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The Atlantic Meridional Transect Programme (1995–2012)
Abstract
The aims of the Atlantic Meridional Transect (AMT) Programme [www.amt-uk.org] are to quantify the nature and causes of ecological and biogeochemical variability in the planktonic ecosystems of the Atlantic Ocean, and to assess the effects of this variability on biological carbon cycling and air–sea exchange of radiatively active gases and aerosols (Fig. 1). Since 1995, marine and atmospheric data have been collected twice a year along a 13,500 km transect between 50°N and 52°S in the Atlantic Ocean. This transect encompasses a range of biogeographical provinces including the subtropical gyres and the equatorial and coastal upwellings. The sampling strategy is described in Robinson et al. (2006a), and for the last six cruises has involved continuous surface seawater and atmospheric sampling, and twice daily deployment of the CTD, bio-optical sensor rig and mesozooplankton nets. Up to 70 determinands were measured on each cruise (see Table 2 in Robinson et al., 2006a). The programme has included 18 research cruises, involving 180 scientists from 11 countries (Fig. 2), contributing to 170 refereed publications and 70 Ph.D. theses. This unique, spatially extensive, decadal dataset continues to be made available to the wider community through the British Oceanographic Data Centre (BODC) (www.bodc.ac.uk).
... The aim of the Atlantic Meridional Transect (AMT) programme (http://www.amt-uk.org/) is to annually monitor the physical and biogeochemical properties of "oceanographic provinces" in the Atlantic ocean, defined by the regulation of phytoplankton distributions by hydrographic properties Reygondeau et al., 2013). Since 1995, atmospheric and oceanic samples have been collected along AMT transects and examined for physical, optical and biogeochemical parameters (Robinson et al., 2009). A typical AMT transect covers 13,500 km, crossing approximately 17 oceanographic provinces with distinct biological and physiochemical characteristics, from temperate regions to oligotrophic gyres and equatorial upwelling . ...
... The Atlantic Meridional Transect (AMT) programme is a series of cruises that began in 1995 during which biological, chemical and physical oceanographic research is carried out between the UK and destinations in the South Atlantic Ocean (Robinson et al. 2009). Despite the collection of zooplankton during many cruises, detailed analyses of the meso-zooplankton net hauls has never been carried out. ...
An in-flow instrument for imaging and identifying meso-zooplankton from a ship’s clean pumped sea water supply, from 6m, is described. Meso-zooplankton abun- dance was measured continuously, from 38 transects and 36 net hauls (0 - 200m), on a 13,500 km passage from the Bay of Biscay to the southern Atlantic Ocean on Atlantic Meridian Transect 21.
A Line scan camera was used to give complete imaging of the in-flow water, in contrast to normal area scan cameras that sub-sample the flow. A total of 474 m3 of water was processed from transects and nearly 600,000 particles were imaged and cate- gorized. Similarly, a total of 1901 m3 were processing from the early morning net hauls and over 300,000 biological specimens were categorized. The small and large copepod categories were dominated by the calanoid copepods although the most abundant har- pacticoid in the 6m transect was Microsetella norvegica.
The value of underway monitoring is demonstrated by the presence of the filamentous cyanobacteria Trichodesmium spp., for the first time, throughout the North and South Atlantic Oceans from 48oN to 46oS. The effort required to complete the AMT 21 data analysis should allow ecological data to be extracted from net hauls and in-flow pumping within a week of the specimen images being available. This is more than a factor of ten faster than is currently possible using purely human effort alone. It is presented as a way of collecting biological samples that is faster, higher volume and at a greatly reduced cost to an entirely manual process. The large number and quality of images recorded provides an opportunity for the genera and species of much plankton to be determined by specialists. A complete set of data and images have been submitted to British Oceanographic Data Centre.
... Moreover, there was a surface and deep PIC maximum at 300m (0.1-0.15μM) in the North Subtropical Gyre (hereafter, referred to as the North Atlantic Gyre, NAG) , Equatorial region (EQ) and STW. The water mass definitions that we use in this work were given elsewhere (Robinson et al., 2006a). A distinct surface minimum in PIC concentration (∼0.06 μM) was found in the upper 50m of the EQ and in the NAG near the 27 sigma theta density anomaly (σ θ ) isopycnal at ∼200m. ...
We present average coccolithophore and associated biogeochemical results from ten Atlantic Meridional Transect (AMT) cruises between 50oN-50oS, results that show a number of unique observations across this massive data set. Lowest concentrations of coccolithophores were consistently found in equatorial waters. Highest concentrations of coccolithophore cells and coccoliths were associated with temperate, sub-polar environments. Concentrations of detached coccoliths were dispersed to ∼300m depth and show low inter-cruise variance. Coccolithophore cell concentrations in the subtropics remain highest at < 200m depth in the S. Atlantic and 100m in the N. Atlantic, near the 26.5 to 27 sigma-theta (σθ) isopycnal, within the Subantarctic Mode Water (SAMW). Diversity and species richness of coccolithophore cells was greater than for the detached coccoliths and generally greater in surface populations than in the deep chlorophyll maximum (DCM). Placolith-bearing coccolithophore species dominated over the umbelliform and floriform coccolithophore groups in surface waters of the Southern Ocean at 50oS, as well as the DCM from 50oS to the equator. The DCM was strongly associated with the σθ, of ∼27 kg m⁻³, coincident with the top of the nitracline, within the SAMW. Concentrations of coccolithophore cells and detached coccoliths were much less related to the density field than chlorophyll was. Highest integrated euphotic coccolith concentrations were observed when the integrated POC:chlorophyll ratio was highest. PIC:POC ratios increased with depth and approached maximum values near the top of the nitracline. Great Calcite Belt waters (upper 300m) showed biogenic silica:particulate inorganic carbon ratios (BSi:PIC) >1 while all other waters showed BSi:PIC ratios <1. Peaks in PIC and BSi were observed within, and above the SAMW, in waters with low phosphate and silicate concentrations. The results of this study suggest that coccolithophore-rich SAMW in the Southern Ocean is being conditioned after its formation, such that by the time it upwells in the Atlantic equatorial region, the water is no longer conducive to coccolithophore growth.
