Deep Sea Research Part II Topical Studies in Oceanography

The abundance and higher taxonomic composition of epizooic metazoan meiobenthic communities associated with mussel and tubeworm aggregations of hydrocarbon seeps at Green Canyon, Atwater Valley, and Alaminos Canyon in depths between 1400 and 2800 m were studied and compared to the infaunal community of non-seep sediments nearby. Epizooic meiofaunal abundances of associated meiobenthos living in tubeworm bushes and mussel beds at seeps were extremely low (usually <100 ind. 10 cm(-2)), similar to epizooic meiofauna at deep-sea hydrothermal vents, and the communities were composed primarily of nematodes, copepods, ostracods, and halacarids. In contrast, epizooic meiobenthic abundance is lower than previous studies have reported for infauna from seep sediments. Interestingly, non-seep sediments contained higher abundances and higher taxonomic diversity than epizooic seep communities, although in situ primary production is restricted to seeps.

Harmful algal blooms (HABs) are a global problem that affects both human and ecosystem health. One of the most serious and widespread HAB poisoning syndromes is paralytic shellfish poisoning, commonly caused by Alexandrium spp. dinoflagellates. Like many toxic dinoflagellates, Alexandrium produces resistant resting cysts as part of its life cycle. These cysts play a key role in bloom initiation and decline, as well as dispersal and colonization of new areas. Information on cyst numbers and identity is essential for understanding and predicting blooms, yet comprehensive cyst surveys are extremely time- and labor-intensive. Here we describe the development and validation of a quantitative real-time PCR (qPCR) technique for the enumeration of cysts of A. tamarense of the toxic North American/Group I ribotype. The method uses a cloned fragment of the large subunit ribosomal RNA gene as a standard for cyst quantification, with an experimentally determined conversion factor of 28,402±6152 LSU ribosomal gene copies per cyst. Tests of DNA extraction and PCR efficiency show that mechanical breakage is required for adequate cyst lysis, and that it was necessary to dilute our DNA extracts 50-fold in order to abolish PCR inhibition from compounds co-extracted from the sediment. The resulting assay shows a linear response over 6 orders of magnitude and can reliably quantify ≥10 cysts/cm³ sediment.

A sediment core collected from an area ∼100 miles south of the ODP Leg 116 (distal Bengal Fan) in the equatorial Indian Ocean was investigated for microfossils, mineralogy, mineral chemistry, magnetic susceptibility, grain size, major, minor and rare-earth element geochemistry, organic carbon and total nitrogen contents in the bulk sediments. Distinct changes in depositional characteristics (including presence of abundant sand-sized micas and other detrital minerals) occur at two sub-surface depths corresponding to ∼0.5 and probably 0.8 Ma time periods. The detrital mineral suite of this core resembles that of turbidite unit I sediments of ODP cores in the distal Bengal Fan. The core site has received an increased supply of terrigenous sediments at these two time periods, the older pulse (0.8 Ma) stronger than the younger pulse. Several lines of evidence such as the nature of the mineral suite, lower magnetic susceptibility values, Si/Al in mica mineral separates; major element composition; discrimination plots of Ca/Ti versus K/Ti and K2O/Al2O3 and La/Yb ratios suggest a highly metamorphosed source such as higher Himalayan crystalline (HHC) series indicating two events of increased physical weathering and erosion in the Himalayan region. While the erosional event of 0.8 Ma is well known, the episode of 0.5 Ma was not reported earlier.

We describe Bathyallogromia weddellensis gen. & sp. nov., a deep-water, monothalamous foraminiferan from bathyal and abyssal sites in the western Weddell Sea. The species is characterised by a delicate, almost spherical, organic-walled test, a low, broad projecting apertural region, and light grey or greenish cytoplasm containing mineral grains and other inclusions. Molecular phylogenetic analyses, based on small subunit rRNA gene sequences, indicate that Bathyallogromia is an independent lineage branching within a clade of monothalamous foraminiferans, which also includes such genera as Saccammina, Gloiogullmia, Cylindrogullmia, Rhabdammina, Toxisarcon and Pilulina (?). Lack of significant genetic differences between specimens collected at depths ranging from 1000 to 6300 m suggests that B. weddellensis is adapted to conditions that span a broad bathymetric range.

The vertical distribution (0–1000 m depth) of macrozooplankton along the northern portion of the Mid-Atlantic Ridge (59°58N, 25°53W to 41°29N, 28°19W) was investigated during the MAR-ECO program (June and July 2004) using the Underwater Video Profiler (UVP). Twelve relatively large (>1 cm) groups were selected from the recorded images: sarcodines (with two sub-groups), crustaceans (excluding copepods), chaetognaths, ctenophores (with two sub-groups cydippids and lobates), siphonophores, medusae (with three sub-groups Aeginura grimaldii, Aglantha spp. and all other medusae), appendicularians, and thaliaceans. The numerically dominant groups over the whole area were crustaceans (26%), medusae (20%) and appendicularians (17%). The gelatinous fauna were consistently most numerous at 400–900 m. Appendicularians, ctenophores and Aeginura grimaldii occurred mostly below 300 m (maximum concentrations of 75, 58, and 30 individuals 100 m−3, respectively). Siphonophores, Aglantha spp. and the other medusae were more uniformly distributed in the water column (maxima of 42, 42 and 300 individuals 100 m−3, respectively). The macrozooplankton community below 200 m varied with the spatial distribution of the water masses, suggesting that the Sub-Polar Front restricts the mixing of macrozooplankton communities down to 1000 m depth.

Distinct profiles of extracellular proteolytic enzyme activity were observed in the water column of the North Atlantic, with maximum potential proteolytic activity occurring in the top 35 m. The proteolytic enzyme Vmax values varied significantly and decreased from 1.46 nM min−1 in surface waters to 0.365 nM min−1 at 100 m. In contrast, Km values increased with depth from about 70 to 360 μM. Cell-associated enzymes accounted for the majority of the observed proteolytic activity. Dissolved enzymes comprised only 30–40% of the total extracellular enzyme activity and exhibited a low substrate affinity (Km=∼1000 μM). These observations indicate clear stratification of bacterial associated extracellular enzyme activity, with the maximum activity in surface waters. This is consistent with some environmental changes in the water column, especially algal biomass and nitrate concentration. Bacterial mediated nitrogen remineralization in surface waters was approximately three times the total nitrogen demand of phytoplankton and bacteria. We determined bacterial population diversity using 16S rRNA sequence analysis and found evidence for stratification, with a higher representation of the Cytophaga/Flexibacter/Bacteriodes group at 5 m compared to 100 m. No similar stratification was observed among the α-proteobacterial SAR11 cluster, which were especially prevalent in the PRIME eddy. However, sequences phylogenetically related to another marine cluster, SAR122, were only observed at 100 m. We suggest that stratification of proteolytic activity within the water column may be explained at least in part, by differences in the composition of the bacterial community.

Ice-rafted debris (IRD) from sediment traps, surficial sediments, and sediment cores were investigated in order to reconstruct the extent history of sea-ice cover during the last 100 kyrs in the Okhotsk Sea. The seasonal IRD buried into the sediments, during and just after sea-ice melting in spring, consists of silt- and sand- size terrigenous particles. The spatial distribution of IRD within surficial sediments clearly corresponds to the extent of sea-ice coverage. Sea-ice expanded to a maximum extent during the glacial periods, although sea-ice cover was not perennial. The Okhotsk Sea Sea-ice Expansion (OSIE) events, which are recognized by millennial-scale-abrupt peaks of IRD content, have occurred 13 times during the past 100 kyrs. These events were amplified during the glacial periods. A significant modification of the polar atmospheric circulation in the northern hemisphere is a key process in explaining these OSIE events.

Laminated diatom ooze samples collected during ODP Leg 177 were analysed using scanning electron microscope (SEM) and optical microscopy to test their potential as high-resolution records of Polar Front hydrography, surface production, and export. SEM analysis from two intervals, marine isotope stage (MIS) 29 and 12/11, respectively, recovered from 50°S in the Atlantic Ocean (ODP Site 1093, Hole A, sections 13H-4 0-18 cm and 23H-4 0-22cm), show abundant and well-preserved Thalassiothrix antarctica mats, thought to be indicative of rapid export from the surface and deposition in the sediment. A preliminary analysis of laminae succession points to a possible annual couplet/triplet succession of laminae, and suggests exceptionally high local sedimentation rates of 57 and 80cm kyr-1 for MIS 12/11 and 29, respectively. Such high accumulation rates imply that local export from the surface layer and sequestration of biogenic silica and organic matter to the sediments may have been much higher than previously suggested.

