Vertical Carbon Flux Research Articles

Vertical migration flux of soil carbon and nitrogen of sloping farmland in <?A3B2 pi6?>typical black soil areas

Vertical migration flux of soil carbon and nitrogen of sloping farmland in <?A3B2 pi6?>typical black soil areas

Impact of aggregate‐colonizing copepods on the biological carbon pump in a high‐latitude fjord

AbstractZooplankton consumption of sinking aggregates affects the quality and quantity of organic carbon exported to the deep ocean. Increasing laboratory evidence shows that small particle‐associated copepods impact the flux attenuation by feeding on sinking particles, but this has not been quantified in situ. We investigated the impact of an abundant particle‐colonizing copepod, Microsetella norvegica, on the attenuation of the vertical carbon flux in a sub‐Arctic fjord. This study combines field measurements of vertical carbon flux, abundance, and size‐distribution of marine snow and degradation rates of fecal pellets and algal aggregates. Female M. norvegica altered their feeding behavior when exposed to aggregates, and ingestion rates were 0.20 μg C ind.−1 d−1 on marine snow and 0.11 μg C ind.−1 d−1 on intact krill fecal pellets, corresponding to 48% and 26% of the females' body carbon mass. Due to high sea surface abundance of up to ~ 50 ind. L−1, the population of M. norvegica had the potential to account for almost all the carbon removal in the upper 50 m of the water column, depending on the type of the aggregate. Our observations highlight the potential importance of abundant small‐sized copepods for biogeochemical cycles through their impact on export flux and its attenuation in the twilight zone.

Does the settling column method underestimate phytoplankton sinking speeds?

Phytoplankton sinking is a major component of vertical ocean carbon and nutrient fluxes, and sinking is an integral component of phytoplankton biology and ecology. Much of our understanding of phytoplankton sinking derives from the settling column method (SETCOL) in which sinking speeds are calculated from the proportion of cells reaching the bottom of a water-filled column after a set time. Video-based methods are a recent alternative to SETCOL in which sinking speeds are measured by tracking the movement of individual cells in a salinity-stratified water column. In this study, we present the results of a meta-analysis showing that SETCOL produces significantly and consistently lower sinking speeds than the video method. Next, we perform a particle image velocimetry analysis, which shows that the observed discrepancy in sinking speeds between the two methods can probably be explained by weak convection currents in the SETCOLs. Finally, we discuss the implications of these results for the interpretation of past and future phytoplankton sinking speed measurements and models that rely on those measurements.

Modeling the Vertical Flux of Organic Carbon in the Global Ocean.

The oceans play a fundamental role in the global carbon cycle, providing a sink for atmospheric carbon. Key to this role is the vertical transport of organic carbon from the surface to the deep ocean. This transport is a product of a diverse range of physical and biogeochemical processes that determine the formation and fate of this material, and in particular how much carbon is sequestered in the deep ocean. Models can be used to both diagnose biogeochemical processes and predict how the various processes will change in the future. Global biogeochemical models use simplified representations of food webs and processes but are converging on values for the export of organic carbon from the surface ocean. Other models concentrate on understanding specific processes and can be used to develop parameterizations for global models. Model development is continuing by adding representations and parameterizations of higher trophic levels and mesopelagic processes, and these are expected to improve model performance.

Reproductive biology, elemental composition and diel vertical migration of the cosmopolitan warm-temperate pelagic tunicate Soestia zonaria

Abstract Pelagic tunicates (appendicularians, doliolids, pyrosomes, salps) are cosmopolitan members of open-ocean food webs that serve as a link to the microbial loop and play a disproportional role in vertical carbon flux. Soestia zonaria is an oceanic salp species studied for more than a century, but little information exists on its ecology. Specimens of Soestia collected between 2008 and 2021 during four research expeditions (three to the Chatham Rise, New Zealand, and one to the Northeast Atlantic) using MOCNESS-1 net and large midwater trawl were analyzed for reproductive biology, stoichiometry and vertical distribution. Populations at the Chatham Rise sampled in early winter were more developed than in spring/early summer and dominated by small and medium-sized sexually reproducing blastozooids. Whole Soestia specimens had high organic content (mean ± SD = 32.8 ± 7.5%) and carbon-to-nitrogen values (6.8 ± 0.9) compared with other salp species, indicating a stronger dependency on carbon to meet its nutritional needs. Depth-stratified sampling showed that Soestia is primarily a low-amplitude diel vertical migrator occurring in the top 150 m of the water column, but also found at depths exceeding 500 m. Soestia is primarily an epipelagic salp species adapted to living in warm-temperate nutrient-depleted and more productive ocean regions.

