Sinking POC Research Articles

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.

Seasonal variability in carbon demand and flux by mesozooplankton communities at subarctic and subtropical sites in the western North Pacific Ocean

We investigated seasonal changes in carbon demand and flux by mesozooplankton communities at subtropical (S1) and subarctic sites (K2) in the western North Pacific Ocean to compare the impact of mesozooplankton communities on the carbon budget in surface and mesopelagic layers. Fecal pellet fluxes were one order higher at K2 than at S1, and seemed to be enhanced by copepod and euphausiid egestion under high chlorophyll a concentrations. The decrease in pellet volume and the lack of any substantial change in shape composition during sink suggest a decline in fecal pellet flux due to coprorhexy and coprophagy. While respiratory and excretory carbon by diel migrants at depth (i.e., active carbon flux) was similar between the two sites, the actively transported carbon exceeded sinking fecal pellets at S1. Mesozooplankton carbon demand in surface and mesopelagic layers was higher at K2 than S1, and an excess of demand to primary production and sinking POC flux was found during some seasons at K2. We propose that this demand was met by supplementary carbon sources such as feeding on protozoans and fecal pellets at the surface and carnivory of migrants at mesopelagic depths.

Simulation of Observed PCBs and Pesticides in the Water Column during the North Atlantic Bloom Experiment.

The dynamics of persistent organic pollutants in the oceans are not well constrained, in particular during a bloom formation and collapse. Polychlorinated biphenyls (PCBs) and some pesticides were measured in air, water, and zooplankton tracking the North Atlantic Bloom in May 2008. Lower weight PCBs were entering the water column from the atmosphere during the main bloom period but reached equilibrium after the bloom collapsed. The PCBs in the lipids of zooplankton Calanus were in equilibrium with those in the dissolved phase. A Lagrangian box model was developed to simulate the dissolved phase PCBs and pesticides by including the following processes: air-water exchange, reversible sorption to POC, changes in mixed layer depth, removal by sinking particles, and degradation. Results suggest that sorption to (sinking) POC was the dominant removal process for hydrophobic pollutants from seawater. Statistical test suggested simulated results were not significantly different from observed values for hydrophobic pollutants (p,p'-DDE).

Analysis of remineralisation, lability, temperature sensitivity and structural composition of organic matter from the upper ocean

Organic carbon (OC) synthesised by plankton is exported out of the surface layer as particulate (POC) and dissolved (DOC) organic carbon. This “biological pump” constitutes a major pathway in the global marine carbon cycle, each year exporting about 10PgC into the ocean interior, where it is subsequently remineralised via biological decomposition. Remineralised inorganic nutrients and carbon are, ultimately, again brought to the surface by advection and turbulent mixing which closes the OC-cycle in the upper ocean. Thus, remineralisation rates of OC are a critical component of the biological pump. These rates are regulated by the lability of the material and the environmental conditions in the ambient water. Temperature is particularly important in regulating the rate of microbial respiration and, thus, remineralisation rates. A significant temperature dependence of the microbial metabolic activity in the ocean interior is expected, as this is a feature observed elsewhere in the biosphere. Such temperature dependence of microbial remineralisation of POC and DOC will alter the amount of material available for transport by the biological pump to the deep ocean. Very few studies on the lability of OC and temperature sensitivity of microbial degradation processes in the mesopelagic zone (∼100–1000m) have, to date, been carried out. Here, we present a comprehensive new experimental data set from all major ocean basins and quantify remineralisation rates of OC and their temperature sensitivity in long-term incubations of water from the upper 350m. Microbial respiration was measured by non-invasive oxygen optodes and oxygen consumption was used as a constraint for determining the remineralisation rates and temperature sensitivity by two complementary methods. First, we analysed the oxygen consumption from a multi-component OC-model where the concentration, remineralisation rates and temperature sensitivity of two bio-available (labile) pools of organic carbon were fitted to the data via a non-linear fitting procedure. Thereafter, a continuous OC-model was fitted to the data through an inverse method and information about lability, temperature sensitivity and structural composition of the OC-pool was analysed together with the results from the two-pool solutions. Median values of remineralisation rates from all experiments on material characterising sinking POC were found to be 0.6 and 0.05days−1 for the two decomposable pools corresponding to turnover times of 2 and 21days, respectively. Accordingly, solutions from the continuous model resulted in median turnover times between 6 and 11days. Similar analyses were carried out for the OC-pool characterising DOC. A significant bio-available OC-pool was found to be present in the surface layer with a median value from all experiments of 30μM TOC. The median values of all remineralisation rates from the two bio-available OC-pools were found to be 0.2 and 0.02days−1, corresponding to turnover times of 5 and 56days, respectively, in good agreement with previous studies. The corresponding temperature sensitivities, characterised by a Q10-value, were found to be about 1.9 for the POC-fraction whereas the DOC fraction was characterised with values above 2.8 for the continuous OC-models. The analysis of the structural composition indicated that the TOC distribution in the surface layer was characterised by more heterogeneous material in terms of lability compared with the POC material sampled from the upper 350m. Finally, we analyse the potential impact of the calculated temperature sensitivity on the general OC-cycling in the upper ocean and show that the implied reduction in OC-flux in a warmer ocean may have significant impact on nutrient cycling in general and also tends to reduce future ocean carbon uptake significantly.

