Particle Sinking Rates Research Articles

Macroscale vertical power-law distribution of bacteria in dark oceans can emerge from microscale bacteria-particle interactions

Microbes in the dark oceans are a key determinant of remineralization of sinking carbon particles. However, most marine ecosystem models overlook how microbes aggregate on particles and the microscale interactions between particle-associated microbes, making it difficult to obtain mechanistic insights on their vertical power-law decay pattern. Here, we present a spatial population model where the attachment and detachment processes of bacterial cells depend on local density of particle-associated bacteria. We show that the power-law relationship can emerge when the non-random aggregated distribution of bacteria is considered without any depth-specific environmental parameters. Furthermore, the comparison between model behavior and empirical patterns in the Pacific and Southern Ocean indicated that temperature-dependent hydrolysis rate and nutrient-dependent sinking rate of particles are key parameters to explain the regional variations of the power-law exponent. The mechanistic approach developed here provides a pathway to link micro-scale interactions between individuals to macro-scale food chain structures and carbon cycle

Relationships Between Plankton Size Spectra, Net Primary Production, and the Biological Carbon Pump

AbstractPhotosynthesis in the surface ocean and subsequent export of a fraction of this fixed carbon leads to carbon dioxide sequestration in the deep ocean. Ecological relationships among plankton functional groups and theoretical relationships between particle size and sinking rate suggest that carbon export from the euphotic zone is more efficient when communities are dominated by large organisms. However, this hypothesis has never been tested against measured size spectra spanning the >5 orders of magnitude found in plankton communities. Using data from five ocean regions (California Current Ecosystem, North Pacific subtropical gyre, Costa Rica Dome, Gulf of Mexico, and Southern Ocean subtropical front), we quantified carbon‐based plankton size spectra from heterotrophic bacteria to metazoan zooplankton (size class cutoffs varied slightly between regions) and their relationship to net primary production and sinking particle flux. Slopes of the normalized biomass size spectra (NBSS) varied from −1.6 to −1.2 (median slope of −1.4 equates to large 1–10 mm organisms having a biomass equal to only 7.6% of the biomass in small 1–10 μm organisms). Net primary production was positively correlated with the NBSS slope, with a particularly strong relationship in the microbial portion of the size spectra. While organic carbon export co‐varied with NBSS slope, we found only weak evidence that export efficiency is related to plankton community size spectra. Multi‐variate statistical analysis suggested that properties of the NBSS added no explanatory power over chlorophyll, primary production, and temperature. Rather, the results suggest that both plankton size spectra and carbon export increase with increasing system productivity.

Spatiotemporal Variability of Microplastics in the Eastern Baltic Sea

Microplastic (MP) pollution is present in all aquatic environments and is gaining critical concern. We have conducted sea surface MP monitoring with a Manta trawl at 16 sampling stations in the eastern Baltic Sea in 2016–2020. The concentrations varied from 0.01 to 2.45 counts/m3 (0.002–0.43 counts/m2), and the mean was 0.49 counts/m3 (0.08 counts/m2). The fibers and fragments had, on average, an approximately equal share in the samples. Correlation between the concentration of fibers and fragments was higher near the land and weaker further offshore. The following spatial patterns were revealed: higher mean values were detected in the Baltic Proper (0.65 counts/m3) (0.11 counts/m2) and the Gulf of Finland (0.46–0.65) (0.08–0.11) and lower values were detected in the Gulf of Riga (0.33) (0.06) and Väinameri Archipelago Sea (0.11) (0.02). The difference between the latter three sub-basins and the meridional gradient in the Gulf of Riga can likely be explained by the degree of human pressure in the catchment areas. The MP concentration was higher in autumn than in summer in all regions and stations, probably due to the seasonality of the biofouling and consequent sinking rate of particles. A weak negative correlation between the wind speed and the MP concentration was detected only in the central Gulf of Finland, and positive correlation in the shallow area near river mouth. We observed a 60-fold difference in MP concentrations during coastal downwelling/upwelling. Divergence/convergence driven by the (sub)mesoscale processes should be one of the subjects in future studies to enhance the knowledge on the MP pathways in the Baltic Sea.

