamics and hydrologic processes that control POM distributions within the Shark River estuary in South Florida. Low tide and wet season conditions coincided with an enhanced signal of the freshwater end-member biomarker abundance, while high tide and dry season conditions resulted in enhanced POM input of marine origin. Incoming tide was observed to be an important factor in the re-suspension and tidal pumping of mangrove-derived POM, which seems to be the dominant source of particulate organic carbon (POC) in the estuary. The three end-member conceptual model was tested to obtain a rough estimate of POC source strength, with the ultimate goal of constraining carbon budgets in this sub-tropical estuary. Mangrove-derived POC flux of ca. 5.3 × 10 5 to 1.0 × 10 6 kg/yr POC from the Shark River to the Gulf of Mexico were estimated, but end-member values used in the assessment need to be better constrained to reduce the degree of variability. [ABSTRACT FROM AUTHOR]

Stream water carbon (C) export is one important pathway for C loss from seasonally snow-covered mountain ecosystems and an assessment of overarching controls is necessary. However, such assessment is challenging because changes in water fluxes or flow paths, seasonal processes, as well as catchment specific characteristics play a role. For this study we elucidate the impact of: (i) changes in water flux (by comparing years of variable wetness), (ii) catchment aspect [north-facing (NF) vs. south-facing (SF)] and (iii) season (snowmelt vs. summer) on all forms of dissolved stream water C [dissolved organic C (DOC), chromophoric dissolved organic matter (CDOM) and dissolved inorganic C (DIC)] in forested catchments within the Valles Caldera National Preserve, New Mexico. The significant correlation between annual water and C fluxes (e.g. DOC r = 0.83, p < 0.02) confirms annual stream water discharge as the overarching control on C efflux, likely from a well-mixed ground water reservoir as indicated by previous research. However, CDOM exhibited a dominantly terrestrial fluorescence signature (59-71 %) year round, signaling a strong riparian and near stream soil control on CDOM composition. During snowmelt, the role of water as C transporter was superimposed on its control as C reservoir, when the NF stream transported significantly more soil C (40 % DOC, 56 % DIC) than the SF stream as a result of hillslope flushing. Inter-annual variations in winter precipitation were paramount in regulating annual stream C effluxes, e.g., reducing C effluxes three-fold after a dry (relative to wet) winter season. During the warmer summer months % dissolved oxygen saturation decreased, δC increased and CDOM assumed a more microbial signature, consistent with heterotrophic respiration in the stream and riparian soils. As a result of stream C incubation and soil respiration, $$ P_{{{\text{CO}}_{2} }} $$ increased up to 12 times atmospheric values leading to substantial degassing. [ABSTRACT FROM AUTHOR]

Abstract: Diatoms are major actors in the export of organic carbon out of the euphotic zone. Yet, the processes linking biogenic silica and carbon sedimentation fluxes to deep oceanic layers remain unclear. Analysing organic fractions in biominerals is challenging because efficient cleaning often led to structural alteration of organic molecules. Hence, although lipids are widely used as biogeochemical markers in ocean flux study, few studies have dealt with the lipids that are associated with frustules. In the present study, a protocol was set up to extract and quantify the fatty acids associated to the frustule of the diatom species Thalassiosira weissflogii. The protocol involves solvent extraction of diatom external lipids, followed by clean frustule dissolution by 4% NaOH during 1h at 95°C and subsequent solvent re-extraction of frustule-associated lipids. Results confirmed that this protocol was efficient first, to isolate the frustule from the rest of the cellular organic carbon and second to extract and quantify fatty acids (FA) associated to frustules of this species. FA composition of the frustules was significantly different from that of the whole cells consisting primarily of 14:0, 16:0 and 18:0 FA, as well as a smaller portion of 16:1 and 18:1 unsaturated FA. Frustule-associated FA constituted 7% of the total FA and 1.8% of the total POC. The 30 days T. weissflogii degradation/dissolution experiment suggested that frustule FA 14:0 and 16:0 were mainly associated with the bSiO2 phase dissolving slowly as no degradation of this pool was measured despite 78% frustule dissolution. At the end of the degradation experiment, this pool constituted 5.8% of the remaining total POC suggesting an effective protection by the frustule through strong interaction with the biogenic silica which is consistent with the correlation observed at depth between Si and POC sedimentation fluxes. [Copyright &y& Elsevier]

