Biogeochemistry of surface sediments off Concepción (∼36°S), Chile: El Niño vs. non-El Niño conditions

We compared the signals of several water column properties (upwelling intensity, sea level anomaly, temperature, nutrients, dissolved oxygen, chlorophyll-a, and surface sediments) of the continental shelf off Concepción (36°S) during the 1997-1998 El Niño with those of a normal year (2002-2003). We found that the primary hydrographic effect of El Niño 1997-1998 was a reduction in the input of nutrient-rich, oxygen-poor Equatorial Subsurface Water over the shelf. This affected the biology of the water column, as evidenced by the reduced phytoplankton biomass. Surface sediment properties (biogenic opal, organic carbon, bulk δ¹⁵N) observed during El Niño 1997-1998 reflected a reduced export production and the sediments failed to show the water column seasonality that occurs under normal conditions. In addition, weakened denitrification and/or upper water column fertilization could be inferred from the sedimentary δ¹⁵N. Although diminished, export production was preserved in the surface sediments, revealing less degraded organic matter in the upwelling period of the El Niño year than in the normal year. We suggest that the fresher organic material on the seafloor was probably associated with a severe reduction in the polychaete Parapronospio pinnata, which is considered to be the most important metazoan remineralizer of organic carbon at the sediment-water interface in the study area.     

Microbial plankton abundance and heterotrophic activity across the Central Atlantic Ocean

The role of microorganisms in the transfer of carbon of marine systems is very important in open oligotrophic oceans. Here, we analyze the picoplankton structure, the heterotrophic bacterioplankton activity, and the predator-prey relationships between heterotrophic bacteria and nanoflagellates during two large scale cruises in the Central Atlantic Ocean (∼29°N to ∼40°S). Latitud cruises were performed in 1995 between March-April and October-November. During both cruises we crossed the regions of different trophic statuses; where we measured different biological variables both at the surface and at the deep chlorophyll maximum (DCM). The concentration of chlorophyll a varied between 0.1 and 0.8 mg m⁻³, the abundance of heterotrophic bacteria varied between <1.0 × 10⁵ and >1.0 × 10⁶ cells ml⁻¹, and that of heterotrophic nanoflagellates between <100 and >1.0 × 10⁴ cells ml⁻¹. The production of heterotrophic bacteria varied more than three orders of magnitude between <0.01 and 24 μgC L⁻¹ d⁻¹; and the growth rates were in the range <0.01-2.1 d⁻¹. In the Latitud-II cruise, Prochlorococcus ranged between <10³ and >3 × 10⁵ cells ml⁻¹, Synechococcus between <100 and >1.0 × 10⁴ cells ml⁻¹, and picoeukaryotes between <100 and >10⁴ cells ml⁻¹.Two empirical models were used to learn more about the relationship between heterotrophic bacteria and nanoflagellates. Most bacterial production was ingested when this production was low, the heterotrophic nanoflagellates could be controlled by preys during Latitud-I cruise at the DCM, and by predators in the surface and in the Latitud-II cruise. Our results were placed in context with others about the structure and function of auto- and heterotrophic picoplankton and heterotrophic nanoplankton in the Central Atlantic Ocean.     

Influence of ENSO variability on sinking-particle fluxes in the northeastern equatorial Pacific

A time-series sediment trap was operated from July 2003 to July 2008 at a station located in the 10°N thermocline ridge of the northeastern equatorial Pacific (10°30′N, 131°20′W), with the aim of understanding variations in natural background sinking-particle flux and the influence on such fluxes of ENSO (El Niño-Southern Oscillation). Each one of weak El Niño, moderate El Niño and moderate La Niña were observed during the monitoring period. During non-ENSO periods, total mass fluxes varied from 4.1 to 36.9 mg m⁻² d⁻¹, with a distinct seasonal variation, ranging from an average flux of 14.0 mg m⁻² d⁻¹ in the warm season (June-November) to 25.3 mg m⁻² d⁻¹ in the cold season (December-May). This seasonal fluctuation was characterized by a distinct difference in CaCO₃ flux between the two seasons. The enhanced particle fluxes during the cold season are attributed to the supply of nutrient-enriched subsurface water by wind-driven vertical mixing, supported by a simultaneous reduction in sea surface temperature and enhanced trade winds. The weak El Niño event occurred in the monitoring period had no recognizable effect on particle fluxes in the study area, but the moderate El Niño event was accompanied by a significant reduction in particle fluxes to 60% of the average background value in the warm season. In contrast, particle fluxes during the moderate La Niña increased to a maximum value of 129.9 mg m⁻² d⁻¹, almost three times the average background value. Organic carbon and biogenic silica fluxes were most sensitive to the El Niño and La Niña conditions. The observed variations of particle fluxes are synchronized with those of chlorophyll-a, suggesting primary productivity for the main cause of flux change. The present data indicate that marked seasonal variability in background fluxes commonly exceeds the variability associated with ENSO and post-ENSO signals, which should be taken into account when evaluating the influence of ENSO on sinking particle fluxes in the 10°N thermocline ridge area.     

