Eastern North Atlantic Subtropical Gyre Research Articles

Recent Trends in SST, Chl‐a, Productivity and Wind Stress in Upwelling and Open Ocean Areas in the Upper Eastern North Atlantic Subtropical Gyre

AbstractThe global upper ocean has been warming during the last decades accompanied with a chlorophyll‐a (Chl‐a) and productivity decrease. Whereas subtropical gyres show similar trends, Eastern Boundary Upwelling Systems are thought to increase in productivity due to increased trade winds. This study analyzes recent trends in sea surface temperature (SST), Chl‐a, net primary production (NPP) and meridional wind stress in the Eastern North Atlantic subtropical gyre (NASE) in order to examine if the global trends can be detected in open ocean and upwelling areas and how the ocean biota responds. Satellite data of such variables of the last 15–40 years were analyzed to calculate mean trends in upwelling areas in the Canary upwelling system and open ocean areas around the Azores, Madeira and the Canary Islands. Our results show significant warming in the area with a maximum of 2.7°C per century for the Azores. Moreover, a general decreasing trend for Chl‐a and NPP seems to be more evident in the permanent upwelling areas, which will be responsible for a loss of 0.13% of the global NPP per century. Our results also highlight a significant expansion of the oceanic desert area of 10% with an increase in unproductive days of up to 84 days in the last 20 years. The competitive relationship of stratification and wind stress in the Canary upwelling system might be a more plausible explanation for the decrease in Chl‐a and NPP in upwelling areas linked to the increase in upwelling favorable wind stress and the surface warming.

Distribution and transport of microplastics in the upper 1150 m of the water column at the Eastern North Atlantic Subtropical Gyre, Canary Islands, Spain

Nowadays it is widely known that pollution by microplastics (MP) at the open ocean covers immense areas. Buoyant plastics tend to accumulate in areas of convergence at the sea surface such as subtropical gyres, while non-buoyant plastics accumulate at the seafloor. However, previous studies have revealed that the total amount of plastic in the different oceans is not well correlated with the concentrations measured at the sea surface and the sea floor, evidencing a significant amount of missing plastic in the oceans. This deviation could be related to an underestimation of the role played by small fragments of plastic and fibers in the oceans. Furthermore, microplastic fragments with a density lower than the density of seawater have been gathered hundreds of meters below the sea surface in the Pacific Ocean due to their size and shape.The main objective of this study is to carry out, for the first time, an equivalent analysis along the water column for the Atlantic Ocean. In that sense, a total number of 51 samples were collected during four different oceanographic cruises between February and December 2019, from the sea surface down to 1150 m depth at the open ocean waters of the Canary Islands region (Spain). For each sample, 72 l of seawater were filtered on board with a mesh size of 100 μm, where the presence of microplastics has been clearly observed. Our results reveal the presence of microplastics at least up to 1150 m depth, at the Northeastern Atlantic Subtropical Gyre with noticeable seasonal differences. The spatial distribution of these small fragments and fibers at the water column is mainly related to the oceanic dynamics and mesoscale convective flows, overcoming the MP motion induced by their own buoyancy.Moreover, these microplastics have being transported by the ocean dynamics as passive drifters.

Variability of the Canary Current Diagnosed by Inverse Box Models

AbstractFour hydrographic cruises carried out between ~26.5 and 31°N in the eastern North Atlantic Subtropical Gyre in fall (2016 and 2017) and spring (2017 and 2018) are used to identify water masses and infer oceanic circulation. Geostrophic velocities are initially adjusted by referencing them to data from a Lower Acoustic Doppler Current Profiler (LADCP) and later to velocities estimated with an inverse box model. The distribution of the intermediate water masses (700 to 1,400 m depth) varies seasonally. Antarctic Intermediate Water (AAIW) comprises the largest contributor to the seasonal cycle in the intermediate water masses. Circulation of the Canary Current (CC) differs in fall and spring. In fall, the CC flows southward through the western islands and recirculates south of the archipelago, subsequently flowing northward through the passage between the eastern islands and Africa. North of Lanzarote, the recirculated CC intensified as it is joined by a southeasterly branch of the CC north of Lanzarote. In spring, the net transport of the CC is southward. High interannual variability in mass transport is evident in both spring and fall as a result of the position of the current, with its easternmost (westernmost) position found in spring (fall) 2018 (2016). At intermediate levels, highly variable northward/southward transport is apparent in fall over the African slope, with the Intermediate Poleward Under Current (IPUC) only present in 2017.