Environmental influences on sinking rates and distributions of transparent exopolymer particles after a typhoon surge at the Western Pacific

A multidisciplinary approach was used to investigate the causes of the distributions and sinking rates of transparent exopolymer particles (TEPs) during the period of September–October (2017) in the Western Pacific Ocean (WPO); the study period was closely dated to a northwest typhoon surge. The present study discussed the impact of biogeophysical features on TEPs and their sinking rates (sTEP) at depths of 0–150 m. During the study, the concentration of TEPs was found to be higher in areas adjacent to the Kuroshio current and in the bottom water layer of the Mindanao upwelling zone due to the widespread distribution of cyanobacteria, i.e., Trichodesmium hildebrandti and T. theibauti. The positive significant regressions of TEP concentrations with Chl-a contents in eddy-driven areas (R2 = 0.73, especially at 100 m (R2 = 0.75)) support this hypothesis. However, low TEP concentrations and TEPs were observed at mixed layer depths (MLDs) in the upwelling zone (Mindanao). Conversely, high TEP concentrations and high sTEP were found at the bottom of the downwelling zone (Halmahera). The geophysical directions of eddies may have caused these conditions. In demonstrating these relations, the average interpretation showed the negative linearity of TEP concentrations with TEPs (R2 = 0.41 ~ 0.65) at such eddies. Additionally, regression curves (R2 = 0.78) indicated that atmospheric pressure played a key role in the changes in TEPs throughout the study area. Diatoms and cyanobacteria also curved the TEPs significantly (R2 = 0.5, P < 0.05) at the surface of the WPO. This study also revealed that TEP concentration contributes less to the average particulate organic carbon in this oligotrophic WPO.

Role of suspended matter in controlling beryllium-7 (7Be) in the Black Sea surface layer

Spatial variability of 7Be activity in both particulate and dissolved phases in the Black Sea was studied using field observations from the 95th cruise of R/V Professor Vodyanitsky in the summer of 2017. The total (particulate + dissolved) 7Be activity varied spatially from 2.8 to 8.3 Bq m−3 and averaged to 5.3 ± 1.9 Bq m−3. The particulate fraction of 7Be activity varied spatially between 5% and 13%, with an average value of 8% ± 2%. This paper provides an analysis of the influence of atmospheric precipitation, mixed layer depth, total suspended matter concentration (TSM), seawater temperature and salinity on the spatial variability of 7Ве activity. It is shown that the variability of the TSM might determine the spatial heterogeneity of total 7Be activity in the study area. Quantitative estimates of the distribution coefficient (Kd) of 7Be for the dissolved and particulate phases were obtained, and the functional dependence of Kd on the TSM was established. The average Kd value was determined to be (1.6 ± 0.3) × 102 m3 kg−1. Kd decreased with an increase in TSM. The obtained estimates suggested that the average 7Be sinking rate on the particulate matter was (1.3 ± 0.7) × 10−5 m s−1. Sinking particles removed approximately 7% of total 7Be activity from the surface layer offshore and reached 29% on the Black Sea shelf. The influence of the TSM, its sinking rate, vertical exchange intensity, and bathymetry on the 7Be sinking flux in the surface mixed layer (SML) were also investigated. A statistically significant correlation was established for both the particle sinking rate and TSM: an increase in the sinking rate and TSM led to an increase in the contribution of the sinking process to the total 7Be flux from SML.

Implications for the mesopelagic microbial gardening hypothesis as determined by experimental fragmentation of Antarctic krill fecal pellets.

