Influence of sex and age on PCBs accumulation in the commercial fish Chelon labrosus

Thicklip grey mullet, Chelon labrosus, is an important commercial fish species and has been studied worldwide. However, no recent studies have been made regarding polychlorinated biphenyls (PCBs) in wild C. labrosus. Due to that, the concentration of 13 PCBs congeners was measured in muscles and livers, of males and females, of C. labrosus of different ages, allowing the estimation of PCB bioaccumulation throughout the species lifespan, in the Mondego estuary, a southern European temperate estuary. Male muscle sample concentrations ranged from 32 to 96 ng g^− 1 (lipid wt.) and in females from 32 to 62 ng g^− 1 (lipid wt.). In male liver sample concentrations ranged from 106 to 158 ng g^− 1 (lipid wt.), while female concentrations ranged from 88 to 129 ng g^− 1 (lipid wt.). The most abundant congeners presenting higher percentages in all samples were CB 138, 153 and 180. No significant differences were found between the concentrations in both sexes, but muscle and liver PCB concentrations in males tended to increase with age whereas in females concentrations remained stable throughout the species lifespan. Significant differences were found between concentrations in muscle and liver.     

Effects of mesoscale phytoplankton variability on the copepods Neocalanus flemingeri and N. plumchrus in the coastal Gulf of Alaska

The copepods Neocalanus flemingeri and N. plumchrus are major components of the mesozooplankton on the shelf of the Gulf of Alaska, where they feed, grow and develop during April–June, the period encompassing the spring phytoplankton bloom. Satellite imagery indicates high mesoscale variability in phytoplankton concentration during this time. Because copepod ingestion is related to food concentration, we hypothesized that phytoplankton ingestion by N. flemingeri and N. plumchrus would vary in response to mesoscale variability of phytoplankton. We proposed that copepods on the inner shelf, where the phytoplankton bloom is most pronounced, would be larger and have more lipid stores than animals collected from the outer shelf, where phytoplankton concentrations are typically low. Shipboard feeding experiments with both copepods were done in spring of 2001 and 2003 using natural water as food medium. Chlorophyll concentration ranged widely, between 0.32 and 11.44 μg l⁻¹ and ingestion rates varied accordingly, between 6.0 and 627.0 ng chl cop⁻¹ d⁻¹. At chlorophyll concentrations<0.50 μg l⁻¹, ingestion is always low, <40 ng cop⁻¹ d⁻¹. Intermediate ingestion rates were observed at chlorophyll concentrations between 0.5 and 1.5 μg l⁻¹, and maximum rates at chlorophyll concentrations>1.5 μg l⁻¹. Application of these feeding rates to the phytoplankton distribution on the shelf allowed locations and time periods of low, intermediate and high daily feeding to be calculated for 2001 and 2003. A detailed cross-shelf survey of body size and lipid store in these copepods, however, indicated they were indistinguishable regardless of collection site. Although the daily ingestion of phytoplankton by N. flemingeri and N. plumchrus varied widely because of mesoscale variability in phytoplankton, these daily differences did not result in differences in final body size or lipid storage of these copepods. These copepods efficiently dealt with small and mesoscale variations in their food environment such that mesoscale structure in phytoplankton did not affect their final body size.     

PCBs in the fish assemblage of a southern European estuary

The Mondego estuary fish assemblage was studied for the accumulation of PCBs. Three sampling stations were visited along an estuarine salinity gradient, and, in total, 15 species were collected. Analysis of PCBs revealed no significant differences among the sampling stations, although differences were observed among the fish assemblages. Fish assemblages could be divided into three groups. The first group comprised those with higher concentration (more than 10 ng g^− 1, dw), included the species Gobius niger, Sardina pilchardus, Anguilla anguilla, Pomatoschistus microps, Chelidonichthys lucerna and Liza ramada; the second group with medium concentration (5–10 ng g^− 1, dw), included the species Pomatoschistus minutus, Dicentrarchus labrax, Atherina presbyter, Chelon labrosus, Diplodus vulgaris, Platichthys flesus and Cilata mustela; and a third group with low concentration (less than 5 ng g^− 1, dw), included the species Solea solea and Callionymus lyra. A positive correlation was found between lipid content and PCB concentrations. To evaluate the influence of the residence time of species on the accumulation of PCBs, species were divided into two groups: species that spend more than 3 years in the estuary, and species that spend less than 3 years in the estuary. Species that spend more than 3 years in the estuary presented higher concentrations than species that spend less than 3 years in the estuary. CBs 138 and 153 had higher concentration, and tended to increase with time spent in the estuary.     