Short-term patterns of vertical particle flux in northern Benguela: a comparison between sinking POC and respiratory carbon consumption

Short-term variability of vertical carbon flux (C-flux) was assessed by two approaches at two stations on the Namibian shelf upwelling system (20°S). The first approach was based on modeling water-column respiratory CO2 production (Fc) by integrating vertical profiles of the respiratory electron transport system. The second approach was based on automated sediment trap sampling daily. In both cases, temporal variability at the same station proved to be greater than the spatial variability between stations. Comparison of the two methods yielded higher C-flux values from the sediment traps in the presence of upwelling filaments (7.85 vs 3.67mmolCm−2d−1). Contrarily, in the post-filament stage of the upwelling system, the values from the Fc models were higher (1.43 vs 2.57mmolCm−2d−1). This difference was attributed to compositional changes in sinking particles and their settling velocities. In combination, both approaches served to increase current understanding of the short-term particle fluxes in this northern Benguela upwelling system. The Fc approach was further applied to zooplankton from the same area in order to quantify their importance in attenuating the particulate organic C-flux. This attenuation, as measured by fractional consumption of the primary productivity, ranged from 2% to 20%.

Carbon dynamics in the western Arctic Ocean: insights from full-depth carbon isotope profiles of DIC, DOC, and POC

Abstract. Arctic warming is projected to continue throughout the coming century. Yet, our currently limited understanding of the Arctic Ocean carbon cycle hinders our ability to predict how changing conditions will affect local Arctic ecosystems, regional carbon budgets, and global climate. We present here the first set of concurrent, full-depth, dual-isotope profiles for dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and suspended particulate organic carbon (POCsusp) at two sites in the Canada Basin of the Arctic Ocean. The carbon isotope composition of sinking and suspended POC in the Arctic contrasts strongly with open ocean Atlantic and Pacific sites, pointing to a combination of inputs to Arctic POCsusp at depth, including surface-derived organic carbon (OC), sorbed/advected OC, and OC derived from in situ DIC fixation. The latter process appears to be particularly important at intermediate depths, where mass balance calculations suggest that OC derived from in situ DIC fixation contributes up to 22% of POCsusp. As in other oceans, surface-derived OC is still a dominant source to Arctic POCsusp. Yet, we suggest that significantly smaller vertical POC fluxes in the Canada Basin make it possible to see evidence of DIC fixation in the POCsusp pool even at the bulk isotope level.

Variability of primary production and air‐sea CO2 flux in the Southern Ocean

Biogeochemical cycling in the Southern Ocean (SO) plays a key role in the global sea‐air CO2balance and in the ocean anthropogenic carbon inventory (Ito et al., 2010; Khatiwala et al., 2009; Sarmiento et al., 2004). Some previous studies suggest a decreasing trend in the Southern Ocean carbon sink (Le Quéré et al., 2007; Lovenduski et al., 2007; Wetzel et al., 2005). We investigate the interannual and decadal variations in sea‐air CO2flux and phytoplankton production in the SO with hindcast simulations by an ocean biogeochemical model. Decreasing trends in sinking POC and primary production are found from 1979 to 2003, concurrent with a decreasing trend in carbon uptake from the atmosphere. Simulations show substantial interannual and decadal variability in productivity. The sea‐air CO2flux is significantly correlated with sinking POC, especially in high productivity regions of the Southern Ocean. Both mixed layer depths and iron concentrations are important to the long‐term trends in production and phytoplankton community structure. Sea ice cover also plays an important role at high latitudes. Variability in dust deposition in recent decades has little influence on total SO productivity and carbon uptake, however, there are regional impacts near dust source regions. Accurately representing mixed layer depths and their impacts on phytoplankton light stress are critical for understanding how climate change impacts SO ecosystems and biogeochemistry.