... Rates of net community production and gross primary production presented in this work were compared with other published studies in the South Atlantic (Table 1). Other methods used to calculate primary production in this region included one study in the ARP also using the O 2 /Ar and triple oxygen isotope tracers as well as dissolved inorganic carbon drawdown and multiple years of coverage across the basin in May and October as part of the Atlantic Meridional Transect program [Robinson et al., 2006[Robinson et al., , 2009, using euphotic zone integrated rates of radiocarbon uptake into particles [Marañón et al., 2003;Tilstone et al., 2009] and oxygen light-dark bottle incubations [Serret et al., 2015]. We additionally compared our results to satellite-derived estimates of NCP based on satellite POC estimates and POC to NCP and temperature relationships derived from the AMT incubations [Tilstone et al., 2015], which provided expanded spatial and temporal coverage. ...
In situ oxygen tracers (triple oxygen isotope and oxygen/argon ratios) were used to evaluate meridional trends in surface biological production and export efficiency across ~8000 km of the tropical and subtropical South Atlantic in March–May 2013. We used observations of picophytoplankton, nanophytoplankton, and microphytoplankton to evaluate community structure and diversity and assessed the relationships of these characteristics with production, export efficiency, and particulate organic carbon (POC) fluxes. Rates of productivity were relatively uniform along most of the transect with net community production (NCP) between 0 and 10 mmol O2 m−2 d−1, gross primary production (GPP) between 40 and 100 mmol O2 m−2 d−1, and NCP/GPP, a measure of export efficiency, ranging from 0.1 to 0.2 (0.05–0.1 in carbon units). However, notable exceptions to this basin-scale homogeneity included two locations with highly enhanced NCP and export efficiency compared to surrounding regions. Export of POC and particulate nitrogen, derived from sediment traps, correlated with GPP across the transect, over which the surface community was dominated numerically by picophytoplankton. NCP, however, did not correlate with POC flux; the mean difference between NCP and POC flux was similar to published estimates of dissolved organic carbon export from the surface ocean. The interrelated rates of production presented in this work contribute to the understanding, building on the framework of better-studied ocean basins, of how carbon is biologically transported between the atmosphere and the deep ocean.
... The second phase, between 2002 and 2006 was funded by a NERC consortium grant which is introduced and summarised in two manuscripts by Robinson (Robinson et al., 2009 which form the introduction to special issues of Deep-Sea Research II. This phase utilised a hypothesis-led approach concerning the biogeochemistry of the different Atlantic Ocean provinces. ...
... The Atlantic Meridional Transect (AMT) programme is a series of cruises that began in 1995 during which biological, chemical and physical oceanographic research is carried out between the UK and destinations in the South Atlantic Ocean (Robinson et al. 2009). Despite the collection of zooplankton during many cruises, detailed analyses of the meso-zooplankton net hauls has never been carried out. ...
An in-flow instrument for imaging and identifying meso-zooplankton from a ship’s clean pumped sea water supply, from 6m, is described. Meso-zooplankton abun- dance was measured continuously, from 38 transects and 36 net hauls (0 - 200m), on a 13,500 km passage from the Bay of Biscay to the southern Atlantic Ocean on Atlantic Meridian Transect 21.
A Line scan camera was used to give complete imaging of the in-flow water, in contrast to normal area scan cameras that sub-sample the flow. A total of 474 m3 of water was processed from transects and nearly 600,000 particles were imaged and cate- gorized. Similarly, a total of 1901 m3 were processing from the early morning net hauls and over 300,000 biological specimens were categorized. The small and large copepod categories were dominated by the calanoid copepods although the most abundant har- pacticoid in the 6m transect was Microsetella norvegica.
The value of underway monitoring is demonstrated by the presence of the filamentous cyanobacteria Trichodesmium spp., for the first time, throughout the North and South Atlantic Oceans from 48oN to 46oS. The effort required to complete the AMT 21 data analysis should allow ecological data to be extracted from net hauls and in-flow pumping within a week of the specimen images being available. This is more than a factor of ten faster than is currently possible using purely human effort alone. It is presented as a way of collecting biological samples that is faster, higher volume and at a greatly reduced cost to an entirely manual process. The large number and quality of images recorded provides an opportunity for the genera and species of much plankton to be determined by specialists. A complete set of data and images have been submitted to British Oceanographic Data Centre.