Ballasting outside port to prevent spread of butyltin from merchant ships

Most international discussions on ballast water management onboard ships have been virtually on the prevention of the transport of biotic alien species. This study focuses on how to improve this limited knowledge regarding the transport of butyltin compounds via ballasting. Butyltin compounds in ballast water, extracted from three sea-going merchant ships with similar voyage routes and berth ports but with different ballasting practices, were determined. Only monobutyltin (8–57 ng l^?1) and dibutyltin (10–32 ng l^?1) onboard two general cargo ships were detected in the ballast water taken outside the ports. In contrast, significant tributyltin (23–93 ng l^?1) was detected onboard the container ship in the ballast water pumped inside the loading/unloading ports. The comparison results from this study indicate that the risk of tributyltin transfer may be effectively prevented by ballasting outside the ports.     

Respiration,mineralization, and biochemical properties of the particulate matter in the southern Nansen Basin water column in April 1981

Determinations of the activity of the respiratory electron transport system (ETS), during the FRAM III expedition permit us to estimate oxygen utilization rates (R_O2) from the surface to 2000 m under the polar pack ice in the Nansen Basin just north of Svalbard (83°N, 7°E) during April 1981. We found R_O2 at in situ temperatures ranging from 20 pM O₂ min⁻¹ just below the ice to 0.2 pM O₂ min⁻¹ at 2000 m. These rates are low compared to most other ocean regions, but they could decrease particulate organic carbon and nitrogen by 76% and 74%, respectively, over a period of ∼6 months. The R_O2 calculations based on measurements made at 0 °C yielded a power function of R_O2 vs. depth (Z) of R_O2=67Z^−0.5534. When this R_O2 profile was superimposed on a more recent oxygen utilization rate profile made using the ³He–³H–AOU method (OUR), in the same vicinity of the Nansen Basin during 1987 (OUR=52Z^–0.4058, [Zheng, Y., Schlosser, P., Swift, J.W., Jones, E.P., 1997. Oxygen utilization rates in the Nansen Basin, Arctic Ocean: implications for new production. Deep Sea Research I 44, 1923–1943]), the agreement of the two profiles was close. On one hand, this was to be expected because R_O2 is the biological basis of OUR, on the other hand, it was a surprise because the methodologies are so different. Nitrate mineralization obtained from ETS activities also compared favorably with calculations based on the data of Zheng et al. [1997. Oxygen utilization rates in the Nansen Basin, Arctic Ocean: implications for new production. Deep Sea Research I 44, 1923–1943]. Chlorophyll ranged from 6 ng L⁻¹ at 5 m to 0.06 ng L⁻¹ at 2000 m. Particulate organic carbon (POC) decreased from 0.93 μM C just below the ice to less than 0.4 μM C at 500 m. Particulate organic nitrogen (PON) was not detectable below 70 m, however in the upper 70 m it ranged from 0.16 to 0.04 μM N. The C/N mass ratio over these depths ranged from 5.8 to 11.3. Annual carbon productivity as calculated to balance the total water column respiration was 27 g C m⁻² y⁻¹. The integrated respiration rate between 50 and 4000 m suggests that exported production and carbon flux from the 50 m level was 24 g C m⁻² y⁻¹. These are minimal estimates for the southern Nansen Basin because they are based on measurements made at the end of the Arctic winter.     

Ectoenzymatic activity in surface waters: A transect from the Mediterranean Sea across the Indian Ocean to Australia