Dissolved organic carbon (DOC) distributions along two Atlantic Meridional Transects conducted in 2005 in the region between 47°N and 34°S showed clear latitudinal patterns. The DOC concentrations in the epipelagic zone (0–100m) were the highest (70–90µM) in tropical and subtropical waters with stable mixed layers, and lowest (50–55µM) at the poleward extremities of the transects due to deep convective mixing supplying low DOC waters to the surface. A decrease in DOC occurred with depth, and lowest DOC concentrations (41–45µM) in the 100–300m depth range were observed in the equatorial region due to upwelling of low DOC waters. A strong relationship between DOC and AOU was observed in the σ–t 26–26.5 isopycnal layer which underlies the euphotic zone and outcrops at the poleward extremities of the North and South Atlantic Subtropical Gyres (NASG and SASG) in the region ventilating the thermocline waters. Our observations reveal significant north–south variability in the DOC–AOU relationship. The gradient of the relationship suggests that 52% of the AOU in the σ–t 26–26.5 density range was driven by DOC degradation in the NASG and 36% in the SASG, with the remainder due to the remineralisation of sinking particulate material. We assess possible causes for the greater contribution of DOC remineralisation in the NASG compared to the SASG. [ABSTRACT FROM AUTHOR]

The export of organic carbon from the surface ocean by sinking particles is an important, yet highly uncertain, component of the global carbon cycle. Here we introduce a mechanistic assessment of the global ocean carbon export using satellite observations, including determinations of net primary production and the slope of the particle size spectrum, to drive a food-web model that estimates the production of sinking zooplankton feces and algal aggregates comprising the sinking particle flux at the base of the euphotic zone. The synthesis of observations and models reveals fundamentally different and ecologically consistent regional-scale patterns in export and export efficiency not found in previous global carbon export assessments. The model reproduces regional-scale particle export field observations and predicts a climatological mean global carbon export from the euphotic zone of ~6 Pg C yr−1. Global export estimates show small variation (typically < 10%) to factor of 2 changes in model parameter values. The model is also robust to the choices of the satellite data products used and enables interannual changes to be quantified. The present synthesis of observations and models provides a path for quantifying the ocean's biological pump. [ABSTRACT FROM AUTHOR]

Abstract: Studies of Middle Atlantic Bight (MAB) shelf water export to the open ocean at Cape Hatteras have produced fairly consistent estimates of ~0.25Sv, with just under half of the total carried within the Shelfbreak Front (SBF), and the remainder carried southward over the MAB shelf inshore of the shelf break and the SBF. Mean northward transport along the northern South Atlantic Bight (SAB) delivers SAB shelf water to the Cape Hatteras, which must also be exported to the open ocean. In the following, the magnitude of year-round SAB shelf water export at Cape Hatteras is estimated for the first time, and is found to be as large as the exported MAB shelf water volumes, with large seasonal variability. In summer, SAB export volume is much larger than the volume of exported MAB shelf water, while in late winter, MAB export volume exceeds that of SAB shelf water. Several aspects of circulation fields at Cape Hatteras that affect export pathways and residence times of both MAB and SAB shelf water are examined. The importance of these large and variable export volumes to carbon budgets in the southern MAB and northern SAB is considered. [Copyright &y& Elsevier]