... Open symbols are the matchup points from latitudes north of the 38°S parallel and closed points are for matchups from south of the 38° parallel (in the GCB). Inset in the bottom right shows the global map of the matchup database, with observations from the GCB cruises described here, plus other cruises where the same technique was applied: Patagonian Shelf COPAS'08[Balch et al., 2014b], Atlantic Meridional Transect[Robinson et al., 2009], Western Arctic[Balch et al., 2014a], and Gulf of Maine North Atlantic Time Series[Balch et al., 2012]. Note that the Atlantic GCB cruise in 2011 was anomalously cloudy and overcast. ...
The Great Calcite Belt (GCB) is a region of elevated surface reflectance in the Southern Ocean (SO) covering ~16% of the global ocean and is thought to result from elevated, seasonal concentrations of coccolithophores. Here, we describe field observations and experiments from two cruises that crossed the GCB in the Atlantic and Indian sectors of the SO. We confirm the presence of coccolithophores, their coccoliths, and associated optical scattering, located primarily in the region of the sub-tropical, Agulhas, and sub-Antarctic frontal regions. Coccolithophore-rich regions were typically associated with high-velocity frontal regions with higher seawater partial pressures of CO2 (pCO2) than the atmosphere, sufficient to reverse the direction of gas exchange to a CO2 source. There was no calcium carbonate (CaCO3) enhancement of particulate organic carbon (POC) export, but there were increased POC transfer efficiencies in high-flux particulate inorganic carbon (PIC) regions. Contemporaneous observations are synthesized with results of trace-metal incubation experiments, 234Th-based flux estimates, and remotely-sensed observations to generate a mandala that summarizes our understanding about the factors that regulate the location of the GCB.
... A hydrographic front at $52.5°N marked the approximate transition point. Pigment analyses conducted on Atlantic Meridional Transect (AMT) cruises 3 and 5 (October 1996 and October 1997 respectively) also indicate a nanoplankton dominance at $50°N giving way to picoplankton dominance in the south (Gibb et al., 2000;Aiken and Bale, 2000;Barlow et al., 2002;Robinson et al., 2006Robinson et al., , 2009. ...
Phytoplankton chemotaxonomic distributions are examined in conjunction with taxon specific particulate biomass concentrations and phytoplankton abundances to investigate the biogeochemical consequences of the passage of an autumn storm in the northeast Atlantic Ocean. Chemotaxonomy indicated that the phytoplankton community was dominated by nanoplankton (2-20 μ), which on average represented 75±8% of the community. Microplankton (20-200 μ) and picoplankton (<2 μ) represented 21±7% and 4±3% respectively with the microplankton group composed of almost equal proportions of diatoms (53±17%) and dinoflagellates (47±17%). Total chlorophyll-a (TCHLa = CHLa + Divinyl CHLa) concentrations ranged from 22 to 677 ng L-1, with DvCHLa making minor contributions of between <1% and 13% to TCHLa. Higher DvCHLa contributions were seen during the storm, which deepened the surface mixed layer, increased mixed layer nutrient concentrations and vertically mixed the phytoplankton community leading to a post-storm increase in surface chlorophyll concentrations. Picoplankton were rapid initial respondents to the changing conditions with pigment markers showing an abrupt 4-fold increase in proportion but this increase was not sustained post-storm. 19’-HEX, a chemotaxonomic marker for prymnesiophytes, was the dominant accessory pigment pre- and post-storm with concentrations of 48-435 ng L-1, and represented 44% of total carotenoid concentrations. Accompanying scanning electron microscopy results support the pigment-based analysis but also provide detailed insight into the nano- and microplankton communities, which proved to be highly variable between pre-storm and post-storm sampling periods. Nanoplankton remained the dominant size class pre- and post-storm but the microplankton proportion peaked during the period of maximum nutrient and chlorophyll concentrations. Classic descriptions of autumn blooms resulting from storm driven eutrophication events promoting phytoplankton growth in surface waters should be tempered with greater understanding of the role of storm driven vertical reorganization of the water column and of resident phytoplankton communities. Crucially, in this case we observed no change in integrated chlorophyll, particulate organic carbon or biogenic silica concentrations despite also observing a ∼50% increase in surface chlorophyll concentrations which indicated that the surface enhancement in chlorophyll concentrations was most likely fed from below rather than resulting from in situ growth. Though not measured directly there was no evidence of enhanced export fluxes associated with this storm. These observations have implications for the growing practice of using chlorophyll fluorescence from remote platforms to determine ocean productivity late in the annual productivity period and in response to storm mixing.
... The Atlantic Meridian Transect programme (AMT [1] , see Fig.1) has been conducting meridional transects of the whole Atlantic Ocean between latitudes 50ºN and 50ºS for 20 years (25 cruises). The Optical Plankton Recorder [2] has been used for sampling zooplankton from vertical net hauls at discrete stations, and for continuous surface underway measurements along 17 of these transects. ...
... The Atlantic Meridian Transect programme (AMT [1] , see Fig.1) has been conducting meridional transects of the whole Atlantic Ocean between latitudes 50ºN and 50ºS for 20 years (25 cruises). The Optical Plankton Recorder [2] has been used for sampling zooplankton from vertical net hauls at discrete stations, and for continuous surface underway measurements along 17 of these transects. ...