Detritivores need to upgrade their food to increase its nutritional value. One method is to fragment detritus promoting the colonization of nutrient‐rich microbes, which consumers then ingest along with the detritus; so‐called microbial gardening. Observations and numerical models of the detritus‐dominated ocean mesopelagic zone have suggested microbial gardening by zooplankton is a fundamental process in the ocean carbon cycle leading to increased respiration of carbon‐rich detritus. However, no experimental evidence exists to demonstrate that microbial respiration rates are higher on recently fragmented sinking detrital particles.Using aquaria‐reared Antarctic krill fecal pellets, we showed fragmentation increased microbial particulate organic carbon (POC) turnover by 1.9×, but only on brown fecal pellets, formed from the consumption of other pellets. Microbial POC turnover on un‐ and fragmented green fecal pellets, formed from consuming fresh phytoplankton, was equal. Thus, POC content, fragmentation, and potentially nutritional value together drive POC turnover rates.Mesopelagic microbial gardening could be a risky strategy, as the dominant detrital food source is settling particles; even though fragmentation decreases particle size and sinking rate, it is unlikely that an organism would remain with the particle long enough to nutritionally benefit from attached microbes. We propose “communal gardening” occurs whereby additional mesopelagic organisms nearby or below the site of fragmentation consume the particle and the colonized microbes.To determine how fragmentation impacts the remineralization of sinking carbon‐rich detritus and to parameterize microbial gardening in mesopelagic carbon models, three key metrics from further controlled experiments and observations are needed; how particle composition (here, pellet color/krill diet) impacts the response of microbes to the fragmentation of particles; the nutritional benefit to zooplankton from ingesting microbes after fragmentation along with identification of which essential nutrients are being targeted; how both these factors vary between physical (shear) and biological particle fragmentation.

Phytoplankton size classes changed oppositely over shelf and basin areas of the South China Sea during 2003–2018

Phytoplankton size classes (PSC) is closely related to the export of surface carbon to the deep ocean, by affecting the phytoplankton photosynthetic efficiency, particle sinking rate, and the structure of the marine food chain, etc. As the largest marginal sea of the western North Pacific, there have been some researches on satellite-retrieved PSC algorithms and PSC-relative POC export in the South China Sea (SCS), however, long-term changes of PSC and its effect on POC export in the SCS are still poorly understood. In this study, we constructed a reliable long-time series PSC dataset in the SCS by evaluating the performances of published empirical PSC models. Our results revealed opposite changes of PSC on shelf and basin areas of the SCS in the past 16 yrs (2003–2018). Over the shelf regions, the proportion of phytoplankton in micro-size class (20–200 µm) raised significantly (0.28 unit·year−1, p < 0.05). While, in the basin, phytoplankton in pico-size class (<2 µm) increased significantly (0.19 unit·year−1, p < 0.05). The driving factors of long-term PSC variability varied by region in the SCS. Processes related to the long-term rise of sea surface temperature were the key factors in the western areas of the Luzon Strait; in the southeastern areas of Vietnam and the northern shelf, local alongshore winds and currents played important role in driving the PSC changes. In the context of PSC shifts, phytoplankton primary production raised on the shelf and decreased in the basin. Meanwhile, the POC export at euphotic depth in the SCS remained stable during the 2003–2018 period. System comparison between shelf and basin would be helpful to improve the understanding of PSC changes in the tropical marginal seas and carbon fixation in the context of climate change.

Concentrations and sinking rates of transparent exopolymer particles (TEPs) in a coastal sea: the Changjiang River (Yangtze River) Estuary

Transparent exopolymer particles (TEPs) are ubiquitous throughout the oceans, and their sedimentation is considered an efficient biological carbon sink pathway. To investigate the role of coastal TEPs in sinking carbon from the upper layer, samples were collected in the spring and summer of 2011 in the Changjiang River (Yangtze River) Estuary, a typical coastal water. The concentrations and sinking rates of TEPs were measured, and potential sedimentation flux of TEPs was estimated. TEPs concentrations ranged from 40.00 µg/L to 1 040.00 µg/L (mean=(209.70±240.93) µg/L) in spring and 56.67 µg/L to 1 423.33 µg/L (mean=(433.33±393.02) µg/L) in summer, and they were higher at bloom stations than at non-bloom stations during both cruises. A significant positive correlation between TEPs concentration and chlorophyll a (Chl a) concentration was detected, suggesting that phytoplankton was the primary source of TEPs in this area. TEPs sinking rates ranged from 0.08 m/d to 0.57 m/d with a mean of (0.28±0.14) m/d in spring and 0.10 m/d to 1.08 m/d with a mean of (0.34±0.31) m/d in summer. The potential sedimentation flux of TEP-C ranged from 4.95 mg/(m2·d) to 29.40 mg/(m2·d) with a mean of (14.66±8.83) mg/(m2·d) in spring and 6.80 mg/(m2·d) to 30.45 mg/(m2·d) with a mean of (15.71±8.73) mg/(m2·d) in summer, which was ∼17.81% to 138.27% (mean=65.15%±31.75%) of sedimentation flux of phytoplankton cells in the study area. Due to the increase of TEPs concentrations and their sinking rates, sedimentation fluxes of TEPs at the bloom station were obviously higher than at the non-bloom station during both cruises. This study indicates that TEPs serve as a carbon sink in the Changjiang River Estuary, especially during bloom events, and their sedimentation should be taken into account when we study the carbon sedimentation in the coastal sea.