Interannual differences in sea ice regime in the north-western Barents Sea cause major changes in summer pelagic production and export mechanisms

The Barents Sea is a highly dynamic and productive marine ecosystem and a hotspot of global warming. Variability in sea ice extent is a common feature in the Barents Sea with substantial movements of the sea ice edge on short-term, seasonal to interannual time scales. Historically the northern Barents Sea (north of 75°N) has been ice-covered in winter, but recently it has become the area with most winter ice loss in the Arctic, and year-round ice-free conditions are predicted for the second half of the 21st century. These environmental changes have significant implications for the marine ecosystem. In this study we used contrasting sea ice regimes in August 2018 and August 2019 to explore the response of phytoplankton and bacterial production, microbial abundance, and vertical carbon flux in the north-western Barents Sea (between 76°N and 83°N) to the variability of sea ice. While the study area was ice-free in August 2018, extensive areas north of 79°N were ice-covered in 2019. When the northern parts of the transect were still ice covered, diatoms and other larger phytoplankton were dominant and highest abundances were observed following the receding ice edge. In contrast, under ice-free conditions in 2018, the pelagic ecosystem resembled a post-bloom stage of the seasonal succession with higher abundance of small phytoplankton and heterotrophic protists and low vertical flux throughout the water column. While phytoplankton biomass, bacterial production and downward vertical flux of particulate organic carbon in the upper 60 m were on average higher in 2019, primary production and carbon export below the euphotic layer were comparable between both years. However, overall highest primary production, bacterial production and abundance of both photosynthetic and heterotrophic microorganisms were observed in surface waters (upper 30 m) in 2019, connected to the retreating ice edge, where also vertical particle flux was higher and characterized by a strong attenuation curve. The results clearly demonstrate that differences in ice cover affect the phenology of pelagic primary production and associated biological processes in the Barents Sea.

The appendicularian Oikopleura dioica can enhance carbon export in a high CO2 ocean.

Gelatinous zooplankton are increasingly recognized to play a key role in the ocean's biological carbon pump. Appendicularians, a class of pelagic tunicates, are among the most abundant gelatinous plankton in the ocean, but it is an open question how their contribution to carbon export might change in the future. Here, we conducted an experiment with large volume insitu mesocosms (~55-60 m3 and 21 m depth) to investigate how ocean acidification (OA) extreme events affect food web structure and carbon export in a natural plankton community, particularly focusing on the keystone species Oikopleura dioica, a globally abundant appendicularian. We found a profound influence of O. dioica on vertical carbon fluxes, particularly during a short but intense bloom period in the high CO2 treatment, during which carbon export was 42%-64% higher than under ambient conditions. This elevated flux was mostly driven by an almost twofold increase in O. dioica biomass under high CO2 . This rapid population increase was linked to enhanced fecundity (+20%) that likely resulted from physiological benefits of low pH conditions. The resulting competitive advantage of O. dioica resulted in enhanced grazing on phytoplankton and transfer of this consumed biomass into sinking particles. Using a simple carbon flux model for O. dioica, we estimate that high CO2 doubled the carbon flux of discarded mucous houses and fecal pellets, accounting for up to 39% of total carbon export from the ecosystem during the bloom. Considering the wide geographic distribution of O. dioica, our findings suggest that appendicularians may become an increasingly important vector of carbon export with ongoing OA.