The development and testing of a new imaging and classification system for mesozooplankton sampling over very large spatial and temporal scales is reported. The system has been evaluated on the Atlantic Meridional Transect (AMT), acquir- ing nearly one million images of planktonic particles over a transect of 13,500 km. These images have been acquired at a flow rate of 12.5 L per minute, in near-con- tinuous underway mode from the ships seawater supply and in discrete mode using integrated vertical net haul samples. The aim of this development is to produce an instrument capable of delivering autonomously acquired and processed data on the biomass and taxonomic distribution of mesozooplankton over ocean-basin scales, in or near real-time, so that data are immediately available without the need for signif- icant amounts of post-cruise processing and analysis. The hardware and image ac- quisition and processing software system implemented to support this development, together with some preliminary results from AMT21, are described. The images ac- quired during this Atlantic cruise comprise microplankton, mesoplankton, fish larvae and sampling artefacts (air bubbles, detritus, etc.), and were classified to one of 7 pre-defined taxonomic categories with 67% success.
... Meridional transits of polar research vessels offer a great opportunity to perform long-distance surface water transects with continuous water and air sampling, since frequent stops for long station work are usually not possible during such transit cruises. Trace metal (Pohl et al., 2011; Helmers, 1996) and plankton studies (Robinson et al., 2009) and a recent GEOTRACES pilot study in the Atlantic Ocean (Rutgers van der Loeff, 2007) are great examples for the importance of such projects. Previous work along the north–south transect also provided a high resolution comparison of bacterial and primary production (Hoppe et al., 2002). ...
... The AMT programme offers a unique opportunity to conduct a basin scale study of phytoplankton dynamics across different environmental settings including subtropical gyres, equatorial and coastal upwelling systems, and temperate/subpolar regions. [17]. From September 1995 to May 1997, two meridional transects between Great Britain and the Falkland Islands were carried out each year during the boreal spring and autumn. ...
Despite enormous environmental variability linked to glacial/interglacial climates of the Pleistocene, we have recently shown that marine diatom communities evolved slowly through gradual changes over the past 1.5 million years. Identifying the causes of this ecological stability is key for understanding the mechanisms that control the tempo and mode of community evolution.
If community assembly were controlled by local environmental selection rather than dispersal, environmental perturbations would change community composition, yet, this could revert once environmental conditions returned to previous-like states. We analyzed phytoplankton community composition across >10(4) km latitudinal transects in the Atlantic Ocean and show that local environmental selection of broadly dispersed species primarily controls community structure. Consistent with these results, three independent fossil records of marine diatoms over the past 250,000 years show cycles of community departure and recovery tightly synchronized with the temporal variations in Earth's climate.
Changes in habitat conditions dramatically alter community structure, yet, we conclude that the high dispersal of marine planktonic microbes erases the legacy of past environmental conditions, thereby decreasing the tempo of community evolution.
Viruses are recognised as the most abundant biological entities on the planet. In addition to their role in disease, they are crucial components of co-evolutionary processes, are instrumental in global biogeochemical pathways such as carbon fluxes and nutrient recycling, and in some cases
act regionally on climate processes. Importantly, viruses harbour an enormous, as of yet unexplored genetic and metabolic potential. Some viruses infecting microalgae harbour hundreds of genes, including genes involved in cellular metabolic pathways. Collectively, these attributes have given rise to new fields of research: environmental virology and viral ecology. While traditionally the potential of viruses was recognised by isolating novel viruses into culture and subsequent sequencing of their genomes in the laboratory, advancements in next-generation sequencing technologies now allow for direct sequencing of viral genomes from their natural setting, bypassing
the need for culturing. Nevertheless, the lack of associated biological reference material with most of these novel environmental genomes is problematic as there are limitations to what can be achieved with sequence data alone. Where aquatic viruses do exist in culture, they are most often kept privately within research institutes and are not available to the wider research community. Many are thus at risk of being lost because research teams rarely have secure long term resources to ensure continued propagation. Culture collections do exist for medically and agriculturally important viruses causing disease, but collections focusing on viruses infecting aquatic algae
and bacteria are non-existent. We therefore highlight here the need for a centralised depository for aquatic viruses and present arguments indicating the benefits such a collection would have for the scientific community of environmental virologists.
The Atlantic Meridional Transect (AMT) series of twenty-five cruises over the past twenty years has produced a rich depth-resolved biogeochemical in situ data resource consisting of a wealth of core variables. These multiple core datasets, key to the operation of AMT, such as temperature, salinity, oxygen and inorganic nutrients, are often only used as ancillary measurements for contextualising hypothesis-driven process studies. In this paper these core in situ variables, alongside data drawn from satellite Earth Observation (EO) and modelling, have been analysed to determine characteristic oceanic province variations encountered over the last twenty years on the AMT through the Atlantic Ocean. The EO and modelling analysis shows the variations of key environmental variables in each province, such as surface currents, the net heat flux and subsequent large scale biological responses, such as primary production. The in situ core dataset analysis allows the variation in features such as the tropical oxygen minimum zone to be quantified as well as showing clear contrasts between the provinces in nutrient stoichiometry. Such observations and relationships can be used within basin scale biogeochemical models to set realistic variation ranges.