Remineralization of particulate organic carbon in an ocean oxygen minimum zone

Biological oceanic processes, principally the surface production, sinking and interior remineralization of organic particles, keep atmospheric CO2 lower than if the ocean was abiotic. The remineralization length scale (RLS, the vertical distance over which organic particle flux declines by 63%, affected by particle respiration, fragmentation and sinking rates) controls the size of this effect and is anomalously high in oxygen minimum zones (OMZ). Here we show in the Eastern Tropical North Pacific OMZ 70% of POC remineralization is due to microbial respiration, indicating that the high RLS is the result of lower particle fragmentation by zooplankton, likely due to the almost complete absence of zooplankton particle interactions in OMZ waters. Hence, the sensitivity of zooplankton to ocean oxygen concentrations can have direct implications for atmospheric carbon sequestration. Future expansion of OMZs is likely to increase biological ocean carbon storage and act as a negative feedback on climate change.

Flexible C : N ratio enhances metabolism of large phytoplankton when resource supply is intermittent

Abstract. Phytoplankton cell size influences particle sinking rate, food web interactions and biogeographical distributions. We present a model in which the uptake, storage and assimilation of nitrogen and carbon are explicitly resolved in different-sized phytoplankton cells. In the model, metabolism and cellular C : N ratio are influenced by the accumulation of carbon polymers such as carbohydrate and lipid, which is greatest when cells are nutrient starved, or exposed to high light. Allometric relations and empirical data sets are used to constrain the range of possible C : N, and indicate that larger cells can accumulate significantly more carbon storage compounds than smaller cells. When forced with extended periods of darkness combined with brief exposure to saturating irradiance, the model predicts organisms large enough to accumulate significant carbon reserves may on average synthesize protein and other functional apparatus up to five times faster than smaller organisms. The advantage of storage in terms of average daily protein synthesis rate is greatest when modeled organisms were previously nutrient starved, and carbon storage reservoirs saturated. Small organisms may therefore be at a disadvantage in terms of average daily growth rate in environments that involve prolonged periods of darkness and intermittent nutrient limitation. We suggest this mechanism is a significant constraint on phytoplankton C : N variability and cell size distribution in different oceanic regimes.

Factors influencing particulate lipid production in the East Atlantic Ocean

Extensive analyses of particulate lipids and lipid classes were conducted to gain insight into lipid production and related factors along the biogeochemical provinces of the Eastern Atlantic Ocean. Data are supported by particulate organic carbon (POC), chlorophyll a (Chl a), phaeopigments, Chl a concentrations and carbon content of eukaryotic micro-, nano- and picophytoplankton, including cell abundances for the latter two and for cyanobacteria and prokaryotic heterotrophs. We focused on the productive ocean surface (2m depth and deep Chl a maximum (DCM)). Samples from the deep ocean provided information about the relative reactivity and preservation potential of particular lipid classes. Surface and DCM particulate lipid concentrations (3.5–29.4μgL−1) were higher than in samples from deep waters (3.2–9.3μgL−1) where an increased contribution to the POC pool was observed. The highest lipid concentrations were measured in high latitude temperate waters and in the North Atlantic Tropical Gyral Province (13–25°N). Factors responsible for the enhanced lipid synthesis in the eastern Atlantic appeared to be phytoplankton size (micro, nano, pico) and the low nutrient status with microphytoplankton having the most expressed influence in the surface and eukaryotic nano- and picophytoplankton in the DCM layer. Higher lipid to Chl a ratios suggest enhanced lipid biosynthesis in the nutrient poorer regions. The various lipid classes pointed to possible mechanisms of phytoplankton adaptation to the nutritional conditions. Thus, it is likely that adaptation comprises the replacement of membrane phospholipids by non-phosphorus containing glycolipids under low phosphorus conditions. The qualitative and quantitative lipid compositions revealed that phospholipids were the most degradable lipids, and their occurrence decreased with increasing depth. In contrast, wax esters, possibly originating from zooplankton, survived downward transport probably due to the fast sinking rate of particles (fecal pellets). The important contribution of glycolipids in deep waters reflected their relatively stable nature and degradation resistance. A lipid-based proxy for the lipid degradative state (Lipolysis Index) suggests that many lipid classes were quite resistant to degradation even in the deep ocean.