Non-redfieldian mesopelagic nutrient remineralization in the eastern North Atlantic subtropical gyre

The eastern boundary of the North Atlantic subtropical gyre (NASTG) receives organic materials produced in the nearby Canary upwelling system (CanUS), which has important implications for the metabolic balance of the region. Here we computed absolute geostrophic, Ekman and eddy diffusive transports and net budgets of oxygen, nitrate, and phosphate, with an inverse model applied to annual climatologies from the World Ocean Atlas 2013, in a large box covering the eastern boundary of the NASTG. The surface waters (<100 m) of the box exported 4.2±1.1 mol O2 m−2 yr−1 to the adjacent ocean and atmosphere, and imported 0.40±0.17 mol N m−2 yr−1 and 22.2±9.5 mmol P m−2 yr−1 of nitrate and phosphate, respectively, indicating that net autotrophy prevailed in the euphotic waters of the eastern NASTG. The central, intermediate and deep waters (100 m-bottom) imported 6.7±3.3 mol O2 m−2 yr−1, and exported 1.05±0.35 mol N m−2 yr−1 and 50.2±21.8 mmol P m−2 yr−1 of nitrate and phosphate, respectively, indicative of net remineralization. Mesopelagic remineralization rates exceeded epipelagic net community production, implying a net deficit of carbon and nutrients in the region. Remineralization occurred at high N:P molar ratios (∼28) in central waters, and thus, a excess nitrate (DINxs = NO3-16·PO4) was produced at a rate of 2.2±1.6×1011 mol N yr−1. This excess nitrate was exported to the gyre interior, influencing the biogeochemistry of the whole NASTG and leaving the eastern margin with a strong nitrogen deficit, relative to carbon and phosphorus. According to the available literature, the organic carbon export from the CanUS could be sufficient to account for the carbon deficit, but the source of nitrogen remains elusive.

Land–sea climatic variability in the eastern North Atlantic subtropical region over the last 14,200 years: Atmospheric and oceanic processes at different timescales

High-temporal resolution analysis of different climatic tracers (pollen, foraminiferal-based winter sea surface temperature (SST), benthic foraminiferal δ18O) from marine core MD95-2042, retrieved off SW Iberia, allows us to directly compare, without any chronological ambiguity, Mediterranean vegetation and eastern North Atlantic winter SST changes for the last 14.2 kyr. We identify on land and in the ocean several climatic phases such as the end of the warm and humid Bølling–Allerød, the cold and dry Younger Dryas, and the warm and humid Holocene with the Mediterranean forest (MF) optimum between 9.6 and 8.1 kyr. This record shows that, at multi-centennial timescale (~800 years), declines in forest cover generally related to dry and cool periods in southern Iberia are synchronous with cold SST in the eastern part of the North Atlantic subtropical gyre. At multi-centennial timescale, changes in thermohaline circulation, via freshwater content fluctuations, appear to be responsible for the coupling between dryness in Iberia and SST cooling in eastern North Atlantic subtropical gyre. In contrast, some Holocene events include centennial-scale oscillations (~100 years) marked by MF declines in southern Iberia concomitant with SST warming in the eastern North Atlantic subtropical gyre. This climatic pattern is similar to that observed at decadal timescale under the influence of the positive mode of the North Atlantic Oscillation (NAO). We suggest, therefore, that synchronous SW Iberian dryness and SST warming at centennial timescale could be explained by atmospheric fluctuations related to NAO changes.