Boundary scavenging processes result in the deposition of particle-reactive chemical substances in ocean margin sediments at rates much greater than their average rate of deposition over the entire ocean. Sediments and samples collected by time-series sediments traps were studied to examine the nature and extent of boundary scavenging in the Middle Atlantic Bight (MAB) region of the NW Atlantic. Here, new results are presented for 230Th, 231Pa and 10Be and compared to older findings, based on studies of 210Pb, where it appeared that the normal enhancement of particle-reactive trace-metal deposition at ocean margins was absent. In contrast to the behavior of 210Pb, deposition rates of 230Th, 231Pa and 10Be exceed their local rates of supply, in some cases by more than an order of magnitude. Enhanced deposition of these tracers is clearly occurring in the MAB. An unusual feature of boundary scavenging, however, is that the pattern of enhanced deposition follows the order Be > Th > Pa > Pb, whereas the expected order be Be > Pb ≥ Pa > Th. Furthermore, unsupported 231Pa/230Th activity ratios are consistently less than their production ratio of 0.093, in contrast to ratios typically found at other ocean margins in the range of 0.2–0.3. Export from the region of fine-grained manganese-rich particles is postulated to explain the low fluxes of 210Pb and the low 231Pa/230Th ratios. Other trace elements which have high affinities for adsorption to Mn oxides may, like Pb and Pa, have much lower rates of deposition in the MAB than in other, more typical, ocean-margin environments.

The scavenging of ²³⁰Th, ²³¹Pa and ¹⁰Be was studied in the Pacific sector of the Southern Ocean along 170°W using measurements from surface sediments, time-series sediment traps, and the water column. All sediment traps collected an annual flux of ²³¹Pa greater than the integrated rate of production by ²³⁵U-decay (up to 5.5 times greater) and a flux of ¹⁰Be greater than the global-average deposition rate of ¹⁰Be (up to 3.4 times greater). Fluxes of ²³⁰Th were on average close to the production rate in the overlying water column. These large fluxes of ²³¹Pa and ¹⁰Be, and high unsupported ²³¹Pa/²³¹Th and ¹⁰Be/²³⁰Th ratios in the sediments, are not associated with depletion of nuclides in the water column, and therefore are not the result of the classical boundary scavenging mechanism. We estimate that of the ²³¹Pa and ²³⁰Th advected into the Southern Ocean as part of the large-scale overturning circulation, only those nuclides associated with the “upper limb” of this circulation, i.e. those that pass through the surface as part of the wind-driven Ekman flow, are scavenged efficiently. The majority of the nuclides advected into the Southern Ocean and associated with the “bottom water limb” are not scavenged to the sediments of the Southern Ocean, but are returned northward.

Phytoplankton dynamics were investigated in the eastern equatorial Pacific at 32 stations sampled during two cruises (December 2004 and September 2005). Based on standing stock analyses from HPLC pigments, flow cytometry and microscopy, we used a modified 2-treatment approach of the seawater dilution method to estimate taxon-specific phytoplankton growth and mortality rates in 8-depth per station profiles. These data were complemented by contemporaneous measurements of dissolved iron (Fe). The stations encompassed an equatorial zonal gradient (110 to 140°W) of diminishing eastward Fe availability in the euphotic zone from upwelling of the Equatorial Undercurrent (EUC). Latitudinal variation was assessed by meridional transects at 110 and 140°W. Overall, euphotic zone averaged growth rates were 0.53±0.17 d−1 (total chlorophyll a), 0.34±0.15 d−1 (divinyl chlorophyll a) and 0.86±0.32 d−1 (fucoxanthin). Microzooplankton grazing accounted for 50-60% of daily production of eukaryotic algae, whereas essentially all growth of phototrophic prokaryotes was consumed daily. Fucoxanthin, representing diatoms, was a minor component of the accessory pigments, but diatom growth rates were both significantly higher than other taxonomically defined groups and dropped off more sharply with depth (low light level). Strikingly, no spatial or temporal trends were seen in the 256 growth rate measurements for each measured pigment. However, the diminishing eastward equatorial Fe gradient was associated with deepening subsurface pigment maxima and decreasing surface-layer pigment stocks (down to the 8% light level). In addition, integrated standing stocks of total chlorophyll a and Prochlorococcus (divinyl chlorophyll a) were strongly correlated with integrated iron at equatorial upwelling stations, yet no correlation with Fe was seen for any of the eukaryotic groups, including diatoms. This latter result is contrary to expectations from previous Fe addition experiments (in situ or in bottles), where diatom biomass increased relative to other phytoplankton. We hypothesize that the natural supply of Fe to the base of the euphotic zone from the EUC is less favorable for diatoms because of light limitation. Rather, new Fe is rapidly incorporated into a small phytoplankton-dominated community in the deep euphotic zone, and tightly coupled grazing control results in a system regulated by return of recycled Fe.

A transition from fully laminated to highly bioturbated sediments on continental margins is thought to derive from increased animal activity associated with increasing bottom-water oxygen concentration. We examined faunal community responses to oxygen and organic matter gradients across the lower oxygen minimum zone (OMZ) on the bathyal Pakistan margin, where sediments grade from fully laminated sediment at 700 m (0.12 mL L−1 O2 [5 μM]) to highly bioturbated sediment at 1100 m (0.23 mL L−1 O2 [10 μM]). High-resolution sampling of the seafloor (every 50 m water depth) was conducted along a single transect during inter- and post-monsoon periods in 2003 to address (a) the existence of oxygen thresholds regulating macrofaunal abundance, composition, diversity and lifestyles, (b) the interactive effects of organic matter quantity and quality, (c) associated community effects on sediment structure, and (d) potential seasonality in these processes. Macrofaunal biomass and bioturbation depth were positively correlated with organic matter availability, which peaked at 850–950 m (3.39–3.53% Org. C). In contrast, macrofaunal diversity (H′), dominance (R1D), and burrow number exhibited threshold responses at oxygen concentrations of 0.12–0.20 mL L−1 [5–9 μM]), with few animals and highly laminated sediments present below this concentration and most taxa present in fully bioturbated sediments above it. The highly mobile, burrowing amphinomid polychaete Linopherus sp. exhibited almost complete dominance and high density at 750–850 m (0.12–0.14 mL L−1 O2 [5–6 μM]), but despite its activity, sediment laminae remained faintly visible. Formation of permanent burrows and detritivory were dominant macrofaunal lifestyles within the OMZ, allowing laminae to persist at surprisingly high animal density and biomass. Results reflect a shift from organic matter to oxygen regulation of body size and biogenic structures following the monsoon. This study suggests that for assemblages evolving under permanent severe hypoxia, food availability remains a significant determinant of animal abundance and biogenic structure depth. Oxygen influences patterns of diversity and dominance and interacts with organic matter to generate abrupt faunal transitions on the Pakistan margin.

Backscattering measurements of 14 live individual Antarctic krill (Euphausia superba) were made at a frequency of 120 kHz in a chilled insulated tank at the Long Marine Laboratory in Santa Cruz, CA. Individual animals were suspended in front of the transducers, were only loosely constrained, had substantial freedom to move, and showed more or less random orientation. One thousand echoes were collected per animal. Orientation data were recorded on video. The acoustic data were analyzed and target strengths determined from each echo. A method was developed for estimating the three-dimensional orientation of the krill based on the video images and was applied to five of them, giving their target strengths as functions of orientation. Scattering models based on a simplified distorted-wave Born approximation (DWBA) method were developed for five animals and compared with the measurements.Both measured and modeled scattering patterns showed that 120 kHz acoustic scattering levels are highly dependent on animal orientation. Use of these scattering patterns with orientation data from shipboard studies of E. superba gave mean scattering levels approximately 12 dB lower than peak levels. These results underscore the need for better in situ behavioral data to properly interpret acoustic survey results. A generic E. superba DWBA scattering model is proposed that is scalable by animal length. With good orientation information, this model could significantly improve the precision and accuracy of krill acoustic surveys.

The EDdy Dynamics, mixing, Export, and Species composition (EDDIES) project provided a unique opportunity to evaluate the response of the microbial community and further understand the biological and biogeochemical consequences of mesoscale perturbation events in an oligotrophic system. In order to characterize microbial dynamics, we performed measurements of bacterial biomass (BB) and production (BP) and phytoplankton pigment analyses in two upwelling eddies in the Sargasso Sea sampled in 2004 and 2005. We also observed a 3-fold increase in BP at the Bermuda Atlantic Time-series Study (BATS) site during the passage of a cyclonic eddy in 2003. Although the integrated BB and BP over 140 m in 2004 and 2005 eddies remained within the climatological range measured at the BATS site, there was systematic variability in bacterioplankton dynamics across both eddies. Cyclonic eddy C1 demonstrated decreased BP at the feature's center relative to its periphery, and BP was not correlated with total chlorophyll a (TChl a) variability. However, BP correlated with prymnesiophyte pigments throughout the feature. In contrast, mode-water eddy A4 showed an enhancement in BP at the eddy center (EC) relative to its edges and was coincident with elevated TChl a, high primary production measurements, and a high concentration of diatoms. In eddy A4, the tight relationship between enhanced BP, TChl a and specific phytoplankton taxa implies that the phytoplankton community structure was an important factor influencing BP variability. While the heterotrophic bacterial response in C1 and A4 was not enhanced relative to BATS summer climatology, these data and the presence of similar nutrient fields across both eddies suggest that BP and BB were influenced by the eddy perturbations and responded to changes in the phytoplankton community.