Phytoplankton community dynamics during late spring coccolithophore blooms at the continental margin of the Celtic Sea (North East Atlantic, 2006–2008)

We determined the spatial and temporal dynamics of major phytoplankton groups in relation to biogeochemical and physical variables during the late spring coccolithophore blooms (May–June) along and across the continental margin in the Celtic Sea (2006–2008). Photosynthetic biomass (chl a) of the dominant plankton groups was determined by CHEMTAX analysis of chromatographic (HPLC) pigment signatures.Phytoplankton standing stock biomass varied substantially between and during the campaigns (areal chl a [mgchl am−2] in June 2006: 63.8±26.5, May 2007: 27.9±8.4, and May 2008: 41.3±21.8), reflecting the different prevailing conditions of weather, irradiance, and sea surface temperature between the campaigns. Coccolithophores, represented mainly by Emiliania huxleyi, and diatoms were the dominant phytoplankton groups, with a maximal contribution of, respectively, 72% and 89% of the total chl a. Prasinophytes, dinoflagellates, and chrysophytes often co-occurred during coccolithophorid blooms, while diatoms dominated the phytoplankton biomass independently of the biomass of other groups. The location of the stations on the shelf or on the slope side of the continental margin did not influence the biomass and the composition of the phytoplankton community despite significantly stronger water column stratification and lower nutrient concentrations on the shelf. The alternation between diatom and coccolithophorid blooms of similar biomass, following the mostly diatom-dominated main spring bloom, was partly driven by changes in nutrient stoichiometry (N:P and dSi:N). High concentrations of transparent exopolymer particles (TEP) were associated with stratified, coccolithophore-rich water masses, which probably originated from the slope of the continental margin and warmed during advection onto the shelf. Although we did not determine the proportion of export production attributed to phytoplankton groups, the abundance of coccolithophores, together with TEP and coccoliths may affect the carbon export efficiency through increased sinking rates of particles formed by aggregation of TEP and coccoliths.

Optical imaging of mesopelagic particles indicates deep carbon flux beneath a natural iron‐fertilized bloom in the Southern Ocean

We recorded vertical profiles of size distributions of particles (ranging from 0.052 to several mm in equivalent spherical diameter) in the natural iron‐fertilized bloom southeast of Kerguelen Island (Southern Ocean) and in surrounding high‐nutrient, low‐chlorophyll (HNLC) waters with an Under Water Video Profiler during the Kerguelen Ocean and Plateau Compared Study cruise (Jan–Feb 2005). Total particle numerical abundance and total particle volume (TPV) in the 0–200‐m layer were respectively 3‐fold and 20‐fold higher in the bloom, and integrated TPV was correlated to integrated chlorophyll concentration. The difference persisted well into the ocean interior with a 10‐fold higher TPV at 400‐m depth beneath the natural iron‐fertilized bloom. Below 400 m, increases in TPV values at the bloom stations reflect the suspension of bottom sediments. Bloom waters had a greater proportion of large particles from the surface to 400 m and also exhibited an increase of this proportion with depth compared to HNLC waters. Multiple visits to the bloom reference Sta. A3, suggest preferential removal of large particles as the bloom declined. Comparing our particle abundance size spectra with those observed previously in polyacrylamide gel‐filled sediment traps allows us to estimate mesopelagic particle sinking rates. These results suggest that particles sink faster in the HNLC waters than beneath the bloom. The fact that sinking speeds were not a simple monotonic function of particle size and varied spatially highlights the need to go beyond parameterizations of sinking rate as a function of size alone.