The fate of production in the central Arctic Ocean - top-down regulation by zooplankton expatriates?

We estimated primary and bacterial production, mineral nutrients, suspended chlorophyll a (Chl), particulate organic carbon (POC) and nitrogen (PON), abundance of planktonic organisms, mesozooplankton fecal pellet production, and the vertical flux of organic particles of the central Arctic Ocean (Amundsen basin, 89-88° N) during a 3 week quasi-Lagrangian ice drift experiment at the peak of the productive season (August 2001). A visual estimate of ≈15% ice-free surface, plus numerous melt ponds on ice sheets, supported a planktonic particulate primary production of 50-150 mg C m⁻² d⁻¹ (mean 93 mg C m⁻² d⁻¹, n = 7), mostly confined to the upper 10 m of the nutrient replete water column. The surface mixed layer was separated from the rest of the water column by a strong halocline at 20 m depth. Phototrophic biomass was low, generally 0.03-0.3 mg Chl m⁻³ in the upper 20 m and <0.02 mg Chl m⁻³ below, dominated by various flagellates, dinoflagellates and diatoms. Bacterial abundance (typically 3.7-5.3 × 10⁵, mean 4.1 × 10⁵ cells ml⁻¹ in the upper 20 m and 1.3-3.7 × 10⁵, mean 1.9 × 10⁵ cells ml⁻¹ below) and Chl concentrations were closely correlated (r = 0.75). Mineral nutrients (3 μmol NO₃ l⁻¹, 0.45 μmol PO₄ l⁻¹, 4-5 μmol SiO₄ l⁻¹) were probably not limiting the primary production in the upper layer. Suspended POC concentration was ∼30-105 (mean 53) mg C m⁻³ and PON ∼5.4-14.9 (mean 8.2) mg N m⁻³ with no clear vertical trend. The vertical flux of POC in the upper 30-100 m water column was ∼37-92 (mean 55) mg C m⁻² d⁻¹ without clear decrease with depth, and was quite similar at the six investigated stations. The mesozooplankton biomass (≈2 g DW m⁻², mostly in the upper 50 m water column) was dominated by adult females of the large calanoid copepods Calanus hyperboreus and Calanus glacialis (≈1.6 g DW m⁻²). The grazing of these copepods (estimated via fecal pellet production rates) was ≈15 mg C m⁻² d⁻¹, being on the order of 3% and 20% of the expected food-saturated ingestion rates of C. hyperboreus and C. glacialis, respectively. The stage structure of these copepods, dominated by adult females, and their unsatisfied grazing capacity during peak productive period suggest allochthonous origin of these species from productive shelf areas, supported by their long life span and the prevailing surface currents in the Arctic Ocean. We propose that the grazing capacity of the expatriated mesozooplankton population would match the potential seasonal increase of primary production in the future decreased ice perspective, diminishing the likelihood of algal blooms.     

Influence of allochthonous organic matter on bacterioplankton biomass and activity in a eutrophic,sub-tropical estuary

Heterotrophic bacterial and phytoplankton biomass, production, specific growth rates, and growth efficiencies were studied in the Northern region of the Cananéia–Iguape estuarine system, which has recently experienced an intense eutrophication due to anthropogenic causes. Two surveys were carried out during spring and neap tide periods of the dry season of 2005 and the rainy season of 2006. This region receives large freshwater inputs with organic seston and phosphate concentrations that reach as high as 1.0 mg l⁻¹ and 20.0 μM, respectively. Strong decreasing gradients of seston and dissolved inorganic nutrients were observed from the river/estuary boundary to the estuary/coastal interface. Gradients were also observed in phytoplankton and bacterial production rates. The production rates of phytoplankton were 5.6-fold higher (mean 8.5 μg C l⁻¹ h⁻¹) during the dry season. Primary production rates (PP) positively correlated with salinity and euphotic depth, indicating that phytoplankton productivity was light-limited. On the other hand, bacterial biomass (BB) and production rates (BP) were 1.9- and 3.7-fold higher, respectively, during the rainy season, with mean values of up to 40.4 μg C l⁻¹ and 7.9 μg C l⁻¹ h⁻¹, respectively. Despite such a high BP, bacterial abundance remained <2 × 10⁶ cells ml⁻¹, indicating that bacterial production and removal were coupled. Mean specific growth rates ranged between 0.9 and 5.5 d⁻¹. BP was inversely correlated with salinity and positively correlated with temperature, organic matter, exopolymer particles, and particulate-attached bacteria; this last accounted for as much as 89.6% of the total abundance. During the rainy season, BP was generally much higher than PP, and values of BP/PP > 20 were registered during high freshwater input, suggesting that under these conditions, bacterial activity was predominantly supported by allochthonous inputs of organic carbon. In addition, BB probably represented the main pathway for the synthesis of high-quality (low C:N) biomass that may have been available to the heterotrophic components of the plankton food web, particularly nanoheterotrophs.     