Monthly atmospheric depositional fluxes of 137Cs, 239,240Pu and 241Am, coupled with measurements of these radionuclides in seawater of the northwest Mediterranean Sea, are presented. Precipitation rates are the dominant factor in controlling the temporal deposition of anthropogenic radionuclides onto the northwest Mediterranean; however, special events such as Saharan dust input bring a significant amount of radionuclides to this area. The average annual deliveries of 137Cs, 238Pu, 239,240Pu and 241Am through the atmosphere into the northwest Mediterranean were 990, 0.9, 22 and 7.9 mBq m−2, respectively. The corresponding amounts delivered annually into the northwest Mediterranean were 0.02 TBq for 239,240Pu, 0.007 for 241Am, and 0.84 for 137Cs. The present atmospheric inputs represent about 0.08% for 239,240Pu, 0.58% for 241Am, and 0.02% for 137Cs of their water-column inventories.While the present surface 239,240Pu concentrations in the west Mediterranean have significantly decreased since 1976, those recorded in deeper layers (below 1000 m) have increased. The observed differences between the 241Am and 239,240Pu profiles have been caused by the different scavenging efficiencies of the two radionuclides, and also by 241Am in-growth from 241Pu. The activity ratios of 241Am/239,240Pu and 239,240Pu/137Cs through the water column appear to be lower than the global fallout ratio, whereas 137Cs/90Sr activity ratios reflect the global fallout ratio. Higher concentrations of 90Sr and 137Cs were found in the intermediate layer (200–600 m), where higher salinity and temperature levels prevail. This indicates the intrusion of Levantine Intermediate Water from the Eastern Basin into the northwest Mediterranean Sea. The estimated radionuclide inventories in the water column for 2001 were around 3700 Bq m−2 for 137Cs, 2200 for 90Sr, 52 for 239,240Pu, and 7.0 for 241Am.

The distribution of anthropogenic 137Cs in surface waters of the NE Atlantic Ocean resulting from discharges from European nuclear reprocessing plants, the Chernobyl accident, and global fallout has shown decreasing concentrations in most regions in the past 2 decades, in proportion to the lessening of its release rates from the Sellafield and La Hague reprocessing plants. In contrast, concentrations increased significantly in the Baltic Sea in 1986, as a result of the Chernobyl accident. The average 137Cs concentrations in surface water have been estimated for the year 2000 to be 60±50 Bq m−3 for the Irish Sea, 3±1 for the Celtic Sea, 2.1±1.2 for the English Channel, 4±2 for the North Sea, 50±20 for the Baltic Sea, 2±1 for the Norwegian Sea, 2.4±0.5 for the Barents Sea, and 4±1 Bq m−3 for the Kara Sea. On the basis of time series data, the corresponding effective half-lives of 137Cs in surface water have been estimated to be 4±1 years for the Irish Sea, 2±1 for the Celtic Sea, 1.2±0.5 for the English Channel, 3±1 for the North Sea, 13±2 for the Baltic Sea, 4±1 for the Norwegian Sea, 5±1 for the Barents Sea, and 13±4 years for the Kara Sea. The mean transit times of water masses have been estimated to be 0.5±0.1 year from the Irish Sea to the North Channel, 2±0.2 years to the north of Scotland, 2.5±0.2 to the east coast of England, 3±0.5 to the southern North Sea, 4±0.5 to the central and eastern North Sea, 1.5±0.2 to the Celtic Sea, 2±0.2 to the English Channel, 5±1 to the Baltic Sea, 4.5±0.5 to the Norwegian Sea, 5±1 to the Barents Sea, and 6±1 years to the Kara Sea.

Geographical and temporal variations of anthropogenic radionuclides, 137Cs and 239,240Pu, in surface waters of the North Pacific Ocean were studied using the HAM database, which is a comprehensive data set on 137Cs, 239,240Pu, and other anthropogenic radionuclides. The geographical distributions suggest that 137Cs and 239,240Pu concentrations in surface waters of the Pacific Ocean in the 1960s were controlled by global fallout from the 1961–1962 US and former USSR atmospheric nuclear weapons tests. The changes in surface 137Cs concentrations in the Pacific that occurred after 1970 were caused by physical processes. In the 1990s, surface 137Cs showed a homogeneous distribution throughout the Pacific. Biogeochemical and physical processes are important factors controlling surface 239,240Pu concentrations. The time-series data on anthropogenic radionuclides suggest that wind-driven circulation, subduction, and upwelling of Pacific surface and subsurface waters control the geographic distributions of anthropogenic radionuclides and their fates.

The North Water Polynya is an area of high biological activity that supports large numbers of higher trophic-level organisms such as seabirds and marine mammals. An overall objective of the Upper Trophic-Level Group of the International North Water Polynya Study (NOW) was to evaluate carbon and contaminant flux through these high trophic-level (TL) consumers. Crucial to an evaluation of the role of such consumers, however, was the establishment of primary trophic linkages within the North Water food web. We used δ15N values of food web components from particulate organic matter (POM) through polar bears (Ursus maritimus) to create a trophic-level model based on the assumptions that Calanus hyperboreus occupies TL 2.0 and there is a 2.4‰ trophic enrichment in 15N between birds and their diets, and a 3.8‰ trophic enrichment for all other components. This model placed the planktivorous dovekie (Alle alle) at TL 3.3, ringed seal (Phoca hispida) at TL 4.5, and polar bear at TL 5.5. The copepods C. hyperboreus, Chiridius glacialis and Euchaeta glacialis formed a trophic continuum (TL 2.0–3.0) from primary herbivore through omnivore to primary carnivore. Invertebrates were generally sorted according to planktonic, benthic and epibenthic feeding groups. Seabirds formed three trophic groups, with dovekie occupying the lowest, black-legged kittiwake (Rissa tridactyla), northern fulmar (Fulmarus glacialis), thick-billed murre (Uria aalge), and ivory gull (Pagophilia eburnea) intermediate (TL 3.9–4.0), and glaucous gull (Larus hyperboreus) the highest (TL 4.6) trophic positions. Among marine mammals, walrus (Odobenus rosmarus) occupied the lowest (TL 3.2) and bearded seal (Erignathus barbatus), ringed seal, beluga whale (Delphinapterus leucas), and narwhal (Monodon monoceros) intermediate positions (TL 4.1–4.6). In addition to arctic cod (Boreogadus saida), we suggest that lower trophic-level prey, in particular the amphipod Themisto libellula, contribute fundamentally in transferring energy and carbon flux to higher trophic-level seabirds and marine mammals. We measured PCB 153 among selected organisms to investigate the behavior of bioaccumulating contaminants within the food web. Our isotopic model confirmed the trophic magnification of PCB 153 in this high-Arctic food web due to a strong correlation between contaminant concentration and organism δ15N values, demonstrating the utility of combining isotopic and contaminant approaches to food-web studies. Stable-carbon isotope analysis confirmed an enrichment in 13C between POM and ice algae (–22.3 vs. –17.7‰). Benthic organisms were generally enriched in 13C compared to pelagic species. We discuss individual species isotopic data and the general utility of our stable isotope model for defining carbon flux and contaminant flow through the North Water food web.

The carbon isotope systematics of marine carbonates, organic matter and dissolved inorganic carbon (DIC) play a critical role in quantifying carbonate dissolution fluxes from modern deep-ocean sediments to paleoocean–atmospheric modeling. However, there is a growing body of evidence that C mass and isotope balances in marine pore waters appear incompatible, suggesting that some processes other than mass transport, carbonate dissolution, and organic matter decomposition have significantly increased the value of δ13C(DIC). We present a comprehensive data set of pore water and sediment geochemistries in biogenic carbonates from well-characterized depositional environments of the South Florida platform. Pore water elemental and δ13C(DIC) values are integrated with δ13C values of carbon sources (seawater, organic and inorganic carbon), sediment mixing rates (210Pb profiles), microbial sulfate reduction rates (SRR) (radiotracer 35SO42−), and incubation experiments spiked with low δ13C(DIC) to estimate the rate and extent of C isotope exchange. Together, these data indicate that biogenic carbonates undergo extensive syndepositional recrystallization at rates comparable to net dissolution rates, permitting significant exchange between isotopically depleted organic C and isotopically enriched inorganic C pools.