A preliminary study of particle dynamics in the water column of Saronikos Gulf, Greece, by using Th-234/U-238 disequilibrium approach

In the present study the particle export flux in the water column of 3 sampling stations (one of them near the pipeline outfall of the Waste Water Treatment Plant—WWTP of Psitalia) in the Eastern Saronikos gulf, Central Greece, is investigated based on the disequilibrium ratio of 234Th with its parent 238U. This approach is based on direct measurements of the activity concentrations of 234Th and calculated ones of 238U from the respective salinity measurements. From the derived results, the values of the activity concentrations of particulate and dissolved fraction of 234Th in the surface layer of the studied area range from 2.7±1.4 to 3.8±1.8 Bq m-3 and 13.2±1.4 to 28.2±1.9 Bq m-3 respectively, which educe fluctuation according to the station environmental features. Regarding the vertical distribution of radionuclides, the observed profile curves from the 3 sampling stations follow the same pattern of dispersion in the water column. However, the calculated export flux (sinking rate) of particles associated with 234Th results to a wide range of values depending on the sampling station. From the calculated residence times in the 3 stations, the station near the WWTP shows the longer residence time for the dissolved fraction, which is attributed to the sewage outflow. Besides, total suspended matter (TSM) and dissolved organic carbon (DOC) have been analyzed in the samples and the correlation coefficients with the particulate and dissolved 234Th have been calculated on the basis of the relation of 234Th cycling with particulate matter and organic substances.

Importance of the different manganese species in the formation of water column redox zones: Observations and modeling

The joint analysis of manganese species distributions (dissolved Mn, particulate Mn, and bound Mn) obtained in the Black Sea, the Baltic Sea (Landsort Deep) and Oslo Fjord (Baerumsbassenget), enabled common features to be revealed that demonstrate the similarity in the mechanisms of pelagic redox-layer formation. Our investigations demonstrate that Mn bound in stable complexes with hypothetical P-containing compounds or organic matter (OM) is observed in the redox zones at significant (average: 0.5–1.0 μM) concentrations and is likely present as Mn (III), an intermediate product of Mn (II) oxidation. This bound Mn (III) can explain phosphate distribution in redox interfaces and the formation of the so-called “phosphate dipole” with a minimum level above the sulfidic boundary, a maximum just below and a rapid increase in concentration within this range. This dipole structure serves as a geochemical barrier that decreases the upward flux of phosphate from the anoxic layer. Modeling results show that the manganese cycle (formation of settling Mn (IV) particles and presence of dissolved Mn (III)) is the main cause of lack of oxygen and hydrogen sulfide contact. According to the model, the formation of the “phosphate dipole” may also be connected with manganese cycling and different factors (i.e. mixing, particle sinking rate, presence of Mn in the water column) should affect the peculiarities of the observed distributions at the different geographical locations.

Offshore advection of particles within the Cape Blanc filament, Mauritania: Results from observational and modelling studies

This article will review major features of the ‘giant’ Cape Blanc filament off Mauritania with regard to the transport of chlorophyll and organic carbon from the shelf to the open ocean. Within the filament, chlorophyll is transported about 400 km offshore. Modelled particle distributions along a zonal transect at 21°N showed that particles with a sinking velocity of 5 m d −1 are advected offshore by up to 600 km in subsurface particle clouds generally located between 400 m and 800 m water depth, forming an Intermediate Nepheloid Layer (INL). It corresponds to the depth of the oxygen minimum zone. Heavier particles with a sinking velocity of 30 m d −1 are transported from the shelf within the Bottom Layer (BL) of more than 1000 m thickness, largely following the topography of the bottom slope. The particles advected within the BL contribute to the enhanced winter–spring mass fluxes collected at the open-ocean mesotrophic sediment trap site CB-13 (∼200 nm offshore), due to a long distance advection in deeper waters. The lateral contribution to the deep sediment trap in winter–spring is estimated to be 63% and 72% for organic carbon and total mass, respectively, whereas the lateral input for both components on an annual basis is estimated to be in the order of 15%. Biogenic opal increases almost fivefold from the upper to the lower mesotrophic CB-13 trap, also pointing to an additional source for biogenic silica from eutrophic coastal waters. Blooms obviously sink in smaller, probably mesoscale-sized patches with variable settling rates, depending on the type of aggregated particles and their ballast content. Generally, particle sinking rates are exceptionally high off NW Africa. Very high chlorophyll values and a large size of the Cape Blanc filament in 1998–1999 are also documented in enhanced total mass and organic carbon fluxes. An increasing trend in satellite chlorophyll concentrations and the size of the Cape Blanc filament between 1997 and 2008 as observed for other coastal upwelling areas is not documented.