Oceans are a net source of molecular hydrogen (H2) to the atmosphere. The production of marine H2 is assumed
to be mainly biological by N2 fixation, but photochemical pathways are also discussed. We present measurements of mole fraction and isotopic composition of dissolved and atmospheric H2 from the southern and northern Atlantic between 2008 and 2010. In total almost 400 samples were taken during 5 cruises along a transect between Punta Arenas (Chile) and Bremerhaven (Germany), as well as at the
coast of Mauritania.
The isotopic source signatures of dissolved H2 extracted from surface water are highly deuterium-depleted and correlate negatively with temperature, showing �D values of (-629.�54) ‰ for water temperatures at(2.7�3)° �C and
(-249.�88)‰ below (19.�1)° �C. The results for warmerwater masses are consistent with the biological production of
H2. This is the first time that marine H2 excess has been directly attributed to biological production by isotope measurements. However, the isotope values obtained in the colder water masses indicate that beside possible biological production, a significant different source should be considered. The atmospheric measurements show distinct differences between both hemispheres as well as between seasons. Results from the global chemistry transport model TM5 reproduce the measured H2 mole fractions and isotopic composition
well. The climatological global oceanic emissions from the GEMS database are in line with our data and previously published flux calculations. The good agreement between measurements and model results demonstrates that both the magnitude and the isotopic signature of the main components of the marine H2 cycle are in general adequately represented in current atmospheric models despite a proposed source different from biological production or a substantial underestimation of nitrogen fixation by several authors.
Oceans are a net source of molecular hydrogen (H2) to the atmosphere. The production of marine H2 is assumed to be mainly biological by N2 fixation, but photochemical pathways are also discussed. We present measurements of mole fraction and isotopic composition of dissolved and atmospheric H2 from the southern and northern Atlantic between 2008 and 2010. In total almost 400 samples were taken during five cruises along a transect between Punta Arenas (Chile) and Bremerhaven (Germany), as well as at the coast of Mauretania. The isotopic source signatures of dissolved H2 extracted from surface water are highly deuterium-depleted and correlate negatively with temperature, showing δD values of (−629 ± 54) ‰ for water temperatures at (27 ± 3) °C and (−249 ± 88) ‰ below (19 ± 1) °C. The results for warmer water masses are consistent with biological production of H2. This is the first time that marine H2 excess has been directly attributed to biological production by isotope measurements. However, the isotope values obtained in the colder water masses indicate that beside possible biological production a significant different source should be considered. The atmospheric measurements show distinct differences between both hemispheres as well as between seasons. Results from the global chemistry transport model TM5 reproduce the measured H2 mole fractions and isotopic composition well. The climatological global oceanic emissions from the GEMS database are in line with our data and previously published flux calculations. The good agreement between measurements and model results demonstrates that both the magnitude and the isotopic signature of the main components of the marine H2 cycle are in general adequately represented in current atmospheric models despite a proposed source different from biological production or a substantial underestimation of nitrogen fixation by several authors.
The AMT (www.amt-uk.org) is a multidisciplinary programme which undertakes biological, chemical, and physical oceanographic research during an annual voyage between the UK and a destination in the South Atlantic such as the Falkland Islands, South Africa, or Chile. This transect of >12,000 km crosses a range of ecosystems from subpolar to tropical, from euphotic shelf seas and upwelling systems, to oligotrophic mid-ocean gyres. The year 2015 has seen two milestones in the history of the AMT: the achievement of 20 years of this unique ocean going programme and the departure of the 25th cruise on the 15th of September. Both of these events were celebrated in June this year with an open science conference hosted by the Plymouth Marine Laboratory (PML) and will be further documented in a special issue of Progress in Oceanography which is planned for publication in 2016. Since 1995, the 25 research cruises have involved 242 sea-going scientists from 66 institutes representing 22 countries. AMT was designed from the outset to be a collaborative programme. It was originally conceived by Jim Aiken, Patrick Holligan, Roger Harris, and Dave Robins with Chuck McClain and Chuck Trees at NASA to test and ground truth satellite algorithms of ocean color. The opportunities offered by this initiative meant that this series of repeated biannual cruises rapidly developed into a coordinated study of ocean biodiversity, biogeochemistry, and ocean/atmosphere interactions.
The factors regulating phytoplankton community composition play a crucial role in structuring aquatic food webs. However, consensus is still lacking about the mechanisms underlying the observed biogeographical differences in cell size composition of phytoplankton communities. Here we use a trait-based model to disentangle these mechanisms in two contrasting regions of the Atlantic Ocean. In our model, the phytoplankton community can self-assemble based on a trade-off emerging from relationships between cell size and (1) nutrient uptake, (2) zooplankton grazing, and (3) phytoplankton sinking. Grazing 'pushes' the community towards larger cell sizes, whereas nutrient uptake and sinking 'pull' the community towards smaller cell sizes. We find that the stable environmental conditions of the tropics strongly balance these forces leading to persistently small cell sizes and reduced size diversity. In contrast, the seasonality of the temperate region causes the community to regularly reorganize via shifts in species composition and to exhibit, on average, bigger cell sizes and higher size diversity than in the tropics. Our results raise the importance of environmental variability as a key structuring mechanism of plankton communities in the ocean and call for a reassessment of the current understanding of phytoplankton diversity patterns across latitudinal gradients.