Spatial and temporal variability of microplankton and detritus, and their export to the shelf sediments in the upwelling area off Concepción, Chile (∼36°S), during 2002-2005

Seasonal variations in diversity and biomass of diatoms, tintinnids, and dinoflagellates and the contribution of microplankton and faecal material to the vertical flux of particulates were investigated at one time series station T (station 18) between 2002 and 2005 and at a grid of stations during November 2004 in the coastal and oceanic area off Concepción (36°S), Chile. The variations were analysed in relation to water column temperature, dissolved oxygen, nutrient concentration, offshore Ekman transport, and chlorophyll-a concentration. Abundance was estimated as cell numbers per litre and biomass in terms of biovolume and carbon units.A sharp decrease with depth was observed in the abundance of both phytoplankton and microzooplankton during the whole annual cycle; over 70% of their abundance was concentrated in the upper 10 m of the water column. Also, a clear seasonality in microplankton distribution was observed at station T, with maxima for diatoms, tintinnids, and dinoflagellates every summer (centred on January) from 2002 to 2005.On the grid of stations, the maximum integrated (0-50 m) micro-phytoplankton abundances (>1 × 10⁹ cells m⁻²) occurred at the coastal stations, an area directly influenced by upwelling. A similar spatial distribution was observed for the integrated (0-200 m) faecal carbon (with values up to 632 mg C m⁻²). Tintinnids were distributed in all the first 300 miles from the coast and dinoflagellates were more abundant in oceanic waters.At station T, the average POC export production (below 50 m depth) was 16.6% (SD = 17%; range 2-67%; n = 16). The biological-mediated fluxes of carbon between the upper productive layer and the sediments of the continental shelf off Concepción depend upon key groups of phytoplankton (Thalassiosira spp., Chaetoceros spp.) and zooplankton (euphausiids) through the export of either cells or faecal material, respectively.     

Nanoplanktonic assemblages in the upwelling area off Concepción (∼36°S), central Chile: Abundance, biomass, and grazing potential during the annual cycle

The structure and functioning of nanoplanktonic assemblages in coastal upwelling areas have usually been overlooked in explorations of the productivity of these areas. As part of a multidisciplinary, time-series station in the coastal area off Concepción, seasonal variations (upwelling and non-upwelling) in the abundance and biomass of these assemblages were investigated. Hydrographic measurements and biological samples were taken monthly over a 2-year period (18 August 2004-28 July 2006). Nanoflagellates dominated the total integrated abundance (3-317 × 10⁹ cells m⁻²; 0-80 m). Diatoms and dinoflagellates usually contributed to a lesser degree (<20%) but sporadically made important contributions to the total integrated nanoplankton biomass (0.02-10.6 g C m⁻²). Most of the nanoplankton was concentrated in surface waters (<30 m) during all the samplings and no seasonal differences in abundance or biomass were found in this layer, although the mean values and dispersions around them were highest during the upwelling period along with maximum integrated (0-80 m) chlorophyll-a values, as total or in the <20 μm fraction. Changes in nanoplankton abundance were significantly but weakly (r < 0.4) correlated with changes in the hydrographic variables; the highest correlation values were positive for temperature and oxygen, factors that varied with depth and date. The potential grazing rates of heterotrophic nano-predators (flagellates and dinoflagellates) on prokaryotic prey, estimated with a generic model, ranged from 3 to 242 bacterioplankton predator⁻¹ h⁻¹ and from 0.1 to 14 cyanobacteria predator⁻¹ h⁻¹. Our results imply a small impact of seasonal hydrographic variability on the abundance and biomass of nanoplanktonic assemblages and suggest that grazing by nanoheterotrophs might control the prokaryotic picoplankton populations in the upwelling area off Concepción.     

Association of picophytoplankton distribution with ENSO events in the equatorial Pacific between 145°E and 160°W

