Surface phytoplankton pigment distributions in the Atlantic Ocean: an assessment of basin scale variability between 50°N and 50°S

We present an overview of the spatial distributions of phytoplankton pigments along transects between the UK and the Falkland Islands. These studies, undertaken as a component of the UK Atlantic Meridional Transect (AMT) programme, provided the first post-launch validation data for the NASA SeaWiFS satellite. Pigment data are used to characterise basin-scale variations in phytoplankton biomass and community composition over 100° of latitude, and to compliment the definition of hydrographic oceanic provinces. A summary of the key pigment characteristics of each province is presented.Concentrations of total chlorophyll a (totCHLa = chlorophyll a, CHLa + divinyl CHLa, dvCHLa) were greatest in high latitude temperate waters (>37°N and >35°S), and in the Canary Current Upwelling system. In these regions, the total carotenoid (totCAR) budget was dominated by photosynthetic carotenoids (PSCs). High accessory pigment diversity was observed of which fucoxanthin (FUC), 19'–hexanoyloxyfucoxanthin (HEX), and diadinoxanthin (DIAD) were most abundant, indicating proliferation of large eukaryotes and nanoflagellates. In contrast, tropical and sub-tropical waters exhibited concentrations of totCHLa below 500 ng l⁻¹, with the North Atlantic Sub-tropical East gyre (NASE, 26.7–35°N), South Equatorial Current (SeqC, 7–14.6°S) and South Atlantic tropical Gyre (SATG, 14.6–26°S) characterised by totCHLa of <100 ng⁻¹. These waters exhibited relatively limited pigment diversity, and the totCAR budget was dominated by photoprotecting pigments (PPCs) of which zeaxanthin (ZEA), a marker of prokaryotes (cyanobacteria and prochlorophytes), was most abundant. DvCHLa, a marker of prochlorophytes was detected in waters at temperatures >15°C, and between the extremes of 48°N and 42°S. DvCHLa accounted for up to two-thirds of totCHLa in oligotrophic provinces demonstrating the importance of prochlorophytes to oceanic biomass.Overall, HEX was the dominant PSC, contributing up to 75% of totCAR. HEX always represented >2% of totCAR and was the only truly ubiquitous carotenoid. Since HEX is a chemotaxonomic marker of prymnesiophytes, this observation reflects the truly cosmopolitan distribution of this algal class. ZEA was found to be the most abundant PPC contributing more than one third of the total carotenoid budget in each transect.Greatest seasonality was observed in highly productive waters at high latitudes and in shallow continental shelf waters and attributed to proliferation of large eukaryotes during spring. Concentrations of the prokaryote pigments (ZEA + dvCHLa) also exhibited some seasonality, with elevated concentrations throughout most of the transect during Northern Hemisphere spring.     

The behaviour of dissolved Cd, Co, Zn, and Pb in North Atlantic near-surface waters (30°N/60°W–60°N/2°W)

In September 1993 (M26) and June/July 1996 (M36), a total of 239 surface samples (7 m depth) were collected on two transects across the open Atlantic Ocean (224 samples) and northwest European shelf edge area. We present an overview of the horizontal variability of dissolved Cd, Co, Zn, and Pb in between the northwest and northeast Atlantic Ocean in relation to salinity and the nutrients. Our data show a preferential incorporation of Cd relative to P in the particulate material of the surface ocean when related to previously published parallel measurements on suspended particulate matter from the same cruise. There is a good agreement with results recently estimated from a model by Elderfield and Rickaby (Nature 405 (2000) 305), who predict for the North Atlantic Ocean a best fit for α_Cd/P=[Cd/P]_POM/[Cd/P]_SW of 2.5, whereas the approach of our transect shows a α_Cd/P value of 2.6. The Co concentrations of our transects varied from <5 to 131 pmol kg⁻¹, with the lowest values in the subtropical gyre. There were pronounced elevations in the low-salinity ranges of the northwest Atlantic and towards the European shelf. The Co data are decoupled from the Mn distribution and support the hypothesis of marginal inputs as the dominant source. Zinc varied from a minimum of <0.07 nmol kg⁻¹ to a maximum of 1.2 and 4.8 nmol kg⁻¹ in regions influenced by Labrador shelf or European coastal waters, respectively. In subtropical and northeast Atlantic waters, the average Zn concentration was 0.16 nmol kg⁻¹. Zinc concentrations at nearly three quarters of the stations between 40°N and 60°N were <0.1 nmol kg⁻¹. This suggests that biological factors control Zn concentrations in large areas of the North Atlantic surface waters. The Pb data indicated that significant differences in concentration between the northwest and northeast Atlantic surface waters presently (1996) do not exist for this metal. The transects in 1993 and 1996 exhibited Pb concentrations in the northeast Atlantic surface waters of 30 to 40 pmol kg⁻¹, about a fifth to a quarter of the concentrations observed in 1981. This decline is supported by our particle flux measurements in deep waters of the same region.     