The activities of two hydrolytic enzymes (leucine aminopeptidase and β glucosidase), belonging to the particle-bound enzymatic fraction, were measured in open-sea surface waters. Samples were collected along a transect crossing the Indian Ocean during the early NW monsoon period (November and December 2001). The latitudinal pattern of the ectoenzymatic activities highlighted a generally increasing trend of glycolysis approaching the equator, with significantly higher β glucosidase activity (0.79–3.00 nmol l⁻¹ h⁻¹) within the latitudinal range from 12°N to 16°S. In this area, the surface waters coming from the Indonesian Throughflow and the Bay of Bengal carry a considerable quantity of carbohydrates (38.9–41.9 μg l⁻¹), which stimulated glycolytic activity and its cell-specific rates scaled to bacterial abundance. On the other hand, in the Central Indian Ocean, the proteolytic activity was considerable (0.91–2.03 nmol l⁻¹ h⁻¹), although the particulate proteins did not show significant increases and the dissolved proteinlike signal was one of the lowest of the entire transect (0.7 mg l⁻¹ on average compared to the 1.4–1.6 mg l⁻¹ of the adjacent areas). Therefore, in this area, the two ectoenzymes studied did not respond to the same stimulatory effect (namely the specific substrate concentrations). The time needed for the hydrolysis of macromolecules within the particulate and dissolved organic substrate fractions, although these measures are affected by a number of assumptions starting with the potential nature of the ectoenzymatic determinations, confirms these observations. The Central Indian Ocean displayed the lowest values, from 8 to 26 days for particulate and dissolved organic carbon, respectively. As observed in the equatorial areas of the Atlantic Ocean, the relevant degradation activity of the central area of the Indian Ocean Basin suggests a notable heterotrophy based on a faster turnover of organic substrates.     

Copper complexation by fulvic acid affects copper toxicity to the larvae of the polychaete Hydroides elegans

Copper toxicity is influenced by a variety of environmental factors including dissolved organic matter (DOM). We examined the complexation of copper by fulvic acid (FA), one of the major components of DOM, by measuring the decline in labile copper by anodic stripping voltammetrically (ASV). The data were described using a one-site ligand binding model, with a ligand concentration of 0.19 μmol site mg⁻¹ C, and a log K′ of 6.2. The model was used to predict labile copper concentration in a bioassay designed to quantify the extent to which Cu–FA complexation affected copper toxicity to the larvae of marine polychaete Hydroides elegans. The toxicity data, when expressed as labile copper concentration causing abnormal development, were independent of FA concentration and could be modeled as a logistic function, with a 48-h EC₅₀ of 58.9 μg l⁻¹. However, when the data were expressed as a function of total copper concentration, the toxicity was dependent on FA concentration, with a 48-h EC₅₀ ranging from 55.6 μg l⁻¹ in the no-FA control to 137.4 μg l⁻¹ in the 20 mg l⁻¹ FA treatment. Thus, FA was protective against copper toxicity to the larvae, and such an effect was caused by the reduction in labile copper due to Cu–FA complexation. Our results demonstrate the potential of ASV as a useful tool for predicting metal toxicity to the larvae in coastal environment where DOM plays an important role in complexing metal ions.     

Phytoplankton chemotaxonomy in the Atlantic sector of the Southern Ocean during late summer 2009

A chemotaxonomic investigation of surface phytoplankton was undertaken on a research cruise to the Atlantic sector of the Southern Ocean during late austral summer 2009. Based on pigment signatures, several distinct regions emerged that were delineated by physical features. CHEMTAX analysis of high performance liquid chromatography (HPLC) pigment data indicated that diatoms generally dominated communities south of the Antarctic Polar Front (APF), particularly in regions of elevated biomass where chlorophyll-a (chl-a) was >1.5 µg l⁻¹ and diatoms comprised >80% of biomass. Pigment signatures representative of haptophytes-8, indicative of Phaeocystis antarctica, were dominant near the ice shelf. Chl-a concentrations were 0.2–0.6 µg l⁻¹ between the APF and the Subtropical Front (STF) and outputs suggested that chlorophytes, haptophytes-8 and haptophyte-6, in the form of coccolithophores, were the major constituents. Very low chl-a levels (<0.2 µg l⁻¹) were observed north of the STF and the prokaryotes Synechococcus spp. and Prochlorococcus spp. were the dominant groups in these oligotrophic waters.     

Community respiration/production and bacterial activity in the upper water column of the central Arctic Ocean