Climatological variability of picophytoplankton populations that consisted of >64% of total chlorophyll a concentrations was investigated in the equatorial Pacific. Flow cytometric analysis was conducted along the equator between 145°E and 160°W during three cruises in November–December 1999, January 2001, and January–February 2002. Those cruises were covering the La Niña (1999, 2001) and the pre-El Niño (2002) periods. According to the sea surface temperature (SST) and nitrate concentrations in the surface water, three regions were distinguished spatially, viz., the warm-water region with >28 °C SST and nitrate depletion (<0.1 μmol kg⁻¹), the upwelling region with <28 °C SST and high nitrate (>4 μmol kg⁻¹) water, and the in-between frontal zone with low nitrate (0.1–4 μmol kg⁻¹). Picophytoplankton identified as the groups of Prochlorococcus, Synechococcus and picoeukaryotes showed a distinct spatial heterogeneity in abundance corresponding to the watermass distribution. Prochlorococcus was most abundant in the warm-water region, especially in the nitrate-depleted water with >150×10³ cells ml⁻¹, Synechococcus in the frontal zone with >15×10³ cells ml⁻¹, and picoeukaryotes in the upwelling region with >8×10³ cells ml⁻¹. The warm-water region extended eastward with eastward shift of the frontal zone and the upwelling region during the pre-El Niño period. On the contrary, these regions distributed westward during the La Niña period. These climatological fluctuations of the watermass significantly influenced the distribution of picophytoplankton populations. The most abundant area of Prochlorococcus and Synechococcus extended eastward and picoeukaryotes developed westward during the pre-El Niño period. The spatial heterogeneity of each picophytoplankton group is discussed here in association with spatial variations in nitrate supply, ambient ammonium concentration, and light field.     

Time-series measurements of macrobenthos abundance and sediment bioturbation intensity on a flood-dominated shelf

Four 70-m stations on the continental shelf offshore from the Eel River (northern California) were occupied at roughly four-month intervals between February 1995 and March 1998, and in August 1999. At each of the stations, profiles of excess ²³⁴Th were used to quantify sediment bioturbation intensity. In addition, at two of the stations macrofaunal abundance, species composition and functional groupings were quantified. During the study period, the Eel River displayed a range of hydrological conditions, with historically significant floods in January 1995 and January 1997 (return periods of 15 and 40 y, respectively), relatively low flows during the winters of 1995-1996 and 1998-1999 and an El Niño year characterized by moderate, but frequent discharges in 1997-1998. The January 1995 and 1997 floods deposited 3-7 cm of fine-grained, high porosity sediment with high C/N ratios and a terrestrial organic carbon signature at the study sites. The following general questions are addressed herein: (1) how do macrofaunal abundance, species composition and functional groupings vary over time? (2) Does the sediment deposition following the January 1997 flood constitute a major disturbance to the Eel shelf macrobenthos? (3) How does sediment bioturbation intensity vary in time/space and what are the main factors controlling this variation?The Eel shelf macrofauna is strongly dominated by subsurface-deposit feeding polychaetes, with anomalously low abundances of surface-deposit feeders and virtually no suspension feeders among the community dominants. The abundance data revealed a clear seasonal pattern, with peak density (∼4.5 × 10⁴ m⁻²) in the fall and a factor of two lower density in the late winter/spring (∼2 × 10⁴ m⁻²). Within this seasonal context there was little evidence for extraordinary mortality caused by the January 1997 flood, in that overall wintertime mortality and the mortality of most community dominants during a year (1995-1996) when there was no flood deposition were comparable to the mortality observed following the January 1997 flood. In contrast, the depth distribution of the macrofauna revealed a distinctive post-flood pattern, whereby a majority (55-70%) of individuals were temporarily found at depths >4 cm. This pattern suggests an active response by the resident fauna to sediment deposition, and supports the idea that the floods did not cause a widespread disturbance. Although there may not be clear evidence for short-term flood effects, the overall species composition and functional groupings do imply that the sedimentary environment (high sediment accumulation rates and abundant terrestrial organic matter) has had a long-term influence on the Eel shelf macrofauna.Model fits to ∼75 profiles of excess ²³⁴Th show that in general the data are consistent with a steady-state, biodiffusive model. The resultant mixing intensities ranged from 3 to 325 cm² y⁻¹, with averages (±standard deviation) of 35 ± 33, 24 ± 19, 37 ± 35, and22 ± 9 cm² y⁻¹ at stations C70, I70, L70, and O70, respectively. The average biodiffusivity for all stations and times was 29 ± 25 cm² y⁻¹ (N = 62). Due to the large amount of variability, which is consistent with other continental margin studies, it was not possible to detect significant spatial or temporal variability, although there is a hint of higher mixing intensities during the late summer - early fall, the period of maximal carbon flux to the seabed. Correlations between total macrofaunal abundance and mixing intensity are notably poor, whereas a slightly better correlation (r² = 0.22) was obtained between the abundance of large animals and bioturbation intensity. By explicitly considering organic carbon flux, or some measure of seabed food resources, and the abundance of larger organisms it may be possible to predict bioturbation intensity better in future studies, although the pervasive small-scale variability detected on the Eel River shelf warrants in-depth theoretical and experimental consideration.     

Latitudinal comparison of spawning season and growth of 0-group sole, Solea solea (L.)

