Oligotrophic Subtropical Gyres Research Articles

Similarity in microbial amino acid uptake in surface waters of the North and South Atlantic (sub-)tropical gyres

The Earth’s most extensive biomes – the oceanic subtropical gyres – are considered to be expanding with current surface ocean warming. Although it is well established that microbial communities control gyre biogeochemistry, comparisons of their metabolic activities between gyres are limited. In order to estimate metabolic activities including production of microbial communities, the uptake rates of amino acids leucine, methionine and tyrosine at ambient concentrations were estimated in surface waters of the Atlantic Ocean using radioisotopically labelled tracers. Data were acquired during six research cruises covering main oceanic provinces herein termed: North and South Atlantic Gyres, Bermuda Atlantic Time-series Study site (BATS), Equatorial region, and Mauritanian Upwelling (off Cape Blanc). Data were divided between provinces, the extents of which were identified by ocean colour data, in order to achieve provincial mean uptake rates. Leucine and methionine uptake rates did not differ between sampling periods, and were comparable between the North and South subtropical gyres. Furthermore, variation in uptake rates measured throughout the two oligotrophic gyres, where sampling covered ∼4 × 10 6 km 2, was considerably lower than that measured within the Mauritanian Upwelling and Equatorial regions, and even at the BATS site. Tyrosine was generally the slowest of the amino acids to be taken up, however, it was assimilated faster than methionine within the Mauritanian Upwelling region. Thus, we propose that one value for leucine (12.6 ± 3.2 pmol L −1 h −1) and methionine (10.0 ± 3.3 pmol L −1 h −1) uptake could be applied to the oligotrophic subtropical gyres of the Atlantic Ocean. However, with the significantly lower uptake rates observed at the BATS site, we would not advise extrapolation to the Sargasso Sea.

Biominerals and the vertical flux of particulate organic carbon from the surface ocean

Particulate inorganic carbon (PIC; calcium carbonate) is thought to be a significant source of light scattering in the sea. It also provides ballast for particulate matter, driving the ocean's biological carbon pump. During three trans‐Atlantic cruises, we measured particle optical properties plus concentrations of the three major components of sinking aggregates [particulate organic carbon (POC), PIC and biogenic silica (BSi)]. PIC contributed 15–23% of particle backscattering in oligotrophic subtropical gyres and temperate waters. Light scattering properties allowed quantification of the surface PIC:POC ratio. The ratio of the two ballast minerals (PIC:BSi) was significantly, inversely, correlated to POC concentration, allowing robust modeling of the density of sinking aggregates. Results showed greater PIC:POC ratios and sinking rates in oligotrophic regions due to greater relative abundance of PIC.

Distributions of putative aerobic methanotrophs in diverse pelagic marine environments

Aerobic methane oxidization in the pelagic ocean serves an important role in limiting methane release to the atmosphere, yet little is known about the identity and distribution of bacteria that mediate this process. The distribution of putative methane-oxidizing marine groups, OPU1, OPU3 and Group X, was assessed in different ocean provinces using a newly developed fingerprinting method (monooxygenase intergenic spacer analysis (MISA)) in combination with pmoA clone library analysis and quantitative PCR (qPCR). The distribution of these three distinct monooxygenase groups, previously reported from pelagic marine environments, was examined in 39 samples including active methane seeps in the Gulf of Mexico and Santa Monica Bay, submarine canyon heads along the California continental margin, an oligotrophic subtropical gyre and areas proximal to a hydrothermal vent in the North Fiji back-arc basin. OPU1 and OPU3 were widely and similarly distributed within the meso- and bathypelagic zone (110 to approximately 2000 m water depth) and showed a >50-fold greater abundance near methane seeps relative to non-seep sites. In contrast, Group X was predominantly recovered from samples along the California margin, at both seep and non-seep sites. All three phylotypes were below detection in the epipelagic zone to depths of 100 m. Several additional deeply branching monooxygenase sequences were also identified in this study, indicating the presence of uncharacterized groups of microorganisms potentially involved in the cycling of methane or ammonium.