Community metabolism (respiration and production) and bacterial activity were assessed in the upper water column of the central Arctic Ocean during the SHEBA/JOIS ice camp experiment, October 1997–September 1998. In the upper 50 m, decrease in integrated dissolved oxygen (DO) stocks over a period of 124 d in mid-winter suggested a respiration rate of ∼3.3 nM O₂ h⁻¹ and a carbon demand of ∼4.5 gC m⁻². Increase in 0–50 m integrated stocks of DO during summer implied a net community production of ∼20 gC m⁻². Community respiration rates were directly measured via rate of decrease in DO in whole seawater during 72-h dark incubation experiments. Incubation-based respiration rates were on average 3-fold lower during winter (11.0±10.6 nM O₂ h⁻¹) compared to summer (35.3±24.8 nM O₂ h⁻¹). Bacterial heterotrophic activity responded strongly, without noticeable lag, to phytoplankton growth. Rate of leucine incorporation by bacteria (a proxy for protein synthesis and cell growth) increased ∼10-fold, and the cell-specific rate of leucine incorporation ∼5-fold, from winter to summer. Rates of production of bacterial biomass in the upper 50 m were, however, low compared to other oceanic regions, averaging 0.52±0.47 ngC l⁻¹ h⁻¹ during winter and 5.1±3.1 ngC l⁻¹ h⁻¹ during summer. Total carbon demand based on respiration experiments averaged 2.4±2.3 mgC m⁻³ d⁻¹ in winter and 7.8±5.5 mgC m⁻³ d⁻¹ in summer. Estimated bacterial carbon demand based on bacterial productivity and an assumed 10% gross growth efficiency was much lower, averaging about 0.12±0.12 mgC m⁻³ d⁻¹ in winter and 1.3±0.7 mgC m⁻³ d⁻¹ in summer. Our estimates of bacterial activity during summer were an order of magnitude less than rates reported from a summer 1994 study in the central Arctic Ocean, implying significant inter-annual variability of microbial processes in this region.     

Export fluxes of particulate organic carbon in the Chukchi Sea: A comparative study using 234Th/238U disequilibria and drifting sediment traps

Large-volume sampling of ²³⁴Th and drifting sediment trap deployments were conducted as part of the 2004 Western Arctic Shelf–Basin Interactions (SBI) spring (May 15–June 23) and summer (July 17–August 26) process cruises in the Chukchi Sea. Measurements of ²³⁴Th and particulate organic carbon (POC) export fluxes were obtained at five stations during the spring cruise and four stations during the summer cruise along Barrow Canyon (BC) and along a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. ²³⁴Th and POC fluxes obtained with in situ pumps and drifting sediment traps agreed to within a factor of 2 for 70% of the measurements. POC export fluxes measured with in situ pumps at 50 m along BC were similar in spring and summer (average = 14.0 ± 8.0 mmol C m^− 2 day^− 1 and 16.5 ± 6.5 mmol C m^− 2 day^− 1, respectively), but increased from spring to summer at the EHS transect (average = 1.9 ± 1.1 mmol C m^− 2 day^− 1 and 19.5 ± 3.3 mmol C m^− 2 day^− 1, respectively). POC fluxes measured with sediment traps at 50 m along BC were also similar in both seasons (31.3 ± 9.3 mmol C m^− 2 day^− 1 and 29.1 ± 14.2 mmol C m^− 2 day^− 1, respectively), but were approximately twice as high as POC fluxes measured with in situ pumps. Sediment trap POC fluxes measured along the EHS transect also increased from spring to summer (3.0 ± 1.9 mmol C m^− 2 day^− 1 and 13.0 ± 6.4 mmol C m^− 2 day^− 1, respectively), and these fluxes were similar to the POC fluxes obtained with in situ pumps. Discrepancies in POC export fluxes measured using in situ pumps and sediment traps may be reasonably explained by differences in the estimated POC/²³⁴Th ratios that arise from differences between the techniques, such as time-scale of measurement and size and composition of the collected particles. Despite this variability, in situ pump and sediment trap-derived POC fluxes were only significantly different at a highly productive station in BC during the spring.     

Kinetic study reveals weak Fe-binding ligand,which affects the solubility of Fe in the Scheldt estuary

