Oxygen dynamics in the North Atlantic subtropical gyre

Dissolved oxygen (DO) in the ocean is a tracer for most ocean biogeochemical processes including net community production and remineralization of organic matter which in turn constrains the biological carbon pump. Knowledge of oxygen dynamics in the North Atlantic Ocean is mainly derived from observations at the Bermuda Atlantic Time-series Study (BATS) site located in the western subtropical gyre which may skew our view of the biogeochemistry of the subtropical North Atlantic. This study presents and compares a 15 yr record of DO observations from ESTOC (European Station for Time-Series in the Ocean, Canary Islands) in the eastern subtropical North Atlantic with the 20 yr record at BATS. Our estimate for net community production of oxygen was 2.3±0.4 mol O₂ m⁻² yr⁻¹ and of oxygen consumption was −2.3±0.5 mol O₂ m⁻² yr⁻¹ at ESTOC, and 4 mol O₂ m⁻² yr⁻¹ and −4.4±1 mol m⁻² yr⁻¹ at BATS, respectively. These values were determined by analyzing the time-series using the Discrete Wavelet Transform (DWT) method. These flux values agree with similar estimates from in-situ observational studies but are higher than those from modeling studies. The difference in net oxygen production rates supports previous observations of a lower carbon export in the eastern compared to the western subtropical Atlantic. The inter-annual analysis showed clear annual cycles at BATS whereas longer cycles of nearly 4 years were apparent at ESTOC. The DWT analysis showed trends in DO anomalies dominated by long-term perturbations at a basin scale for the consumption zones at both sites, whereas yearly cycles dominated the production zone at BATS. The long-term perturbations found are likely associated with ventilation of the main thermocline, affecting the consumption and production zones at ESTOC.     

Dissolved organic carbon export and subsequent remineralization in the mesopelagic and bathypelagic realms of the North Atlantic basin

Dissolved organic carbon (DOC) data are presented from three meridional transects conducted in the North Atlantic as part of the US Climate Variability (CLIVAR) Repeat Hydrography program in 2003. The hydrographic sections covered a latitudinal range of 6°S to 63°N along longitudes 20°W (CLIVAR line A16), 52°W (A20) and 66°W (A22). Over 3700 individual measurements reveal unprecedented detail in the DOC distribution and systematic variations in the mesopelagic and bathypelagic zones of the North Atlantic basin. Latitudinal gradients in DOC concentrations combined with published estimates of ventilation rates for the main thermocline and North Atlantic Deep Water (NADW) indicate a net DOC export rate of 0.081 Pg C yr⁻¹ from the epipelagic zone into the mesopelagic and bathypelagic zones. Model II regression and multiple linear regression models applied to pairwise measures of DOC and chlorofluorocarbon (CFC-12) ventilation age, retrieved from major water masses within the main thermocline and NADW, indicate decay rates for exported DOC ranging from 0.13 to 0.94 μmol kg⁻¹ yr⁻¹, with higher DOC concentrations driving higher rates. The contribution of DOC oxidation to oxygen consumption ranged from 5 to 29% while mineralization of sinking biogenic particles drove the balance of the apparent oxygen utilization.     

Tracing dissolved organic matter cycling in the eastern boundary of the temperate North Atlantic using absorption and fluorescence spectroscopy

Apparent oxygen utilization (AOU), dissolved organic carbon (DOC), coloured dissolved organic matter (CDOM) absorption spectra, and CDOM fluorescence characteristic of aromatic amino acids (excitation/emission 280 nm/320 nm; F(280/320)) and marine-humic like substances (320 nm/410 nm; F(320/410)) were measured in full depth during a cruise in the temperate Eastern North Atlantic (ENA). An optimum multi-parameter (OMP) inverse method was run to calculate water mass proportion-weighted average (archetypal) concentrations of these chemical parameters for all water masses and samples. Archetypal concentrations retain the variability due to water mass mixing and basin scale mineralization from the water mass formation sites to the study area. Conversely, the difference between measured and archetypal concentrations, retain the variability due to dissimilarities in mineralization processes within the study area. Our analysis indicates that DOC supported 26±3% of the AOU in the dark temperate ENA and that basin scale processes occurring at and from the formation area of the water masses explained 63% of the total DOC variability. Our data also suggests that DOC remineralized at the basin scale was of lower molecular weight, and with a lower proportion of fluorescent aromatic amino acids than found within the study area. The relationship between the absorption coefficient at 254 nm (a_CDOM(254)) and AOU indicates that a_CDOM(254) was consumed during organic matter remineralization in the dark ocean, with 55% of the variability being explained by basin scale processes. The relationships of F(320/410) with AOU and DOC confirmed that marine humic-like substances are produced by microbial degradation processes, at a rate of 6.1±0.9×10⁻³ mg equivalents of QS mol AOU⁻¹. Our results also indicate that basin-scale remineralization processes account for 85% of the total variability of F(320/410), emphasizing that large scale processes control the formation of humic-like substance in the dark ENA.     