Particle fluxes in the deep Eastern Mediterranean basins: the role of ocean vertical velocities

Abstract. This paper analyzes the relationship between deep sedimentary fluxes and ocean current vertical velocities in an offshore area of the Ionian Sea, the deepest basin of the Eastern Mediterranean Sea. Sediment trap data are collected at 500 m and 2800 m depth in two successive moorings covering the period September 1999–May 2001. A tight coupling is observed between the upper and deep traps and the estimated particle sinking rates are more than 200 m day−1. The current vertical velocity field is computed from a 1/16°×1/16° Ocean General Circulation Model simulation and from the wind stress curl. Current vertical velocities are larger and more variable than Ekman vertical velocities, yet the general patterns are alike. Current vertical velocities are generally smaller than 1 m day−1: we therefore exclude a direct effect of downward velocities in determining high sedimentation rates. However we find that upward velocities in the subsurface layers of the water column are positively correlated with deep particle fluxes. We thus hypothesize that upwelling would produce an increase in upper ocean nutrient levels – thus stimulating primary production and grazing – a few weeks before an enhanced vertical flux is found in the sediment traps. High particle sedimentation rates may be attained by means of rapidly sinking fecal pellets produced by gelatinous macro-zooplankton. Other sedimentation mechanisms, such as dust deposition, are also considered in explaining large pulses of deep particle fluxes. The fast sinking rates estimated in this study might be an evidence of the efficiency of the biological pump in sequestering organic carbon from the surface layers of the deep Eastern Mediterranean basins.

Sinking rates and ballast composition of particles in the Atlantic Ocean: implications for the organic carbon fluxes to the deep ocean

Abstract. The flux of materials to the deep sea is dominated by larger, organic-rich particles with sinking rates varying between a few meters and several hundred meters per day. Mineral ballast may regulate the transfer of organic matter and other components by determining the sinking rates, e.g. via particle density. We calculated particle sinking rates from mass flux patterns and alkenone measurements applying the results of sediment trap experiments from the Atlantic Ocean. We have indication for higher particle sinking rates in carbonate-dominated production systems when considering both regional and seasonal data. During a summer coccolithophorid bloom in the Cape Blanc coastal upwelling off Mauritania, particle sinking rates reached almost 570 m per day, most probably due the fast sedimentation of densely packed zooplankton fecal pellets, which transport high amounts of organic carbon associated with coccoliths to the deep ocean despite rather low production. During the recurring winter-spring blooms off NW Africa and in opal-rich production systems of the Southern Ocean, sinking rates of larger particles, most probably diatom aggregates, showed a tendency to lower values. However, there is no straightforward relationship between carbonate content and particle sinking rates. This could be due to the unknown composition of carbonate and/or the influence of particle size and shape on sinking rates. It also remains noticeable that the highest sinking rates occurred in dust-rich ocean regions off NW Africa, but this issue deserves further detailed field and laboratory investigations. We obtained increasing sinking rates with depth. By using a seven-compartment biogeochemical model, it was shown that the deep ocean organic carbon flux at a mesotrophic sediment trap site off Cape Blanc can be captured fairly well using seasonal variable particle sinking rates. Our model provides a total organic carbon flux of 0.29 Tg per year down to 3000 m off the NW African upwelling region between 5 and 35° N. Simple parameterisations of remineralisation and sinking rates in such models, however, limit their capability in reproducing the flux variation in the water column.

In situ measurement of mesopelagic particle sinking rates and the control of carbon transfer to the ocean interior during the Vertical Flux in the Global Ocean (VERTIGO) voyages in the North Pacific