Sources of molecularly uncharacterized organic carbon in sinking particles from three ocean basins: A coupled Δ 14C and δ 13C approach

Abstract Several recent studies have suggested dramatically different ideas about the source and nature of molecularly uncharacterized organic carbon (MUC) in sinking marine particles (POC). Carbon isotope data coupled with hydrolysis has indicated MUC is lipid-like material, suggesting selective preservation [Hwang, J. and Druffel, E.R.M., 2003. Lipid-like material as the source of the uncharacterized organic carbon in the ocean? Science, 299: 881–884.]. In contrast, NMR-based work has strongly indicated non-selective degradation, with amino acid dominating resistant material [Hedges, J.I. et al., 2001. Evidence for non-selective preservation of organic matter in sinking marine particles. Nature, 409: 801–804.]. This study set out to explore this seeming paradox, and to examine the hypothesis that the nature of MUC may vary strongly between margin and open ocean regions. We examined the coupled elemental, stable and radiocarbon isotopic compositions of three fractions of sinking POC: lipids, acid soluble (AS) material (a proxy for hydrolyzable biomolecules), and acid insoluble (AI) material (a proxy for the MUC). Δ 14C and δ 13C measurements were made on three time-series of sediment trap samples in widely separate ocean regions: Santa Barbara Basin, Cariaco Basin, and an oceanic site in the Eastern Subtropical Atlantic off Dakar, Africa. Δ 14C compositions of AI fractions at all sites indicated substantial contribution by pre-aged marine carbon sources (40–60% of total sinking POC), not derived from direct export of surface productivity. Comparison of δ 13C and Δ 14C values with coexisting lipid and AS signatures also suggested widely different AI compositions from different ocean environments. AI material in coastal California waters consistently appeared lipid-like, in agreement with several studies near the same region [Hwang, J. and Druffel, E.R.M., 2003. Lipid-like material as the source of the uncharacterized organic carbon in the ocean? Science, 299: 881–884.; Hwang, J., Druffel, E.R.M., Eglinton, T.I. and Repeta, D.J., 2006b. Source(s) and cycling of the nonhydrolyzable organic fraction of oceanic particles. Geochimica et Cosmochimica Acta, 70: 5162–5168.]. In contrast, our oceanic site's AI isotopic signatures were more consistent with non-selective preservation of a range of biochemical classes. AI compositions in Cariaco Basin proved to be variable with time, suggesting a complex and variable mixture of AI sources. These results suggest a major divergence between coastal and oceanic AI sources and composition. Taken together, we propose that the dominant mechanism influencing molecularly uncharacterized material is non-selective preservation during initial water column transit; influenced to varying degrees by subsequent addition of allochthonous and 14C-depleted organic carbon sources. In our coastal margin sites, all AI properties point to resuspended sediment as the most likely source of total uncharacterizable POC. In our more oceanic site, where particles have much longer transit times, incorporation of old DOC into POC, in addition to resuspended sediment, may have a substantial influence on overall Δ 14C ages. We propose that major differences between AI compositions in different locations is tied to variation in availability of such pre-aged OC sources, linked to regional oceanographic conditions and strongly influenced by proximity to continental shelves. The strong correlations we observe between %AI vs. %AS composition and elemental and isotopic values also implies that substantial amounts of POC collected via sediment traps in some locations is not exported directly from surface production, but added from other pre-aged reservoirs.