13C contents of total dissolved inorganic carbon (δ13C-DIC) and particulate organic matter (δ13Corg) were determined to examine the factors influencing phytoplankton 13C contents and carbon export from the SOIREE iron-induced algal bloom. Suspended particles sieved into 200, 70, 20, 5, and 1 micrometer (μm) size classes displayed an extremely large range in δ13Corg of 8‰. δ13Corg values increased from −28‰ for the 1–5 μm class to a maximum of −20‰ for the 20–70 μm class, which was dominated by the large pennate diatom Fragilariopsis kerguelensis. Larger particles (70–200 and >200 μm) had similar δ13Corg to the smaller (1–5 and 5–20 μm) particles, reflecting both the presence of long narrow Thalassiothrix antarctica diatoms and zooplankton that grazed on small phytoplankton. Comparison of results inside and outside the bloom identified cell surface/volume ratio (mainly reflecting cell size) as the dominant control of phytoplankton δ13Corg, with subsidiary roles for growth rate and seawater [CO2]aq. The SOIREE iron fertilization provoked an increase in the proportion of large (>20 μm) diatoms. This increased the δ13Corg of the bulk suspended particles within the mixed layer, but there was minimal increase in the δ13Corg of sub-surface suspended particles and negligible change in the δ13Corg of particles obtained with sediment traps suspended below the bloom. This suggests that there was no increase in carbon export over the ∼13 day observation period. However, comparison to δ13Corg results from previous voyages, and to vertical changes in δ13C-DIC, suggests that large diatoms control carbon export from the Antarctic Zone over the summer growth season. This result must be viewed with great caution as it is based on very sparse data and involves several assumptions.

Spatial distribution of both abundance and biomass of phytoplankton <13 μm were analysed by flow cytometry, distinguishing eukaryotic picoplankton, nanoplankton, cryptomonads, Synechococcus and Prochlorococcus. These groups were related to different circulation patterns observed in two successive mesoscale samplings in the Gulf of Cádiz. The first sampling was characterized by westerly-driven upwelling processes around Cape Santa Maria and a surface signal of the Huelva front. During the second sampling easterlies blew and warm waters covered the shelf between Cádiz and Cape Santa Maria although a reminiscent upwelling was still observed west of the cape. Under the first wind regime, lowest phytoplankton abundance was measured in the upwelling area probably due to time lag of phytoplankton response, whereas the highest abundance occurred associated with the Huelva front (T∼16 °C). During the second cruise and under easterly wind regime, higher phytoplankton abundance and biomass of all groups were found as compared to the first period. Phytoplankton accumulation was mainly associated with the 16 °C isotherm in upwelled waters and in warm shelf waters. In the mixing area between warm shelf waters and cold upwelled waters, phytoplankton size spectra were found to be less negative in the warm stratified side than in the cold mixed side. This unusual feature was attributed to the warming and nutrient enrichment of shelf water during each tidal cycle in river delta and marshes located around the Gulf of Cádiz, leading to elevated biomass around these structures. On TS diagrams, eukaryotic nanoplankton was associated with warm shelf water, eukaryotic picoplankton and Synechococcus with upwelled water, while Prochlorococcus reached highest abundances in deeper water (50–75 m) associated with the mixing line between North Atlantic Central Water and Surface Atlantic Water.

Bacterial abundance and production were studied in different zones in the Southern Ocean (39–62°S, 4–14°E) during a cruise in December–January 1997/1998. The role of potential growth limitation of bacteria due to limited availability of organic carbon (glucose) or inorganic N and P was studied in parallel. A positive correlation between surface water temperatures (−2 to 18 °C) and bacterial abundance (<0.1×106–1.5×106 cells ml−1) was observed. Bacteria were studied in vertical profiles, concentrated to three areas close to 6°E: the former Spring Ice Edge (SIE, 60°S, high chlorophyll a), the former Winter Ice Edge (WIE, 56°S, low chlorophyll a) and the Antarctic Polar Front at 51°S (APF, moderate chlorophyll a levels). Bacterial abundance was uniformly low south of the APF, and for the upper 50 m generally below 0.3×106 bacteria ml−1. In deeper water, bacterial abundance decreased dramatically for WIE and APF stations, but less markedly for SIE stations. The average volumetric bacterial production in the mixed layer was highest for APF stations (0.04 μg C l−1 h−1), but only half of this value for SIE stations (0.02 μg C l−1 h−1), with WIE in between (approximately 0.03 μg C l−1 h−1). Below 100 m, bacterial production decreased to values close to the detection limit. None of the three areas demonstrated any systematic diurnal variations in bacterial production in surface water (2 m) or at the chlorophyll maximum (situated between 30 and 66 m). We observed a positive correlation between bacterial production and in vivo chlorophyll a fluorescence, but there was no correlation between this parameter and bacterial abundance, possibly indicating different control mechanisms for these two parameters. Unfiltered water samples from 20 m depth were incubated at in situ temperatures and amended with ammonium, phosphate or glucose. In all the three experiments, from warm waters (relatively poor in inorganic nitrogen and phosphorus) south of Cape Town (38°S, +18.6 °C), in the colder and inorganic nutrient-rich waters north of the APF (45°S, +7.0 °C) as well as in the cold, nutrient-rich waters at the SIE (61°S, −0.13 °C), organic carbon additions resulted in a significant increase in bacterial production. Bacterial growth rates were very different between the three regions, and the growth response in the bacterial communities to the carbon additions was very slow at low temperatures.

Total organic carbon (TOC) was analyzed on four transects along 140°W in 1992 using a high temperature combustion/discrete injection (HTC/DI) analyzer. For two of the transects, the analyses were conducted on-board ship. Mixed-layer concentrations of organic carbon varied from about 80 μM C at either end of the transect (12°N and 12°S) to about 60 μM C at the equator. Total organic carbon concentrations decreased rapidly below the mixed-layer to about 38–40 μM C at 1000 m across the transect. Little variation was observed below this depth; deep water concentrations below 2000m were virtually monotonic at about 36 μM C. Repeat measurements made on subsequent cruises consistently found the same concentrations at 1000 m or deeper, but substantial variations were observed in the mixed-layer and the upper water column above 400 m depth.Linear mixing models of total organic carbon versus σθ exhibited zones of organic carbon formation and consumption. TOC was found to be inversely correlated with apparent oxygen utilization (AOU) in the region between the mixed-layer and the oxygen minimum. In the mixed-layer, TOC concentrations varied seasonally. Part of the variations in TOC at the equator was driven by changes in the upwelling rate in response to variations in physical forcing related to an El Niño and to the passage of tropical instability waves. TOC export fluxes, calculated from simple box models, averaged 8±4 mmol C m−2day−1 at the equator and also varied seasonally. These export fluxes account for 50–75% of the total carbon deficit and are consistent with other estimates and model predictions.

Micro-phytoplankton (>20 um cell size) was sampled in the upper 200 m of the water column at the Pacific equator, 140°W during two JGOFS EqPac Time Series (TS) Studies, in order to determine the changes in the micro-phytoplankton assemblage between March–April and October 1992, to find the vertical distribution of micro-phytoplankton taxa, and to relate any changes in the environmental factors to micro-phytoplankton structure. Cell abundance and carbon biomass of three major taxonomic classes: diatoms, dinoflagellates and coccolithophorids were examined. During the abnormal warmth of El Niño 1992 (SST = 28−29°C), low abundance (<3000 cells liter−1) and carbon biomass (<1,μg C liter−1) characterized the micro-phytoplankton structure, accompanied by low numbers of diatoms and coccolithophorids. Pennate diatoms, Pseudonitzschia delicatissima and thecate dinoflagellate, Oxytoxum variabile, were the most abundant organisms observed during March–April 1992 Time Series study. The microphytoplankton assemblage during El Niño conditions in March–April 1992 exhibited lower microphytoplankton species richness and abundance compared with October 1992. Also in contrast to the spring, in October 1992 the micro-phytoplankton assemblage showed large variability, mainly due to the passage of an instability wave through the study site. During this period, the coldest temperatures (SST = 25°C) were associated with increased abundance (range = 2 × 102 to 12 × 103 cells liter−1) and richness in micro-phytoplankton species assemblage, which was again dominated by a colonial pennate diatom P. delicatissima. On the average, micro-phytoplankton carbon ranged from 0.5 to 4.0 μg C liter−1, where the diatom group consistently comprised the major part of the micro-phytoplankton autotrophic biomass in the upper 60 m. Large centric diatoms, such as Rhizosolenia species, as well as chains of P. delicatissima united in stepped colonies, and heavily silicified species of the Thalassionema/Thalassiothrix spp. complex were important groups contributing to the total micro-phytoplankton carbon biomass. At the equator, diatoms and dinoflagellates were restricted to the surface and to the upper 60 m, respectively, during both Time Series cruises, while coccolithophorid cells were concentrated at 90 m during TS I. The presence of an El Niño event and a instability wave during March–April and October 1992, respectively, may explain most of the variability in abundance and species richness found in the equatorial Pacific at 140°W during the study periods.