Picocyanobacteria are the most abundant primary producers in the ocean and play a fundamental role in marine carbon cycling. Quantification of picocyanobacteria on sinking particles and in sediments is essential to understanding their contribution to the biological carbon pump. We designed a primer set targeting the 16S-23S rRNA internal transcribed spacer (ITS) sequence of cyanobacteria and established a quantitative PCR (qPCR) method for quantifying the ITS sequence abundance. High-throughput sequencing confirmed that this primer set can cover broad diversities of marine picocyanobacteria and avoid amplification of other marine cyanobacteria such as Trichodesmium and Crocosphaera . Amplification efficiencies were slightly different when seven marine Synechococcus and Prochlorococcus strains were assayed. The qPCR results were comparable with flow cytometry for water samples. Using this method, we found that, in the dark ocean, picocyanobacterial ITS sequence abundances were 10 to 100 copies/mL in the size fraction of 0.2 to 3 μm, which were 1 to 3 orders of magnitude more abundant than on the >3-μm particles. We also found that picocyanobacterial ITS abundance in sediment ranged from 10 5 to 10 7 copies/g along two nearshore-to-offshore transects in the northern South China Sea. These results further explain the important role of picocyanobacteria in carbon export. Collectively, we provide a qPCR method quantifying the total abundance of marine picocyanobacteria on water column particles and in sediments. Moreover, this newly designed primer set can be also applied to investigate the community of picocyanobacteria via high-throughput sequencing. IMPORTANCE Picocyanobacteria are the most abundant primary producers in the ocean. However, quantification of picocyanobacteria on the sinking particles and in sediments remains challenging using flow cytometry or epifluorescence microscopy. Here, we developed a real-time PCR method to quantify picocyanobacteria using a newly designed primer set specifically targeting the 16S-23S rRNA ITS sequence of cyanobacteria. We showed that in the dark ocean, picocyanobacteria are 1 to 3 orders of magnitude more abundant in small particles (0.2 to 3 μm) than in larger particles (>3 μm). This result supports the important role of direct sinking free-living picocyanobacteria cells in the carbon export to deep ocean. We also found that the picocyanobacterial ITS sequence abundance were 10 5 to 10 7 copies per gram in sediments, suggesting significant accumulation of sinking picocyanobacteria in the benthic ecosystem. This qPCR method can be used to quantify the contribution of picocyanobacteria to the biological carbon pump.

This study analyzed the diversity and abundance of diatom frustules including the ancillary parameters using the core top sediments from five locations (21, 19, 15, 13, and 11°N) along the central Arabian Sea (64°E), an area profoundly influenced by atmospheric forcing (monsoons) and oxygen minimum zone (OMZ) with high spatial variability. Significantly higher organic carbon (0.97 ± 0.05%) and diatom frustules (5.92 ± 0.57 × 10 4 valves g -1 ) were noticed in the north (21, 19, 15°N) where natural nutrient enrichment via open-ocean upwelling, winter convection, and lateral advection support large diatom-dominated phytoplankton blooms and intense OMZ. Conversely, the south (13, 11°N) depicted significantly lower organic carbon (0.74 ± 0.08%) as well as frustules (4.02 ± 0.87 × 10 4 valves g -1 ) as this area mostly remains nutrient-poor dominated by small-medium-sized phytoplankton. The north was dominated by large-sized diatoms like Coscinodiscus that could escape grazing and sink consequently due to higher ballasting. Furthermore, the presence of the intense OMZ in the north might reduce grazing pressure (low zooplankton stock) and mineralization speed facilitating higher phytodetritus transport. Relatively smaller chain-forming centric (Thalassiosira) and pennate diatoms (Pseudo-nitzschia, Fragilaria, Nitzschia, etc.) were found throughout the transect with higher abundance in the south. The euphotic diatom diversity from the existing literature was compared with the frustule diversity from the sediments suggesting not all diatoms make their way to the abyss. Such distinct spatial north-south variability in diatom frustule size as well as abundance could be attributed to cell size, grazing, and water column mineralization rates related to OMZ., (© 2022. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)

• Two new gelatinous zooplankton groups were added to an ocean biogeochemical model. • GZ-COBALT simulates large scale distributions of small and large pelagic tunicates. • It accounts for an expanded view of the abundance and role of GZ within food webs. • Pelagic tunicates produce 11% of the detritus in the upper 100 m (0.98 Pg C y-1) • Tunicates slightly shift the upper oceans away from recycling and towards export. The pelagic tunicates, gelatinous zooplankton that include salps, doliolids, and appendicularians, are filter feeding grazers thought to produce a significant amount of particulate organic carbon (POC) detritus. However, traditional sampling methods (i.e., nets), have historically underestimated their abundance, yielding an overall underappreciation of their global biomass and contribution to ocean biogeochemical cycles relative to crustacean zooplankton. As climate change is projected to decrease the average plankton size and POC export from traditional plankton food webs, the ecological and biogeochemical role of pelagic tunicates may increase; yet, pelagic tunicates were not resolved in the previous generation of global earth system climate projections. Here we present a global ocean study using a coupled physical-biogeochemical model to assess the impact of pelagic tunicates in the pelagic food web and biogeochemical cycling. We added two tunicate groups, a large salp/doliolid and a small appendicularian to the NOAA-GFDL Carbon, Ocean Biogeochemistry, and Lower Trophics version 2 (COBALTv2) model, which was originally formulated to represent carbon flows to crustacean zooplankton. The new GZ-COBALT simulation was able to simultaneously satisfy new pelagic tunicate biomass constraints and existing ecosystem constraints, including crustacean zooplankton observations. The model simulated a global tunicate biomass of 0.10 Pg C, annual tunicate production of 0.49 Pg C y-1 in the top 100 m, and annual tunicate detritus production of 0.98 Pg C y-1 in the top 100 m. Tunicate-mediated export flux was 0.71 Pg C y-1, representing 11% of the total export flux past 100 m. Overall export from the euphotic zone remained largely constant, with the GZ-COBALT pe-ratio only increasing 5.3% (from 0.112 to 0.118) compared to the COBALTv2 control. While the bulk of the tunicate-mediated export production resulted from the rerouting of phytoplankton- and mesozooplankton-mediated export, tunicates also shifted the overall balance of the upper oceans away from recycling and towards export. Our results suggest that pelagic tunicates play important trophic roles in both directly competing with microzooplankton and indirectly shunting carbon export away from the microbial loop. [ABSTRACT FROM AUTHOR]