Zooplankton grazing in the Atlantic Ocean: A latitudinal study

Mesozooplankton and 63–200 ?m net-collected microzooplankton grazing on phytoplankton and protozoans was evaluated by 24-h incubations on a latitudinal transect in the Atlantic Ocean, from 35°N to 38°S (AMT-15; September–October 2004). The sampling area comprised contrasting ecosystems, including upwelling zones and oligotrophic subtropical gyres. Grazing impacts of mesozooplankton and 63–200 ?m microzooplankton on total chlorophyll a (Chl a), >5 ?m Chl a, ciliates, and dinoflagellates were low for both zooplankton size fractions, always removing<1.5% of the standing stocks of these groups. Grazing had a slightly greater impact upon primary production (up to 10% of primary production consumed daily), although on most occasions grazing removed<1% of primary production per day. To account for the reduction of micrograzers by predators in the experimental bottles and the consequent reduction of grazing pressure, the data were corrected with knowledge on the decrease of microzooplankton during incubations and global estimates of microzooplankton grazing. The corrected grazing rates for mesozooplankton ranged from 4% to 28% of the primary production consumed daily, and from 1% to 2% of the standing stock of Chl a removed every day. The 63–200 ?m microzooplankton corrected grazing impact was always<5% of the primary production and standing stock consumed per day. The corrected grazing activity of 63–200 ?m microzooplankton and mesozooplankton rendered daily rations ranging from 3% to 38% of the body carbon consumed daily, not sufficient for basal metabolism in most of the areas studied. Finally, the data on mesozooplankton grazing on primary production confirm the recent hypothesis of a decline of the relative importance of mesozooplankton grazing on primary producers with increasing primary production [Calbet, A., 2001. Mesozooplankton grazing effect on primary production: a global comparative analysis in marine ecosystems. Limnology and Oceanography 46, 1824–1830].

Comparative study of picoplankton biomass and community structure in different provinces from subarctic to subtropical oceans

Picoplankton biomass and community structure in the subtropical and subarctic Pacific Oceans were investigated during November 2003, April-August 2005 and July-August 2005. The sampling covered the subarctic K2 station, the Western North Pacific subtropical gyre (WNPG1 and 2 stations) and the Eastern North Pacific subtropical area (ENP1, 2, 3 and 4 stations). Distinct differences in community structure and autotrophic and heterotrophic picoplankton biomass were observed among the above provinces. In subtropical areas, the picoplankton community comprised Prochlorococcus, Synechococcus, picoeukaryotes and heterotrophic bacteria. While in the subarctic area (K2 station), Prochlorococcus were absent. Prochlorococcus were numerically dominant in the subtropical oceans, their abundance tended to decrease with increasing nutrient levels, which is the opposite of the other picoplankton populations. Although the aerobic anoxygenic phototrophic heterotrophic bacteria (AAPB), accounted for only a small proportion of total heterotrophic bacterial abundance, their potential contribution to carbon export may be important due to their larger cell size and higher cell turnover rates compared with other heterotrophic bacteria. Biomass contribution of the AAPB increased distinctly along the oligotrophic to relatively eutrophic gradient. Vertically, AAPB generally followed the phytoplankton except in the subtropical WNPG. Spatial variability of biomass in the autotrophic picoplankton was distinctly larger than that in the heterotrophic bacteria. Changes in the picoplankton community were more closely associated with latitude while nutrient availability was more important for differences in picoplankton biomass. The biomass of autotrophic picoplankton in the upper mixed layer, and also the depth attenuation, were higher in eutrophic relative to oligotrophic waters. Picoplankton seemed to be an important source of new organic carbon for higher trophic level organisms and for detritus production, especially in the oligotrophic subtropical gyre.