High temporal resolution measurements of physical and bio-optical variables were made in the upper ocean using a mooring located at 0°, 140°W from 9 February 1992 to 15 March 1993 as part of the equatorial Pacific Ocean (EgPac) study. Chlorophyll and primary productivity time-series records were generated using the mooring data. Primary productivity varied by about 50% around the mean on time scales of weeks and by over a factor of four within our observational period. The mooring observations encompassed both El Niho and cool conditions. Kelvin waves were evident during the El Nifio phase, and tropical instability waves (TIWs) were dominant during the cool phase. The two extreme conditions also were observed concurrently with complementary ship-based measurements. In addition, bio-optical drifters provided simultaneous spatial data concerning net phytoplankton growth rates during passage of a TIW. The collective data sets have been used to examine the causes of the observed variability in phytoplankton biomass and productivity. Our joint results and analyses appear to support the hypothesis that the vertical transport of iron into the upper layer and primary production rates are modulated by variability of the depth of the Equatorial Undercurrent and by equatorial longwaves. In particular, our results are consonant with the suggestion of Barber et al. (1996) that passage of a TIW may be considered to be a natural analog of a small iron enrichment experiment. Predicting primary productivity and, thus, carbon flux in the equatorial Pacific requires continuous, long-term observations of a few physical, biological, and optical properties that can be used to parameterize the biological variability.

The role of trace metal limitation in the maintenance of the high nutrient and low chlorophyll condition in eastern equatorial Pacific surface waters remains controversial. On the EqPac Survey II cruise, photosynthesis versus irradiance (P-I) and phytoplankton spectral absorption were measured along 140°W from 12°N to 12°S in the eastern tropical Pacific to determine the spatial variability of P-I parameters in relation to the light, macronutrient, and trace metal regime. The latitudinal patterns of the maximum quantum yield of photosynthesis (φm) and carotenoid-corrected spectral absorption covaried with the concentration of N03 at 60 m, a proxy for the macronutrient supply rate. At the equator, the maximum quantum yield of phytoplankton photosynthesis was less than half that expected for nutrient-replete phytoplankton. This indicates that phytoplankton photosynthesis was nutrient limited even though N03 concentrations at the equator exceeded 6 μmol kg−1. When φm was corrected for absorption by photoprotective carotenoids, maximum values were found at 12°N and values declined linearly with latitude to 12°S. This pattern is coincident with the putative atmospheric iron flux and the distribution of dissolved iron. These observations support the hypothesis that the photosynthesis of equatorial phytoplankton is regulated by iron availability. Phytoplankton photosynthetic performance and in situ chlorophyll-normalized primary production along 140W appeared to be regulated by the interaction of macronutrient and trace metal supply rates on phytoplankton photoacclimation processes.

The calcite standing stock, calcification rate, concentrations of detached coccoliths and plated coccolithophore cells were determined in the equatorial Pacific along 140°W, between 12°N and 12°S latitude, during August and September 1992. Continuous surface optical and fluorescence measurements were also taken along this transect. Integrated calcification ranged between 3 and 12% of the total carbon fixed into particulate matter. Calcification exceeded 50% of the total fixed carbon (per unit volume) at specific depths from the northern-most oligotrophic stations. A pronounced subsurface peak in suspended calcite was noted near the equator. Calcification was considerably more patchy than photosynthesis. Normalizing the calcification rates to the surface area of calcite-producing species provided an estimate of the extracellular calcite flux rates. These results showed that the populations from the equator to 3°N at 60 m depth, and near the surface from the equator to 9°S were the most active calcite producers. Underway estimates of light scattering showed the importance of upwelling for bringing cold, clear, relatively particle-free water to the surface, followed by growth and calcite production as the water warmed. When temperatures reached their upper range (about 28.8°C), light scattering decreased again, presumably as growth slowed and particles sank. Integrated calcification estimates averaged over the equatorial region were compared to sediment trap data: the results suggest significant disappearance of calcite particles in the top 1000 m, above the lysocline. One hypothesis to explain this is that dissolution occurred in microzones where decomposition of reduced organic matter lowered the pH sufficiently to dissolve calcite.

The vertical distributions of zooplankton biomass and community composition were measured on the equator at 140°W during March/April (19 days) and October 1992 (21 days). El Niño conditions prevailed during the March/April time series. The average integrated (200 m) zooplankton (>64 μm) biomass was 32 mM C m−2 in March/April and 41 mM C m−2 in October. The overall cruise means were not significantly different; however during October zooplankton were more aggregated in the upper euphotic zone, there were more copepods >200 μm, and there were more day/night differences in the vertical distribution of zooplankton biomass as a result of vertical migration behavior. Cyclopoid copepods were more abundant during the El Niño conditions of March compared to October. There were no apparent trends in integrated zooplankton biomass during the March/April time series. However, the species composition of the zooplankton community changed towards the end of the time series when meridional currents flowed to the south. Zooplankton biomass during the October cruise increased nearly five-fold with the passage of a tropical instability wave. A lag in the increase of zooplankton compared to nitrate and chlorophyll as well as changes in the zooplankton community composition over the time series reflect the response times of zooplankton to upwelling events.

We construct a one-dimensional ecosystem model (nitrate, ammonium, phytoplankton, zooplnakton and detritus) with simple physics and biology in order to focus on the structural relations and intrinsic properties of the food web that characterizes the biological regime in the central equatorial Pacific at 140°W. When possible, data collected during the EgPac and other cruises were used to calibrate model parameters for two simulations that differ in the limiting nutrient, i.e. nitrogen or iron. Both simulations show annual results in good agreement with the data, but phytoplankton biomass and primary production show a more pronounced annual variability when iron is used as the limiting nutrient. This more realistically reproduces the variability of biological production and illustrates the greater coupling between vertical physical processes and biological production when the limiting nutrient is iron rather than nitrogen. The iron simulation also illustrates how iron supply controls primary production variability, how grazingbalances primary production and controls phytoplankton biomass, and how both iron supply and grazingcontrol primary production. These results suggest that it is not possible to capture primary production variability in the central equatorial Pacific with biological models using nitrogen as the limiting nutrient. Other indirect results of this modeling study were: (1) partitioning of export production between dissolved and particulate matter is almost equal, suggesting that the importance of DOC export may have been previously overestimated; (2) lateral export of live biomass has to be taken into account in order to balance the nitrogen budget on the equator at 140°W; and (3) preferential uptake of ammonium (i.e. nitrate uptake inhibition by ammonium) associated with high regeneration of nitrogen (low f ratio as a consequence of the food web structure imposed by iron limitation) largely accounts for the surface build-up of upwelled nitrate.

Abundances and distributions of picoplankton were studied on two cross-equatorial transect cruises (12°N, 140°W-12°S, 135°W) during February–March (TT007) and August–September 1992 (TT011). Samples were collected in the upper 200 m from early-morning and late-afternoon hydrocasts at 15 stations on each cruise (60 depth profiles, 820 samples). Populations of heterotrophic bacteria, Prochlorococcus, Synechococcus and small autotrophic eukaryotes were enumerated by dual-beam flow cytometry. At the northern end of the transect (7–12°N), abundances and vertical distributions were similar to those reported for the oligotrophic North Pacific gyre, with Prochlorococcus and heterotrophic bacteria dominating in the upper euphotic zone, and Synechococcus and eukaryotes exhibiting peaks in cell abundance at depth. All populations were abundant throughout the euphotic zone close to the equator and at the southern end of the transect. Heterotrophic bacteria and Synechococcus were generally more abundant in late-afternoon hydrocasts. The diel enhancement followed the temporal pattern in beam c and suspended particulates, and was particularly strong in the equatorial upwelling region where it averaged 13.6% of the morning population for heterotrophic bacteria and 22.3% for Synechococcus. Conservative estimates of daily growth rates from these data are 0.25 and 0.40 day−1, respectively, for the two populations. Near-surface maxima in heterotrophic bacteria were symmetrical around the equator, centered around 5°S and 5°N. Prochlorococcus was most abundant during local summer conditions at the respective ends of the transect. A minimum occurred in association with a dense aggregation of buoyant diatoms at the convergent front of a tropical instability wave (2°N, TT011). The ratio of Prochlorococcus to total bacteria was generally in the range of 0.15-0.2 for the upper water column, but varied during TT011 from > 0.3 for the most northern stations to < 0.1 at the 2°N front. At higher latitudes, Synechococcus was more numerous during El Niño conditions (TT007) on both sides of the equator and at southern stations on both cruises. Autographic eukaryotes were more abundant during local winters at the ends of the transect and during the “cold tongue” conditions (TT011) at the equator. Picoplankton account for most of the chlorophyll biomass and primary production in the central equatorial Pacific. Nonetheless, their abundances and distributions are relatively stable and conservative while other populations, such as diatoms, respond more dramatically to environmental forcing.