Spectral absorption coefficient of photosynthetically active pigments in the equatorial Pacific Ocean (165°11–150°W)

Spectral absorption coefficients of total particulate material and detritus were measured throughout the euphotic zone along the equator between 165°E and 150°W and during time-series for each of these two longitudes in October 1994 (JGOFS-FLUPAC cruise). The sum of pigments obtained by spectrofluorometry (tChla=DV−chla+Chla) was used for normalization (and was also compared to fluorometric and HPLC measurements as an intercalibration study). In order to assess the specific absorption coefficient of photosynthetically active pigments (aps) from the pigment-specific absorption coefficient for phytoplankton (a_ph^*), we made a multiple regression analysis of measured phytoplankton absorption spectra onto publishedin vivo spectra of pure pigments. This made it possible to calculate the concentrations of photoprotective carotenoids (tPPC) when HPLC measurements were not available and thus to subtract their contribution to absorption from the total phytoplanktonic absorption coefficient (a_ph). Methodological uncertainties in both coefficients used for calculating absorption coefficients and in pigment measurements are discussed. Pigments and absorption measurements made during the cruise enabled us to describe two typical trophic regimes in the equatorial Pacific ocean: oligotrophic waters of the ”warm pool“ west of 170°W and high-nutrient, low-chlorophyll waters (HNLC) of the upwelling east of 170°W. The vertical decreasing gradient of a_ph^* from the surface to the deep chlorophyll maximum (DCM) was due to a high tPPC/tChla ratio at the surface and was higher in the oligotrophic (0.14-0.065 m² mg (tChla)⁻¹ biomass dominated byProchlorococcus, rich in zeaxanthin) than in the mesotrophic area (0.07-0.06 m2 mg (tChl a)-' biomass dominated by picoeucaryotes). Below the DCM,a_ph^* reached a similar minimum value in both oligotrophic and mesotrophic areas.a^*_ps varied less than a^*_ph from the surface layer to the DCM in both oligotrophic and mesotrophic areas. The difference in a^*_ph and a^*_ps from west to east of the transect could be interpreted as a shift in the phytoplankton composition, with a dominance of procaryotes in the west and a dominance of eucaryotes in the upwelling area. Higher aps in well-lit typical oligotrophic waters indicated that phytoplankton communities dominated byProclorococcus might be more efficient for capturing light usable for photosynthesis than those present in the HNLC situation.     