Major contribution of autotrophy to microbial carbon cycling in the deep North Atlantic’s interior

Current estimates point to a mismatch of particulate organic carbon supply derived from the surface ocean and the microbial organic carbon demand in the meso- and bathypelagic realm. Based on recent findings that chemoautotrophic Crenarchaeota are abundant in the mesopelagic zone, we quantified dissolved inorganic carbon (DIC) fixation in the meso- and bathypelagic North Atlantic and compared it with heterotrophic microbial activity. Measuring ¹⁴C-bicarbonate fixation and ³H-leucine incorporation revealed that microbial DIC fixation is substantial in the mesopelagic water masses, ranging from 0.1 to 56.7 μmol C m⁻³ d⁻¹, and is within the same order of magnitude as heterotrophic microbial activity. Integrated over the dark ocean’s water column, DIC fixation ranged from 1–2.5 mmol C m⁻² d⁻¹, indicating that chemoautotrophy in the dark ocean represents a significant source of autochthonously produced ‘new organic carbon’ in the ocean’s interior amounting to about 15–53% of the phytoplankton export production. Hence, chemoautotrophic DIC fixation in the oxygenated meso- and bathypelagic water column of the North Atlantic might substantially contribute to the organic carbon demand of the deep-water microbial food web.     

Year-long float trajectories in the Labrador Sea Water of the eastern North Atlantic Ocean

Year-long Lagrangian trajectories within the Labrador Sea Water of the eastern North Atlantic Ocean are analysed for basic flow statistics. Root-mean-square velocities at 1750 m depth are about 2 cm/s, except within the North Atlantic Current, where they are twice as large. These values are consistent with previous Eulerian measurements and extend those results to a much larger domain of the eastern basin. Mean flow estimates in boxes large enough to contain about 1 float-year of data indicate that Labrador Sea Water, having crossed the Mid- Atlantic Ridge (not resolved) near 50–55^°N, presumably with the North Atlantic Current, partially recirculates to the north in the subpolar gyre, as well as entering the subtropical gyre and continuing south and west. The circulation of this water mass, as defined by the 1 yr average velocities, is stronger than traditional models of deep circulation would suggest, with an interior flow of roughly 1 cm/s. Mean speeds up to 3 cm/s were observed, with the highest values near the Azores Plateau. North of 45°N–55°N, mean eastward speeds closer to 0.2 cm/s were observed. Wind-generated barotropic fluctuations may be responsible for some part of the transport at this depth.     

Downward particle fluxes within different productivity regimes off the Mauritanian upwelling zone (EUMELI program)

A 2-yr record of downward particle flux was obtained with moored sediment traps at several depths of the water column in two regions characterized by different primary production levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean. Particle fluxes, of ∼71–78% biogenic origin (i.e. consisting of CaCO₃, organic matter and opal) on average, decrease about six-fold from the mesotrophic site (highest fluxes in the North Atlantic) nearer the Mauritanian margin (18°30′N, 21°00′W) to the remote, open-ocean, oligotrophic site (21°00′N, 31°00′W). This decrease largely reflects the difference in total primary production between the two sites, from ∼260 to ∼110 g organic C m⁻² yr⁻¹. At both sites, temporal variability of the downward particle flux seems to be linked to westward surface currents, which are likely to transport seaward biomass-rich water masses from regions nearer the coast. The influence of coastal upwelling is marked at the mesotrophic site. The large differences between the 1991 and 1992 records at that site, where carbon export is large, underscore the interest of long-term studies for export budget estimates. The different productivity regimes at the two sites seem to induce contrasting downward modes of transport of the particulate matter, as shown in particular by the faster settling rates and the higher E ratio (particulate organic carbon export versus total primary production) estimated at the mesotrophic site.     