0-Group sole, Solea solea (Linnaeus, 1758) were sampled in four nursery grounds: two on the Northern French coast and two on the Portuguese coast. Juvenile sole were collected at the Vilaine estuary (Northern Bay of Biscay) in 1992, in the Authie estuary (Eastern English Channel) in 1997, and in the Douro and Tagus estuary (Northern and central Portugal, respectively) in 2005. Left lapilli otoliths were used to estimate age and investigate variability in growth rates and hatch dates. In the French study areas nursery colonisation ended in early June in the Vilaine estuary and in late June in the Authie estuary. In the Portuguese estuaries nursery colonisation ended in May in the Douro estuary and in late June in the Tagus estuary. Growth rates were higher in the Portuguese estuaries, 0.767 mm d⁻¹ in the Tagus estuary and 0.903 mm d⁻¹ in the Douro estuary. In the French nurseries, growth rates were estimated to be 0.473 mm d⁻¹ in the Villaine estuary and 0.460 mm d⁻¹ in the Authie estuary. Data on growth rates from other studies shows that growth rates are higher at lower latitudes, probably due to higher water temperature. Spawning took place between early January and early April in the Villaine estuary's coastal area in 1992. In 1997, in the Authie estuary spawning started in late January and ended in early April. On the Douro estuary's adjacent coast spawning started in mid-January and ended in late March, in 2005, while on the Tagus estuary's adjacent coast spawning started in mid-February and ended in mid-April, in the same year. Literature analysis of the spawning period of sole along a latitudinal gradient ranging from 38°N to 55°N in the Northeast Atlantic indicated that there is a latitudinal trend, in that spawning starts sooner at lower latitudes. Results support that local conditions, particularly hydrodynamics, may overrule general latitudinal trends.     

Picoplankton composition related to thermohaline circulation: The Albanian boundary zone (southern Adriatic) in late spring

Picoplankton distribution at the boundary zone of the southern Adriatic in May 2009 on a 75 km long shelf-continental slope transect was assessed by combining epifluorescence microscopy, flow cytometry and high-performance liquid chromatography data with hydrographic observations. The picoplankton distribution was greatly influenced by the hydrographic conditions prevailing in the southern Adriatic because of the influence of the Levantine Intermediate Water (LIW) and East Adriatic Current (EAC) forcing. Heterotrophic bacteria numerically dominated the picoplankton community through the entire transect with no significant accumulation. By contrast, picophytoplankton accumulated in the 50–75 m layer, forming a pronounced deep chlorophyll maximum. Synechococcus dominated the photosynthetic picoplankton, whereas picoeukaryotes were the least abundant. The intrusion of warm LIW observed in the layer between 100 and 350 m was followed by Prochlorococcus and Synechococcus peaks (10 × 10³ cells mL⁻¹ and 90 × 10³ cells mL⁻¹, respectively), as well as by the appearance of two Synechococcus ecotypes. Most picoeukaryotes were observed at the offshore stations, where geostrophic current calculation revealed the strongest EAC influence. A strong EAC spread over the central and eastern basin created a barrier for Prochlorococcus, whereas the picoeukaryote maxima coincided with the core of the EAC, suggesting its persistence to hydrological instabilities.     

Dynamics of heterotrophic dinoflagellates off the Pearl River Estuary,northern South China Sea

Variations in abundance, biomass, vertical profile and cell size of heterotrophic dinoflagellates (HDFs) between summer and winter and its controlling factors were studied in the northern South China Sea (SCS). It was found that HDF abundance and carbon biomass were 4–102 × 10³ cells L⁻¹ and 0.34–12.3 mg C L⁻¹ in winter (February 2004), respectively, while they were 2–142 × 10³ cells L⁻¹ and 0.22–31.4 μg C L⁻¹ in summer (July, 2004), respectively, in the northern SCS. HDF abundance and carbon biomass decreased from the estuary to inshore and then offshore. Vertical profiles of HDF abundance were heterogeneous, which accorded well with that of chlorophyll a (Chl.a). Higher abundance of HDFs was often observed at a depth of 30–70 m offshore waters, matching well with the Chl.a maximum, while it showed high abundance at the surface in some coastal and estuary stations. Small HDFs (≤20 μm) dominated the assemblage in term of abundance accounting for more than 90%. However, large HDFs (>20 μm) generally contributed equally in terms of carbon biomass, accounting for 47% on average. HDFs showed different variation patterns for the different study regions; in the estuarine and continental shelf regions, abundance and biomass values were higher in summer than those in winter, while it was the reverse pattern for the slope waters. Hydrological factors (e.g. water mass, river outflow, monsoon and eddies) associated with biological factors, especially the size-fractionated Chl.a, seemed to play an important role in regulating HDF distribution and variations in the northern South China Sea.     