The chemistry of dissolved Fe(III) was studied in the Scheldt estuary (The Netherlands). Two discrete size fractions of the dissolved bulk (< 0.2 μm and < 1 kDa) were considered at three salinities (S = 26, 10 and 0.3).Within the upper estuary, where fresh river water meets seawater, the dissolved Fe concentration decreases steeply with increasing salinity, for the fraction < 0.2 μm from 536 nM at S = 0.3 to 104 nM at S = 10 and for the fraction < 1 kDa from 102 nM to 36 nM Fe. Further downstream, in the middle and lower estuary, this decrease in the Fe concentration continues, but is far less pronounced. For all samples, the traditionally recognised dissolved strong organic Fe-binding ligand concentrations are lower than the dissolved Fe concentrations.Characteristics of dissolved Fe-binding ligands were determined by observing kinetic interactions with adsorptive cathodic stripping voltammetry. From these kinetic experiments we concluded that apart from the well-known strong Fe-binding organic ligands (L, logK′ = 19–22) also weak Fe-binding ligands (P) existed with an α value (binding potential = K′ · [P]) varying between 10^11.1 and 10^11.9. The presence of this relatively weak ligand explained the high concentrations of labile Fe present in both size fractions in the estuary. This weak ligand can retard or prevent a direct precipitation after an extra input of Fe.The dissociation rate constants of the weak ligand varied between 0.5 × 10^− 4 and 4.3 × 10^− 4 s^− 1. The rate constants of the strong organic ligand varied between k_d = 1.5 × 10^− 3–17 × 10^− 2 s^− 1 and k_f = 2.2 × 10⁸–2.7 × 10⁹ M^− 1 s^− 1. The dissociation rate constant of freshly amorphous Fe-hydroxide was found to be between 4.3 × 10^− 4 and 3.7 × 10^− 3 s^− 1, more labile or equal to the values found by Rose and Waite [Rose, A.L., Waite, T.D., 2003a. Kinetics of hydrolysis and precipitation of ferric iron in seawater. Environ. Sci. Technol., 37, 3897–3903.] for freshly precipitated Fe in seawater.Kinetic rate constants of Fe with the ligand TAC (2-(2-Thiazolylazo)-p-cresol) were also determined. The formation rate constant of Fe(TAC)₂ varied between 0.1 × 10⁸ and 3.6 × 10⁸ M^− 1 s^− 1, the dissociation rate constant between 0.2 × 10^− 5 and 17 × 10^− 5 s^− 1 for both S = 26 and S = 10. The conditional stability constant of Fe(TAC)₂ (β_Fe(TAC)2′) varied between 22 and 23.4 for S = 10 and S = 26 more or less equal to that known from the literature (logβ_Fe(TAC)2′ = 22.4; [Croot, P.L., Johansson, M., 2000. Determination of iron speciation by cathodic stripping voltammetry in seawater using the competing ligand 2-(2-Thiazolylazo)-p-cresol (TAC). Electroanalysis, 12, 565–576.]). However, at S = 0.3 the logβ_Fe(TAC)2′ was 25.3, three orders of magnitude higher. Apparently the application of TAC to samples of low salinity can only be done when the correct β_Fe(TAC)2′ is known.     

Basin-scale variability of phytoplankton biomass,production and growth in the Atlantic Ocean

The latitudinal distributions of phytoplankton biomass, composition and production in the Atlantic Ocean were determined along a 10,000-km transect from 50°N to 50°S in October 1995, May 1996 and October 1996. Highest levels of euphotic layer-integrated chlorophyll a (Chl a) concentration (75–125 mg Chl m⁻²) were found in North Atlantic temperate waters and in the upwelling region off NW Africa, whereas typical Chl a concentrations in oligotrophic waters ranged from 20 to 40 mg Chl m⁻². The estimated concentration of surface phytoplankton carbon (C) biomass was 5–15 mg C m⁻² in the oligotrophic regions and increased over 40 mg C m⁻² in richer areas. The deep chlorophyll maximum did not seem to constitute a biomass or productivity maximum, but resulted mainly from an increase in the Chl a to C ratio and represented a relatively small contribution to total integrated productivity. Primary production rates varied from 50 mg C m⁻² d⁻¹ at the central gyres to 500–1000 mg C m⁻² d⁻¹ in upwelling and higher latitude regions, where faster growth rates (μ) of phytoplankton (>0.5 d⁻¹) were also measured. In oligotrophic waters, microalgal growth was consistently slow [surface μ averaged 0.21±0.02 d⁻¹ (mean±SE)], representing <20% of maximum expected growth. These results argue against the view that the subtropical gyres are characterized by high phytoplankton turnover rates. The latitudinal variations in μ were inversely correlated to the changes in the depth of the nitracline and positively correlated to those of the integrated nitrate concentration, supporting the case for the role of nutrients in controlling the large-scale distribution of phytoplankton growth rates. We observed a large degree of temporal variability in the phytoplankton dynamics in the oligotrophic regions: productivity and growth rates varied in excess of 8-fold, whereas microalgal biomass remained relatively constant. The observed spatial and temporal variability in the biomass specific rate of photosynthesis is at least three times larger than currently assumed in most satellite-based models of global productivity.     