Intercomparison and coupling of magnesium-induced co-precipitation and long-path liquid-waveguide capillary cell techniques for trace analysis of phosphate in seawater

Currently, two common techniques for nanomolar-level phosphate measurements in seawater are magnesium-induced co-precipitation (MAGIC) and long-path liquid-waveguide capillary cell (LWCC) spectrophotometry. These techniques have been applied in the open ocean, and our understanding of phosphate distributions in oligotrophic subtropical gyres is based on those data. However, intercomparison of these methods has not previously been performed at nanomolar levels. Here, we report experimental results directly comparing the MAGIC and LWCC techniques. We also evaluated the impact of various commonly employed filters on phosphate determinations, as well as interferences from dissolved organic phosphorus (DOP) and arsenate. Our results find agreement between these methods at phosphate concentrations <100 nM. We found that filter selection is important for accurate determinations of phosphate, and that DOP hydrolysis affects both techniques similarly. Finally, we demonstrate the advantage of combining MAGIC preconcentration and LWCC spectrophotometry for analysis of very low nanomolar concentrations.

Global prediction of planktic foraminiferal fluxes from hydrographic and productivity data

Abstract. Understanding and quantifying the seasonal and spatial distribution of planktic foraminiferal fluxes reflected in sedimentary assemblages is key to interpret foraminifera-based proxies in paleoceanography. Towards this goal we present an empirical model to predict foraminiferal fluxes on a global scale. A compilation of planktic foraminiferal flux and export production data from globally distributed sediment traps together with environmental data of sea-surface temperature and mixed-layer depth from online databases is used to calibrate the model that calculates monthly foraminiferal fluxes for the 18 most common species. The calibrated model is then forced with a global data set of hydrographic and productivity data to predict monthly foraminiferal fluxes worldwide. The predictive skills of the model are assessed by comparing the model output with planktic foraminiferal assemblages from globally distributed surface sediments as well as with measured foraminiferal fluxes of sediment traps not included in the calibration data set. Many general distribution patterns of foraminiferal species recognized from the model output compare favorably with observations from coretops or sediment traps, even though the model still produces problematic results in some places. Among others, meridional gradients in species richness and diversity, increased relative abundances of Neogloboquadrina pachyderma (dex.) in upwelling areas, and peak abundances of Globigerinella siphonifera in oligotrophic subtropical gyres show good agreement between model and coretops. Absolute foraminiferal fluxes are significantly underestimated in most cases, while seasonal variations can be reproduced for some species. Interannual differences in foraminiferal fluxes are not reflected by the model which might partly be due to a lack of actual environmental data for the calibration and model experiments. The limited predictive skills of the model suggest that additional parameters should be considered. Export production should probably be replaced by a more realistic representation of food availability for planktic foraminifera. This could be achieved by adding a dynamic component to the model and linking it to an ecosystem model.

Primary production in the eastern tropical Pacific: A review

The eastern tropical Pacific includes 28 million km 2 of ocean between 23.5°N and S and Central/South America and 140°W, and contains the eastern and equatorial branches of the north and South Pacific subtropical gyres plus two equatorial and two coastal countercurrents. Spatial patterns of primary production are in general determined by supply of macronutrients (nitrate, phosphate) from below the thermocline. Where the thermocline is shallow and intersects the lighted euphotic zone, biological production is enhanced. In the eastern tropical Pacific thermocline depth is controlled by three interrelated processes: a basin-scale east/west thermocline tilt, a basin-scale thermocline shoaling at the gyre margins, and local wind-driven upwelling. These processes regulate supply of nutrient-rich subsurface waters to the euphotic zone, and on their basis we have divided the eastern tropical Pacific into seven main regions. Primary production and its physical and chemical controls are described for each. Enhanced rates of macronutrient supply maintains levels of primary production in the eastern tropical Pacific above those of the oligotrophic subtropical gyres to the north and south. On the other hand lack of the micronutrient iron limits phytoplankton growth (and nitrogen fixation) over large portions of the open-ocean eastern tropical Pacific, depressing rates of primary production and resulting in the so-called high nitrate-low chlorophyll condition. Very high rates of primary production can occur in those coastal areas where both macronutrients and iron are supplied in abundance to surface waters. In these eutrophic coastal areas large phytoplankton cells dominate; conversely, in the open-ocean small cells are dominant. In a ‘shadow zone’ between the subtropical gyres with limited subsurface ventilation, enough production sinks and decays to produce anoxic and denitrified waters which spread beneath very large parts of the eastern tropical Pacific. Seasonal cycles are weak over much of the open-ocean eastern tropical Pacific, although several eutrophic coastal areas do exhibit substantial seasonality. The ENSO fluctuation, however, is an exceedingly important source of interannual variability in this region. El Niño in general results in a depressed thermocline and thus reduced rates of macronutrient supply and primary production. The multi-decadal PDO is likely also an important source of variability, with the ‘El Viejo’ phase of the PDO resulting in warmer and lower nutrient and productivity conditions similar to El Niño. On average the eastern tropical Pacific is moderately productive and, relative to Pacific and global means, its productivity and area are roughly equivalent. For example, it occupies about 18% of the Pacific Ocean by area and accounts for 22–23% of its productivity. Similarly, it occupies about 9% of the global ocean and accounts for 10% of its productivity. While representative, these average values obscure very substantial spatial and temporal variability that characterizes the dynamics of this tropical ocean.