Approximately a quarter of the carbon dioxide (CO2) that we emit into the atmosphere is absorbed by the ocean. This oceanic uptake of CO2 leads to a change in marine carbonate chemistry resulting in a decrease of seawater pH and carbonate ion concentration, a process commonly called 'Ocean Acidification'. Salinity data are key for assessing the marine carbonate system, and new space-based salinity measurements will enable the development of novel space-based ocean acidification assessment. Recent studies have highlighted the need to develop new in situ technology for monitoring ocean acidification, but the potential capabilities of space-based measurements remain largely untapped. Routine measurements from space can provide quasi-synoptic, reproducible data for investigating processes on global scales; they may also be the most efficient way to monitor the ocean surface. As the carbon cycle is dominantly controlled by the balance between the biological and solubility carbon pumps, innovative methods to exploit existing satellite sea surface temperature and ocean color, and new satellite sea surface salinity measurements, are needed and will enable frequent assessment of ocean acidification parameters over large spatial scales.
Significance
Low concentrations of fixed nitrogen restrict phytoplankton growth in much of the low-latitude surface oceans. Diazotrophic cyanobacteria are capable of fixing atmospheric dinitrogen, thereby replenishing the overall pool of fixed nitrogen. As a consequence, spatial–temporal variability in diazotrophy can potentially influence the global nitrogen cycle. Here we show that movement in the region of elevated iron concentrations tied to the seasonal migration of the intertropical convergence zone drives a shift in the latitudinal distribution of dinitrogen fixation and corresponding phosphate depletion in surface waters. Surface nutrient concentrations and diazotrophic activity divide the (sub)tropical Atlantic into a high-phosphate, low-iron system in the south, and a low-phosphate, high-iron system in the north.
Aim
Develop a biogeographical classification of phytoplankton size distributions for the Atlantic Ocean and predict the global phytoplankton size composition based on prevailing environmental conditions.
Location Atlantic Ocean and Global Ocean
Methods
Using phytoplankton size composition data, nutrient concentrations (nitrite+nitrate, phosphate, and silicate), irradiance, temperature and zooplankton abundances of the Atlantic Meridional Transect programme, we derived and tested an environmental classification method of phytoplankton size distribution with a k-means clustering technique. We then used principal component and Dirichlet multivariate regression analyses to disentangle the relative influence of different environmental conditions on the phytoplankton size composition. Subsequently, we evaluated different probabilisitic models and selected the most parsimonious one to estimate the global phytoplankton size distributions in the world oceans based on global climatology data of the World Ocean Atlas 2009.
Results
Based only on prevailing environmental conditions and without a priori knowledge concerning, for example, the position of oceanic fronts, the primary productivity, the distribution of organisms or any geographical information, our classification method captures the size structures of phytoplankton communities across the Atlantic. We find a strong influence of temperature and nitrite+nitrate concentration on the prevalence of the different size classes, and we present evi- dence that both factors may act independently on structuring phytoplankton com- munities. While at low nitrite+nitrate concentrations temperature has a major structuring impact, at high nitrite+nitrate concentrations its influence is reduced. Finally, we show that the global distribution of phytoplankton community size structure can be predicted by a probabilistic model based only on temperature and nitrite+nitrate.
Main conclusion
The global distribution of phytoplankton community size structure can be predicted with good approximation using a parsimonious proba- bilistic model forced by only temperature and nitrite+nitrate data.
The uptake rates and concentrations of nitrate and ammonium were determined during three research cruises in the Atlantic Ocean: At 20∘W between 60∘N and 37∘N during June and July 1996, between the UK and Falkland Islands (48∘N–50∘S) during September and October 1997, and between South Africa and the UK (33∘S–48∘N) during May and June 1998. Euphotic zone concentrations of NO3- varied between 0.005μmoll-1 in the northern and southern gyres to a maximum of 33μmoll-1 within the Benguela upwelling system. At 70% of stations occupied in oligotrophic conditions (nominally defined as where NO3-0.05μmoll-1), surface elevations of NO3- were attributed to the photoinhibition of uptake mechanisms. NH4+ concentrations were much less variable (0.109±0.150μmoll-1) than NO3-, and showed the same general trend of minima within the gyres and maximum concentration (of 5.2μmoll-1) in the Benguela upwelling system. The microbial uptake of NO3- was significantly correlated to NO3- and chlorophyll concentrations, whilst NH4+ uptake was less dependent on NH4+ concentration and showed no association with other environmental variables. Variability in the nitrogen uptake:concentration ratio between oceanographic provinces was associated with physical conditions including the mixed-layer depth and the rate of diapycnal mixing; therefore, uptake rates cannot be predicted simply from the nutrient concentration. In contrast, the relationship between f-ratio (where the NO3- source is undefined, and may have both new and regenerated components) and NO3- concentration is defined robustly, so that the f-ratio can be predicted (within 95%) by either surface or depth integrated concentrations of NO3-.
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.