Seasonal changes in the sinking export of particulate material under first-year sea ice on the Mackenzie Shelf (western Canadian Arctic)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 353:13-25 (2008) - DOI: https://doi.org/10.3354/meps07165 Seasonal changes in the sinking export of particulate material under first-year sea ice on the Mackenzie Shelf (western Canadian Arctic) Thomas Juul-Pedersen1,2,4,*, Christine Michel1, Michel Gosselin2, Lena Seuthe3 1Freshwater Institute, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada 2Institut des sciences de la mer (ISMER), Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, Québec G5L 3A1, Canada 3Norwegian College of Fisheries Science, University of Tromsø, 9037 Tromsø, Norway 4Present address: Greenland Institute of Natural Resources, PO Box 570, 3900 Nuuk, Greenland *Email: thpe@natur.gl ABSTRACT: The sinking export of particulate material under landfast first-year sea ice was studied from the winter period to spring melt on the Mackenzie Shelf, western Canadian Arctic. Short-term particle interceptor traps were deployed at 1, 15, and 25 m under the ice on 16 consecutive occasions from 23 February to 20 June 2004. The sinking material was analyzed for chlorophyll (chl) a, phaeopigments, total particulate carbon (TPC), particulate organic carbon and nitrogen (POC and PON), and biogenic silica (BioSi). The sinking fluxes of chl a and BioSi increased steadily after 19 March and until the onset of spring melt (26 May), after which they increased considerably. The contribution of large algae (>5 µm) to the total chl a sinking flux also increased after 19 March, reflecting an increasing contribution of diatoms to the sinking export of algal material. Accordingly, chl a sinking fluxes at 1 m showed a significant linear relationship with bottom ice chl a biomass. On average, 46% of the chl a exported at 1 m was lost in the upper 25 m. POC was the main component of the TPC sinking fluxes throughout the study. POC sinking fluxes remained fairly stable until the onset of spring melt, after which a considerable increase was observed. High POC:chl a ratios indicated a significant contribution of non-algal material to the sinking POC. The daily sinking loss rate of chl a, POC, and PON from the sea ice and interfacial layer (top 1 m of the water column) varied seasonally and was highest during the winter period. Our results illustrate the continuous downward sinking export of organic material under landfast ice, from winter throughout late spring. KEY WORDS: Arctic · Sea ice · Sedimentation · Sea ice-water coupling · Organic material · Carbon flux · Ice algae · Biogenic silica Full text in pdf format PreviousNextCite this article as: Juul-Pedersen T, Michel C, Gosselin M, Seuthe L (2008) Seasonal changes in the sinking export of particulate material under first-year sea ice on the Mackenzie Shelf (western Canadian Arctic). Mar Ecol Prog Ser 353:13-25. https://doi.org/10.3354/meps07165 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 353. Online publication date: January 17, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Dissolved and particulate organic carbon in the Sea of Okhotsk: Transport from continental shelf to ocean interior

Dissolved and particulate organic carbon (DOC and POC) were measured in water columns of the Sea of Okhotsk together with other hydrological and chemical properties, including phytoplankton pigments (Chl‐a) and stable carbon isotopic ratios (δ13C) of POC. Transects from shelf to slope off the northeast coast of Sakhalin showed the existence of a cold‐water mass in the intermediate layer on the slope area having a density of 26.7–27.0 σθ, which is equivalent to the cold and dense water on the shelf (dense shelf water: DSW). The cold intermediate water mass was rich in DOC, POC, and Chl‐a, and the δ13C of POC was high, similar to that in DSW, indicating that the organic matter in the cold intermediate water is exported from the highly productive shelf area by the outflow of DSW. On the other hand, surface waters near the coast of northern Sakhalin had a very low salinity, which must be affected by the fresh water discharge from the Amur River. The low‐salinity water contained a large amount of DOC that was more than 3 times greater than the pelagic surface waters. The linear regression line between the salinity and the DOC in the surface layer of the studied area indicates that the Amur River water contains approximately 690 μM of DOC, and the input of DOC from the Amur to the Sea of Okhotsk was estimated to be approximately 2.5 TgC/yr. The relationships between the temperature and the DOC and POC along a density surface from 26.7 to 27.0 σθ suggest that DOC and POC in the DSW are transported into the intermediate layer of the pelagic region without being removed at the shelf edge. Simple estimates for additions of organic carbon into the intermediate layer of the area off the east coast of Sakhalin suggests that DSW outflow from the shelf brings into this layer approximately 13.6 and 0.9 TgC/yr of DOC and POC, respectively. These fluxes are much larger than the sinking POC flux from the in situ surface water, and so are fundamental in characterizing the biogeochemical cycles in the Sea of Okhotsk.