Contrasting life-histories,secondary production,and trophic structure of Peracarid assemblages of the bathyal suprabenthos from the Bay of Biscay (NE Atlantic) and the Catalan Sea (NW Mediterranean)

The life-histories and the secondary production of four dominant peracarid crustaceans (the mysids Boreomysis arctica and Parapseudomma calloplura, the amphipod Rhachotropis caeca, and the isopod Ilyarachna longicornis) in bathyal depths of the Bay of Biscay (NE Atlantic; between 383 and 420 m) and the Catalan Sea (Northwestern Mediterranean; between 389 and 1355 m) were established. Both the Atlantic and the Mediterranean populations of the major part of the target-species had two generations/year with mean cohort-production intervals (CPI) ranging from 5.5 mo for Ilyarachna longicornis to 6.3 mo for Parapseudomma calloplura. The Hynes method showed secondary production to vary in the Bay of Biscay between 0.113 mg DW m⁻² yr⁻¹ for I. longirostris and 3.069 mg DW m⁻² yr⁻¹ for P. calloplura, with P/B ratios between 4.57 (I. longirostris) and 7.93 (Boreomysis arctica). In the Catalan Sea, production varied between 0.286 mg DW m⁻² yr⁻¹ for I. longirostris and 1.096 mg DW m⁻² yr⁻¹ for P. calloplura, with P/B between 5.72 (I. longirostris) and 6.66 (P. calloplura). Application of two different empiric models to the whole peracarid assemblage gave similar levels of secondary production in both study areas (between 29.26 and 32.14 mgDWm⁻² yr⁻¹ in the Bay of Biscay; between 26.23 and 26.54 mg DW m⁻² yr⁻¹ in the Catalan Sea). From the analysis of gut contents of 22 species the dominant species in each study area were assigned to two basic trophic levels, detritus feeders and predators. Also, cumulative curves of dominance showed high diversity (low dominance) for peracarid assemblages distributed at mid-bathyal depths (524–693 m) both in the Bay of Biscay off Arcachon and in the Catalan Sea off Barcelona. We also discuss and compare, both within and between areas, how environmental features may explain the observed diversity patterns, the trophic structure, and the production results obtained for the suprabenthos assemblages.     

Time-series observation of POC fluxes estimated from 234Th in the northwestern North Pacific

Time-series measurements of ²³⁴Th activities and particulate organic carbon (POC) concentrations were made at time-series stations (K1, K2, K3, and KNOT) in the northwestern North Pacific from October 2002 to August 2004. Seasonal changes in POC export fluxes from the surface layer (∼100 m) were estimated using ²³⁴Th as a tracer. POC fluxes varied seasonally from approximately 0 to 180 mg C m⁻² d⁻¹ and were higher in spring–summer than in autumn–winter. The export ratio (e-ratio) ranged from 6% to 55% and was also higher in spring–summer. Annual POC fluxes were estimated to be 31 g C m⁻² y⁻¹ in the subarctic region (station K2) and 23 g C m⁻² y⁻¹ in the region between the subarctic and subtropical gyres (station K3). POC fluxes and e-ratios in the northwestern North Pacific were much higher than those in most other oceans. The annual POC flux corresponded to 69% of annual new production estimated from the seasonal difference of the nutrient in the Western Subarctic Gyre (45 g C m⁻² y⁻¹). These results indicate that much of the organic carbon assimilated in the surface layer of the northwestern North Pacific is transferred to the deep ocean in particulate form. Our conclusions support previous reports that diatoms play an important role in the biological pump.     

Biogenic fluxes in the Cariaco Basin: a combined study of sinking particulates and underlying sediments