Seasonal patterns of vertical flux in the northwestern Barents Sea under Atlantic Water influence and sea-ice decline

The northern Barents Sea is a productive Arctic inflow shelf with a seasonal ice cover and as such, a location with an efficient downward export of particulate organic matter through the biological carbon pump. The region is under strong influence of Atlantification and sea-ice decline, resulting in a longer open water and summer period. In order to understand how these processes influence the biological carbon pump, it is important to identify the seasonal and spatial dynamics of downward vertical flux of particulate organic matter. In 2019 and 2021, short-term sediment traps were deployed between 30 and 200 m depth along a latitudinal transect in the northwestern Barents Sea during March, May, August and December. Vertical flux of particulate organic carbon, δ13C and δ15N values, Chl-a, protists and fecal pellets were assessed. We identified a clear seasonal pattern, with highest vertical flux in May and August (178 ± 202 and 159 ± 79 mg C m−2 d−1, respectively). Fluxes in December and March were < 45 mg C m−2 d−1. May was characterized by diatom- and Chl a-rich fluxes and high spatial variability, while fluxes in August had a higher contribution of fecal pellets and small flagellates, and were spatially more homogenous. Standing stocks of suspended particulate organic matter were highest in August, suggesting a more efficient retention system in late summer. The strong latitudinal sea-ice gradient and the influence of Atlantic Water probably led to the high spatial variability of vertical flux in spring, due to their influence on primary productivity. We conclude that the efficiency of the biological carbon pump in a prolonged open-water period depends on the reworking of small, slow sinking material into efficiently sinking fecal pellets or aggregates, and the occurrence of mixing.

Spatial distribution of vertical carbon fluxes on the Agulhas Bank and its possible implication for the benthic nepheloid layer

Vertical particle fluxes of particulate organic carbon (POC), chlorophyll a (Chl a) and biogenic silica (bSi) were measured on the productive shelf of southern Africa, the Agulhas Bank (AB), in March 2019. Sinking particulate material in the form of aggregates is hypothesized to form the benthic nepheloid layer (BNL) which is a turbid layer found near the seabed. This layer is known to affect the spawning success of squid as it is linked to high turbidity which reduces visibility during mating. To determine the distribution of fluxes and particle composition in the AB, we collected water samples below the surface mixed layer (‘export’) and near the seabed (‘bottom’) using a Marine Snow Catcher. POC export fluxes were significantly higher inshore than offshore (mean ± SD: 944.6 ± 302.0 & 461.1 ± 162.1 mg POC m−2 d−1, respectively). There was no significant difference in the cross-shelf distribution of Chl a and bSi export fluxes, however the inshore fluxes of Chl a and bSi were higher than offshore, suggesting a link between export fluxes and sinking organic matter derived from the more productive inshore surface waters. All bottom fluxes were significantly higher inshore, suggesting the contribution of sinking organic particles and resuspended bottom sediments to inshore fluxes. POC export efficiency (ratio of exported POC flux relative to net primary production (NPP)) was higher on the AB (range: 0.58–9.56) compared to the global shelf seas ratio of 0.18 and not related to NPP, suggesting an export of standing stock of carbon biomass, likely produced before the cruise. Transfer efficiency (i.e., the amount of exported flux that reaches the bottom) was also high (max: 0.99, 1.0 and 33.04 for POC, Chl a and bSi, respectively) but did not show a clear spatial pattern. We observed a significant positive correlation between bottom turbidity (a proxy for BNL presence) and export POC flux, suggesting the possibility that sinking organic matter is contributing to BNL formation on the AB.

The Impact of Sea Ice Loss on Benthic Communities of the Makarov Strait (Northeastern Barents Sea).