Microbial community composition and growth rates in the NW Pacific during spring 2002

The IOC North Pacific expedition in spring (May 2002) sailed from Osaka, Japan, to Honolulu, Hawaii, and surveyed the region from roughly 22 to 50°N and 147°E to 158°W. Nine stations were chosen to characterize three distinct water masses, as well as their boundary regions: the Kuroshio Current, the North Pacific subarctic gyre, and the North Pacific subtropical gyre. Grazing largely balanced growth at all stations, with an average net growth rate of 0.11 ± 0.16 d−1 (cell‐based) and 0.06 ± 0.15 d−1 (chlorophyll‐based). The stations could be distinguished, however, by phytoplankton community composition and growth response to nutrient additions (ammonium, phosphate, manganese, and iron). Nutrients were undetectable in surface waters of the Kuroshio Current, where a centric diatom‐dominated bloom showed a significant growth response to added nutrients. Iron limitation was observed for the cyanobacteria Synechococcus at the two subarctic gyre stations; however, the photosynthetic eukaryotes, which dominated the photosynthetic biomass at these stations, were not nutrient‐limited. Four oligotrophic subtropical gyre stations were dominated by photosynthetic bacteria (Prochlorococcus and Synechococcus) and small (&lt;5 μm) eukaryotic autotrophs, which exhibited a dramatic increase in growth rate with macronutrient additions but displayed little or no increased growth with iron additions alone. Overall, the ratio of heterotroph consumer biomass to autotroph biomass was higher in the Prochlorococcus‐dominated subtropical gyre sites (0.5) than the subarctic or Kuroshio Current sites (0.2–0.3).

High concentrations of marine snow and diatom algal mats in the North Pacific Subtropical Gyre: Implications for carbon and nitrogen cycles in the oligotrophic ocean

A video plankton recorder (VPR) and a remotely operated vehicle (ROV) were utilized on three cruises in the oligotrophic North Pacific Subtropical Gyre (NPSG) between 1995 and 2002 to quantify the size and abundance of marine snow and Rhizosolenia diatom mats within the upper 305 m of the water column. Quantitative image analysis of video collected by the VPR and an ROV-mounted particle imaging system provides the first transect of marine snow size and abundance across the central North Pacific Gyre extending from 920 km NW of Oahu to 555 km off Southern California. Snow abundance in the upper 55 m was surprisingly high for this oligotrophic region, with peak values of 6.0–13.0×10 3 aggregates m −3 at the western- and easternmost stations. At stations located in the middle of the transect (farthest from HI and CA), upper water column snow abundance displayed values of ∼0.5–1.0×10 3 aggregates m −3. VPR and ROV imagery also provided in situ documentation of the presence of nitrogen-transporting, vertically migrating Rhizosolenia mats from the surface to >300 m with mat abundances ranging from 0 to 10 mats m −3. There was clear evidence that Rhizosolenia mats commonly reach sub-nutricline depths. The mats were noted to be a common feature in the North Pacific Gyre, with the lower salinity edge of the California Current appearing to be the easternmost extent of their oceanic distribution. Based on ROV observations at depth, flux by large (⩾1.5 cm) mats is revised upward 4.5-fold, yielding an average value of 40 μmol N m −2 d −1, a value equaling previous estimates that included much smaller mats visible only to towed optical systems. Our results suggest that the occurrence across a broad region of the NPSG of particulate organic matter production events represented by high concentrations of Rhizosolenia mats, associated mesozooplankton, and abundant detrital marine aggregates may represent significant stochastic components in the overall carbon, nitrogen, and silica budgets of the oligotrophic subtropical gyre. Likewise, their presence has important implications for the proposed climate-driven, ecosystem reorganization or domain shift occurring in the NPSG.