238U–234Th disequilibrium is a powerful tool for investigating particle cycling and carbon export associated with the ocean's biological carbon pump. However, the interpretation of this method is complicated by multiple processes that can modify carbon:thorium ratios over small spatial scales. We investigated seasonal variability in the thorium and carbon cycles at a coastal site in the Western Antarctic Peninsula. Throughout the ice-free summer season, we quantified carbon and 234Th vertical flux, total water column 234Th, particulate 234Th, and the C:234Th ratios of sinking material and bulk suspended material. Simultaneous identification and separation of fecal pellets from sinking material showed that fecal pellets (primarily from krill) contributed 56% of carbon flux and that as a result of lower C:234Th ratios than suspended particles, these fecal pellets were primary drivers of variability in the C:234Th ratios of sinking material. Bulk suspended particles had highly variable C:234Th ratios and were consistently elevated in the euphotic zone relative to deeper waters. The fraction of 234Th adsorbed onto particles was positively correlated with chlorophyll and particulate organic carbon (POC) concentrations. The C:234Th ratios of suspended particles were positively correlated with POC, although during the spring diatom bloom C:234Th ratios were lower than would have been predicted based on POC concentrations alone. We hypothesize that diatom production of transparent exopolymers may have led to enhanced rates of thorium adsorption during the bloom, thus decreasing the C:234Th ratios. We used a Bayesian model selection approach to develop and parameterize mechanistic models to simulate thorium sorption dynamics. The best model incorporated one slowly-sinking POC pool and rapidly-sinking fecal pellets, with second-order sorption kinetics. The model accurately simulated temporal patterns in the C:234Th ratios of sinking and suspended particles and the fraction of 234Th adsorbed to particles. However, it slightly over-estimated C:234Th ratios during the spring (diatom-dominated) bloom and underestimated C:234Th ratios during the fall (mixed-assemblage) bloom. Optimized model parameters for thorium sorption and desorption were 0.0047 ± 0.0002 m3 mmol C−1 d−1 and 0.017 ± 0.008 d−1, respectively. Our results highlight the important role that specific taxa can play in modifying the C:234Th ratio of sinking and suspended particles and provide guidance for future studies that use 234Th measurements to investigate the functional relationships driving the efficiency of the biological pump. • Investigated thorium and carbon cycling over full ice-free season. • C:234Th ratios of sinking particles were controlled by low C:234Th of fecal pellets. • C:234Th ratios of suspended particles were correlated with chlorophyll and POC. • Diatom abundance may have led to high particulate thorium during spring bloom. • Second-order thorium sorption kinetics model accurately simulates C:234Th ratios. [ABSTRACT FROM AUTHOR]