This study was conducted as part of two JGOFS transects along 140°W between 12°N and 12°S during February–March 1992 and August–September 1992. Although its purpose was to investigate seasonal variability in nitrogenous nutrient availability and biological utilization in support of primary production, the occurrence of the 1992 El Niño during the first transect permitted us to compare El Niño and post-El Niño conditions. We had hypothesized that an El Niño-related reduction in upwelling of cold nutrient-rich water would lead to a reduction in surface nutrient concentrations and rates of new and primary production in the vicinity of the equator. However, during the height of the El Niño, NO3− concentrations from 2°N to 7°S remained high enough (> 2 μmol kg−1) to preclude nitrogen-limited primary production.

As part of the U.S. JGOFS EqPac program, transmissometer profiles were made from 12°N to 12°S at 140°W during February–March (cruise TT007) and August–September (cruise TT011) 1992. Meridional sections of cp (beam attenuation due to particles) were prepared by selecting profiles made at a specific time during the day (∼ 18:00 h) to reduce the influence of diel variability and to facilitate point-to-point comparisons with other variables (e.g. T, nitrate, bioabundance, etc.). A tight correlation between beam cp and total scattering cross-section of micro-organisms was observed, suggesting that heterotrophic bacteria, prochlorophytes, cyanobacteria and small autotrophic eukaryotes (all < 3 μm) were dominant contributors to beam cp. Size-filtration experiments also confirmed that small particles (< 8 μm) accounted for 41–89% of the cp signal in the equatorial Pacific. Contributions of the bacterial fraction and detrital material were assessed.Three biohydrographic regimes [northern (7°–12°N), equatorial (5°N–5°S) and southern (7°–12°S)] could be distinguished from characteristic profiles of cp and other variables. While the northern and southern regimes remained relatively constant in cp between El Niño (TT007) and cold surface water (TT011) conditions, the equatorial regime showed > 30% increase in surface beam cp and IBAC (integrated beam attenuation coefficient) during TT011 compared to TT007. This suggests that only the equatorial regime responds sensitively to the hydrodynamic factors (e.g. upwelling, currents, El Niño, tropical instability waves, etc.) regulating particle distributions. The cp:chlorophyll a ratio, a proxy for the C:chlorophyll a ratio, also was calculated to obtain insight into biogeochemical cycles in the upper waters of the equatorial Pacific.

Biological productivity in the western Arabian Sea was higher during interglacial than glacial times. In the eastern Arabian Sea productivity was higher during the glacials compared to interglacials, which is in sharp contrast to the southwest monsoon intensity variations. To examine temporal changes in productivity in the eastern Arabian Sea over the last 140 ka, oxygen isotopes, calcium carbonate and organic carbon on three cores (SL-1 & 4 and SK 129-CR05) were analyzed. Oxygen isotope records display distinct glacial and interglacial transitions. In the northeastern (Core SL-1) and eastern Arabian Sea (Core SL-4) both calcium carbonate and organic carbon variations show no significant systematic relationship with glacial and interglacials periods. In the southeastern Arabian Sea (Core SK-129-CR05) calcium carbonate shows high and low values during interglacial and glacials, respectively, and temporal changes in organic carbon concentration are significant only during MIS 5. Differential variation of calcium carbonate and organic carbon concentration at the northeastern and southeastern Arabian Sea, and between glacials and interglacials, are attributed to regional differences in sedimentation rates, dilution and preservation, which modify the signal of carbonate and carbon production.

Seven filtered seawater samples (depths between 30 and 55 m) collected during the SAZ project of the Austral summer of 1997–1998 were used for a simultaneous study of the picophytoplankton pigments based on high-performance liquid chromatography (HPLC) analyses and flow cytometry, and of the molecular diversity of the picophytoplankton based on their rDNA sequences. The sampling sites could be divided into three temperature zones, distinguished by their proximity to the Sub-Antarctic and Polar Fronts.HPLC analysis of total chlorophylls and carotenoids showed fairly low phytoplankton concentrations (77–262 ng chl a l−1), with minimal values of the pigments in the two samples of the Polar Front Zone around 54°S (water temperature of 4°C at time of collection). In this zone, a similar decrease of particles, identified as cyanobacteria on the basis of their fluorescence, was observed by flow cytometry.Sequences very similar to the 16S rDNA sequence of Synechococcus WH8103 were present in all samples. This Synechococcus genotype is thus found in the Southern Ocean in addition to the Atlantic and Pacific locations where it has been previously observed. The yield of PCR products was lower in the two samples taken in the Polar Front Zone, showing a good agreement between molecular and pigment data.16S rDNA sequences of plastids of eukaryotic algae also were retrieved, mostly related to those of an environmental clone called OM164, which has not been cultivated but has phylogenetic affinities to the Raphidophyceae.

Primary production in the oceanshas been estimated mainly on the basis of in vitro incubation measurements. An implicit assumption is that the growth rate of phytoplankton observed in vitro represents that occurring in the freely circulating water of the euphotic zone. We have tested this assumption at 47°N-20°W in the eastern North Atlantic Ocean during the initial stages of a spring phytoplankton bloom. The daily primary production was measured by means of the 14C assimilation method, in which the incubation bottles were suspended in the ocean from dawn to dusk daily (about 14 h). The mean daily carbon assimilation rate in the photic zone and in the mixed layer was (where daylight hours), respectively, during the 12-day period between 26 April and 7 May 1989. The mixed layer carbon assimilation data are found to be consistent with the in situ CO2 utilization rate of estimated on the basis of the thickness of surface mixed layer, the CO2 concentration in it, and the air-sea CO2 flux. We conclude that primary production in the open ocean appears to be well represented by the in vitro measurements, if the samples are incubated under the in situ light and temperature conditions. The mean daily reduction rate of the total CO2 concentration observed in the mixed layer over the 12-day period is 2.3 μmol kg−1 day−1, about 75% of the rate, 3.1 μmol kg−1 day∗−1, expected from the rate of primary production. About 8.5% of this difference is explained by the atmospheric CO2 flux, and the remaining 16.5% may be attributed to the respiration and the influx of CO2-rich waters from the mixed layer.

Estimates of photosynthesis based on the incorporation of -labeled inorganic carbon into particulate carbon were compared to estimates of gross photosynthesis based on net O2 production and the production of from during the US Joint Global Ocean Flux Study (US JGOFS) Arabian Sea process cruises. For samples incubated below the surface and at optical depths<3, the uptake : gross photosynthesis ratio averaged 0.45±0.1. This result is in accord with theoretical considerations of the combined effects of the Mehler reaction, photorespiration, dark respiration, excretion, and grazing effects on the two estimates of photosynthesis. The uptake : gross photosynthesis ratio was distinctly higher (0.62) for samples incubated at the surface. This result is likely due to UV light effects, since the O2 and incubations were done in quartz and polysulfone bottles, respectively. The uptake : gross photosynthesis ratio was lower (0.31) for bottles incubated at optical depths>3. This result probably reflects an increase in the ratio of dark respiration to net photosynthesis in the vicinity of the compensation light level.

As part of the Ocean Margins Program (OMP), organic carbon 14C measurements have been made on benthic fauna and kasten core sediments from the North Carolina continental slope. These analyses are used to evaluate the nature and burial flux of organic matter in the OMP study area off Cape Hatteras. Despite the fact that surface sediment 14C contents ranged from −41 to −215 per mil, the benthic fauna (primarily polychaetes) all contained significant amounts of bomb-14C (body tissue 14C contents ranging from +20 to +82 per mil). Bomb-14C clearly is reaching the seabed on the North Carolina slope, and the labile planktonic material carrying this signal is a primary source of nutrition to the benthic ecosystem. The enrichment of 14C in benthic faunal tissue relative to the 14C content of bulk surface-sediment organic matter (a difference of ∼150 per mil) is attributed to a combination of particle selection and selective digestive processes. Organic carbon burial rates from 12 stations on the North Carolina slope varied from 0.02 to 1.7 mol of C m−2 yr−1, with a mean value of 0.7 mol of C m−2 yr−1. The accumulation of organic matter on the upper slope accounts for <1% of the primary production in the entire continental margin system. The North Carolina margin was deliberately selected because of its potential for offshore transport and high sediment deposition rates, and even in this environment, burial of organic carbon accounts for a very small fraction of the primary production occurring in surface waters.