Silver in the far North Atlantic Ocean

Total (unfiltered) silver concentrations in higher latitudes of the North Atlantic (52–68°N) are reported for the second Intergovernmental Oceanographic Commission (IOC) Global Investigation of Pollutants in the Marine Environment (GIPME) baseline survey of 1993. These silver concentrations (0.69–7.2 pM) are oceanographically consistent with those (0.24–9.6 pM) previously reported for lower latitudes in the eastern North and South Atlantic (Flegal et al., 1995). However, surface (⩽200 m) water concentrations of silver (0.69–4.6 pM) in the northern North Atlantic waters are, on average, ten-fold larger than those (0.25 pM) considered natural background concentrations in surface waters of the central Atlantic. In contrast, variations in deep far North Atlantic silver concentrations are associated with discrete water masses. Consequently, the cycling of silver in the far North Atlantic appears to be predominantly controlled by external inputs and the advection of distinct water masses, in contrast to the nutrient-like biogeochemical cycling of silver observed in the central Atlantic and Pacific oceans.     

Nitrous oxide and methane in the Atlantic Ocean between 50°N and 52°S: Latitudinal distribution and sea-to-air flux

We discuss nitrous oxide (N₂O) and methane (CH₄) distributions in 49 vertical profiles covering the upper ∼300 m of the water column along two ∼13,500 km transects between ∼50°N and ∼52°S during the Atlantic Meridional Transect (AMT) programme (AMT cruises 12 and 13). Vertical N₂O profiles were amenable to analysis on the basis of common features coincident with Longhurst provinces. In contrast, CH₄ showed no such pattern. The most striking feature of the latitudinal depth distributions was a well-defined “plume” of exceptionally high N₂O concentrations coincident with very low levels of CH₄, located between ∼23.5°N and ∼23.5°S; this feature reflects the upwelling of deep waters containing N₂O derived from nitrification, as identified by an analysis of N₂O, apparent oxygen utilization (AOU) and NO₃⁻, and presumably depleted in CH₄ by bacterial oxidation. Sea-to-air emissions fluxes for a region equivalent to ∼42% of the Atlantic Ocean surface area were in the range 0.40–0.68 Tg N₂O yr⁻¹ and 0.81–1.43 Tg CH₄ yr⁻¹. Based on contemporary estimates of the global ocean source strengths of atmospheric N₂O and CH₄, the Atlantic Ocean could account for ∼6–15% and 4–13%, respectively, of these source totals. Given that the Atlantic Ocean accounts for around 20% of the global ocean surface, on unit area basis it appears that the Atlantic may be a slightly weaker source of atmospheric N₂O than other ocean regions but it could make a somewhat larger contribution to marine-derived atmospheric CH₄ than previously thought.     

Particulate organic carbon in the global ocean derived from SeaWiFS ocean color

Satellite remote sensing offers new means of quantifying particulate organic carbon, POC, concentration over large oceanic areas. From SeaWiFS ocean color, we derived 10-year data of POC concentration in the surface waters of the global ocean. The 10-year time series of the global and basin scale average surface POC concentration do not display any significant long-term trends. The annual mean surface POC concentration and its seasonal amplitude are highest in the North Atlantic and lowest in the South Pacific, when compared to other ocean basins. POC anomalies in the North Atlantic, North Pacific, and global concentrations seem to be inversely correlated with El Niño index, but longer time series are needed to confirm this relationship. Quantitative estimates of POC reservoir in the oceanic surface layer depend on the choice of what should represent this layer. Global average POC biomass is 1.34 g m^?2 if integrated over one optical depth, 3.62 g m^?2 if integrated over mixed layer depth, and up to 6.41 g m^?2 if integrated over 200-m layer depth (when assumed POC concentration below MLD is 20 mg m^?3). The global estimate of total POC reservoir in the surface 200-m layer of the ocean is 228.61×10¹³ g. We expect that future estimates of POC reservoir may be even larger, when more precise calculations account for deep-water organic-matter maxima in oligotrophic regions, and POC biomass located just below the seasonal mixed layer in spring and summer in the temperate regions.     