Decadal water mass variations along 20°W in the Northeastern Atlantic Ocean

Water mass variations in the northeastern Atlantic Ocean along 20°W are analyzed with pentadal resolution over the past 15 years using data from four repeat occupations of a meridional hydrographic section running south from Iceland. The section was sampled in 1988, 1993, 1998, and 2003. The results are interpreted in the context of changes in air–sea forcing, ocean circulation, and water properties associated with the North Atlantic Oscillation (NAO). The NAO index oscillated around zero from 1984 to 1988, was strongly positive from 1989 to 1995, after which it shifted to lower positive, and occasionally negative values from 1996 to 2003. Previously published studies suggest that after the 1995–1996 shift of the NAO, the subpolar gyre largely retreated to the northwest in the northeastern Atlantic Ocean, resulting in an increasingly southeastern character of local water masses with time. Water property changes extending from the SubPolar Mode Water (SPMW) just below the seasonal pycnocline through the density range shared by Mediterranean Outflow Water and SubArctic Intermediate Water (SAIW) along 20°W are consistent with changes in wind-driven ocean circulation and air–sea heat flux associated with shifts in the NAO, especially after accounting for ocean memory. After periods of lower NAO index the SPMW is warmer, saltier, and lighter. At these same times, large increases of apparent oxygen utilization (AOU) and potential vorticity are found at the SPMW base, consistent with SPMW ventilation to lighter densities during lower NAO index periods. Deeper and denser in the water column, the cold, fresh, and dense SAIW signature within the permanent pycnocline that was most strongly present in 1993, near the culmination of a period of high NAO index, is much reduced in 1988 and 1998. In 2003, after a prolonged period of lower NAO index, increasing influence of warmer, saltier subtropical waters is clear within the permanent pycnocline. The deep penetration of the changes implies that they are caused primarily by circulation changes resulting from NAO-associated wind shifts, but changes in air–sea heat flux could also have played a role.     

Observations on the ecology of deep-sea bottom-living fishes collected off northwest Africa (08°–27°N)

The ecology of bottom-living fishes was investigated from a series of collections off northwest Africa in the area 08°–27°N and 14°–30°W from sounding of 261–6059 m. The variety of sampling gear included epibenthic sledges, a semi-balloon otter trawl, benthic traps and longlines. A total of 92 operations collected more than 4600 specimens of at least 148 species. The catches of both types towed gear were shown to have a high degree of similarity, but sampled a different spectrum of species to the static baited gears. An overall consistency by latitude was found in the composition of the net catches. Despite possible indications of faunal boundaries at around 1100 m and 2100 m, the salient feature of an analysis of catch composition by sounding was the generally steady faunal change to a depth of about 3000 m. The only obvious discontinuity was evident at this level, beyond which the fish fauna was more uniform. This lack of marked species assemblages, characteristic of other regions, may result from the high primary productivity in the area. Features distinguishing the fish fauna of the area from that of the non-upwelling temperate western North Atlantic were found to be the smaller mean size of the dominant species, the abundance of bathygadine macrourids and the evident lack of the ‘bigger-deeper’ phenomenon. The relative density of fishes was estimated and shown to decrease by 2 orders of magnitude from the upper slope to 2000 m soundings. Below this the decline was less marked and relative densities of around 0.5 fish/1000 m² were maintained to at least 4000 m soundings.The abundance of slope fishes in the area was tentatively estimated to be greater than in the non-upwelling temperate western North Atlantic. Few species were abundant; only 25% were represented by more than 17 specimens. Yet the dominant family, the Macrouridae, was represented by 48% of the specimens and 18%of the species. A total of 14 species from 6 families, represented by 60 or more specimens, were examined in detail to provide information on vertical distribution, population structure, breeding biology and feeding. Niche availability and resource partitioning among co-occurring species of macrourids are briefly discussed. Station and species data are given, together with a note on the likely conspecificity of Coryphaenoides colon with C. zaniophorus.     

Factors governing abundance of hydrolyzable amino acids in the sediments from the N.W. European Continental Margin (47–50°N)

Fifty-six samples representing 6 sediment cores taken along the N.W. European Continental Margin from the shelf, slope and abyssal plain of the Goban Spur and Meriadzek Terrace were quantitatively analysed for total hydrolyzable amino acids (THAA) and clay minerals. In descending order, the five most abundant amino acids making up more than 70% of the total were: aspartic acid, glycine, serine, alanine and glutamic acid. Clay mineral proportions were typical for the N.E. Atlantic, in order of descending abundance: illite, kaolinite, chlorite, smectite and mixed layers.The Meriadzek Terrace area is characterised by fine grain suspension sedimentation with a low pelagic carbonate input and the lowest content of THAA. In contrast, the Goban Spur transect is characterised by much higher carbonate inputs and more vigorous hydrodynamics as judged from granulometry and the high abundance of minerals of shelf and continental origin and a generally higher THAA content. The pelagic portion of THAA deposited at the sea floor is more readily mineralised during early diagenesis than the more ‘refractory', clay mineral-associated continental portion. Along this margin the average mineralization of THAA down to 25 cm in the sediment is about 54%. There is a significant affinity between chlorites and amino acids which we suggest may involve the formation of ionic bonds between the octahedral layers of the clay and the amino acids.     