The continental shelf of the northeastern Barents Sea is presently experiencing a weak influx of Atlantic water from the west. In recent times, warming in Arctic regions has led to an increase in extended ice-free periods in this area, instead of significantly elevating water temperatures. The implications of this phenomenon on the structure and functioning of benthic communities were investigated during the autumn of 2019 within the Makarov Strait, located in the southwestern part of the St. Anna Trough. The macrozoobenthic communities exhibited a clear connection with the duration of ice-free periods. This variable influenced a vertical carbon flux, which subsequently served as the primary predictor for faunal abundance and diversity, as demonstrated by redundancy and correlation analyses. Two faunal groups were identified, corresponding to short and long open-water periods. Both groups had similar alpha diversity (65 ± 6 and 61 ± 9 species per station) and biomasses (39 ± 13 and 47 ± 13 g m-2) but displayed differing abundances (1140 ± 100 vs. 4070 ± 790 ind. m-2) and other diversity indices. We observed a decline in the proportion of polychaetes, accompanied by an increase in the proportion and diversity of bivalves, as well as a rise in the abundance of infaunal species, sub-surface deposit feeders, and mobile suspension feeders, in response to the increasing vertical carbon flux. The potential increase in anthropogenic pressures related to oil development in the northeastern Barents Sea highlights the importance of our study for conservation and monitoring efforts in the region.

Temporal and vertical dynamics of diatoms and dinoflagellates in the southeastern Mediterranean Sea

AbstractThe temporal distributions of microphytoplankton were studied monthly over 2 years in the shelf and the offshore waters of the “low nutrients low chlorophyll” (LNLC) southeastern (SE) Mediterranean Sea. The microphytoplankton were comprised mostly of diatoms (0–542 cells L−1, ~4 × 106–44 × 106 cells m−2) of the genera Rhizosolenia, Thalassionema, Chaetoceros, Leptocylindrus and Pseudo-nitzschia, and dinoflagellates (1–89 cells L−1, ~1 × 106–10 × 106 cells m−2) of the genera Tripos (formally known as Ceratium), Ornithocercus, Protoperidinium, Ceratocorys and Dinophysis. During the winter mixing, microphytoplankton abundance was ~2-fold higher than during the stratified summer months, in accordance with the higher inorganic nutrient levels. Diatoms were mostly found in the upper illuminated layers (0–100 m), while dinoflagellate distribution was patchy and spread from surface down to 200 m. We also calculated that diatoms and dinoflagellates contribute negligibly (~4%) to the total vertical carbon flux in the offshore water, suggesting fast recycling of organic matter at the photic layer. Our results provide a baseline to better understand carbon and biogenic silica fluxes at the LNLC SE Mediterranean Sea.

Coastal upwelling drives ecosystem temporal variability from the surface to the abyssal seafloor

Long-term biological time series that monitor ecosystems across the ocean's full water column are extremely rare. As a result, classic paradigms are yet to be tested. One such paradigm is that variations in coastal upwelling drive changes in marine ecosystems throughout the water column. We examine this hypothesis by using data from three multidecadal time series spanning surface (0 m), midwater (200 to 1,000 m), and benthic (~4,000 m) habitats in the central California Current Upwelling System. Data include microscopic counts of surface plankton, video quantification of midwater animals, and imaging of benthic seafloor invertebrates. Taxon-specific plankton biomass and midwater and benthic animal densities were separately analyzed with principal component analysis. Within each community, the first mode of variability corresponds to most taxa increasing and decreasing over time, capturing seasonal surface blooms and lower-frequency midwater and benthic variability. When compared to local wind-driven upwelling variability, each community correlates to changes in upwelling damped over distinct timescales. This suggests that periods of high upwelling favor increase in organism biomass or density from the surface ocean through the midwater down to the abyssal seafloor. These connections most likely occur directly via changes in primary production and vertical carbon flux, and to a lesser extent indirectly via other oceanic changes. The timescales over which species respond to upwelling are taxon-specific and are likely linked to the longevity of phytoplankton blooms (surface) and of animal life (midwater and benthos), which dictate how long upwelling-driven changes persist within each community.