Abstract: The magnitude and composition of the sinking-particle flux were studied over the northern Mid-Atlantic Ridge (MAR) from June 2007 to July 2010. Four moorings equipped with dual sediment traps, 100m and 1000m above the sea floor, sampled regions north and south of the Charlie Gibbs Fracture Zone (between 49°N and 54°N) and east and west of the MAR. Biogenic data were coupled with satellite estimates of primary production and modelled particle source to assess the variability in export flux. Large variations were found in the seasonality, chemical composition, magnitude and source of sinking particulate material between mooring sites. The northern moorings recorded both greater mean primary production and greater particle mass flux than the southern moorings, although, the large inter-annual variability within the sites exceeded inter-site differences. While estimates of primary production and organic carbon fluxes are comparable to other investigations of this type, they are notably lower than previous estimates for the abyssal plain of the North Atlantic. The deeper traps consistently recorded a higher mass flux compared to the shallower traps. However, we suggest that the overall flux recorded by the shallower traps was reduced by trapping inefficiency, which in the light of the low current velocities, may largely be due to the physical nature of the sinking material. Although deep-trap flux estimates may be more susceptible to errors due to re-suspended and advected material from nearby topography, mass flux and current velocity are not linked. In addition, the relatively low aluminium concentration of the deep-trap material indicates that this contribution is relatively small. The organic carbon flux to the NE, NW, SE and SW station was 0.8, 1.2, 1.1 and 1.1gm−2 y−1 respectively, corresponding to an export flux of 0.6% over this region of the MAR. [Copyright &y& Elsevier]

Abstract: Assuming steady-state over seasonal to annual timescales, and limited horizontal export of dissolved nutrients, the vertical fluxes of limiting nutrients into the euphotic zone should be balanced by particle export. Sediment traps and 234Th budgets have both been used extensively throughout the oceans as a means to measure this particulate flux from the upper ocean. One main goal of these efforts has been to determine the amount of CO2 fixed by primary producers in the surface ocean that is exported as particulate organic carbon (POC) and conversely, the decrease of particle flux with depth has been used to estimate remineralization rates of nutrients. Although disagreement between trap-derived and 234Th-derived fluxes has often been noted, the possible reasons for the imbalance are numerous, and thus often it is difficult to assign causes. Here, we examine many commonly implicated contributors to the disagreement, allowing us to assess data from a recent 2-year study in the ETSP that shows systematic disagreement between the two methods. Averaging results from both years, sediment traps collected 0.2–1.5mmolCm−2 d−1 (mean: 0.74mmolCm−2 d−1) of POC, while the thorium-based method estimated an average POC flux of 1.5–14mmolCm− d−1 (mean: 6.2mmolCm−2 d−1). The study area spans regions of differing ecological structure, as inferred from trap mineralogy, and the flux disagreement coincides with this ecological range. We interpret the difference as undercollection of poorly ballasted, slowly sinking particles by the sediment traps. Using both methods simultaneously offers insight into ecosystem structure and resulting particle flux dynamics. The thorium deficit-based flux is 5–10% of previously published estimates of primary productivity based on 14C incubations (Pennington et al., 2006), and 8–20% of concurrent estimates based on 14C incubations and oxygen supersaturation (Capone et al., personal communication; Prokopenko et al., personal communication). [Copyright &y& Elsevier]

Highlights: [•] A new physically based model for carbon cycling in flooded wetlands is proposed. [•] Model fully reflects the dynamics of sediment–water interactions in flooded wetlands. [•] Model was validated using field collected data from a from a small wetland, collecting agricultural runoff. [•] Model behavior was scrutinized through in-depth sensitivity and uncertainty analysis. [•] Study wetland removed/retained equivalent to 47±12% of the OC intake over two years. [ABSTRACT FROM AUTHOR]

Abstract: For over a decade the relationship between iron addition and carbon export has been an active topic of Southern Ocean biogeochemical investigation. To study the iron-carbon interaction, a number of natural iron sources within the Antarctic Circumpolar Current have been targeted to quantify the resultant particulate organic carbon (POC) export. Three studies in particular near the Crozet Islands, Kerguelen Plateau, and Antarctic Peninsula have produced estimates of both dissolved iron (DFe) supply and POC export. For each area, we present a detailed synthesis of 234Thorium-derived POC export and of the DFe budgets that fuel the substantial phytoplankton blooms observed in these areas. Furthermore, we discuss the nuances of calculating seasonal POC export, which is required to estimate the seasonal response of POC export to DFe fertilisation. To conclude, we review the relationship between DFe supply and POC export (C:Fe ratio) for these areas, which provides an estimate of POC export efficiency. Daily rates of POC export from the mixed layer of naturally Fe-enriched Southern Ocean blooms range from 15 to 32mmol Cm−2 d−1, with associated control sites typically exporting ∼3-times less POC, (5 to 12mmol Cm−2 d−1). Within each project, the 3-fold trend is also observed in estimates of seasonal POC export; however, variation between study regions shows up to a 4-fold difference (range 1.4–5.0molm−2). DFe supply is dominated by horizontal processes and spans an order of magnitude depending on the location (190–2700nmolm−2 d−1). Where the calculation of seasonal C:Fe ratios is possible, almost an order of magnitude variation (16,790–154,000) is observed between different Southern Ocean regions. [Copyright &y& Elsevier]