Abstract Among the parameters affecting carbon transfer to the ocean interior, particle sinking rates vary three orders of magnitude and thus more than primary production, f -ratios, or particle carbon contents [e.g., Boyd, P.W., Trull, T.W., 2006. Understanding the export of marine biogenic particles: is there consensus? Progress in Oceanography 4, 276–312, doi:10.1016/j.pocean.2006.10.007 ]. Very few data have been obtained from the mesopelagic zone where the majority of carbon remineralization occurs and the attenuation of the sinking flux is determined. Here, we report sinking rates from ∼300 m depth for the subtropical (station ALOHA, June 2004) and subarctic (station K2, July 2005) North Pacific Ocean, obtained from short (6.5 day) deployments of an indented rotating sphere (IRS) sediment trap operating as an in situ settling column [Peterson, M.L., Wakeham, S.G., Lee, C., Askea, M.A., Miquel, J.C., 2005. Novel techniques for collection of sinking particles in the ocean and determining their settling rates. Limnology and Oceanography Methods 3, 520–532] to separate the flux into 11 sinking-rate fractions ranging from >820 to >2 m d −1 that are collected by a carousel for further analysis. Functioning of the IRS trap was tested using a novel programming sequence to check that all particles have cleared the settling column prior to the next delivery of particles by the 6-hourly rotation cycle of the IRS. There was some evidence (from the flux distribution among the cups and photomicroscopy of the collected particles) that very slow-sinking particles may have been under-collected because they were unable to penetrate the brine-filled collection cups, but good evidence for appropriate collection of fast-settling fractions. Approximately 50% of the particulate organic carbon (POC) flux was sinking at greater than 100 m d −1 at both stations. At ALOHA, more than 15% of the POC flux sank at >820 m d −1 , but low fluxes make this uncertain, and precluded resolution of particles sinking slower than 137 m d −1 . At K2, less than 1% of the POC flux sank at >820 m d −1 , but a large fraction (∼15–45%) of the flux was contributed by other fast-sinking classes (410 and 205 m d −1 ). PIC and BSi minerals were not present in higher proportions in the faster sinking fractions, but the observations were too limited to rule out a ballasting contribution to the control of sinking rates. Photographic evidence for a wide range of particle types within individual sinking-rate fractions suggests that biological processes that set the porosity and shape of particles are also important and may mask the role of minerals. Comparing the spectrum of sinking rates observed at K2 with the power-law profile of flux attenuation with depth obtained from other VERTIGO sediment traps deployed at multiple depths [Buesseler, K.O., Lamborg, C.H., Boyd, P.W., Lam, P.J., Trull, T.W., Bidigare, R.R., Bishop, J.K.B., Casciotti, K.L., Dehairs, F., Elskens, M., Honda, M., Karl, D.M., Siegel, D., Silver, M., Steinberg, D., Valdes, J., Van Mooy, B., Wilson, S.E., 2007b. Revisiting carbon flux through the Ocean's twilight zone. Science 316(5824), 567–570, doi: 10.1126/science.1137959] emphasizes the importance of particle transformations within the mesopelagic zone in the control of carbon transport to the ocean interior.

Formation and behavior of composite CO 2 hydrate particles in a high-pressure water tunnel facility

Sinking CO 2 composite particles consisting of seawater, liquid CO 2 , and CO 2 hydrate were produced by a coaxial flow injector fed with liquid CO 2 and artificial seawater. The particles were injected into a high-pressure water tunnel facility to permit determination of their settling velocities and dissolution rates. Injections were performed at fixed pressures approximately equivalent to 1200-m, 1500-m, and 1800-m depths and at temperatures varying from approximately 2 to 5 °C. Immediately after injection, the cylindrical particles were observed to break away from the injector tip and often aggregated into sinking clusters. The seawater flow in the tunnel was then adjusted in a countercurrent flow mode to suspend the particles in an observation window so that images of the particles could be recorded for later analysis. The flow would often break or cause rearrangement of some of the clusters. Selected individual particles and some clusters were studied until they became too hydrodynamically unstable to follow. In general, the flow required to suspend clusters or individual particles decreased with time as the particles dissolved. For example, one particle was produced and observed for over 6 min at an average pressure of 15.022 MPa and an average temperature of 5.1 °C. Its sinking rate, determined from the flow required for stabilization, changed from 37.2 to 3.3 mm/s over this time. Particle sinking rates were compared to correlations from the literature for uniform cylindrical objects. Reasonable agreement was observed for short times; however, the observed decrease in sinking velocity with time was greater than that predicted by the correlations for longer times. Particle dissolution rates, based on changes in diameter, were also determined and varied from 5 to 11 μ m / s . A pseudo-homogeneous mass transfer model was used to predict single-particle dissolution rates. Good agreement was achieved between experimental dissolution data and the modeling results.