Seasonal variability of dissolved oxygen,percent oxygen saturation,and apparent oxygen utilization in the Atlantic and Pacific Oceans

Using objectively analyzed seasonal fields of dissolved oxygen content, percent oxygen saturation, and apparent oxygen utilization (AOU), we describe the large-scale seasonal variability of oxygen for the Atlantic and Pacific Oceans in the upper 400 m. The winter minus summer basin zonal averages of AOU reveal a two-layer feature in both the Atlantic and the Pacific, for both hemispheres. Biological activity and seasonal stratification in the summer give the upper 50–75 m of the water column in each basin a lower AOU in summer than winter. Greater mixing of upper ocean waters in winter gives the 75–400 m layer lower AOU values in that season. The basin integral seasonal volumes of oxygen for both the North Atlantic and the North Pacific mirror what is occurring in the atmosphere, indicating that there is a seasonal flux of oxygen across the air–sea interface. Winter total O₂ volume in the ocean is above the annual mean; the summer volume is below. Larger seasonal differences in the total O₂ content are observed in the North Atlantic Ocean than the North Pacific Ocean. A seasonal net outgassing (SNO) of 8.3×10¹⁴ moles O₂ is calculated from basin means, which is 25% higher than previous results.     

Interannual to decadal Gulf Stream variability in an eddy-resolving ocean model

Meridional shifts of the Gulf Stream (GS) jet on interannual to decadal timescales and the corresponding oceanic changes around the GS are investigated using a near global eddy-resolving ocean model hindcast from 1960 to 2003. The simulated variability in the shifts of the GS jet axis shows good agreement with observations, and lags atmospheric fluctuations characterized by the North Atlantic Oscillation by about 2 years. This lagged response of the GS jet to the atmospheric variations is attributed to the westward propagation of the undulation of the jet axis from 45°W to 75°W, which has a wavelength of about 4000 km and a displacement of 0.5°. The propagation direction and phase speed of about 2.8 cm s⁻¹ are consistent with the thin-jet theory. The shifts of the jet axis in the downstream region are likely induced by wind fluctuations through Ekman convergence over the central North Atlantic. Associated with the northward (southward) shift of the jet axis, sea surface temperature is warming (cooling) around and north of the jet, and the former warming has a deep and meridionally narrow subsurface structure, consistent with the northward shift of the jet. The meridional shifts of the jet accompany coherent meridional shifts of energetic eddy activity regions around the GS. Our numerical results suggest that the GS jet brings the atmospheric signals from the central to the western North Atlantic, and the resultant meridional shift of the jet induces the notable oceanic changes around the GS.     

Declining nutrient concentrations in the northeast Atlantic as a result of a weakening Subpolar Gyre

Between 1996 and the mid-2000s the upper waters (200–700 m) of the Rockall Trough became warmer (+0.72 °C), saltier (+0.088) and reduced in nitrate and phosphate (−2.00 µM and −0.14 µM respectively). These changes, out-with calculated errors, can be explained by the varying influence of southern versus subpolar water masses in the basin as the Subpolar Gyre weakened and contracted. Upper water properties strongly correlate with a measure of the strength of the Subpolar Gyre (the first principal component of sea surface height over the Subpolar North Atlantic) prior to the mid-2000s. As the gyre weakens, the upper layers of the trough become warmer (r−0.85), more saline (r−0.86) and reduced in nitrate and phosphate (r+0.81 and r+0.87 respectively). Further the proportion of subpolar waters in the basin decreases from around 50% to less than 20% (r+0. 88). Since the mid−2000s the Subpolar Gyre has been particularly weak. During this period temperatures decreased slightly (−0.21 °C), salinities remained near constant (35.410±0.005) and phosphate levels low and stable (0.68±0.02 µM). These relative lack of changes are thought to be related to the maximum proportion of southern water masses within the Rockall Trough having been reached. Thus the upper water properties are no longer controlled by changes in the relative importance of different water masses in the basin (as prior to the mid-2000s), but rather a different process. We suggest that when the gyre is particularly weak the interannual changes in upper water properties in the Rockall Trough reflect changes in the source properties of the southern water masses. Since the early-2000s the Subpolar Gyre has been weaker than observed since 1992, or modelled since 1960–1970. Hence upper waters within the Rockall Trough may be warmer, saltier and more depleted in nitrate and phosphate than at any time in the last half century.     