The production rates of DOC and POC by phytoplankton during the FRUELA 95 cruise were estimated by means of time-course experiments of 14C-bicarbonate incorporation. A three-compartment carbon exchange model was used to avoid the artifacts that may appear in end-point experiments due to heterotrophic removal of recently released DOC. The study area was classified into three regions with different ecological characteristics: Bransfield Strait, Gerlache Strait and the Gerlache–Bransfield Confluence (GB Confluence). Percent extracellular release (PER) for all stations ranged from 3% to 47%, with an average of 24%. In surface (5 m depth) waters, POC and DOC production rates were higher in Gerlache Strait than in the GB Confluence and Bransfield Strait. PER values followed an opposite trend, with an average of 26% in Bransfield Strait, 17% in the GB Confluence and 13% in Gerlache Strait. In Gerlache Strait, PER at 10 m depth was significantly higher than at the surface. With pooled data from all experiments, there was a positive relationship between DOC and POC production rates (log–log), but the slope significantly smaller than 1.0 indicated an inverse trend between PER and primary production rate. Phytoplanktonically produced DOC appeared to meet carbon requirements of heterotrophic prokaryotes in the whole area. A positive relationship between prokaryotic heterotrophic production and DOC production rate was found only in Bransfield Strait. These differences are discussed in relationship with the ecological characteristics of the different regions

Long-term variability in zooplankton 15N/14N was investigated in two species of calanoid copepods (Calanus pacificus and Eucalanus californicus) and two chaetognaths (Sagitta bierii and Sagitta euneritica) sampled in the spring of selected years from 1951 to 2001 off the central California coast. No statistically significant trend in 15N/14N was detected for any of the four species, with isotopic ratios in 2001 resembling those in copepods and chaetognaths sampled five decades earlier. Zooplankton body lengths also showed no long-term trends. With respect to proposed regime shifts in this region, heterogeneity in 15N/14N was detected only for S. bierii when comparing the periods 1951–1975, 1978–1998, and 1999–2001. In this species the 15N/14N in the most recent, brief period (1999–2001) averaged slightly lower than in the previous period. Three of the four species (C. pacificus, S. bierii, and S. euneritica) showed significant increases in 15N/14N during major El Niños. El Niño-related enrichment in 15N could arise as a consequence of increased nitrate demand:supply at the base of the food web or advection of 15N-enriched nitrate from more southerly waters. While a range of physical and climate indices were evaluated, anomalies of 15N/14N from the long-term mean were found to be significantly related only to: (i) the Southern Oscillation Index in the case of both chaetognath species, (ii) a regional surface water temperature record (S. bierii only), (iii) an index of wind-driven coastal upwelling for the surface-dwelling C. pacificus, and (iv) variability in the Pacific Decadal Oscillation for the somewhat deeper-dwelling E. californicus. The relationships among each species’ 15N/14N averaged over the total sampling period was: E. californicus≈C. pacificus⪡S. euneritica < S. bierii, consistent with trophic 15N biomagnification and the predatory nature of Sagitta.

The circulation and water masses in the region between 80 and 150°E and from the Antarctic continental shelf to the Southern Boundary of the Antarctic Circumpolar Current (ACC) (∼62°S) are described from hydrographic and surface drifter data taken as part of the multi-disciplinary experiment, Baseline Research Oceanography Krill and the Environment (BROKE). Two types of bottom water are identified, Adelie Land Bottom Water, formed locally between 140 and 150°E, and Ross Sea Bottom Water. Ross Sea Bottom Water is found only at 150°E, whereas Adelie Land Bottom Water is found throughout the survey region. The bottom water mass properties become progressively warmer and saltier to the west, suggesting a westward flow. All of the eight meridional CTD sections show an Antarctic Slope Front of varying strength and position with respect to the shelf break. In the water formation areas (between 140 and 150°E) and 104°E, the Antarctic Slope Front is more “V” shaped, while elsewhere it is one-sided. The shape of the slope front, and the presence or absence of water formation there, are consistent with other meridional sections in the Weddel Sea and simple theories of bottom-water formation (Gill, 1973. Deep-Sea Research 20, 111–140; Whitworth et al., 1998. In: Jacobs and Weiss (Eds.), Ocean, Ice and Atmosphere: Interactions at the Antartic Continental Margin, Antarctic Research Series. American Geophysical Union, Washington, pp. (1−27). ADCP surface velocities and buoy drift tracks show a strong westward flow over the shelf and slope regions. In the region 90–100°E there is a strong eastward flow of the waters just south of the Southern Boundary of the ACC, suggesting a recirculation of the westward slope current and the presence of a weak cyclonic gyre. Using the ADCP velocities as a reference for the CTD data, the average westward transport in this region is 29.4±14.7 Sv.

A large-scale biological/oceanographic survey (BROKE - Baseline Research on Oceanography, Krill and the Environment) was conducted off east Antarctica in the Austral summer of 1995/96. The prime focus was on describing the distribution and abundance of Antarctic krill (Euphausia superba) and on determining possible sources of Antarctic bottom water in the region. A range of other studies were also carried out on BROKE including: measurements of primary production, surveys of Zooplankton and phytoplankton community structure, and sighting surveys of seabirds and cetaceans. The large dataset collected over an area of 873 000 km2 has led to a greater understanding of the marine ecosystem off East Antarctica and the environmental features that determine productivity in this region.

From August 2002 to September 2004 a high-resolution mooring array was maintained across the western Arctic boundary current in the Beaufort Sea north of Alaska. The array consisted of profiling instrumentation, providing a timeseries of vertical sections of the current. Here we present the first-year velocity measurements, with emphasis on the Pacific water component of the current. The mean flow is characterized as a bottom-intensified jet of O (15 cm s−1) directed to the east, trapped to the shelfbreak near 100 m depth. Its width scale is only 10–15 km. Seasonally the flow has distinct configurations. During summer it becomes surface-intensified as it advects buoyant Alaskan Coastal water. In fall and winter the current often reverses (flows westward) under upwelling-favorable winds. Between the storms, as the eastward flow re-establishes, the current develops a deep extension to depths exceeding 700 m. In spring the bottom-trapped flow advects winter-transformed Pacific water emanating from the Chukchi Sea. The year-long mean volume transport of Pacific water is 0.13±0.08 Sv to the east, which is less than 20% of the long-term mean Bering Strait inflow. This implies that most of the Pacific water entering the Arctic goes elsewhere, contrary to expected dynamics and previous modeling results. Possible reasons for this are discussed. The mean Atlantic water transport (to 800 m depth) is 0.047±0.026 Sv, also smaller than anticipated.

Temporal changes of prokaryoplankton in three different provinces of the Atlantic Ocean were examined between 1996 and 2004. The abundance and integrated biomass of three prokaryote groups (Prochlorococcus spp., Synechococcus spp. and other prokaryoplankton) were used to detect standing stock changes in the northern and southern oligotrophic gyres and in the equatorial region. Mean cell concentrations (±standard error of the mean) of Prochlorococcus spp., Synechococcus spp. and other prokaryoplankton above the nitracline in the northern oligotrophic gyre were 1.2×105 (±0.08), 5.0×103 (±1.22) and 0.9×106 (±0.03) cells mL−1, respectively. Similar concentrations of 1.2×105 (±0.06) Prochlorococcus mL−1, 1.9×103 (±0.29) Synechococcus mL−1 and 0.7×106 (±0.03) other prokaryoplankton mL−1 were measured in the southern oligotrophic gyre, with higher concentrations of all prokaryote groups in equatorial waters. Integrated biomass (±standard error of the mean) of Prochlorococcus spp. above the nitracline was 173 (±21) mg C m−2 in the northern oligotrophic gyre, 190 (±14) mg C m−2 in the southern oligotrophic gyre and 141 (±15) mg C m−2 in the equatorial region. Synechococcus spp. biomass was lower in each of the three provinces (18 (±2), 17 (±4) and 32 (±5) mg C m−2, respectively). The data showed no statistically significant inter-annual variability in Prochlorococcus or Synechococcus abundance or integrated biomass above the nitracline in any of the provinces. The abundance and biomass of the remaining prokaryoplankton were variable, but these variations could not be ascribed to seasonal differences and did not follow a clear inter-annual trend. In light of results presented here, recommendations on the frequency and spatial resolution of sampling needed to characterise province-scale temporal variability of prokaryoplankton communities have been suggested.

Dissolved barium (Ba) concentrations are reported for water samples collected during six oceanographic cruises to the Arctic in 1993 and during the 1994 Arctic Ocean Section. Upper Arctic (<200 m) values ranged widely (19–168 nmol Ba 1−1) in a manner geographically consistent with identified sources and sinks. Although surface waters in the eastern part of the Bering Strait and the southern Chukchi Sea were highly enriched in Ba (reflecting the influence of the Yukon River), surface waters over most of the Chukchi Sea were generally depleted in Ba due to the intense seasonal biological activity which occurs in the area. The highest Ba concentrations (>-75 nmol Ba 1−1) observed in the surface mixed layer of the Arctic interior (i.e. beyond the 200 m isobath) occurred in the Canada Basin. As a result of the biological removal of Ba from surface waters of the Chukchi Sea and the tendency of the Bering inflow to enter the Arctic interior at depths below the relatively fresher mixed layer, we hypothesize that the Mackenzie River is the dominant source of the high Ba observed in the surface waters of the Canada Basin.