The fluxes of total mass, organic carbon (OC), biogenic opal, calcite (CaCO₃) and long-chain C₃₇ alkenones (ΣAlk₃₇) were measured at three water depths (275, 455 and 930 m) in the Cariaco Basin (Venezuela) over three separate annual upwelling cycles (1996–1999) as part of the CARIACO sediment trap time-series. The strength and timing of both the primary and secondary upwelling events in the Cariaco Basin varied significantly during the study period, directly affecting the rates of primary productivity (PP) and the vertical transport of biogenic materials. OC fluxes showed a weak positive correlation (r²=0.3) with PP rates throughout the 3 years of the study. The fluxes of opal, CaCO₃ and ΣAlk₃₇ were strongly correlated (0.6<r²<0.8) with those of OC. The major exception was the lower than expected ΣAlk₃₇ fluxes measured during periods of strong upwelling. All sediment trap fluxes were significantly attenuated with depth, consistent with marked losses during vertical transport. Annually, strong upwelling conditions, such as those observed during 1996–1997, led to elevated opal fluxes (e.g., 35 g m⁻² yr⁻¹ at 275 m) and diminished ΣAlk₃₇ fluxes (e.g., 5 mg m⁻² yr⁻¹ at 275 m). The opposite trends were evident during the year of weakest upwelling (1998–1999), indicating that diatom and haptophyte productivity in the Cariaco Basin are inversely correlated depending on upwelling conditions.The analyses of the Cariaco Basin sediments collected via a gravity core showed that the rates of OC and opal burial (10–12 g m⁻² yr⁻¹) over the past 5500 years were generally similar to the average annual water column fluxes measured in the deeper traps (10–14 g m⁻² yr⁻¹) over the 1996–1999 study period. CaCO₃ burial fluxes (30–40 g m⁻² yr⁻¹), on the other hand, were considerably higher than the fluxes measured in the deep traps (∼10 g m⁻² yr⁻¹) but comparable to those obtained from the shallowest trap (i.e. 38 g m⁻² yr⁻¹ at 275 m). In contrast, the burial rates of ΣAlk₃₇ (0.4–1 mg m⁻² yr⁻¹) in Cariaco sediments were significantly lower than the water column fluxes measured at all depths (4–6 mg m⁻² yr⁻¹), indicating the large attenuation in the flux of these compounds at the sediment–water interface. The major trend throughout the core was the general decrease in all biogenic fluxes with depth, most likely due to post-depositional in situ degradation. The major exception was the relatively low opal fluxes (∼5 g m⁻² yr⁻¹) and elevated ΣAlk₃₇ fluxes (∼2 mg m⁻² yr⁻¹) measured in the sedimentary interval corresponding to 1600–2000 yr BP. Such compositions are consistent with a period of low diatom and high haptophyte productivity, which based on the trends observed from the sediment traps, is indicative of low upwelling conditions relative to the modern day.     

Compositional and fluorescence characteristics of the giant diatom Ethmodiscus along a 3000 km transect (28°N) in the central North Pacific gyre

The giant diatom Ethmodiscus was examined along an east–west transect at 28–30°N during 2002 and 2003 to determine if abundance, chemical composition or physiological status of this largest of diatoms varied on the scale of 100's–1000's of km in North Pacific gyre. Abundance ranged from <0.1–>2.0 cells m⁻³ and supported the notion of an abundance mosaic reported previously. However, there was only minimal support for the relationship between abundance and nutrient concentration at 125 m reported previously. Cellular chlorophyll varied little along the transect (7.3–10.9 ng chl cell⁻¹) except at the westernmost station. Cellular N and P quotas co-varied 3–4.5 fold (mean=50.8±3.7 and 3.7±0.8 nmol N and P cell⁻¹) and yielded N:P ratios that closely clustered around the Redfield ratio (average=14.6±1.1). Only low levels of chlorophyll-normalized alkaline phosphatase (APase) activity were observed (0.4–2.5 nmol P μg chl⁻¹ h⁻¹) with APase activity lower than that in either the bulk water, or co-occurring Trichodesmium spp. and Pyrocystis noctiluca. The active fluorescence parameter F_v:F_m, a property sensitive to Fe stress, was uniformly high at all stations (average=0.73±0.04 for 2003, and 0.69±0.05 for 2002), indicating sufficient Fe for optimum photosynthetic competence. These results contrasted sharply with results from Rhizosolenia mats reported along the same transect where there was a significant decline westward in F_v:F_m. Both ferredoxin (Fd) and flavodoxin accumulated in cells of Ethmodiscus, resulting in Fd Index values of<0.6. Iron cell quotas ranged from 0.7–5.1 pmol Fe cell⁻¹. When normalized to cytoplasmic volume, the Fe μm⁻³ was comparable to that of Escherichia coli. We note that the disproportionate contribution of the vacuole (with its high organic content) to total volume typical of large diatoms is a potentially significant source of error in Fe:C ratios and suggest that Fe should be normalized to cytoplasmic volume whenever possible to permit valid intercomparisons between studies. The composition, F_v:F_m data and Fe:C ratio suggest a relatively uniform population experiencing little N, P or Fe stress. The uncoupling of the Fd Index from these measures is consistent with previous findings showing that the expression of flavodoxin can be characterized as an early stress response and that its accumulation is not necessarily correlated with physiological deficit. Ethmodiscus appears to be well adapted to some of the most oligotrophic waters in the ocean. Because it is an important sedimentary marker, the biology of living Ethmodiscus provides insights into the source of extensive Ethmodiscus oozes. Mass sedimentation after frontal accumulation has been suggested as a source for these oozes. Our data contain no evidence that the flux is linked directly to Fe, N or P stress.     