Close coupling between ammonium uptake by phytoplankton and excretion by Antarctic krill, Euphausia superba

In this study we examined the hypothesis that, under conditions of replete macronutrients and iron in the Southern Ocean, phytoplankton abundance and specific N uptake rates are influenced strongly by the processes of grazing and NH₄ regeneration. NH₄ and NO₃ uptake rates by marine phytoplankton were measured to the northeast and northwest of the island of South Georgia during January-February 1998. Mean specific uptake rate for NO₃ (vNO₃) was 0.0026 h⁻¹ (range 0.0013-0.0065 h⁻¹) and for NH₄ (vNH₄) was 0.0097 h⁻¹ (0.0014-0.0376 h⁻¹). vNH₄ was related positively with NH₄ availability, which ranged from 0.1 to 1.5 mmol m⁻³ within the upper mixed layer. Ambient NH₄ concentrations and vNH₄ were both positively related to local krill biomass values, computed from mean values along acoustic transect segments within 2 km of the uptake measurement stations. These biomass values ranged from ∼1 g krill fresh mass m⁻² in the northwest to >4 kg krill wet mass m⁻² in the northeast. In contrast to the variability found with NH₄ concentrations and uptake rates, vNO₃ was more uniform across the sampling sites. Under these conditions, increasing NH₄ concentration appeared to represent an additional N resource. However, high vNH₄ tended to be found for stations with lower phytoplankton standing stocks, across a total range of 0.24-20 mg chlorophyll a m⁻³. These patterns suggest a coupling between phytoplankton biomass, vNH₄ and krill in this region of variable but high krill biomass. Locally high concentrations of krill in parts of the study area appeared to have two opposing effects. On the one hand they could graze down phytoplankton stocks, but on the other hand, their NH₄ excretion supported enhanced uptake rates by the remaining, ungrazed cells.     

Particulate organic carbon fluxes to the ocean interior and factors controlling the biological pump: A synthesis of global sediment trap programs since 1983

Particulate organic carbon (POC) is vertically transported to the oceanic interior by aggregates and their ballasts, mainly CaCO₃ and biogenic opal, with a smaller role for lithogenic aerosols through the mesopelagic zone. Diel migrating zooplankton communities effect vertical transport and remineralization of POC in the upper layers of the ocean. Below 1.5 km, the presence of zooplankton is reduced and thus the aggregates travel mainly by gravitational transport. We normalized the fluxes of POC, CaCO₃, and biogenic opal from data published on samples collected at 134 globally distributed, bottom-tethered, time-series sediment trap (TS-trap) stations to annual mole fluxes at the mesopelagic/bathypelagic boundary (m/b) at 2 km and defined them as F_m/bC_org, F_m/bC_inorg, and F_m/bSi_bio. Using this global data set, we investigated (1) the geographic contrasts of POC export at m/b and (2) the supply rate of ∑CO₂ to the world mesopelagic water column. F_m/bC_org varies from 25 (Pacific Warm Pool) to 605 (divergent Arabian Sea) mmolC m⁻² yr⁻¹; F_m/bC_inorg varies from >8 (high latitude Polar Oceans) or 15 (Pacific Warm Pool) to 459 (divergent Arabian Sea) mmolC m⁻² yr⁻¹; and F_m/bSi_bio, the most spatially/temporally variable flux, ranges from 6 (North Atlantic Drift) to 1118 (Pacific Subarctic Gyre) mmolSi m⁻² yr⁻¹. The oceanic region exhibiting the highest POC flux over a significantly large region is the area of the North Pacific Boreal Gyres where the average F_m/bC_org = 213, F_m/bC_inorg = 126, and F_m/bSi_bio = 578 mmol m⁻² yr⁻¹. F_m/bC_org and F_m/bC_inorg are particularly high in large upwelling margins, including the divergent Arabian Sea and off Cape Verde. One of the data sets showing the lowest flux over a significant region/basin is F_m/bC_org = 39, F_m/bC_inorg = 69, and F_m/bSi_bio = 22 mmol m⁻² yr⁻¹ in the North Pacific subtropical/tropical gyres; Pan-Atlantic average fluxes are similar except F_m/bSi_bio fluxes are even lower. Where C_org/C_inorg and Si_bio/C_inorg are <1 defines the “Carbonate Ocean”, and where these ratios are ?1 defines the “Silica Ocean”. The Carbonate Ocean occupies about 80% of the present world pelagic ocean between the two major oceanographic fronts, the North Pacific Polar Front and the Antarctic Polar Front, and the Silica Ocean is found on the polar sides of these fronts. The total global annual fluxes of F_m/bC_org, F_m/bC_inorg, and F_m/bSi_bio at m/b calculated by parameterizations of the export flux data from 134 stations are surprisingly similar; 36.2, 33.8, and 34.6 teramol yr⁻¹ (120, 112, and 114 mmol m⁻² yr⁻¹), respectively, resulting in a near uniform binary ratio between the above three elements of about one. The global ternary % ratios estimated from 152 TS-trap samples of the three elements are 35:32:33. From our global F_m/bC_org and a published model estimate of the global export production, we estimate the regeneration rate of CO₂ through the mesopelagic zone by the biological pump is 441 teramolC yr⁻¹. Based on our global F_m/bC_inorg and recently estimated global primary production of PIC, 36-86 teramolC yr⁻¹ of PIC is assumed to be dissolved within the upper 2 km of the water column.     