Kinematic elements of Antarctic Intermediate Water in the western South Atlantic

The northward flowing Antarctic Intermediate Water (AAIW) is a major contributor to the large-scale meridional circulation of water masses in the Atlantic. Together with bottom and thermocline water, AAIW replaces North Atlantic Deep Water that penetrates into the South Atlantic from the North. On the northbound propagation of AAIW from its formation area in the south-western region of the Argentine Basin, the AAIW progresses through a complex spreading pattern at the base of the main thermocline. This paper presents trajectories of 75 subsurface floats, seeded at AAIW depth. The floats were acoustically tracked, covering a period from December 1992 to October 1996. Discussions of selected trajectories focus on mesoscale kinematic elements that contribute to the spreading of AAIW. In the equatorial region, intermittent westward and eastward currents were observed, suggesting a seasonal cycle of the AAIW flow direction. At tropical latitudes, just offshore the intermediate western boundary current, the southward advection of an anticyclonic eddy was observed between 5^°S and 11°S. Farther offshore, the flow lacks an advective pattern and is governed by eddy diffusion. The westward subtropical gyre return current at about 28°S shows considerable stability, with the mean kinetic energy to eddy kinetic energy ratio being around one. Farther south, the eastward deeper South Atlantic Current is dominated by large-scale meanders with particle velocities in excess of 60 cm s^-1. At the Brazil–Falkland Current Confluence Zone, a cyclonic eddy near 40°S 50°W seems to act as injector of freshly mixed AAIW into the subtropical gyre. In general, much of the mixing of the various blends of AAIW is due to the activity of mesoscale eddies, which frequently reoccupy similar positions.     

Methane and methane carbon isotope ratios in the Northeast Atlantic including the Mid-Atlantic Ridge (50°N)

Previous work has shown that methane anomalies frequently occur within the rift valley of the Mid-Atlantic Ridge (MAR). The plumes appear confined within the high, steep walls of the valley, and it is not known whether methane may escape to the open ocean outside. In order to investigate this question, the concentration and ¹³C/¹²C ratio of methane together with CCl₃F concentration were measured in the northeastern Atlantic including the rift valley near 50°N. This segment contained methane plumes centered several 100 m above the valley floor with δ¹³C values mostly between –15‰ and –10‰. A limited number of helium isotope measurements showed that δ³He increased to 17% at the bottom of the valley, which suggests the helium and methane sources may be spatially separated. In the eastern Atlantic away from the ridge (48°N, 20°W), the methane concentration decreased monotonically from the surface to the bottom, but the methane δ¹³C exhibited a mid-water maximum of about –25‰. The bottom water methane contained a significantly lower δ¹³C of about –36‰. Thus, it appears that isotopically heavy methane escapes from the MAR into North Atlantic Deep Water (NADW) that contacts the ridge crest while circulating to the east. The formation of NADW supplies isotopically light methane that dilutes the input of heavy carbon from the ridge. We employed a time-dependent box model to calculate the extent of isotope dilution and thereby the flux of MAR methane into the NADW circulation. The degree of methane oxidation, which affects the ¹³C/¹²C of methane through kinetic isotope fractionation, was estimated by comparing methane and CFC-11 model results with observations. The model calculations indicate a MAR methane source of about 0.06×10⁻⁹ mol L⁻¹ yr⁻¹ to waters at the depth of the ridge crest. Assuming this extends to a 500 m thick layer over half of the entire Atlantic, the amount of methane escaping from the MAR to the open ocean is estimated to be about 1×10⁹ mol yr⁻¹. The total production of methane within the rift valley is likely much greater than the flux from the valley to the outside because of local oxidation. This implies that serpentinization of ultramafic rocks supports much of methane production in the rift valley because the amount expected from basalt degassing in association with mantle helium (<0.6×10⁹ mol CH₄ yr⁻¹) is less than even the net amount escaping from the valley. The model results also indicate the methane specific oxidation rate is about 0.05 yr⁻¹ in open waters of the northern Atlantic.     