A 1998–1992 comparison of inorganic carbon and its transport across 24.5°N in the Atlantic

In January and February 1998, when an unprecedented fourth repetition of the zonal hydrographic transect at 24.5°N in the Atlantic was undertaken, carbon measurements were obtained for the second time in less than a decade. The field of total carbon along this section is compared to that provided by 1992 cruise which followed a similar path (albeit in a different season). Consistent with the increase in atmospheric carbon levels, an increase in anthropogenic carbon concentrations of 8±3 μmolkg⁻¹ was found in the surface layers. Using an inverse analysis to determine estimates of absolute velocity, the flux of inorganic carbon across 24.5° is estimated to be −0.74±0.91 and −1.31±0.99 PgCyr⁻¹ southward in 1998 and 1992, respectively. Estimates of total inorganic carbon flux depend strongly upon the estimated mass transport, particularly of the Deep Western Boundary Current. The 1998 estimate reduces the large regional divergence in the meridional carbon transport suggested by previous studies and brings into question the idea that the tropical Atlantic constantly outgasses carbon, while the subpolar Atlantic sequesters it. Uncertainty in the carbon transports themselves, dominated by the uncertainty in the total mass transport estimates, are a hindrance to determining the “true” picture.The flux of anthropogenic carbon (C_ANTH) across the two transects is estimated as northward at 0.20±0.08 and 0.17±0.06 PgCyr⁻¹ for the 1998 and 1992 sections, respectively. The net transport of C_ANTH across 24.5°N is strongly affected by the difference in concentrations between the northward flowing shallow Florida Current and the mass balancing, interior return flow. The net northward transport of C_ANTH is opposite the net flow of total carbon and suggests, as has been found by others, that the pre-industrial southward transport of carbon within the Atlantic was stronger than it is today. Combining these flux results with estimates of atmospheric and riverine inorganic carbon input, it is determined that today's oceanic carbon system differs from the pre-industrial system in that today there is an uptake of anthropogenic carbon to the south that is advected northward and stored within the North Atlantic basin.     

Introduction to ‘The 1997–8 El Niño Atlas of oceanographic conditions along the west coast of North America (23°N–50°N)’

Hydrographic data collected along the West Coast of North America between January 1997 and January 1999 have been compiled into a web-based Atlas of the 1997–8 El Niño event. This paper discusses the organization of the Atlas, describes the data that were incorporated into the Atlas and explains how vertical and horizontal distributions for the different properties were constructed. Examples of figures from the Atlas are shown.     

Recent sediments, sediment accumulation and carbon burial at Goban Spur, N.W. European Continental Margin (47–50°N)