Impact of sea ice transport on Beaufort Gyre liquid freshwater content

The Arctic Ocean’s Beaufort Gyre (BG) is a wind-driven reservoir of relatively fresh seawater, situated beneath time-mean anticyclonic atmospheric circulation, and is covered by mobile pack ice for most of the year. Liquid freshwater accumulation in and expulsion from this gyre is of critical interest due to its potential to affect the Atlantic meridional overturning circulation and due to the importance of freshwater in modulating vertical fluxes of heat, nutrients and carbon in the ocean, and exchanges of heat and moisture with the atmosphere. Here, we investigate the hypothesis that wind-driven sea ice transport into/from the BG region influences the freshwater content of the gyre and its variability. To test this hypothesis, we use the results of a coordinated climate response function experiment with four ice-ocean models, in combination with targeted experiments using a regional setup of the MITgcm, in which we rotate the surface wind forcing vectors (thereby changing the ageostrophic component of these winds). Our results show that, via an effect on the net thermodynamic growth rate, anomalies in sea ice transport into the BG affect liquid freshwater adjustment. Specifically, increased ice import increases freshwater retention in the gyre, whereas ice export decreases freshwater in the gyre. Our results demonstrate that uncertainty in the ageostrophic component of surface winds, and in the dynamic sea ice response to these winds, has important implications for ice thermodynamics and freshwater. This sensitivity may explain some of the observed inter-model spread in simulations of Beaufort Gyre freshwater and its adjustment in response to wind forcing.

Carbon budgets of copepod communities in the northern Humboldt Current System off Peru

Abundance, biomass and respiration rates of dominant medium- to larger-sized copepod species (ML class) from the upwelling system off Peru (8.5-16°S) were determined along with their carbon ingestion and egestion rates. Small copepods (S class) were included for comparisons of community rates. Overall, abundance/biomass was highest in the upper 50 m and decreased with depth and thus also community ingestion and egestion. Ingestion of the ML class (0-50 m) in shelf regions (14-515 mg C m-2 d-1) was lower in the south compared to the north and central study areas, while their offshore ingestion (11-502 mg C m-2 d-1) was comparable across regions (8.5-16°S). Ingestion rates (0-50 m) of the S class were in a range similar to those of the ML class in shelf regions (100-417 mg C m-2 d-1) but were higher offshore (177-932 mg C m-2 d-1). Calanus chilensis and the S class contributed most to total ingestion in the north, while in the south, Centropages brachiatus had the highest community ingestion aside from the S class. Egestion varied from 3-155 mg C m-2 d-1 for the ML class and 30-280 mg C m-2 d-1 for the S class. The high community rates highlight the crucial role of both size classes for carbon budgets in the northern Humboldt Current System off Peru and indicate that the ML class may enhance passive vertical carbon flux, whereas the S class may support carbon remineralization rates in surface waters.

Detection of foray behaviour in zooplankton of the Antarctic Polar Front region

In addition to diel vertical migration, individual zooplankton may also make a number of shorter-term migrations, or forays, into the surface layers from deeper depths. Direct observation of these forays is limited, particularly in the open ocean, which hinders our understanding of carbon flux via the biological carbon pump (BCP). We designed a novel net device capable of trapping zooplankton during such forays. The motion-compensated upward and downward looking (MUDL) net device consisted of 2 conical nets, one looking upwards and the other looking downwards, designed for stationary deployment at a set depth, into which migrating individuals must swim to be captured. The device was deployed at different time points within the diel cycle and at contrasting environments across the Antarctic Polar Front region in the southwestern Atlantic sector of the Southern Ocean. A range of zooplankton species were captured, with differences in abundance and species composition between times of day, net directions and sites. Of particular note was the large contribution of the copepod Oithona spp. to catches of both the upward and downward looking nets. Our study demonstrates the utility of our MUDL net for future ecosystem studies in the open ocean, particularly in relation to quantifying vertical carbon flux via the BCP.