Phytoplankton Growth and Microzooplankton Grazing in the Subtropical Northeast Atlantic

Dilution experiments were performed to estimate phytoplankton growth and microzooplankton grazing rates during two Lagrangian surveys in inner and eastern locations of the Eastern North Atlantic Subtropical Gyre province (NAST-E). Our design included two phytoplankton size fractions (0.2–5 µm and >5 µm) and five depths, allowing us to characterize differences in growth and grazing rates between size fractions and depths, as well as to estimate vertically integrated measurements. Phytoplankton growth rates were high (0.11–1.60 d−1), especially in the case of the large fraction. Grazing rates were also high (0.15–1.29 d−1), suggesting high turnover rates within the phytoplankton community. The integrated balances between phytoplankton growth and grazing losses were close to zero, although deviations were detected at several depths. Also, O2 supersaturation was observed up to 110 m depth during both Lagrangian surveys. These results add up to increased evidence indicating an autotrophic metabolic balance in oceanic subtropical gyres.

Production and respiration control the marine microbial metabolic balance in the eastern North Atlantic subtropical gyre

Two main contrasted hypotheses have arisen during the last decades about the factors controlling the planktonic net metabolic balance in oligotrophic waters: gross primary production controls net community production vs. variability of net community production is also influenced by changes in microbial respiration. This work discusses both hypotheses analyzing the variability of metabolic rates along a gradient from the margin to the centre of the North Atlantic oligotrophic gyre, i.e. from relatively productive to more oligotrophic conditions. Net community production (NCP) was close to zero (between −3.34 and −11.77 mmol O 2 m −2 d −1) at the margin of the gyre and tended towards net heterotrophy (−44.03 mmol O 2 m −2 d −1) to the centre of the gyre as both gross primary production (GPP) and community respiration (CR) decreased. The strong relationships found between nutrient availability and both NCP and GPP suggest that factors controlling GPP are prevalent in determining NCP variability in this biogeographic region. However implementation of existing models to predict NCP from the measured GPP indicates that the precise estimation of NCP in different oligotrophic systems requires consideration of the magnitude and variability of microbial respiration rates.

Inverse modeling of salinity–temperature–depth relationships: Application to the upper eastern North Atlantic subtropical gyre

We test the skill of a polynomial fit to reproduce the upper ocean (down to 750 m) salinity in the eastern North Atlantic (from the Canary Islands to the Iberian Peninsula, approximately 12° × 12°) as a function of temperature and depth. A historical database, constructed by merging several regional datasets, is used. An ANOVA test is performed to determine the optimum degree of temperature and depth in the polynomial fit. The polynomial coefficients are estimated by solving an inverse model where we control the size of both coefficients and residuals. We divide the basin in 21 zones (2° × 2°) and four regions (each comprising several zones), and run the inversion for the whole basin, as well as for each individual region and zone. This allows us to assess the sensitivity of the model to changes in the spatial domain, and to investigate the spatial variability of the polynomial coefficients. Regions are defined by applying a cluster analysis to objectively group those zones with similar oceanographic properties. The seasonality of the coefficients is addressed with data from the whole basin and individual regions. We find that, for either the whole basin or individual regions, seasonal coefficients predict salinity more accurately than annual ones, but annual coefficients per zone yet provide the best results. The depth-averaged error estimating salinity is less than 0.086 psu.

Trichodesmium spp. population structure in the eastern North Atlantic subtropical gyre

Trichodesmium spp. population structure was studied based on a transect in the eastern subtropical North Atlantic in October 2006, covering a gradient from near-temperate conditions up to fully subtropical, oligotrophic waters. Trichodesmium spp. trichomes were counted and measured, distinguishing free trichomes from those forming macroscopic colonies (“puffs” or “tufts”). Moreover, for both free and colonial individuals, abundance and size structure variation were quantified from single trichome-level data. Nitrogen fixation rates were estimated using an empirical approach based on abundance observations and a heuristic approach to incorporate variation in trichome size and environmental temperature. A gradual change towards greater sizes and increased abundances was observed as physical conditions became subtropical. An extreme response was detected at one station (29.8°W, 26.3°N), in which both in situ and remote sensing data revealed the presence of a Trichodesmium bloom. Despite the presence of a mesoscale anticyclonic eddy, a close examination of prior surface conditions using altimeter data indicated that the bloom was related to the advection of southern waters. In general, our results highlighted the importance of free trichomes in Trichodesmium spp. populations in this part of the North Atlantic. Furthermore, they suggested a possible role of both size and population structure on the wide range of N 2 fixation estimates currently available in the literature.