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

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

Interhemispheric exchanges of mass and heat in the Atlantic Ocean in January–March 1993

Hydrographic, geochemical, and direct velocity measurements along two zonal (7.5°N and 4.5°S) and two meridional (35°W and 4°W) lines occupied in January–March, 1993 in the Atlantic are combined in an inverse model to estimate the circulation. At 4.5°S, the Warm Water (potential temperature θ>4.5°C) originating from the South Atlantic enters the equatorial Atlantic, principally at the western boundary, in the thermocline-intensified North Brazil Undercurrent (33±2.7×10⁶ m³ s⁻¹ northward) and in the surface-intensified South Equatorial Current (8×10⁶ m³ s⁻¹ northward) located to the east of the North Brazil Undercurrent. The Ekman transport at 4.5°S is southward (10.7±1.5×10⁶ m³ s⁻¹). At 7.5°N, the Western Boundary Current (WBC) (17.9±2×10⁶ m³ s⁻¹) is weaker than at 4.5°S, and the northward flow of Warm Water in the WBC is complemented by the basin-wide Ekman flow (12.3±1.0×10⁶ m³ s⁻¹), the net contribution of the geostrophic interior flow of Warm Water being southward. The equatorial Ekman divergence drives a conversion of Thermocline Water (24.58⩽σ₀<26.75) into Surface Water (σ₀<24.58) of 7.5±0.5×10⁶ m³ s⁻¹, mostly occurring west of 35°W. The Deep Water of northern origin flows southward at 7.5°N in an energetic (48±3×10⁶ m³ s⁻¹) Deep Western Boundary Current (DWBC), whose transport is in part compensated by a northward recirculation (21±4.5×10⁶ m³ s⁻¹) in the Guiana Basin. At 4.5°S, the DWBC is much less energetic (27±7×10⁶ m³ s⁻¹ southward) than at 7.5°N. It is in part balanced by a deep northward recirculation east of which alternate circulation patterns suggest the existence of an anticyclonic gyre in the central Brazil Basin and a cyclonic gyre further east. The deep equatorial Atlantic is characterized by a convergence of Lower Deep Water (45.90⩽σ₄<45.83), which creates an upward diapycnal transport of 11.0×10⁶ m³ s⁻¹ across σ₄=45.83. The amplitude of this diapycnal transport is quite sensitive to the a priori hypotheses made in the inverse model. The amplitude of the meridional overturning cell is estimated to be 22×10⁶ m³ s⁻¹ at 7.5°N and 24×10⁶ m³ s⁻¹ at 4.5°S. Northward heat transports are in the range 1.26–1.50 PW at 7.5°N and 0.97–1.29 PW at 4.5°S with best estimates of 1.35 and 1.09 PW.     

Spatial and Temporal Distribution of Coloured Dissolved Organic Matter (CDOM) in Narragansett Bay,Rhode Island: Implications for Phytoplankton in Coastal Waters

One indicator of health in estuarine and coastal ecosystems is the ability of local waters to transmit sunlight to planktonic, macrophytic, and other submerged vegetation for photosynthesis. The concentration of coloured dissolved organic matter (CDOM) is a primary factor affecting the absorption of incident sunlight in coastal and estuarine waters. In estuaries, CDOM concentrations vary due to changes in salinity gradients, inflows of industrial and domestic effluents, and the production of new dissolved organic matter from marine biologic activity. CDOM absorption data have been collected from a variety of waters. However, there are a limited number of measurements along the US east coast and a general lack of data from New England waters.This study characterized the temporal and spatial variability of CDOM absorption over an annual cycle in Narragansett Bay and Block Island Sound (Rhode Island). Results suggested that, in Narragansett Bay, the magnitude of CDOM absorption is related to the seasonal variability of freshwater input from surrounding watersheds and new CDOM production from in situ biologic activity. The data show that the average CDOM absorption coefficient at 412 nm was 0·45 m⁻¹ and the average spectral slope was 0·020 nm⁻¹.     