A regional-scale approach for modeling primary production and biogenic silica export in the Southern Ocean

Persistent uncertainties in the representations of net primary production (NPP) and silicate in the Southern Ocean have been noted in recent assessments ofthe ocean biogeochemical components of Earth system models (ESMs). Consequently, more mechanistic studies at the regional scale are required. To reduce these uncertainties, we applied a one-dimensional (1D) marine ecosystem model to different bioregions in the Southern Ocean: the Polar Frontal Zone in the Pacific sector, the seasonal sea ice zone in the northwestern Ross Sea, and the inner shelf of Terra Nova Bay. To make the existing ecosystem model applicable to the Southern Ocean, we modified the phytoplankton physiology (stoichiometry depending on species) and the silicate cycle (dissolution rate of biogenic silica (BSi) depending on latitude) in the model. We quantified and compared seasonal variations in several limitation factors of NPP, namely, iron, irradiance, silicate and temperature, in the three regions. The simulation results showed that dissolved iron plays the most significant role in determining the magnitude of NPP and the phytoplankton community structure during summer. Additionally, the modified model successfully reproduced the vertical flux of BSi and particulate organic carbon (POC). The POC export efficiency was high in the inner shelf zone, which had high levels of iron concentration, NPP, and Phaeocystis biomass. In contrast, BSi export occurred most efficiently in the Polar Frontal Zone, where diatoms are dominant, the BSi dissolution rate is low, and NPP is extremely low. Our results from the integrated mechanistic framework at the regional scale demonstrate which specific processes should be urgently included in ESMs for better representation of the biogeochemical dynamics in the Southern Ocean.

Biogeochemical‐Argo data suggest significant contributions of small particles to the vertical carbon flux in the subpolar North Atlantic

AbstractThe biological carbon pump exerts a strong control on atmospheric CO2 levels. It includes a range of processes that generate organic carbon in the surface ocean and transport this organic matter from the surface to the deep ocean where it is remineralized and sequestered as inorganic carbon for decades to millennia. While ocean productivity is relatively well observed through a combination of approaches including remote sensing, the magnitude of vertical carbon transport remains poorly constrained by observations and the detailed processes involved are insufficiently understood. In particular, attention to the contribution of small particles has increased in recent years, but previous estimates of the associated vertical carbon flux have ignored remineralization and particle fragmentation. The resulting estimates are likely biased. In this study, we present a method for estimating (1) vertical carbon flux of two different size classes of organic particles and (2) the effect of remineralization and particle fragmentation on mesopelagic flux attenuation using Biogeochemical‐Argo profiles of backscattering and dissolved oxygen. We applied this method to observations from the subpolar North Atlantic and found that, on annual timescales, gravitational settling of large organic particles is dominating the vertical flux through the lower mesopelagic zone. However, small particles contribute significantly to the vertical carbon flux at 100 m (around 36% but can be up to 63%), via different mechanisms, and at 600 m (0–25%) since they can be produced by the fragmentation within the mesopelagic zone.

Cascading effects of calanoid copepod functional groups on the biological carbon pump in the subtropical South Atlantic

Life strategies, ecophysiological performances and diel vertical migration (DVM) of zooplankton key species affect the efficiency and strength of the biological carbon pump (BCP). However, it is unclear to what extent different functional groups affect the BCP. Depth-stratified day and night samples (0-800 m) from the subtropical South Atlantic were analyzed focusing on the calanoid copepod community. Calanoid abundance, biomass distribution and species-specific impact on the passive (fecal pellets) and active (via DVM) vertical flux of carbon were determined. Species were assigned to different migrant groups where, their contributions were estimated by using the proportion of the migratory community instead of simple day-night differences in biomass. This novel approach leads to more robust flux estimates, particularly for small sample sizes. According to migration ranges and day/night residence depth, functional groups were characterized, i.e. small- and large-scale epipelagic and mesopelagic migrants. Epipelagic small-scale migrants transported respiratory (1.5 mg C m-2 d-1) and fecal pellet (1.1 mg C m-2 d-1) carbon from the upper to the lower epipelagic zone, where the latter can fuel the microbial loop, and thus deep chlorophyll maxima, or be ingested by other zooplankton. Large-scale migrants actively transported up to 10.5 mg C m-2 d-1 of respiratory carbon from the epipelagic layer into the twilight zone. The majority was transported by Pleuromamma borealis (5.7 mg C m-2 d-1) into the upper mesopelagic. In addition, up to 8.0 mg C m-2 d-1 was potentially egested as fecal material by large-scale zone shifters. Mesopelagic migrants transported respiratory (0.2 mg C m-2 d-1) and fecal pellet carbon (0.1 mg C m-2 d-1) even deeper into the ocean. Community consumption of migrants in the epipelagic layer during the night was 98 mg C m-2 d-1, while non-migrants consumed 98-208 mg C m-2 d-1 in the epipelagic zone, with a potential subsequent egestion of 29-62 mg C m-2 d-1. This carbon may fuel omnivorous-detritivorous feeding, the microbial loop and/or may sink as fecal pellets. This case study shows how calanoid functional groups mediate carbon fluxes in the subtropical South Atlantic Ocean and demonstrates how detailed community analyses can elucidate the complexity of pelagic carbon budgets.