Organic matter distributions in the Eastern North Atlantic–Azores Front region

Temperate, transitional and subtropical waters of the remote Azores Front region east of Azores (24–40°N, 22–32°W) were sampled during three cruises conducted under increasing stratification conditions (April 1999, May 1997 and August 1998). Despite the temporal increase of surface temperature (by 5 °C) and stratification (by 2.1 min −2), as well as the thermocline shoaling (by ∼15 m), dissolved organic carbon (DOC) and nitrogen (DON) in the surface layer were not significantly different for the early spring, late spring and summer periods, with average concentrations of 69±2 μM-C and 5.2±0.4 μM-N, respectively. The surface excess of semi-labile DOC, compared with the baseline DOC concentration in the deep ocean (47±2 μM-C), represents 33% of the bulk DOC concentration and as much as 85% of the TOC (=POC+DOC) excess. When compared with the winter baseline (56±2 μM-C), the seasonal surface DOC excess is 20% of the bulk DOC concentration and 87% of the seasonal TOC excess. These results confirm the major role played by DOC in the carbon cycle of surface waters of the Azores Front region. The total amount of bioreactive DOC transported from the temperate to the subtropical North Atlantic by the Ekman flux between March and December represents only ∼15% of the average annual primary production, and ∼15% and ∼30% of the measured sinking POC flux+vertical DOC eddy diffusion during early spring and summer, respectively. Vertical eddy diffusion is 35% and 2% of the spring and summer sinking POC flux, respectively. On the other hand, DOC only contributes 13% to the local oxidation of organic matter in subsurface waters (between the pycnocline and 500 m) of the study region.

Quality of Particulate Matter and Its Potential Sinking Rates in Pelagic Samples of Inshore and Bodden Waters of the Southern Baltic Sea

In situ measurements of vertical particle transport are strongly influenced by water currents and resuspension. For mass balances, especially for calculations of POM im- and export in certain areas, or for estimating the significance of aggregate formation in the context of organisms' removal from the euphotic zone, a potential sedimentation of organic particles preferably in combination with particle quality parameters influencing sinking velocity is needed. Sedimentation of POM can be measured by small scale laboratory experiments in settling tubes. We used different parameters that describe the quantity and quality of POM in combination with the SETCOL method, Sinking rates of POC from 3 different nearshore and inner coastal waters of the Southern Baltic Sea were low with 1-32 cm d -1 , but comparable to rates obtained from several phytoplankton species, e.g. diatoms, which are important parts of POM and have a similar size range compared to the particles investigated in this study. We did not find any correlation between the sinking rate and the quantity or quality of particulate organic matter. Obviously, the differences in organic matter composition of particles smaller than 200 μm, investigated with this miniscale experimental method, did not affect the sinking rates considerably. Calculated from sinking rates and POC concentrations 19-1,430 mg C could sediment per squaremetre and day, if horizontal matter transfer and resuspension were ignored as it was done during the settling procedure. Concerning the application of the SETCOL method to plankton samples we must stress the importance of appropriate laboratory incubation conditions. Special attention has to be paid to an equal light climate and oxygen supply for the reference and settling vessels.

Zooplankton vertical migration and the active transport of dissolved organic and inorganic carbon in the Sargasso Sea

The least known component of the “biological pump” is the active transport of carbon and nutrients by diel vertical migration of zooplankton. We measured CO 2 respiration and dissolved organic carbon (DOC) excretion by individual species of common vertically migrating zooplankton at the US JGOFS Bermuda Atlantic Time-series Study (BATS) station. The inclusion of DOC excretion in this study builds on published research on active transport by respiration of inorganic carbon and allows a direct assessment of the role of zooplankton in the production of dissolved organic matter used in midwater microbial processes. On average, excretion of DOC makes up 24% (range=5–42%) of the total C metabolized (excreted+respired) and could represent a significant augmentation to the vertical flux that has already been documented for respiratory CO 2 flux by migrant zooplankton. Migratory fluxes were compared to other transport processes at BATS. Estimates of combined active transport of CO 2 and DOC by migrators at BATS averaged 7.8% and reached 38.6% of mean sinking POC flux at 150 m, and reached 71.4% of mean sinking POC flux at 300 m. DOC export by migrator excretion averaged 1.9% and reached 13.3% of annual DOC export by physical mixing at this site. During most of the year when deep mixing does not occur, diel migration by zooplankton could provide a supply of DOC to the deeper layers that is available for use by the microbial community. A carbon budget comparing migrant zooplankton transport to the balance of fluxes in the 300–600 m depth strata at BATS shows on average that the total migrant flux supplies 37% of the organic carbon remineralized in this layer, and that migrant DOC flux is more than 3 times the DOC flux gradient by diapycnal mixing. New estimates of active transport of both organic and inorganic carbon by migrants may help resolve observed imbalances in the C budget at BATS, but the magnitude is highly dependent on the biomass of the migrating community.