Interactions of the iron and phosphorus cycles: A three‐dimensional model study

We use an ocean circulation, biogeochemistry, and ecosystem model to explore the interactions between ocean circulation, macro‐ and micro‐nutrient supply to the euphotic layer, and biological productivity. The model suggests a tight coupling between the degree of iron limitation in the upwelling subpolar and tropical oceans and the productivity of the adjacent subtropical gyres. This coupling is facilitated by lateral Ekman transfer of macro‐nutrients in the surface ocean. We describe a coarse resolution configuration of the MIT ocean circulation and biogeochemistry model in which there are fully prognostic representations of the oceanic cycles of phosphorus, iron, and silicon. The pelagic ecosystem is represented using two functional groups of phytoplankton and a single grazer. Using present‐day forcing, the model qualitatively captures the observed basin and gyre scale patterns of nutrient distributions and productivity. In a suite of sensitivity studies we find significant regional variations in response to changes in the aeolian iron supply. In a dustier (model) world, the Southern Ocean and Indo‐Pacific upwelling regions are more productive, but there is a decrease in productivity in the subtropical gyres and throughout the Atlantic basin. These results can be described most easily by a simple conceptual classification of the Southern and Indo‐Pacific oceans into two regimes: (1) upwelling, iron limited regions and (2) macro‐nutrient limited, oligotrophic subtropical gyres. Enhancing the aeolian iron supply to the upwelling regions relieves iron limitation and increases local primary and export production, but reduces the Ekman transfer of phosphate to the neighboring subtropical gyres. Consequently, over time, the gyres become further depleted in macro‐nutrients and productivity decreases in response to global scale iron fertilization. In a large‐scale analogy, the macro‐nutrient budget of the Atlantic is maintained by lateral transfer of nutrients in the upper ocean. Enhanced aeolian supply of iron leads to increased productivity in the Southern Ocean and Indo‐Pacific upwelling regions, reducing the lateral transfer of macro‐nutrients to the Atlantic basin, which becomes increasingly macro‐nutrient limited throughout.

Seasonal variations in settling fluxes of major components in the oligotrophic Shikoku Basin, the western North Pacific: coincidence of high biogenic flux with Asian dust supply in spring

Time-series sediment traps were deployed at ca. 500 m below the sea surface (upper) and ca. 500 m above the sea bottom (lower) at station SHIBT (29°30′N, 135°15′E, ca. 4600 m water depth) near the center of oligotrophic subtropical gyre of the Shikoku Basin in the western North Pacific from February to December 1999 and from June to September 2000 to collect settling particle samples with high time resolution (2.5 days in spring 1999). Major components of settling particles, i.e., CaCO 3, Fe–Mn oxyhydroxide (ERFe-Mn), organic matter (OM), opal and lithogenic aluminosilicate (clay), were measured by stepwise leaching. Strontium isotope ratio and grain size of clay fraction were also determined. Average total mass fluxes were 18.9 and 57.4 mg m −2 day −1 in the upper and lower traps from February to early December 1999, respectively, and 3.8 and 58.0 mg m −2 day −1 in the upper and lower traps from June to September 2000, respectively. Total biogenic flux (CaCO 3+OM+opal) in the upper trap accounted for 93% of total mass flux in 1999. CaCO 3 was the dominant component in the upper and lower traps. Seasonal variations in settling fluxes of major components were observed for almost 1 year. High biogenic fluxes with four peaks were observed in spring, i.e., four times of spring blooms occurred in the euphotic zone from February to May 1999. Estimated export flux of particulate organic carbon from the euphotic zone was 6.2 mg C m −2 day −1 on average in 1999, and 9.0 mg C m −2 day −1 on average during the high biological production period in spring. 87Sr/ 86Sr ratio of clay fraction fell within the range of 0.7163–0.7188 in spring, which was consistent with that of Asian loess and dust. This indicates that the clay fraction was mainly composed of Asian dust via eolian transport in spring 1999. The high biogenic flux accompanying Asian dust coincided with Asian dust supply with time lag in early spring 1999. The coincidence of high biogenic flux with Asian dust supply suggests the possibility that high biological production in the oligotrophic subtropical Shikoku Basin might be affected by the Asian dust supply in spring.