We measured spatial and temporal variations in carbon concentrations, isotopic compositions and exports during a complete hydrological cycle in nine watercourses draining a lowland forested podzolized catchment, flowing into the Arcachon lagoon (France). In addition, integrated fluxes of CO across the water-atmosphere interface were estimated to assess the relative importance of CO evasion versus lateral carbon transport at the catchment scale. Watercourse similarities and specificities linked to the local catchment characteristics are discussed and compared with other riverine systems. Low concentrations of suspended particulate matter and particulate organic carbon (POC) were generally measured in all the watercourses (8.4 ± 3.4 and 1.6 ± 0.6 mg L, respectively), reflecting limited mechanical soil erosion. The generally high POC content in the suspended matter (20 %), low Chl a concentrations (1.3 ± 1.4 μg L) and the relatively constant δC-POC value (near −28 ‰) throughout the year reveal this POC originates from terrestrial C plant and soil detritus. The presence of podzols leads to high levels of dissolved organic carbon (DOC; 6.6 ± 2.2 mg L). Similarly, high dissolved inorganic carbon (DIC) concentrations were measured in the Arcachon lagoon catchment (5.9 ± 2.2 mg L). The δC-DIC value around −20 ‰ throughout the year in many small watercourses reveals the predominance of terrestrial carbon mineralisation and silicate rock weathering in soils as the major DIC source. With pCO between 1,000 and 10,000 ppmv, all watercourses were a source of CO to the atmosphere, particularly during the low river stage. Organic carbon parameters remained relatively stable throughout the year, whereas DIC parameters showed strong seasonal contrasts closely linked to the hydrological regime and hyporheic flows. In total, the carbon export from the Arcachon watershed was estimated at 15,870 t C year or 6 t C km year, mostly exported to the lagoon as DOC (35 %), DIC (24 %) and lost as CO degassing to the atmosphere (34 %). [ABSTRACT FROM AUTHOR]

Abstract: We characterise the sources, pathways and export fluxes of particulate organic carbon (POC) in a headwater catchment in the Swiss Alps, where suspended sediment has a mean organic carbon concentration of 1.45%±0.06. By chemically fingerprinting this carbon and its potential sources using carbon and nitrogen elemental and isotopic compositions, we show that it derives from binary mixing between bedrock and modern biomass with a soil-like composition. The hillslope and channel are strongly coupled, allowing runoff to deliver recent organic carbon directly to the stream beyond a moderate discharge threshold. At higher flows, more biomass is mobilised and the fraction of modern carbon in the suspended load reaches 0.70, increased from 0.30 during background conditions. Significant amounts of non-fossil organic carbon are thus transferred from the hillslope without the need for extreme events such as landsliding. Precipitation is key: as soon as the rain stops, biomass supply ceases and fossil carbon again dominates. We use rating curves modelled using samples from five storm events integrated over 29-year discharge records to calculate long-term export fluxes of total POC and non-fossil POC from the catchment of 23.3±5.8 and 14.0±4.4tkm−2 yr−1 respectively. These yields are comparable to those from active mountain belts, yet the processes responsible are much more widely applicable. Such settings have the potential to play a significant role in the global drawdown of carbon dioxide via riverine biomass erosion, and their contribution to the global flux of POC to the ocean may be more important than previously thought. [Copyright &y& Elsevier]