To quantify recent sediment accumulation, carbon fluxes and cycling, three N.W. European Continental Margin transects on Goban Spur and Meriadzek Terrace were extensively studied by repeated box- and multicore sampling of bottom sediments. The recent sediment distribution and characteristics appear directly related to the near-bed hydrodynamic regime on the margin, which at the upper slope break on the Goban Spur results in along-slope and periodic off-slope directed transport of particles, possibly by entrainment of particles in a detached bottom or intermediate nepheloid layer. From the shelf to the abyssal plain the surface sediments on the Goban Spur change from terrigenous sandy shelf sediments into clayey silts. ²¹⁰Pb activity decreases exponentially down core, reaching a stable background value at 10 cm (shallower stations) to 5 cm (deeper stations) sediment depth. ²¹⁰Pb profiles of repeatedly sampled stations indicate negligible annual variability of mixing and flux. The ²¹⁰Pb_xs flux to the sediment shows a decreasing trend with increasing water depth. Below about 2000 m the average ²¹⁰Pb_xs flux is about 0.3 dpm cm⁻² y⁻¹, a third of the fluxes measured on the shelf and upper slope stations. Sediment mixing rates (D_b) correlate with macro- and meiofaunal density changes and are within the normal oceanic ranges. Lower mixing rates on the lower slope likely reflect lower organic carbon fluxes there. Mass accumulation rates on Meriadzek Terrace are at maximum 80 g m⁻² y⁻¹, almost twice as high as at Goban Spur stations of comparable depth. A minimum accumulation rate of 16.6 g m⁻² y⁻¹ is found at the Goban Spur upper slope break. Organic carbon burial rates are low compared to other margins and range from a lowest value of 0.05 g m⁻² y⁻¹ at the upper slope break to 0.11 g m⁻² y⁻¹ downslope. A maximum organic carbon burial rate of 0.41 g m⁻² y⁻¹ is found on Meriadzek Terrace. Carbonate burial rates increase along the northern transect from the shelf (13 g m⁻² y⁻¹) via a low (9.3 g m⁻² y⁻¹) on the upper slope break to the deep sea (30.7 g m⁻² y⁻¹). Carbonate burial is highest on Meriadzek Terrace (44.5 g m⁻² y⁻¹). The N.W. European Margin at Goban Spur and Meriadzek Terrace cannot be considered a major carbon depocenter.     

Transport of South Atlantic water through the passages south of Guadeloupe and across 16°N, 2000–2004

CTD, vessel-mounted ADCP and LADCP measurements in the Caribbean passages south of Guadeloupe (three repeats) and along 16°N (five repeats) were carried out between December 2000 and July 2004. The CTD data were used to calculate the contribution of South Atlantic water (SAW) in the upper 1200 m between the isopycnals σ_θ=24.5 and 27.6. Northern and southern source water masses are defined and an isopycnal mixing approach is applied. The SAW fractions are then combined with the ADCP flow field to calculate the transport of SAW into the Caribbean and across 16°N. The SAW inflow into the Caribbean through the passages south of Guadeloupe ranges from 7.6 to 11.6 Sv, which is 50–75% of the total inflow. The mean (9.1±2.2 Sv) is in the range of previous estimates. Ambiguities in the northern and southern source water masses of the salinity maximum water permitted us only to calculate the contribution of SAW from the eastern source in this water mass. We estimated the additional SAW transport by the western source to be of the order of 1.9±0.7 Sv. The calculation of the SAW transport across 16°N was hampered by the presence of several anticyclonic rings from the North Brazil Current (NBC) retroflection region, some of the rings were subsurface intensified. Provided that the rings observed at 16°N are typical rings and that all rings which are annually produced in the NBC retroflection area (6.5–8.5 per year) reach 16°N, the SAW ring transport across 16°N is calculated to 5.3±0.7 Sv. From the 5 repeats at 16°N, only two showed a net northward flow, suggesting that the mean northward SAW transport is dominated by ring advection. The joint SAW transports of the Caribbean inflow (9.1 Sv) and the flow across 16°N (5.3 Sv) sum up to 14.4 Sv. The transport increases to 16.3 Sv if the additional SAW transport from the western source of SMW (1.9±0.7 Sv) is included. These transport estimates and the following implications depend strongly on the assumption that the surface water in the Caribbean inflow is of South Atlantic origin. The transport estimates are, however, in the range of the inverse model calculations for the net cross-hemispheric flow. About 30–40% of this transport is intermediate water from the South Atlantic, presumably supporting studies which found the contributions of intermediate and upper warm water to be of a comparable magnitude. For the upper warm water (σ_θ<27.1), the Caribbean inflow seems to be the major path (7.9±1.6 Sv), the ring induced transport across 16°N is about 30% of that value. The intermediate water transport across 16°N was calculated to be 2.3–3.6 Sv, the inflow into the Caribbean is slightly smaller (1.5–2.4 Sv).     