Distribution of water masses and diapycnal mixing in the Cape Verde Frontal Zone

The Cape Verde Frontal Zone separates North and South Atlantic Central Waters in the eastern North Atlantic Subtropical Gyre. CTD‐O2 and shipboard ADCP data from three hydrographic sections carried out in September 2003 are used to study the structure of the front. Results show the relation between spatial variations of water masses and currents, demonstrating the importance of advection in the distribution of water masses. Diapycnal diffusivities due to double diffusion and vertical shear instabilities are also estimated. Existence of competition between the two processes through the water column is shown. Depth‐averaged diffusivities suggest that salt fingering dominates diapycnal mixing, except areas of purest South Atlantic Central Water. Here, double diffusion processes are weak and, consequently, shear of the flow is the main process. Results also show that strong mixing induced by vertical shear is associated with a large intrusion found near the front.

Correction to “Decadal analysis of hydrography and in situ nutrient budgets in the western and eastern North Atlantic subtropical gyre”

[1] In the paper “Decadal analysis of hydrography and in situ nutrient budgets in the western and eastern North Atlantic subtropical gyre” (Journal of Geophysical Research, 112, C07025, doi:10.1029/2006JC003788), a number of errors were introduced during processing. [2] Susanne Neuer’s sole affiliation is School of Life Sciences, Arizona State University, Tempe, Arizona, USA. The corresponding authors are Andrés Cianca ([email protected]) and Susanne Neuer ([email protected]). [3] The word “at” was left out of the following sentence in the abstract, corrected here: “However, significant differences in input mechanisms existed between both stations; eddy pumping constituted the main new nutrient source at BATS, whereas wintertime convection was the main nutrient supply mechanism at ESTOC.” [4] In paragraph 5 in section 1, the final sentence should read as follows: “The authors hypothesized that this difference was due to a smaller new nutrient input into the mixed layer in the eastern NASTG, especially due to lower eddy-induced mixing inferred from eddy-resolving models [McGillicuddy et al., 2003; Oschlies, 2002b] and tracer-based observations [Jenkins, 1988; Lewis et al., 1986].” [5] In paragraph 29 in section 4.2, the final sentence should read as follows: “The direction of the isopycnal displacement in the upper ocean is the same as for a cyclonic eddy, causing upwelling of nutrients into the euphotic zone when they are forming or intensifying [Sweeney et al., 2003].” [6] In paragraph 53 in section 5, the first sentence should read as follows: “In addition, we would like to note the high nutrient concentrations during 1999 at ESTOC in the subsurface water at 300 m of up to 17 μmol kg−1.” [7] In paragraph 54 in section 6, the first sentence should read as follows: “Using a 10-yr time series of in situ data and satellite altimetry, we compared the hydrography and quantified new nutrient budgets of the two oligotrophic time series stations BATS and ESTOC located at about the same latitude in the western and eastern subtropical North Atlantic gyre, respectively.”

Planktonic carbon budget in the eastern subtropical North Atlantic

We used data collected at >60 stations over a 10 yr period to build the carbon budget of the plankton community in the euphotic layer of the Eastern North Atlantic Subtropical Gyre (NASE). Autotrophic biomass exceeded microbial heterotrophic biomass by a factor of 1.7. Mean (±SE), inte- grated chlorophyll a concentration and net particulate primary production (PP) were 17 ± 1 mg m -2 and 271 ± 29 mgC m -2 d -1 , respectively. Protist grazing on phytoplankton represented > 90% of PP. Bacterial production (BP) was 17 ± 3 mgC m -2 d -1 . In vitro O2-evolution experiments indicated that net community production was -65 ± 16 mmolO2 m -2 d -1 , while community respiration (CR) averaged 124 ± 13 mmolO2 m -2 d -1 , equivalent to 1324 ± 142 mgC m -2 d -1 . However, the sum of the respiration rates by each microbial group, estimated from their biomass and metabolic rates, ranged from 402 to 848 mgC m -2 d -1 . Therefore, CR could not be reconciled with the respiratory fluxes sustained by each microbial group. Comparison between estimated gross photosynthesis by phytoplankton (481 to 616 mgC m -2 d -1 ) and the sum of respiration by each group suggests that the microbial community in the NASE province is close to metabolic balance, which would agree with the observed O2 supersat- uration in the euphotic layer. Taking into account the mean open-ocean values for PP, BP, CR and bacterial growth efficiency, we show that bacteria account for approximately 20% of CR. Our results suggest that the view that bacteria dominate carbon cycling in the unproductive ocean must be reconsidered, or else that in vitro incubations misrepresent the real metabolic rates of one or several microbial groups.