Natural soil pipes, which have been widely reported in peatlands, have been shown to contribute significantly to total stream flow. Here, using measurements from eight pipe outlets, we consider the role of natural pipes in the transport of fluvial carbon within a 17.4-ha blanket-peat-covered catchment. Concentrations of dissolved and particulate organic carbon ( DOC and POC) from pipe waters varied greatly between pipes and over time, ranging between 5.3 and 180.6 mg L−1 for DOC and 0.08 and 220 mg L−1 for POC. Pipes were important pathways for peatland fluvial carbon export, with fluxes varying between 0.6 and 67.8 kg yr−1 ( DOC) and 0.1 and 14.4 kg yr−1 ( POC) for individual pipes. Pipe DOC flux was equivalent to 20% of the annual DOC flux from the stream outlet while the POC flux from pipes was equivalent to 56% of the annual stream POC flux. The proportion of different forms of aquatic carbon to total aquatic carbon flux varied between pipes, with DOC ranging between 80.0% and 91.2%, POC from 3.6% to 17.1%, dissolved CO2- C from 2.4% to 11.1% and dissolved CH4- C from 0.004% to 1.3%. The total flux of dissolved CO2- C and CH4- C scaled up to all pipe outlets in the study catchment was estimated to be 89.4 and 3.6 kg yr−1 respectively. Overall, pipe outlets produced discharge equivalent to 14% of the discharge in the stream but delivered an amount of aquatic carbon equivalent to 22% of the aquatic carbon flux at the catchment outlet. Pipe densities in blanket peatlands are known to increase when peat is affected by drainage or drying. Hence, environmental change in many peatlands may lead to an increase in aquatic carbon fluxes from natural pipes, thereby influencing the peatland carbon balance and downstream ecological processes. [ABSTRACT FROM AUTHOR]

Abstract: Budgets of nitrate, dissolved oxygen, and particulate organic carbon (POC) were constructed from data collected on-board a Lagrangian, profiling float deployed between April 4 and May 25, 2008, as part of the North Atlantic Bloom Experiment. These measurements were used to estimate net community production (NCP) and apparent export of POC along the float trajectory. A storm resulting in deep mixing and temporary suspension of net production separated the bloom into early (April 23–27) and main (May 6–13) periods over which ∼264 and ∼805mmolCm−2 were produced, respectively. Subtraction of the total POC production from the NCP yielded maximum estimates of apparent POC export amounting to ∼92 and 574 mmolCm−2 during the early and main blooms, respectively. The bloom terminated the following day and ∼282 mmolCm−2 were lost due to net respiration (70%) and apparent export (30%). Thus, the majority of the apparent export of POC occurred continuously during the main bloom and a large respiration event occurred during bloom Termination. A comparison of the POC flux during the main bloom period with independent estimates at greater depth suggest a rapid rate of remineralization between 60 and 100m. We suggest the high rates of remineralization in the upper layers could explain the apparent lack of carbon overconsumption (C:N>6.6) in the North Atlantic during the spring bloom. [Copyright &y& Elsevier]

Abstract: Wetland processes are strongly influenced by hydrologic factors such as precipitation, surface runoff, and flooding dynamics. Anthropogenic disturbances to flooding regimes can thus substantially alter wetland habitat and biogeochemistry. The Kafue Flats, a large floodplain (∼6500km2) along the Kafue River in South-Central Zambia, is a wetland impacted by upstream and downstream hydropower dams. The main purpose of this study was to develop a water budget for the Kafue Flats under current conditions, quantify nutrient and organic carbon concentrations in the river, and use the combined information to estimate biogeochemical budgets. A water balance was developed for the Kafue Flats at a subcatchment scale for the years 2002–2009 using daily hydrological data. In addition, bi-monthly flow and chemical measurements were performed over 1year (May 2008–May 2009) at multiple stations. Evapotranspiration was an important process in the Flats, accounting for up to 49% of total hydrologic outputs in some subcatchments. Direct precipitation contributes substantial to water inputs to the flats: runoff from the upstream catchment accounted for 45% of water inputs to the Kafue Flats, while the remaining 55% came from direct precipitation to the Kafue Flats from its subcatchment. Estimates from the wet season suggest that ∼75% of the water flowing in the river’s main channel as it exits the Flats spent some time within the highly productive floodplain. This exchange between the floodplain and the river appeared to play an important role in nutrient and carbon export to the river’s main channel and out of the wetland. The floodplain was a net source of phosphate (220t/year), total nitrogen (1300tN/year, of which ∼90% was organic nitrogen) and total organic carbon (50,000tC/year) to downstream systems. Thus, when considering dam impacts and altered flooding dynamics in this system, potential changes to carbon and nutrient cycling also need to be taken in to consideration, which may have implications for nutrient availability within the Kafue Flats and nutrient export to downstream systems. [Copyright &y& Elsevier]