Dianeutral mixing,transformation and transport of Antarctic Intermediate Water in the South Atlantic Ocean

Recently obtained World Ocean Circulation Experiment (WOCE) sections combined with a specially prepared pre-WOCE South Atlantic data set are used to study the dianeutral (across neutral surface) mixing and transport achieving Antarctic Intermediate Water (AAIW) being transformed to be part of the North Atlantic Deep Water (NADW) return cell. Five neutral surfaces are mapped, encompassing the AAIW from 700 to 1100 db at the subtropical latitudes.Coherent and significant dianeutral upwelling is found in the western boundary near the Brazil coast north of the separation point (about 25°S) between the anticyclonic subtropical and cyclonic south equatorial gyres. The magnitude of dianeutral upwelling transport is 10^-3 Sv (1 Sv=10⁶ m³ s^-1) for 1°×1° square area. It is found that the AAIW sources from the southwestern South Atlantic and southwestern Indian Ocean do not rise significantly into the Benguela Current. Instead, they contribute to the NADW return formation by dianeutral upwelling into the South Equatorial Current. In other words, the AAIW sources cannot obtain enough heat/buoyancy to rise until they return to the western boundary region but north of the separation point. The basin-wide integration of dianeutral transport shows net upward transports, ranging from 0.25 to 0.6 Sv, across the lower and upper boundary of AAIW north of 40°S. This suggests that the equatorward AAIW is a slow rising water on a basin average. Given one order of uncertainty in evaluating the along-neutral-surface and dianeutral diffusivities from the assumed values, K=10³ m² s^-1 and D=10^-5 m² s^-1, the integrated dianeutral transport has an error band of about 10–20%. The relatively weak integrated dianeutral upwelling transport compared with AAIW in other oceans implies much stronger lateral advection of AAIW in the South Atlantic.Mapped Turner Angle in diagnosing the double-diffusion processes shows that the salty Central Water can flux salt down to the upper half of AAIW layer through salt-fingering. Therefore, the northward transition of AAIW can gain salt either through along-neutral-surface advection and diffusion or through salt fingering from the Central Water and heat through either along-neutral-surface advection and diffusion or dianeutral upwelling. Cabbeling and thermobaricity are found significant in the Antarctic frontal zone and contribute to dianeutral downwelling with velocity as high as −1.5×10^-7 m s^-1. A schematic AAIW circulation in the South Atlantic suggests that dianeutral mixing plays an essential role in transforming AAIW into NADW return formation.     

Factors influencing particulate lipid production in the East Atlantic Ocean

Extensive analyses of particulate lipids and lipid classes were conducted to gain insight into lipid production and related factors along the biogeochemical provinces of the Eastern Atlantic Ocean. Data are supported by particulate organic carbon (POC), chlorophyll a (Chl a), phaeopigments, Chl a concentrations and carbon content of eukaryotic micro-, nano- and picophytoplankton, including cell abundances for the latter two and for cyanobacteria and prokaryotic heterotrophs. We focused on the productive ocean surface (2 m depth and deep Chl a maximum (DCM)). Samples from the deep ocean provided information about the relative reactivity and preservation potential of particular lipid classes. Surface and DCM particulate lipid concentrations (3.5–29.4 μg L⁻¹) were higher than in samples from deep waters (3.2–9.3 μg L⁻¹) where an increased contribution to the POC pool was observed. The highest lipid concentrations were measured in high latitude temperate waters and in the North Atlantic Tropical Gyral Province (13–25°N). Factors responsible for the enhanced lipid synthesis in the eastern Atlantic appeared to be phytoplankton size (micro, nano, pico) and the low nutrient status with microphytoplankton having the most expressed influence in the surface and eukaryotic nano- and picophytoplankton in the DCM layer. Higher lipid to Chl a ratios suggest enhanced lipid biosynthesis in the nutrient poorer regions. The various lipid classes pointed to possible mechanisms of phytoplankton adaptation to the nutritional conditions. Thus, it is likely that adaptation comprises the replacement of membrane phospholipids by non-phosphorus containing glycolipids under low phosphorus conditions. The qualitative and quantitative lipid compositions revealed that phospholipids were the most degradable lipids, and their occurrence decreased with increasing depth. In contrast, wax esters, possibly originating from zooplankton, survived downward transport probably due to the fast sinking rate of particles (fecal pellets). The important contribution of glycolipids in deep waters reflected their relatively stable nature and degradation resistance. A lipid-based proxy for the lipid degradative state (Lipolysis Index) suggests that many lipid classes were quite resistant to degradation even in the deep ocean.