Ocean margins as a significant source of organic matter to the deep open ocean

Continental shelves and slopes comprise less than 20% of the world ocean area, yet they are proposed to be quantitatively important sources of the organic matter that fuels respiration in the open ocean's interior1,2. At least certain regions of the coastal ocean produce more organic carbon than they respire3, suggesting that some fraction of this non-respired, unburied organic carbon is available for export from the coastal to the open ocean4. Previous studies of carbon fluxes in ocean margins1,5,6 have not considered the potential roles of dissolved organic carbon (DOC) and suspended particulate organic carbon (POCsusp), even though both pools are quantitatively far larger than sinking POC. Here we report natural radiocarbon (14C) abundance measurements that reveal continental slope and rise waters to contain both DOC and POCsusp that are concurrently older and in higher concentrations than DOC and POCsusp from the adjacent North Atlantic and North Pacific central gyres. Mass-balance calculations suggest that DOC and POCsusp inputs from ocean margins to the open ocean interior may be more than an order of magnitude greater than inputs of recently produced organic carbon derived from the surface ocean. Inputs from ocean margins may thus be one of the factors contributing to the old apparent age of organic carbon observed in the deep North Atlantic and Pacific central gyres7,8,9.

Distribution of particulate organic carbon and radiocarbon in the water column from the upper slope to the abyssal NE Pacific Ocean

We report profiles of concentrations and radiocarbon contents of suspended particulate organic carbon (POC susp) and sedimentary organic carbon from an abyssal site (Stn M) in the northeast (NE) Pacific collected in September 1994 (a period of very high flux of particulate carbon in the deep sea) and June 1995, as well as from stations on the continental rise and slope off the coast of California in June 1995. We show that during a period of anomalously high sinking POC flux to the deep sea (September 1994), Δ 14C of suspended POC did not decrease detectably between 85 and 1600 m depth. This is in contrast to depth profiles during low and moderate fluxes of sinking POC at this station where Δ 14C-POC susp decreased 50–60‰ in this depth range. One explanation for the constant Δ 14C values of POC susp between 85 and 1600 m is that large quantities of sinking POC could continuously release labile, 14C-enriched POC susp during biological and chemical alteration of the sinking POC. The radiocarbon evidence further suggests that resuspension of organic carbon from the sediment surface, either locally or laterally transported from the slope to the deep sea, is likely, but is probably limited to depths within a few hundred meters of the bottom. Sorption of ‘old’ DOC by suspended particulate matter in the water column is also possible, especially at shallower depths (<3500 m), though proof of this mechanism cannot be demonstrated at this time.

Transformations of biogenic particles during sedimentation in the northeastern Atlantic

The vertical flux and transformation of biogenic particles are im portant processes in the oceanic carbon cycle. Changes in the magnitude of the biological pump can occur in the north eastern Atlantic on both a seasonal and interannual basis. For example, seasonal variations in vertical flux at 47° N 20° W are linked to seasonal ocean productivity variations such as the spring bloom. The size and organic and inorganic content of phytoplankton species, their development and succession also play a role in the scale and composition of the biological pump. The majority of flux is in the form of fast sinking aggregates. Bacteria and transparent exopolymer particle production by phytoplankton have been implicated in aggregate production and mass flux events. Zooplankton grazing and faecal pellet production, their size and composition and extent of their vertical migration also influence the magnitude of vertical flux. Aggregates are formed in the upper ocean, often reaching a maximum concentration just below the seasonal thermocline and can be a food resource to mesozooplankton as well as to the high concentrations of attached bacteria and protozoa. Attached bacteria remineralize and solubilize the aggregate particulate organic carbon. The degree of particle solubilization is likely to be affected by factors controlling enzyme activity and production, for example temperature, pressure or concentration of specific organic molecules, all of which may change during sinking. Attached bacterial growth is greatest on particulate organic matter collected at 500 m which is the depth where studies of 210 Po reveal that there is greatest break-up of rapidly sinking particles. Break-up of particles by feeding zooplankton can also occur. The fraction of sinking POC lost between 150-3100 m at one station in the north eastern Atlantic could supply about 90% of the bacterial carbon demand. Some larger, faster sinking aggregates escape solubilization and disaggregation in the upper 1000 m and arrive in the deep ocean and on the deep-sea bed. Seasonally varying rates of sedimentation are reflected at the deep-sea floor by deposition of phytodetrital material in summer. Approximately 2-4% of surface water primary production reaches the sea floor in 4500 m depth at 47° N 20° W after a sedimentation time of about 4-6 weeks. In this region, concentrations of chloroplastic pigments increased in summer by an order of magnitude, whereas seasonal changes in activity or biomass parameters were smaller. Breakdown of the generally strongly degraded organic matter deposited on deep-sea sediments is mainly accomplished by bacteria. Rates of degradation and efficiency of biomass production depend largely on the proportion of biologically labile material which decreases with advancing decay. It is likely that different levels of organic matter deposition influence the bioturbation rates of larger benthos, which has an effect on transport processes within the sediment and presumably also on microbial degradation rates.