Decadal analysis of hydrography and in situ nutrient budgets in the western and eastern North Atlantic subtropical gyre

The current debate about the mechanisms and magnitude of new nutrient input to the euphotic zone in subtropical gyres calls for studies which consider large and mesoscale perspectives by combining in situ time series and remote observations. We carried out a first of its kind comparative analysis of hydrography and sea level anomaly (SLA) at the oligotrophic time series stations BATS (Bermuda Atlantic Time Series Study) and ESTOC (European Station for Time Series, Canary Islands) using concomitant 10‐yr in situ and satellite altimetry data. The stations are located at about the same latitude in the western and eastern boundaries of the subtropical North Atlantic gyre, respectively, and provide the opportunity to study differences that may exist between both regions. Observed SLA was 0.25 m at BATS, compared with 0.12 m at ESTOC, a consequence of the higher eddy kinetic energy in the western compared with the eastern subtropical gyre. We quantified a detailed in situ nutrient budget for both time series stations; ESTOC received about 75% of the nutrients available for new production at BATS (in average 0.28 mol N m−2 yr−1 compared with 0.38 mol N m−2 yr−1, respectively), but the difference was not significant. However, significant differences in input mechanisms existed between both stations; eddy pumping constituted the main new nutrient source BATS, whereas wintertime convection was the main nutrient supply mechanism at ESTOC. In addition, the nutricline was significantly shallower at ESTOC compared with BATS, partly compensating for shallower mixed‐layer depths and SLA variability at the western station. We found considerable interannual variability in both eddy pumping and wintertime convection which may be related to NAO‐induced changes in the pattern of the subtropical gyre.

Analytic salinity-temperature relations for the upper-thermocline waters of the eastern North Atlantic subtropical gyre

We study the dependence of salinity on temperature in two by two degrees latitude-longitude boxes, for surface and upper-thermocline waters of the eastern North Atlantic subtropical gyre. The initial data set, from historical databases as well as from recent hydrographic cruises in the region, is carefully scrutinized to reject dubious measurements. We search for polynomial relations of variable degree between salinity and temperature, the optimal fit is selected as the polynomial with the lowest degree that satisfies several statistical criteria. An independent hydrographic cruise is used to confirm that the method performs substantially better than estimates from climatological data, and leads to relatively low deviations in geopotential anomaly and other derived quantities. An error propagation analysis using the Monte Carlo method shows equally good results.

Wind‐driven circulation for the eastern North Atlantic Subtropical Gyre from Argo data

A trans‐oceanic section at 24.5°N in the North Atlantic has been sampled at a decadal frequency. This work demonstrates that the wind‐driven component of the Meridional Overturning Circulation (MOC) may be monitored using autonomous profiling floats deployed in the eastern North Atlantic Subtropical Gyre. More than 500 CTD vertical profiles from the surface to 2000 m depth, spanning one year (from April 2002 to March 2003), are used to compute the geostrophic transport stream function at 24.5°N. The baroclinic transport obtained from the autonomous profiling floats is not statistically different than that from three hydrographic cruises carried out in 1957, 1981 and 1992. A good agreement is found between the geostrophic transport stream function and the transport derived from the wind field through the Sverdrup relation.