Size-fractionated mesozooplankton biomass, metabolism and grazing along a 50 N-30 S transect of the Atlantic Ocean

Size-fractionated mesozooplankton grazing and metabolism were investigated along the wide latitudinal range (50°N-30°S) covered doring the Atlantic Meridional Transect (AMT) 11 cruise. Five different oceanic provinces were traversed in this cruise: North Atlantic Drift (NADR), North Atlantic Subtropical Gyral (NAST), Canary Coastal (CARY), Eastern Tropical Atlantic (ETRA), and South Atlantic Gyral (SATL). CNRY and ETRA were affected by the upwelling Mauritaian and equatorial respectively and primary production in these provinces was higher than in the oligotrophic subtropical gyres (NAST and SATL). Both mesozooplankton and phytoplankton biomass were highest around the equator. The amount of chlorophyll a ingested daily by copepods was noticeably higher in mesotrophic than in oligotrophic provinces as shown by the spatial distribution of gut content values and the high abundances of copepods recorded at the equator. Grazing impact along the transect ranged from 0.2 to 5.6% of the phytoplankton standing stock and from 1.6 to 14.5% of primary production. If only phytoplankton >2 μm are considered, the ranges are 1.0-19.4% (stock) and 3.4-44.7% (primary production). Grazing impact upon both phytoplankton biomass and primary production followed a spatial distribution similar to that of chlorophyll a ingestion, with higher values in upwelling zones than in the gyres. Weight-specific rates of respiration and NH 4 + and PO 4 3- excretion showed large variability both along the transfect and within provinces, but did not differ between provinces. Therefore, zooplankton assemblages inhabiting the different provinces visited in the AMT 11 seem to be adapted to the prevailing thermal conditions. Given the substantial proportion of nitrogen and phosphorus that are supplied to primary producers through the excretory activity of mesozooplankton (the percentage of nitrogen and phosphorus requirements of phytoplankton accounted for by mesozooplankton excretion was >30% in all the provinces) it follows that they may play a crucial role as nutrient regenerators, especially in the oligotrophic gyres where regenerated production dominates.

Mesoscale variability of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) satellite ocean color: Global patterns and spatial scales

Using the semivariogram approach from geostatistics, we characterize for the first time the global patterns of mesoscale (approximately 10–200 km and days to a few weeks) ocean biological variability. The magnitude of the variability and spatial length scale fields are reported for a full year of Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) ocean color data. The analysis shows a number of coherent geographical patterns across a wide range of biological and physical environments. Submonthly, small spatial scale variance dominates ocean color variability in oligotrophic regimes and is about a third of the total variance even in regions with strong seasonality. When normalized to the mean chlorophyll as a coefficient of variation, the resolved mesoscale variance (relative sill) is approximately uniform in the open‐ocean tropics and subtropics (0.1–0.2), increasing to 0.3–1.0 in western boundary current, upwelling (Equatorial Pacific, coastal) and subpolar/polar regions, approximately scaling with the mean chlorophyll concentration. The unresolved variance (nugget), which includes submesoscale geophysical variability as well as instrument/algorithm noise, is inversely correlated with mean chlorophyll concentration in the oligotrophic subtropical gyres, reducing to a low background value (0.1) elsewhere. The high fraction of unresolved variance in the subtropics is consistent with other studies showing substantial speckling due to noise in the atmospheric correction methods. The mesoscale spatial scale (range) is approximately zonal, with values of 200–350 km near the Equator to less than 50 km close to the poles similar to the physical scales estimated from altimeter sea surface height variability data and the first baroclinic Rossby deformation radius. Our results demonstrate the global generality of previous local and regional findings that small‐scale variability in ocean biology and physics occurs on comparable spatial length scales. Regional deviations from these large‐scale variance and spatial length scales trends, for example, lower range and higher resolved variance in coastal upwelling regimes, are explicable by the characteristics of the biological‐physical interaction in those environments.