Lateral carbon export has low impact on the net ecosystem carbon balance of a polygonal tundra catchment

Abstract. Permafrost-affected soils contain large quantities of soil organic carbon (SOC). Changes in the SOC pool of a particular ecosystem can be related to its net ecosystem carbon balance (NECB) in which the balance of carbon (C) influxes and effluxes is expressed. For polygonal tundra landscapes, accounts of ecosystem carbon balances in the literature are often solely based on estimates of vertical carbon fluxes. To fill this gap, we present data regarding the lateral export rates of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) from a polygonal tundra site in the north Siberian Lena River delta, Russia. We use water discharge observations in combination with concentration measurements of waterborne carbon to derive the lateral carbon fluxes from one growing season (2 June–8 September 2014 for DOC, 8 June–8 September 2014 for DIC). To put the lateral C fluxes into context, we furthermore present the surface–atmosphere eddy covariance fluxes of carbon dioxide (CO2) and methane (CH4) from this study site. The results show cumulative lateral DIC and DOC fluxes of 0.31–0.38 and 0.06–0.08 g m−2, respectively, during the 93 d observation period (8 June–8 September 2014). Vertical turbulent fluxes of CO2-C and CH4-C accumulated to −19.0 ± 1.2 and 1.0 ± 0.02 g m−2 in the same period. Thus, the lateral C export represented about 2 % of the net ecosystem exchange of (NEE) CO2. However, the relationship between lateral and surface–atmosphere fluxes changed over the observation period. At the beginning of the growing season (early June), the lateral C flux outpaced the surface-directed net vertical turbulent CO2 flux, causing the polygonal tundra landscape to be a net carbon source during this time of the year. Later in the growing season, the vertical turbulent CO2 flux dominated the NECB.

Functional responses of aggregate‐colonizing copepods

AbstractZooplankton consumption of aggregates, such as marine snow, is an important factor in determining the efficiency of the biological carbon pump. However, the feeding rates of aggregate‐associated small harpacticoid and poecilostomatoid copepods are largely unknown, as are the factors that influence these rates. We measured the functional responses of pellet production (PP) of aggregate‐feeding copepods on appendicularian houses, algal and detrital aggregates, and on Trichodesmium spp. tufts. The PP rates of all copepods increased with increasing aggregate concentrations, but the response varied depending on the aggregate type. The pelagic harpacticoid copepod Microsetella norvegica had the highest PP rates on algal aggregates, while the poecilostomatoid copepod Oncaea spp. and the benthic harpacticoid copepod Amonardia normanni PP rates were highest on appendicularian houses. The ingestion rates of M. norvegica and Oncaea spp. were typically 0.04–0.13 μg C ind.−1 d−1, with the exception of ca. three times higher rates of Oncaea spp. on appendicularian houses and ca. 10 times higher rates of M. norvegica on algal aggregates. The ingestion rates of the larger species, A. normanni, were generally higher, 0.3–1.3 μg C ind.−1 d−1 on algal aggregates, and even ca. 10‐fold higher on appendicularian houses. Our results suggest that the aggregate degradation rates by copepods can vary many‐fold depending on the quality (origin) of the aggregate and the copepod species. This can have large consequences for the attenuation of vertical carbon flux.