Temporal changes of prokaryoplankton in three different provinces of the Atlantic Ocean were examined between 1996 and 2004. The abundance and integrated biomass of three prokaryote groups (Prochlorococcus spp., Synechococcus spp. and other prokaryoplankton) were used to detect standing stock changes in the northern and southern oligotrophic gyres and in the equatorial region. Mean cell concentrations (±standard error of the mean) of Prochlorococcus spp., Synechococcus spp. and other prokaryoplankton above the nitracline in the northern oligotrophic gyre were 1.2×105 (±0.08), 5.0×103 (±1.22) and 0.9×106 (±0.03) cells mL?1, respectively. Similar concentrations of 1.2×105 (±0.06) Prochlorococcus mL?1, 1.9×103 (±0.29) Synechococcus mL?1 and 0.7×106 (±0.03) other prokaryoplankton mL?1 were measured in the southern oligotrophic gyre, with higher concentrations of all prokaryote groups in equatorial waters. Integrated biomass (±standard error of the mean) of Prochlorococcus spp. above the nitracline was 173 (±21) mg C m?2 in the northern oligotrophic gyre, 190 (±14) mg C m?2 in the southern oligotrophic gyre and 141 (±15) mg C m?2 in the equatorial region. Synechococcus spp. biomass was lower in each of the three provinces (18 (±2), 17 (±4) and 32 (±5) mg C m?2, respectively). The data showed no statistically significant inter-annual variability in Prochlorococcus or Synechococcus abundance or integrated biomass above the nitracline in any of the provinces. The abundance and biomass of the remaining prokaryoplankton were variable, but these variations could not be ascribed to seasonal differences and did not follow a clear inter-annual trend. In light of results presented here, recommendations on the frequency and spatial resolution of sampling needed to characterise province-scale temporal variability of prokaryoplankton communities have been suggested.
The Atlantic Meridional Transect (AMT) Programme (1995-2006) [http://www.pml.ac.uk/amt/index.html] aims to quantify the nature and causes of ecological and biogeochemical variability in the planktonic ecosystems of the tropical and temperate Atlantic Ocean, and the effects of this variability on biological carbon cycling and air-sea exchange of radiatively active gases and aerosols. Marine and atmospheric data are collected from more than 14 biogeochemical provinces during the bi-annual passage of RRS James Clark Ross between the UK (50 oN) and the Falkland Islands (52 oS) including the undersampled subtropical gyres and the NW African upwelling. Eleven cruises have taken place since 1995 involving 80 scientists from 10 countries collecting measurements of 31 determinands. These data represent the most coherent set of repeated biogeochemical observations over ocean basin scales and have led to several important discoveries concerning the validation of ocean colour algorithms, distributions of picoplankton, and variability in rates of primary production and respi- ration. Six further cruises are planned for 2003 to 2005 with the specific objectives to 1) de- termine how the structure, functional properties and trophic status of the major plank- tonic ecosystems vary in space and time, 2) determine the role of physical processes in controlling the rates of nutrient supply, including DOM to the planktonic ecosystem and 3) determine the role of atmosphere-ocean exchange and photo-degradation in the formation and fate of organic matter. This presentation will summarise the results obtained thus far and describe the ratio- nale and methods relating to the future hypotheses to be tested.
Recent experimental data from the subtropical NE Atlantic appear to support the prevalence of net heterotrophy in unproductive pelagic ecosystems. However, the proximity of these studies to the NW African upwelling does not exclude the possibility that remote areas of the oceanic gyres are in metabolic balance. Here we present measurements of plankton gross photosynthesis (GPP) and community respiration (CR) made during a latitudinal transect (Atlantic Meridional Transect (AMT) cruise 11, 491N-331S) that traversed five biogeochemical provinces of the Atlantic Ocean, including both the subtropical NE Atlantic and the central part of the South Atlantic subtropical gyre. In these oligotrophic provinces, the euphotic zone average chlorophyll a concentration (0.1570.01 and 0.1670.02 mg m� 3, respectively), the relative contribution of picoplankton (%Chlao2m m7 571 and 7671%, respectively), and GPP (0.4170.12 and 0.4770.09 mmol O2 m � 3 d� 1, respectively) were almost identical. However, net heterotrophy prevailed in the euphotic zone of the tropical NE Atlantic (� 0.2870.13 mmol O2 m � 3 d� 1, n ¼ 3), while the net metabolism of the plankton community in the central S Atlantic gyre was autotrophic (0.2070.02 mmol O2 m � 3 d� 1, n ¼ 5). These results therefore suggest the existence of more than one type of unproductive open-ocean situation, that may be characterised by differences in the relative importance of local vs. allochthonous sources of organic matter. r 2006 Elsevier Ltd. All rights reserved.
The aims of the Atlantic Meridional Transect (AMT) programme [www.amt-uk.org] are to quantify the nature and
causes of ecological and biogeochemical variability in the planktonic ecosystems of the Atlantic Ocean, and to assess the effects of this variability on biological carbon cycling and air–sea exchange of radiatively active gases and aerosols. Marine and atmospheric data have been collected twice a year along a 13,500km transect in the Atlantic Ocean between 501N and 521S since 1995. The cruise track enables biogeochemical measurements to be made within the poorly studied North and South Atlantic oligotrophic gyres as well as within equatorial and coastal upwelling regions. The range of ecosystems sampled during AMT has facilitated the calibration and validation of newly developed optical, microbiological, molecular and analytical techniques and provided a testbed for comparative ecology and the development of atmospheric and oceanographic models. This paper describes the rationale and methodology of the programme. Upper-ocean measurements of density, nitrate and chlorophyll a (Chl a) are presented to illustrate seasonal, inter-annual and decadal
variability in hydrography. Seasonal distributions of dissolved nitrous oxide (N2O) and methane (CH4) are used to derive estimates of the sea–air flux of these gases in the South Atlantic Gyre. Observations made during AMT and published since 2000 are reviewed, and the key findings are highlighted. The extent to which the programme aims have been achieved is discussed and improvements for the future suggested.