Temporal and spatial variability of biogenic particles fluxes during the JGOFS northeast Atlantic process studies at 47°N, 20°W

Particle fluxes to 3100 m depth at 45°50′N, 19°30′W were measured using time-series sediment traps during a 17 month period encompassing 1989 and 1990 JGOFS spring bloom process studies in the northeast Atlantic. There was a marked intra-annual variability in fluxes of mass, particulate inorganic carbon (PIC), particulate organic carbon (POC) and opal, appearing as two major flux events in each year. In 1989, the first flux event represented the settlement of spring bloom-type material, whereas the second, in autumn, was heavily enriched in mucopolysaccharides. In 1990, in contrast, the two flux events comprised spring bloom-type material and arrived at depth at different times relative to the 1989 events. The intra- and interannual variability evident for all three biogenic components was most notable for POC: (i) the autumn 1989 event supplied twice as much POC to 3100 m as the earlier spring bloom settlement—a quite unexpected observation—and (ii) the annual average POC flux in 1989 was 3–4 times more than in 1990. A synthesis of process study datasets with sediment trap data enables an evaluation of the coupling of deep fluxes with surface-water events. Spatial variability of the 1989 deep flux events is assessed by comparing the sediment trap data reported here with those from a second site 100 km away (Honjo and Manganini,Deep-Sea Research II,40, 587–607, 1993). The timing and magnitude of the 1989 spring bloom settlement was indistinguishable in the two datasets, indicating no spatial variability in flux between these sites. In contrast, the autumn 1989 flux event was barely recorded at the second site. Given the biogeochemical importance of this latter event to deep waters, most notable in terms of its contribution to POC flux, this observation of deep-water mesoscale flux variability indicates a significant problem in determining regional carbon budgets. Construction of basin-scale budgets is a central goal of JGOFS and for this to be achieved further studies of mesoscale variability of particle flux are essential.     

Surface drift in the South-East Atlantic Ocean

Surface drift in the South-East Atlantic Ocean is described using historical shipdrift data. The Benguela Current has a width of 200 km in the south and 750 km in the north. The mean speeds of the current vary from <11 cm·s^?1 to a maximum of 23 cm·s^?1 The highest current speeds occur during summer in the southern regions and during winter in the northern regions, and this seasonality corresponds well with seasonal wind speeds. Eddy kinetic energy is enhanced in the Subtropical Convergence zone and is highest in the general vicinity of the Agulhas Current retroflection. The Subtropical Convergence is evident as a line where northward Ekman drift terminates.     

Variability of physico-chemical conditions in 9°50′N EPR diffuse flow vent habitats

The physico-chemical characteristics of habitats have been considered to be one of the prime determinants of animal distribution within hydrothermal vent sites. However, the relative importance of abiotic to biotic influences is still debated. The primary aim of this study is twofold. The first is to determine and compare the ranges of physico-chemical conditions that characterize mussel-dominated and tubeworm-dominated communities at different sites within the vent field of the 9°50′N segment on the East Pacific Rise. The second is to better understand the processes that determine the variability of physico-chemical conditions in these habitats. In situ chemical and temperature measurements confirmed the high variability on small spatial and temporal scales within single aggregations of animals. The correlation of temperature and sulfide or pH revealed substantial differences between similar habitat-types at different sites, which cannot be attributed to changes in the extent of fluid dilution in the subseafloor. Further investigation of habitat variability within individual sites highlighted specific chemical features for the four sites studied, emphasizing the importance of an extensive in situ chemical analyses survey before using temperature as a proxy for chemical conditions. At Tica and Biovent, the fluid source characteristics were shown to vary only slightly within the sites, among aggregations discretely distributed over several meters distance. The variability of the total sulfide concentration and chemical speciation with temperature in the various habitats at each of these sites, can be reasonably simulated from the conservative mixing model of a single source fluid with seawater. The small variation of the empirical trends between different types of faunal assemblages at Tica suggests that changes in the fluid chemistry are not a prime determinant of the temporal succession of mussels and tubeworms. The findings of very similar sulfide–temperature relationships in two sites of different age and faunal compositions, Biovent and Tica, further support this idea. The two other sites, Mussel Bed and Riftia Field, differ both in the animal communities present and their chemistries show significant discrepancies from the predictions of the conservative mixing model. At Riftia Field, elevated iron concentrations and relatively low sulfide levels were correlated to unusually low pH, which could not be fully explained by conservative mixing of a typical diffuse vent fluid with seawater. The increased acidity results in a dramatic reduction in the anionic form, HS⁻. This is the form preferentially assimilated by the tubeworms and could explain the decline of the tubeworms at this site. At Mussel Bed the relationship between the sulfide concentration and temperature varied substantially among the aggregations surveyed and may reflect the influence of associated free-living microbial communities on the chemistry of mussel habitats. This study emphasizes the complex interplay of diffuse fluid formation in subsurface, chemical reactivity in the mixing zone, and biological activity in controlling of characteristics of vent habitats.     