Mesozooplankton grazing and metabolism at the equator in the central Pacific: Implications for carbon and nitrogen fluxes

Primary productivity and chlorophyll in the equatorial Pacific are lower than expected based on ambient nutrient concentrations. We tested the hypothesis that these conditions are due to a balance between phytoplankton growth and mesozooplankton grazing. Grazing rates and biomasses of three size classes of zooplankton in the size range of 200-200 μm were measured during March-April and October of 1992 at the equator (140°W). El Niño conditions prevailed in March–April, whereas in October a tropical instability wave (TIW) passed through the study area. Weight-specific pigment ingestion rates of mesozooplankton tended to be higher in March–April than in October while the opposite was the case for biomass. There were no discernible temporal trends in mesozooplankton community grazing rates in March–April, but there were changes in October associated with the passage of the TIW. Mesozooplankton grazing removed 1–9% day −1 of the total chlorophyll and 1–12% of the primary production within the euphotic zone. Therefore, the grazing hypothesis as stated above is rejected, although a simple chlorophyll budget suggests that grazing (mostly by microzooplankton) and phytoplankton growth are roughly in balance. Most of the phytoplankton was < 2 μm and presumably unavailable as food for mesozooplankton. If we assume that mesozooplankton only fed on > 2-μm phytoplankton, we estimate removal rates of the > 2-μm chlorophyll standing stock of up to 27% day −1. Another question of biogeochemical importance is the rate of the production of large diatoms. Mesozooplankton grazing could have balanced growth of the large (> 20-μm) diatoms in March–April, but not in October. From estimates of respiration and excretion based on water temperature and body size, we infer that: (1) > 80% of the carbon ingested by mesozooplankton is not phytoplankton; (2) mesozooplankton excretion supports < 7% of the nitrogen demands of phytoplankton; (3) the flux of carbon passing through the mesozooplankton would be equivalent to 23% of the primary production; (4) mesozooplankton fecal carbon could account for 100% of the sinking POC flux in this region. Finally, a simple food chain analysis suggests that a significant fraction of the microzooplankton production probably passes through mesozooplankton.

Organic carbon turnover time in deep-sea benthos

The ratio of organic carbon supply to the deep-sea floor estimated with sediment traps to total benthic biomass (in terms of organic carbon concentration) was used to calculate “average” turnover or residence times for the benthic community over a broad depth interval in the western North Atlantic. Ratios indicate that the biomass carbon turns over on average time scales of months, with little predictable variation over the depth interval (70m to 5300m). Detrital organic carbon in sediments at the same stations turned over on scales of years to centuries. The shortest detrital carbon turnover was just over 8 years in a sandy continental shelf environment versus 609 years at 5.3km depth on the Hatteras Abyssal Plain. The turnover time estimates calculated with POC fluxes were assessed by comparing POC fluxes with estimates of total community respiration. At each location the POC flux rates were greater than the sediment oxygen demand flux rates (in mg Carbon m −2d −1), and the disparity was much greater in the deep sea than on the continental margin. We suggest that this difference is primarily the result of a decline in the quality of the organic compounds in the rain of sinking POC, rather than physiological responses to cold or pressure. This implies that caution must be used when interpreting the relationships between the total organic carbon fluxes measured with sediment traps at great depths and rates of metabolic processes.