Bacterial production,glucosidase activity and particle-associated carbohydrates in Dona Paula bay,west coast of India

Size-fractionated bacterial production, abundance and α- and β- glucosidase enzyme activities were studied with respect to changes in hydrography, total suspended matter (TSM), chlorophyll a, particulate organic carbon and nitrogen ratio (POC:PON), 1.5 M NaCl-soluble and 10 mM EDTA-soluble carbohydrates (Sal-PCHO and CPCHO) and transparent exopolymeric particles (TEP) in the surface waters from July 1999–2000 at a shallow coastal station in Dona Paula Bay, west coast of India. The bulk of the total bacterial production and glucosidase activity were associated with particles (75% and >80%, respectively). Total bacterial production was linearly correlated to chlorophyll a (r = 0.513; p < 0.05) whereas enzyme activity was significantly correlated to TSM (α-glucosidase: r = 0.721 (p < 0.001); β-glucosidase: r = 0.596 (p < 0.01)). Both α-glucosidase (r = 0.514; p < 0.05) and β-glucosidase enzymes (r = 0.598; p < 0.01) appeared to be involved in the degradation of CPCHO and Sal-PCHO, respectively. Changes in α-glucosidase/β-glucosidase ratios highlighted the varying composition of particulate organic matter. The bacterial uptake of ¹⁴C-labeled bacterial extracellular carbohydrate measured over 11 days showed a strong linear correlation between ¹⁴C-uptake and bacterial production using tritiated thymidine. The turnover rate of ¹⁴C-labeled carbohydrate-C was 0.52 d⁻¹, higher than the estimated annual mean potential carbohydrate carbon turnover rate of 0.33 ± 0.2 d⁻¹. Our study suggests that carbohydrates derived from sediments may serve as an important alternative carbon source sustaining the bacterial carbon demand in the surface waters of Dona Paula Bay.     

Export and mesopelagic particle flux during a North Atlantic spring diatom bloom

Spring diatom blooms are important for sequestering atmospheric CO₂ below the permanent thermocline in the form of particulate organic carbon (POC). We measured downward POC flux during a sub-polar North Atlantic spring bloom at 100 m using thorium-234 (²³⁴Th) disequilibria, and below 100 m using neutrally buoyant drifting sediment traps. The cruise followed a Lagrangian float, and a pronounced diatom bloom occurred in a 600 km² area around the float. Particle flux was low during the first three weeks of the bloom, between 10 and 30 mg POC m⁻² d⁻¹. Then, nearly 20 days after the bloom had started, export as diagnosed from ²³⁴Th rose to 360-620 mg POC m⁻² d⁻¹, co-incident with silicate depletion in the surface mixed layer. Sediment traps at 600 and 750 m depth collected 160 and 150 mg POC m⁻² d⁻¹, with a settled volume of particles of 1000-1500 mL m⁻² d⁻¹. This implies that 25-43% of the 100 m POC export sank below 750 m. The sinking particles were ungrazed diatom aggregates that contained transparent exopolymer particles (TEP). We conclude that diatom blooms can lead to substantial particle export that is transferred efficiently through the mesopelagic. We also present an improved method of calibrating the Alcian Blue solution against Gum Xanthan for TEP measurements.     

Dynamics and sources of suspended particulate organic matter in the Marennes-Oléron oyster farming bay: Insights from stable isotopes and microalgae ecology

The aim of this study was to distinguish between sources of the complex variety of Marennes-Oléron Bay suspended particulate organic matter (SPOM) contributing to the tropho-dynamics of the Marennes-Oléron oyster farming bay. Basic biomarkers (Chl a, C/N and POC/Chl a ratios), carbon and nitrogen stable isotopes from SPOM were analyzed and the microalgae community was characterized. The sampling strategy was bimonthly from March 2002 to December 2003; samples were taken from an intertidal mudflat. Four main sources contributed to the SPOM pool: terrigenous input from rivers, neritic phytoplankton, resuspended microphytobenthos and periodic inputs from intertidal Zostera noltii meadows. Seasonal fluctuations were observed in both years of the study period: (1) SPOM collected in the spring of 2002 (δ¹³C = −25‰ to −23‰) was mainly composed of fresh estuarine inputs; (2) SPOM from the summer and fall of 2002 and 2003 was predominantly neritic phytoplankton (δ¹³C = − 22‰ to −19‰); (3) SPOM from the winter of 2002, spring of 2003 and winter of 2003 (δ¹³C = −21 to −23‰) was composed of a mixture of decayed terrigenous river inputs and pelagic phytoplankton, which was predominantly resuspended microphytobenthos. In the summer of 2003—the warmest summer on record in southern France and Europe—SPOM was particularly enriched for ¹³C, with δ¹³C values ranging from −14‰ to −12‰. Pulses in δ¹³C values, indicative of ¹³C-enriched decaying materials, extended into the fall. These were attributed to benthic intertidal inputs, including both resuspended microphytobenthos and Z. noltii detritus. Changes in SPOM sources in Marennes-Oléron Bay may lead to differences in the quality of the trophic environment available for reared oysters.     