Can eddies make ocean deserts bloom?

An eddy‐resolving coupled ecosystem‐circulation model of the North Atlantic is used to investigate the impact of mesoscale variability on the nitrate supply to the light‐lit euphotic zone. The focus is on the oligotrophic subtropical gyre where eddies have been suggested to reconcile apparently contradictory observational estimates of nutrient supply and export production. Comparison with observations indicates that the numerical model provides a realistic description of the subtropical eddy field and its interaction with biogeochemical tracers. The model results illustrate that the eddy‐induced nitrate flux into the euphotic zone is largest near the margins of the oligotrophic gyre where both vertical and lateral nutrient supply by eddies are effective. Typical values of simulated eddy‐induced nitrate supply are 0.05 mol m−2 yr−1, which is much lower than has been suggested previously. This new estimate of eddy‐induced nitrate supply is not sufficient to reconcile seemingly contradictory observational estimates of biological production in the subtropics. Alternative sources of fixed nitrogen, deviations from standard elemental stoichiometry, and possible effects of interannual variability will have to be considered in order to resolve apparent observational discrepancies in the oligotrophic subtropical gyres.

Factors governing the oceanic nitrous oxide distribution: Simulations with an ocean general circulation model

A global model of the oceanic nitrous oxide distribution is developed to evaluate current understanding of the processes governing nitrous oxide formation and distribution in the open ocean. N2O is treated as a nonconserved tracer in a global ocean general circulation model subject to biological sources in the oceanic interior and gas exchange at the ocean surface. A simple scalar parameterization linking N2O production to oxygen consumption (and based on observed correlations between excess N2O and apparent oxygen utilization) is successful in reproducing the large‐scale features of the observed distribution, namely, high surface super‐saturations in regions of upwelling and biological productivity, and values close to equilibrium in the oligotrophic subtropical gyres. The majority of the oceanic N2O source is produced in the upper water column (over 75% above 600 m) and effluxes directly to the atmosphere in the latitude band of formation. The observed structure at depth is not as well reproduced by this model, which displays excessive N2O production in the deep ocean. An alternative source parameterization, which accounts for processes which result in a depth variation in the relationship between N2O production and oxygen consumption, yields an improved representation of the deep distribution. The surface distribution and sea‐air flux are, however, determined primarily by the upper ocean source and, therefore, are relatively insensitive to changes in the nature of deep oceanic N2O production.

In vitro and in situ derived rates of gross production, net community production and respiration of oxygen in the oligotrophic subtropical gyre of the North Pacific Ocean

Primary production rates were determined in the central North Pacific Gyre from in vitro and in situ changes in dissolved oxygen; the measurements were made over periods of 12 and 24 h. In situ rates were measured at three depths within the surface mixed layer and compared to in vitro changes in bottles. Clean techniques were used for the collection and incubation of samples. Rates of gross production, net community production and respiration determined from in vitro incubations compared favourably with in situ derived rates, with no evidence of major (i.e. 10-fold) systematic discrepancies between the two approaches. High chlorophyll-normalized rates of photosynthesis were measured both in vitro and in situ, which was taken as strong evidence that in vitro techniques were not seriously in error. The in situ rates of production were shown to be sufficient to produce the observed sub-surface oxygen maximum of ca 107–110% and to sustain the calculated losses of oxygen to the atmosphere from the mixed layer and particulate carbon export from the euphotic zone.