Phytoplankton biomass and primary productivity were measured as a part of the station KNOT (44°N, 155°E) time-series observation program in the western North Pacific, which began in June 1998 and continued on a nearly monthly basis until February 2000. Primary productivity was measured by the 13C uptake method using a clean sampling technique. The surface temperature changed from 2°C to 15°C, and the minimum and maximum temperatures were in February and August, respectively. The minimum and maximum concentrations of surface-layer nutrients were 2.2 and 19.6 μM for nitrate+nitrite, 0.32 and 1.64 μM for phosphate, and 3.3 and 33.3 μM for silicate. The averaged depth of euphotic zone was about 50 m, which did not show clear seasonality. Integrated chlorophyll a in the euphotic zone ranged between 20 and 40 mg Chl a m−2, except for the high values (51–64 mg Chl a m−2) in early June and middle October 1998 and May 1999, and the low value (17 mg Chl a m−2) in late June 1998. The integrated primary productivities in the upper part of the euphotic zone for spring (May), summer (June–August), autumn (October–November), and winter (December–February) were 517–535 (av. 526), 153–304 (av. 227), 101–307 (av. 173), and 25–121 (av. 48) mg C m−2 d−1, respectively. The annual productivity was estimated to be 90 g C m−2 yr−1. The results showed that primary productivity had a distinct seasonal variation, with a 10-fold range during winter–spring, which is far larger than that at Ocean Station Papa (OSP) in the eastern North Pacific. However, the annual productivity at station KNOT was lower than the at OSP. It was found that the light utilization index was constant during all seasons, and the annual average (±1 SD) was 0.30±0.10 g C (g Chl a)−1 m2 (mol quanta)−1.

Dissolved inorganic carbon (DIC) and related chemical species have been measured since June 1998 at the western subarctic time-series station KNOT (44°N, 155°E). Seasonal changes in hydrography, concentrations of nutrients and DIC, and total alkalinity (TAlk) were observed above the pycnocline, which exists at 150–300 m. DIC in the surface mixed layer showed a maximum in February 2000 (2093 μmol kg–1) and minima in August 1998 and 1999 (1980 and 1987 μmol kg–1, respectively). The seasonal amplitude of DIC was 107 μmol kg–1 in the 1999–2000 time series, which is larger than those at other pelagic ocean time-series sites. This large variation is due mainly to biological production in spring to fall and strong vertical mixing in winter. Surface fugacity of CO2 (fCO2) ranged from 300 to 390 μatm in the 1999–2000 time series, generally decreasing in spring and summer and increasing in fall and winter. The seasonal change in fCO2 was controlled mainly by the large variation in surface DIC. The invasion of atmospheric CO2 into the surface seawater continues from June to January, showing a maximum in October. The decrease in carbon in summer was explained by biological uptake on the basis of Redfield stoichiometry and air–sea CO2 exchange. Net community production in the surface mixed layer estimated from the change in nutrients ranged from 250 to 600 mg C m–2 day–1 and was higher in spring.

Algal species from the ice, the water directly below the ice (the sub-ice area), and the water column from 21 stations in the Arctic Ocean were examined using epifluorescence and inverted light microscopy. Biomass of autotrophic dinoflagellates and other miscellaneous autotrophic flagellates was determined for the first time in the central Arctic basins. Together these two groups dominated phytoplankton biomass in 74% of samples from the central Arctic, with diatom biomass predominant in the remainder. Picophytoplankton at selected stations in the Canada and Makarov Basins contributed 93% to autotroph cell numbers and 36% to autotroph biomass. Diatom species achieved high biomass in ice and sub-ice samples. The centric diatom Melosira arctica dominated the sub-ice area, while pennate diatoms were major contributors to the ice samples. Despite ample silicate concentrations in the water, diatom frustules were often lightly silicified.

We used a 17-year time series of shipboard observations to address the hypothesis that marine birds associate with persistent hydrographic features in the southern California Current System (CCS). Overall, approximately 27,000 km of ocean habitat were surveyed, averaging 1600 km per cruise. We identified mesoscale features (eddy centers and the core of the California Current), based on dynamic height anomalies, and considered habitat associations for seven migratory seabird species: black-footed albatross (Phoebastria nigripes), Cook's petrel (Pterodroma cookii), Leach's storm-petrel (Oceanodroma leucorhoa), dark shearwaters (mainly sooty shearwater Puffinus griseus, with a few short-tailed shearwaters Puffinus tenuirostris), northern fulmar (Fulmarus glacialis), red phalarope (Phalaropus fulicaria), and red-necked phalarope (Phalaropus lobatus). We explored associations (presence/absence and density relationships) of marine birds with mesoscale features (eddies, current jet) and metrics of primary productivity (chlorophyll a and nitrate concentrations). Mesoscale eddies were consistently identified in the study region, but were spatially and temporally variable. The resolved eddies were large-scale features associated with meanders of the equatorward-flowing California Current. Cook's petrel was found offshore with no specific habitat affinities. Black-footed albatross, red phalarope, and Leach's storm petrel were found in association with offshore eddies and/or the core of the California Current, but the functional relationship for these species varied, possibly reflecting differences in flight capabilities. The more coastal species, including the shearwaters, fulmar, and red-necked phalarope, were positively associated with proxies of primary productivity. Of the hydrographic habitats considered, the upwelling region of Point Conception appears to be an important “hotspot” of sustained primary production and marine bird concentrations. Point Conception and other similar coastal locations (upwelling cells) may warrant protection as key foraging grounds for seabirds.

During a spring and a summer cruise, we determined the amounts of biogenic silica (bSi) and particulate organic carbon (POC) encapsulated into fecal pellets by the community of large copepods in the Antarctic Polar Front region, in order to estimate the potential contribution of sinking pellets to bSi and POC flux from the surface layer. Grazing by large copepods can be an important mechanism for enhancing vertical flux of POC and bSi because particles in the diatom size range are excellent foods for mesozooplankton, bSi is inert in copepod guts, and copepod fecal pellets (which have a chitinous membrane that retards dissolution of pellet contents) have sinking rates of 100s m d−1. Rates of pellet production were measured in deck incubations with natural seawater as food medium. BSi and POC contents of pellets were measured in the laboratory. These data were applied to measured abundances of large copepods and the resultant rates of bSi and POC pellet production were compared to flux measurements made by other investigators. Comparison between pellet production by the community of large copepods and flux at 100 m determined by thorium isotopes indicated that pellet production was equivalent to 22–63% of POC flux and 42–107% of bSi flux during the spring cruise. The potential contribution of pellets to flux at 100 m in the summer was considerably smaller, between 2% and 7% for POC and between 1% and 5% for bSi (with one exception at the southernmost station where there was a diatom bloom). We conclude that, in the spring especially, fecal pellets can contribute significantly to the bSi and POC sinking from the surface layer within the Antarctic Polar Front region. However, sediment traps deployed at a depth of 1000 m during the spring cruise indicated total fluxes of both bSi and POC were less than pellet production in the upper 100 m, indicating consumption or degradation of sinking material is significant between 100 and 1000 m. Fecal pellet production of POC was 4–10 times greater than the flux of POC at 1000 m and pellet production of bSi was 0.4–5 times the flux of bSi at 1000 m. Although pellets can contribute significantly to total flux, a large fraction of pellet material must be remineralized during sinking.

An array of five bottom-tethered moorings with 19 PARFLUX time-series sediment trap at three depths (1 and 2 km below the surface, and 0.7 km above the sea-floor) was deployed in the western Pacific sector of the Southern Ocean, along 170°W. The five stations were selected to sample settling particles in the main hydrological zones of the Southern Ocean. The sampling period spanned 425 days (November 28, 1996–January 23, 1998) and was divided into 13 or 21 synchronized time intervals. A total of 174 sequential samples were recovered and analyzed to estimate fluxes of total mass (TMF), organic carbon, carbonate, biogenic silica, and lithogenic particles. The fluxes of biogenic material were higher than anticipated, challenging the notion that the Southern Ocean is a low-productivity region. Organic carbon fluxes at 1 km depth within the Polar Frontal Zone and the Antarctic Zone were relatively uniform (1.7–2.3 g m−2 yr−1), and about twice the estimated ocean-wide average (ca. 1 g m−2 yr−1). Carbonate fluxes were also high and uniform between the Subantarctic Front and ca. 64°S (11–13 g m−2 yr−1). A large fraction of the carbonate flux in the Antarctic Zone was due to the presence of pteropod shells. Coccoliths were found only to the north of the Polar Front, and calcium carbonate became the dominant phase in the Subantarctic Zone. In contrast, carbonate particles were nearly absent near 64°S. Latitudinal variations in biogenic silica fluxes were substantial. The large opal flux (57 g m−2 yr−1) measured in the Antarctic Zone suggests that opal productivity in this region has been previously underestimated and helps to explain the high sedimentary opal accumulation often found south of the Polar Front. Unlike biogenic material, fluxes of lithogenic particles were among the lowest measured in the open-ocean (0.12–0.05 g m−2 yr−1), reflecting a very low dust input.