Distribution coefficient of Mg ions between calcite and solution at 10–50°C

The distribution coefficient (λ_Mg) of Mg²⁺ ions between calcite and solution was found to be 0.012 ± 0.001 (10°C), 0.014 ± 0.001 (15°C), 0.019 ± 0.001 (25°C), 0.024 ± 0.001 (30°C), 0.027 ± 0.001 (35°C) and 0.040 + 0.003 (50°C). This indicates a remarkable dependence on temperature. The effect of the Mg²⁺/Ca²⁺ molar ratio in a parent solution on λ_Mg for calcite is small, where the molar ratio lies in the range 0.04-2. However, the λ_Mg value for aragonite tends to decrease with increasing Mg²⁺/Ca²⁺ ratio in the parent solution. The largest Mg content of calcite in the Ca(HCO₃)₂-Mg²⁺ → calcite system is around 2 mol% in the temperature range 10–50°C. Neither homogeneous nor heterogeneous distribution laws hold for aragonite precipitation, and the temperature effect on the coprecipitation of Mg²⁺ ions with aragonite is very small.     

Dynamics of carbohydrates in the Norwegian Sea inferred from monthly profiles collected during 3 years at 66°N, 2°E

Water samples were collected monthly for 3 years at 66°N, 2°E in the Norwegian Sea, 250 nautical miles off the Norwegian coast. Concentrations of mono- and polysaccharides were measured with the 2,4,6-tripyridyl-s-triazine (TPTZ) spectroscopic method. Total dissolved carbohydrates varied from 3.4 to 28.2 μM C of all samples and the ratio of carbohydrate to dissolved organic C (DOC) varied from an average of 14% at 0–25 m depth to 11% at 800–2000 m depth. This indicates that dissolved carbohydrates were a significant constituent of DOC in the Norwegian Sea. Polysaccharides varied from 0.4 to 21.5 μM C and monosaccharides from 0.7 to 11.7 μM C at all depths. The level of monosaccharides was relatively constant at 2.8–3.2 μM C below the euphotic zone, whereas polysaccharides showed more varying concentrations. Dissolved carbohydrates accumulated during the productive season, reaching maximum concentrations during summer although interannual differences were observed. A significant positive correlation between Chl a and soluble carbohydrate was found in one growing season with nutrient analyses. Average values for total carbohydrates were highest in the surface – 0 to 25 m – with 13.3 μM C and decreased to 8.4 μM C at 800–2000 m depth. The ratio of monosaccharides to polysaccharides exhibited a marked seasonal variation, increased from January to a maximum in June of 1.1, and declined to 0.5 in July.     

The Gibbs Fracture Zone: A double fracture zone at 52°30′N in the Atlantic Ocean

A bathymetric survey of the offset in the Mid-Atlantic Ridge Crest at approximately 53°N revealed an east-west offset of 190 nautical miles and north-south offset of 75 nautical miles. The offset is filled with two valleys separated by a sill below 1900 fm. The valley strend approximately 95° east of north and are inconsistent with spreading poles calculated for the north Atlantic. Their trends have been used by earlier authors to calculate poles of rotation. It is proposed to name the offset The Gibbs Fracture Zone after the ship that made the survey.Woods Hole Oceanographic Institution Contribution No. 2443.     