Neogene history of the calcite compensation depth and lysocline in the South Pacific Ocean

The depth–time pattern of calcite accumulation recorded at Deep Sea Drilling Project (DSDP) sites beneath the oligotrophic subtropical gyre in the south-east Pacific Ocean allows us to define here both the calcite compensation depth (CCD) and the lysocline. The Neogene CCD history is one of shoaling before 15 or 20 Myr and subsequent deepening to the present level of 4,100 m. A broad zone of increased calcite dissolution, 600 m thick, formed during early to middle Miocene time and has remained. This zone, the lysocline, denotes an important change in ocean chemistry and may reflect either an increase in the volume of more corrosive deep water beginning about 18 Myr and reaching steady state by 14 Myr, the time of rapid ice build-up in Antarctica, or an increase in calcite rain rate caused by an increase in surface-water productivity.

Dominant Distributions and Niches of Polycystine Radiolarians in Modern Oceans: ABSTRACT

Approximately 400 to 500 relatively common polycystine radiolarian live in the modern oceans. About 200 species live in shallow (0 to 200 m; 0 to 656 ft), temperate and tropical waters (the upper warm water sphere); 40 to 50 in high latitude (poleward of subtropical and polar convergences) shallow waters; 150 to 200 in deep (greater than 200 m; 656 ft) waters, most of which appear to be tropical submergent (shallow in high latitudes and diving as tropical submergent or tropical avoidant forms); and about 40 to 50 eurybathyal forms. In general, spumellarian of the warm water sphere (surface to subsurface) include the beloids, collosphaerids, saturnulids, artiscids, phacodiscids, spongasterids, dictyocoryids, spongurids, pylonids, lithellids (loo ely coiled), and stylodictids; with the spongotrochids, spongopylids, tholonids, lithellids (tightly coiled), and orosphaerids being dominantly intermediate and deep water (cold water sphere) forms. In general, nassellarian family group of the warm water sphere and transition-central waters include the lophophaenids; sethoperids, sethophormids (complex), carpocanids, eucyrtidids, pterocoryids, cannobotryids, spyroids, and artostrobids (not robust). Plectopyramids, artostrobids (robust), sethoperids, sethophormids (simple), and theocalyptrids dominate in either the transition-central (lower warm water sphere) or cold water sphere. Polycystine radiolarian species and family level groups appear to fit into four basic broad nutrition niches: nannoherbivore-carnivore; bacterivore; detritivore, in association with symbiotic algae; and, perhaps an osmotrophic niche. At the family level these groups dominate the following oceanic habitats: nannoherbivore-carnivore the broad shallow warm water sphere, with algal symbiotes dominating the subtropical anticyclonic gyres, eastern equatorial, and shelf subregions; bacterivores in subsurface and deeper (nutricline and deeper); and, detritivores and perhaps osmotrophs the deep ocean. A positive relationship exists between the degree of eutrophism in the epipelagic region, and underlying mesopelagic and deeper radiolarian standing crops. Radiolarians are usually sparse in waters overlying the continental shelves. However, those radiolarians harboring symbiotic algae (collosphaerids, artiscids, spyroids, spongasterids, dictyocoryids, etc) are more tolerant of shelf waters. Radiolarians harboring symbiotic algae are also dominant in the oligotrophic subtropical anticyclonic gyres and the eutrophic eastern tropical regions. Acantharian and diatom bloom conditions exclude polycystine radiolarians from these bloom areas (many nearshore areas and the photic zones poleward of polar convergences). These modern day dominant patterns of radiolarian distribution appear to have been initiated in the early Neogene and End_Page 437------------------------------ related to the development of the Neogene (and modern) water mass regimes at that time. One of the major radiolarian zoogeographic anomalies, since the initiation of the Neogene water mass regimes and their contained radiolarian faunas, has been the isolation of relict radiolarian species in the warm water sphere of the North Atlantic. This oceanographic realm has been semi-isolated from the world ocean for about the last 3 to 3.5 m.y. and contains a relict (and expatriated) radiolarian fauna. Of special interest in the relict fauna are the presence of the species Lamprocyrtis heteroporos, Didymocyrtis penultimus, D. avitus, and Spongaster pentas--all, until recently, thought to be extinct. End_of_Article - Last_Page 438------------