Grazing impact of the copepod community in the Oyashio region of the western subarctic Pacific Ocean

The role of copepod grazing on the ecosystem dynamics in the Oyashio region, western subarctic Pacific was investigated during six cruises from June 2001 to June 2002. In situ grazing rates of the copepod community (CGR) were measured by the gut fluorescence method in respect to developmental stages of dominant species. In terms of biomass, more than 80% of the copepod community was dominated by six large calanoid species (Neocalanus cristatus, Neocalanus flemingeri, Neocalanus plumchrus, Eucalanus bungii, Metridia pacifica and Metridia okhotensis) throughout the year. Resulting from the observed pattern of the interzonal migrating copepods, the CGR in the Oyashio region was divided into three phases, i.e. spring (bloom), summer (post-bloom) and autumn-winter phase. During the spring bloom, late copepodites of the interzonal migrating species, N. cristatus, N. flemingeri and E. bungii appeared in the surface layer (0-50 m) to consume the production of the bloom, resulting in a high grazing rate of the copepod community (7.9 mg Chl m⁻² d⁻¹), though its impact on phytoplankton community was low due to the high primary productivity. During the post-bloom period, although the copepod community which was dominated by N. cristatus, N. plumchrus, M. pacifica and newly recruited E. bungii still maintained a high biomass, the CGR was generally lower (1.8-2.6 mg Chl m⁻² d⁻¹ for June and August 2001), probably due to the lower availability of phytoplankton. Nevertheless, the highest CGR was also observed during this period (10.5 mg Chl m⁻² d⁻¹ in June 2002). The high CGR on autotrophic carbon accounted for 69% of the primary production, suggesting that the copepod community in the Oyashio region potentially terminates the phytoplankton bloom. Abundant occurrence of young E. bungii, which is a characteristic phenomenon in the Oyashio region, was largely responsible for the high grazing pressure in June 2002 suggesting that success of reproduction, growth, and survival in E. bungii during the spring bloom is an important factor in controlling phytoplankton abundance during the post-bloom season. During autumn and winter, CGR was the lowest in the year (0.29-0.38 mg Chl. m⁻² d⁻¹) due to the disappearance of the interzonal migrating copepods from the surface layer. Diel migrant M. pacifica was the most important grazer during this period. The annual ingestion of the copepod community is estimated as 37.7 gC m⁻² on autotrophic carbon (converted using C:Chl ratio of 30) or 137.9 gC m⁻² on suspended particles (using C:Chl ratio of in situ value, 58-191), accounting for 13% and 46% of annual primary production, respectively. This study confirms that copepod grazing is an important pathway in carbon flow in the Oyashio region and in particular their role in the phytoplankton dynamics is significant for the termination of the spring bloom.     

The influence of prolonged mouth closure on selected components of the hyperbenthos in the littoral zone of the temporarily open/closed Kasouga Estuary,South Africa

The influence of prolonged mouth closure on the population dynamics of the caridian shrimp, Palaemon peringueyi and the estuarine isopod, Exosphaeroma hylocoetes, in the littoral zone of temporarily open/closed Kasouga Estuary located on the south-eastern coastline of southern Africa was assessed monthly over the period October 2007 to September 2008. Prolonged mouth closure of the estuary contributed to hypersaline conditions (psu > 35) prevailing throughout the estuary for the last four months of the study. The high salinities coincided with a decrease in the areal extent (up to 80%) of the submerged macrophytes, mainly Ruppia maritima, within the littoral zone of the estuary. Total abundance and biomass values of the shrimp and isopod over the period of investigation ranged from 0 to 14.6 ind m⁻², from 0 to 13.3 mg dwt m⁻², from 12 to 1540 ind m⁻² and from 0.1 to 2.16 mg dwt m⁻², respectively. Maximum values of both the shrimp and isopod were recorded in the upper reaches of the estuary in close association with R. maritima. Over the course of the investigation, both the abundance and biomass values of the shrimp decreased significantly (P < 0.05 in both cases) which could be related to reduced habitat availability, R. maritima, that acts as a refuge against fish predation. Additionally, the decrease in abundance and biomass values could be attributed to reduced recruitment opportunities for the shrimp and the cessation of reproduction in the estuarine isopod. The establishment of a link to the marine environment following an overtopping event in September 2008 contributed to a decrease in salinity within the system although no recruitment of either the isopod or shrimp was recorded.