Variability of chlorophyll and primary production in the Eastern North Atlantic Subtropical Gyre: potential factors affecting phytoplankton activity

Size-fractionated chlorophyll- a and carbon incorporation rates were determined on a series of 13 cruises carried out from 1992 to 2001with the aim of investigating the patterns and causes of variability in phytoplankton chlorophyll and production in the Eastern North Atlantic Subtropical Gyral Province (NASE). Averaged (±SE) integrated chlorophyll- a concentration and primary production rate were 17±1 mg m −2 and 253±22 mg C m −2 d −1. Small-sized cells (<2 μm) formed the bulk of phytoplankton biomass (71%) and accounted for 54% of total primary production. A clear latitudinal gradient in these variables was not detected. By contrast, large seasonal variability was detected in terms of primary production, although integrated phytoplankton biomass, as estimated from chlorophyll- a concentration, remained rather constant and did not display significant changes with time. Variability in primary production (PP) was related mainly to variability in surface temperature and surface chlorophyll- a concentration. The control exerted by surface temperature was related to nutrient availability. By contrary, euphotic-zone depth, depth of maximum concentration of chlorophyll- a and integrated chlorophyll- a did not contribute significantly to the high variability in primary production observed in this oligotrophic region.

Seasonal sea-surface carbon dioxide in the Azores area

The seasonal evolution of total inorganic carbon and CO 2 air–sea fluxes in the Eastern North Atlantic Subtropical Gyre (Azores area) was investigated by means of studying a data set from 10 cruises covering a seasonal cycle. Monthly CO 2 fugacity was modelled as a function of surface temperature and month for 1998. So, the seasonal cycle of CO 2 and its air–sea fluxes were obtained using monthly average surface data in the area. Over the year, the Azores area (2.25·10 12 m 2) acts as a weak net sink of CO 2 (0.38 mmol m −2 day −1). From December to May, the zone is a rather strong sink for CO 2 (10.3 mmol m −2 day −1), while between June and November, it behaves as a CO 2 source (9.9 mmol m −2 day −1), August presents the highest outgassing (3.88 mmol m −2 day −1). Moreover, a box budget was established to evaluate the relative contribution of the physical and biological processes affecting the seasonal CO 2 variability in the mixed layer of the Azores area. The most important contributor to the average mass balance of CO 2 was the mixing with the lower layer (7.8 mmol m −2 day −1) and biological activity (−8.9 mmol m −2 day −1). Conversely, air–sea exchange (0.17 mmol m −2 day −1) and advection (1.7 mmol m −2 day −1) contribute with a very small input. There is a strong coupling between biological activity, advection, and mixing in the mixed layer depth. The biological activity is supported by mixing and advection that provide CO 2 and nutrients to the mixed layer, so we combine the three processes in only one term (ΔC AMB) that represents the net biology production in the water column, and re-evaluated the CO 2 mass balance to discriminate the importance of the physical and biological contributions. The effect of temperature, wind, and net biological process contribute in 42%, 12%, and 46%, respectively, to the explained variance of total CO 2 mass balance in the upper layer.

Phytoplankton growth rates in the Atlantic subtropical gyres

Reported estimates of phytoplankton growth rate (m) in the subtropical gyres range widely from 0.1-0.2 to 1-2 d 21 . Dividing chlorophyll a (Chl a)-normalized photosynthesis (P B ) and .40 determinations of phytoplankton C : Chl a obtained in surface waters of the eastern North Atlantic Subtropical Gyre (NAST-E), the western NAST (NAST-W), and the South Atlantic Tropical Gyre (SATL). Means (95% confidence intervals) of m were 0.26 (0.19-0.36), 0.51 (0.42-0.62), and 0.17 (0.13-0.22) d 21 at NAST-E, NAST-W, and SATL, respectively. These values are all significantly lower than the expected maximum m for phytoplankton living in the warm, surface waters of the subtropical ocean. Thus, phytoplankton growth rates, and not only their biomass, seem to be nutrient-limited in the oligotrophic ocean. A low phytoplankton m in the open ocean is consistent with our knowledge of the relationship between elemental composition of the cells and their physiological state and agrees with other ecological and biogeochemical observations in the subtropical gyres.