Monsoonal influence on the distribution of phytoplankton pigments in the Arabian Sea

Variations in the distribution of chemotaxonomic pigments were monitored in the Arabian Sea and the Gulf of Oman at the end of the SW monsoon in September 1994 and during the inter-monsoon period in November/December 1994 to determine the seasonal changes in phytoplankton composition. The Gulf of Oman was characterized by sub-surface chlorophyll maxima at 20-40 m during both seasons, and low levels of divinyl chlorophyll a indicated that prochlorophytes did not contribute significantly to the total chlorophyll a. Prymnesiophytes (19′-hexanoyloxyfucoxanthin), diatoms (fucoxanthin) and chlorophyll b containing organisms accounted for most of the phytoplankton biomass in September, while prymnesiophytes dominated in November/December. In the Arabian Sea in September, high total chlorophyll a concentrations up to 1742 ng l^-1 were measured in the coastal upwelling region and a progressive decline was monitored along the 1670 km offshore transect to oligotrophic waters at 8°N. Divinyl chlorophyll a was not detected along this transect except at the two most southerly stations where prochlorophytes were estimated to contribute 25–30% to the total chlorophyll a. Inshore, the dominance of fucoxanthin and/or hexanoyloxyfucoxanthin indicated that diatoms and prymnesiophytes generally dominated the patchy phytoplankton community, with zeaxanthin-containing Synechococcus also being important, especially in surface waters. At the southern oligotrophic localities, Synechococcus and prochlorophytes dominated the upper 40 m and prymnesiophytes were the most prominent at the deep chlorophyll maximum. During the inter-monsoon season, total chlorophyll a concentrations were generally half those measured in September and highest levels were found on the shelf (1170 ng l^-1). Divinyl chlorophyll a was detected at all stations along the Arabian Sea transect, and we estimated that prochlorophytes contributed between 3 and 28% to the total chlorophyll a, while at the two oligotrophic stations this proportion increased to 51–52%. While procaryotes were more important in November/December than September, eucaryotes still accounted for >50% of the total chlorophyll a. Pigment/total chlorophyll a ratios indicated that 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes were the dominant group, although procaryotes accounted for 65% at the two southerly oligotrophic stations.     

The natural diet of a hexactinellid sponge: Benthic–pelagic coupling in a deep-sea microbial food web

Dense communities of shallow-water suspension feeders are known to sidestep the microbial loop by grazing on ultraplankton at its base. We quantified the diet, rates of water processing, and abundance of the deep-sea hexactinellid sponge Sericolophus hawaiicus, and found that, like their demosponge relatives in shallow water, hexactinellids are a significant sink for ultraplankton. S. hawaiicus forms a dense bed of sponges on the Big Island of Hawaii between 360 and 460 m depth, with a mean density of 4.7 sponges m⁻². Grazing of S. hawaiicus on ultraplankton was quantified from in situ samples using flow cytometry, and was found to be unselective. Rates of water processing were determined with dye visualization and ranged from 1.62 to 3.57 cm s⁻¹, resulting in a processing rate of 7.9±2.4 ml sponge⁻¹ s⁻¹. The large amount of water processed by these benthic suspension feeders results in the transfer of approximately 55 mg carbon and 7.3 mg N d⁻¹ m⁻² from the water column to the benthos. The magnitude of this flux places S. hawaiicus squarely within the functional group of organisms that link the pelagic microbial food web to the benthos.     

Latitudinal distribution of microbial plankton abundance,production, and respiration in the Equatorial Atlantic in autumn 2000

Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll-a (Chl-a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (10°N–10°S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From 2°N to 5°S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (>1 μM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated ¹⁴C primary production ranged from ∼200 mg C m⁻² d⁻¹ in the TTS region to ∼1300 mg C m⁻² d⁻¹ in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl-a m⁻², respectively). Picophytoplankton were the main contributors (>60%) to both Chl-a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m⁻² d⁻¹) three-fold larger than net production measured in the TTS region (370 mg C m⁻² d⁻¹). The entire region under study showed net autotrophic community metabolism, since respiration accounted on average for 51% of gross primary production integrated over the euphotic layer.