Origin of iron and aluminium in large particles (> 53 µm) in the Crozet region, Southern Ocean

Natural iron fertilization processes are occurring around the Crozet Islands (46°26′S–52°18′E), thus relieving the water masses from the normally encountered High Nutrients Low Chlorophyll (HNLC) conditions of the Southern Ocean. During austral summers 2004/2005 and 2005/2006, iron and aluminium concentrations were investigated in large particles (> 53 µm) collected from just below the mixed layer at stations under the influence of island inputs, and also in adjacent HNLC waters. These large particles are anticipated to sink out of the mixed layer, and to reflect the net effects of input and cycling of these elements in the overlying mixed layer. Labile and refractory fractions were determined by a two-stage leaching technique. Data showed that water masses downstream of the islands were enriched in total iron and aluminium (0.25–2.68 nmol L^− 1 and 0.34–3.28 nmol L^− 1 respectively), relative to the southern HNLC control sites (0.15–0.29 nmol L^− 1 for Fe and 0.12–0.29 nmol L^− 1 for Al), with only a small fraction (typically < 1%) being acid leachable in both environments. Particulate iron predominantly derived from the island system represents a significant fraction of the total water column iron inventory and may complement dissolved Fe inputs that help support the high summer productivity around the Crozet islands.     

Hydroid assemblages from the Southwestern Atlantic Ocean (34–42° S)

This paper provides updated taxonomic knowledge about hydrozoan species and provides ecological information including geographical and bathymetric distributions and biological substrata for the various hydroid assemblages from the Sub-Antarctic Biogeographical Region, mainly Buenos Aires and the Uruguayan coasts. Five of the 41 species found are new records for the study region. Thirty-one species (75.6%), all found at depths of less than 80 m, have cosmopolitan distributions. Biodiversity decreased markedly below 80 m depth, where nine species (≈22%) distributed in the Southern hemisphere and one endemic species (2.4%) were found. Most species were non-specific epizoites, occurring on diverse substrata. A non-parametric multivariate similarity analysis revealed nine species groups that were correlated with large-scale and local oceanographic patterns and with the availability of suitable substrata. The main hydroid substrata for eight of the groups were cnidarians, molluscs (mainly blue mussels), quartzite rocks and sponges. In a single group found in Patagonian scallop beds, the main biological substrata were polychaete tubes, other hydroids and scallops.     

The distribution and preferential biological uptake of cadmium at 6°W in the Southern Ocean

The spatial and temporal distribution of cadmium (Cd) and phosphate in the Southern Ocean are related to biology and hydrography. During a period of 18 days between transects 5/6 and 11, a phytoplankton spring bloom developed in the Polar Frontal region. Upper water Cd concentrations were not depleted and ranged from 0.2 to 0.8 nM at about 10 m depth. These relatively high Cd concentrations are attributed to upwelling of Upper Circumpolar Deep Water (0.5–1.2 nM in the core) in combination with low biological productivity (0.2 to 0.3 mg m⁻³ chlorophyll-a, 0.3 g C m⁻² d⁻¹). Total particulate Cd concentrations at 40 m depth were between 0.02 and 0.14 nM with the maximum in concentration in the Polar Frontal region. Most of the particulate Cd at this depth (85–94%) was detected in the first phase of a sequential chemical leaching treatment which includes adsorbed Cd as well as Cd incorporated in algae. The Polar Frontal region was characterized by minima in Cd concentration and Cd/phosphate ratio of seawater at both transects; values were the lowest at transect 11 after development of the spring bloom which was dominated by diatoms. This decreasing Cd/phosphate ratio in seawater during spring bloom development was attributed to preferential Cd gross uptake which more than compensated the process of preferential Cd recycling. Within the Upper Circumpolar Deep Water, Cd showed a maximum in concentration similar to that of the major nutrients. Both the Cd concentration and the Cd/phosphate ratio of the deeper water increased in southern direction, from 0.4 to 0.7 nM and from 0.2 to 0.3 nM/μM, respectively. Antarctic Intermediate Water has a Cd concentration of 0.21 nM with a Cd/phosphate ratio of 0.10 nM/μM. In Antarctic Bottom Water, Cd concentrations ranged from 0.60 to 0.82 nM.