Rates of dissolved organic carbon production and bacterial activity in the eastern North Atlantic Subtropical Gyre during summer

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 249:53-67 (2003) - doi:10.3354/meps249053 Rates of dissolved organic carbon production and bacterial activity in the eastern North Atlantic Subtropical Gyre during summer E. Teira1,5,*, M. J. Pazó1, M. Quevedo2, M. V. Fuentes3, F. X. Niell4, E. Fernández1 1Departamento de Ecología y Biología Animal, Universidad de Vigo, 36200 Vigo, Spain 2Departamento de Biología de Organismos y Sistemas, Universidad de Oviedo, 33071 Oviedo, Spain 3Departamento de Microbiología, and 4Departamento de Ecología, Universidad de Málaga, 29071 Málaga, Spain 5Present address: Royal Netherlands Institute for Sea Research (NIOZ), Department of Biological Oceanography, PO Box 59, 1790 AB Den Burg, The Netherlands *Email: teira@nioz.nl ABSTRACT: Rates of particulate organic carbon (POC) production, dissolved organic carbon (DOC) production and bacterial production were measured at 8 stations in the eastern North Atlantic Subtropical Gyre during August 1998. The euphotic-depth-integrated POC rate was on average 27 mgC m-2 h-1. The corresponding averaged integrated DOC production rate was 5 mgC m-2 h-1, i.e. about 20% of the total primary production. No statistically significant relationship was found between the rates of POC and DOC production, suggesting that other processes besides phytoplankton exudation, such as cell lysis or protist grazing, could be implied in the release of dissolved organic materials to the oceanic environment. Euphotic-depth-integrated bacterial biomass and production were on average 214 mgC m-2 and 1.4 mgC m-2 h-1, respectively. The lack of correlation between the rates of DOC release and bacterial activity and a bacterial carbon demand (BCD, calculated using an estimated bacterial growth efficiency ranging from 11 to 18%) in excess of DOC production suggested the existence of additional organic carbon sources (both allochthonous and/or autochthonous reservoirs), apart from in situ phytoplankton-derived DOC production, for the maintenance of bacterial activity in this region during summer 1998. This uncoupling between phytoplankton and bacterioplankton could partially explain the heterotrophic metabolic balance in this oligotrophic region during summer 1998. KEY WORDS: Phytoplankton · Bacteria · Primary production · DOC production · Bacterial activity · Coupling · North Atlantic Subtropical Gyre Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 249. Online publication date: March 10, 2003 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2003 Inter-Research.

Observations of temporal changes of tritium-&lt;SUP&gt;3&lt;/SUP&gt;He age in the eastern North Atlantic thermocline: Evidence for changes in ventilation?

A compilation of fifteen years of tritium and 3 He measurements is used to examine the ventilation of the eastern North Atlantic subtropical gyre with specific emphasis on the temporal character of the tracer age field. A multivariate regression analysis in the form of a spatiotemporal Taylor expansion is applied to observations interpolated onto isopycnal surfaces. The time-dependent component of the tracer age field is found to be statistically significant, explaining approximately 10% of the variance of the tracer age observations in the upper thermocline (σ θ = 26.5) and increasing to roughly 50% of the variance in the lower thermocline (σ θ = 27.0). The observed transient tracer age increases over the 15 years of observations with the fractional rate of change of the age field varying between 2% and 5% per year. The largest observed changes occur on the deepest, most slowly ventilated isopycnal surfaces. The second derivative of the tritium- 3 He age with time suggests that the tracer age field may be approaching a steady state. If tritium- 3 He age is interpreted as a true measure of the advective ventilation age, the temporal changes in age would imply a slackening of the ventilation of the lower main thermocline of greater than 50% from the late 1970's to the early 1990's. However, consideration of the full advective-diffusive balance of tritium- 3 He age reveals that the changes in tracer age field represent a time-dependent adjustment of the transient tracer concentrations in conjunction with a steady local circulation field. Integral approximations of the upstream evolution of the tracer field also fail to demonstrate evidence for decadal changes in ventilation. The integral balance along the path of subduction yields an improved estimate of the true ventilation age based on the temporal tendency of the age field along the path of ventilation. An approximation of this integral suggests that actual ventilation ages can be up to 40% larger than the measured tracer age in the deeper portions of the North Atlantic thermocline. Proper accounting of the time-dependent biases of the tracer age dating technique are a prerequisite for examining transient tracer measurements for evidence of changes in the physical ventilation of the upper ocean.