The latitudinal distributions of picoeukaryote phytoplankton (PEUK), coccolithophores (COCCO), cryptophytes (CRYPTO) and other nanoeukaryote phytoplankton (NEUK) were studied in the Atlantic Ocean between 49°N and 46°S in September–October 2003 and April–June 2004 by flow cytometry. Phytoplankton abundance and carbon (C) biomass varied considerably with latitude and down through the water column. Abundance and C biomass of all eukaryotic groups studied were highest in North and South Atlantic temperate waters and in the Mauritanian Upwelling off the west coast of Africa, where the total C biomass of eukaryotic phytoplankton smaller than 10 μm reached almost 150 mg C m−3. Phytoplankton in the Equatorial Upwelling region was concentrated well below the surface at 50–80 m, with total C biomass in this layer being approximately 4 times that in the mixed layer. The North and South Atlantic Gyres supported much lower eukaryotic phytoplankton C biomass, with total eukaryote C biomass only reaching 2–3 mg C m−3, peaking below 100 m. Of the four eukaryote groups studied, the PEUK were the most abundant, reaching densities of up to 40,000 cells cm−3. They often contributed between 25% and 60% of total C biomass, particularly in the deep chlorophyll maxima of the different oceanic regions and also in the South Atlantic temperate waters, both in austral spring and autumn. NEUK also contributed significantly to C biomass. They generally dominated in the mixed layer, where they contributed 65–85% of total C biomass in the subtropical gyres and in North Atlantic temperate waters. CRYPTO and COCCO were generally less abundant. CRYPTO attained highest abundance in the Southern Temperate waters of over 500 cells cm−3 on both cruises. COCCO were often undetectable but on the European continental shelf abundance reached up to 2600 cells cm−3 during AMT 14. The C biomass standing stock of eukaryotic phytoplankton (<10 μm) for the Atlantic Ocean as a whole was estimated to be 80 million tonnes C during AMT 13, approximately one-third of total phytoplankton C biomass in the Atlantic Ocean.
Primary production (PP) was determined using 14C uptake at 117 stations in the Atlantic Ocean to validate three PP satellite algorithms of varying complexity. An empirical satellite algorithm based on log chlorophyll-a had the highest bias and root-mean square error compared with measured 14C PP and tended to under-estimate PP. The vertical generalised production model improved PP estimates and was the most accurate algorithm in the Eastern Tropical Atlantic (ETRA) and Western Tropical Atlantic (WTRA), but tended to over-estimate PP in eutrophic provinces. A photosynthesis-light wavelength-resolved model was the most accurate over the Atlantic basin, having the lowest mean log-difference error, root-mean square error and bias, and exhibited a superior performance in six out of the nine ecological provinces surveyed. Using this algorithm and mean monthly SeaWiFS fields, a PP time series was generated for the Atlantic Ocean from 1998 to 2005 which was compared with Advanced Very High Resolution Radiometer (AVHRR) sea-surface temperature (SST) data. There was a significant negative correlation between SST and PP in the North Atlantic Subtropical Gyre Province (NAST), North Atlantic Tropical Gyre (NATR), and WTRA suggesting that recent warming trends in these provinces are coupled with a decrease in phytoplankton production.
The size distribution of the pelagic community has the potential to compare ecosystems with different species composition as well as to identify the main functional properties of the system. Plankton size spectra are an effective approach of summarising the size structure of the community and have the potential to indicate the transfer of energy up the trophic food web. However, data on which open ocean plankton biomass-size structure can be constructed are scarce. Here we present the latitudinal distribution in the Atlantic Ocean of normalised biomass-size spectra (NB-S), plankton biomass and abundance, as well as mean zooplankton size between 70°N and 50°S. Samples were collected from three Atlantic Meridional Transect (AMT) cruises during May/June 2003, September/October 2003 and April/June 2004, as well as from a Marine Productivity (MarProd) cruise farther north in the Irminger Sea during spring 2002. The biomass-size distribution covered a body size range from nano- to mesoplankton and was based on a depth range of 50–0 m on the AMT and 120–0 m on the Irminger Sea cruise. The distribution of normalised biomass versus size was linear on a double log plot at all of the 95 stations. The slopes of the NB-S spectra ranged from −0.93 to −1.26 and −1.12 to −1.46 on the AMT and Irminger Sea cruises, respectively. A “dome-shaped” pattern in the slopes of community size spectra was observed in the Atlantic, indicating a decrease in the trophic transfer efficiency of energy with increasing latitude and phytoplankton biomass. Mesozooplankton biomass, abundance, and mean size followed a distribution similar to ecosystem productivity.
Euphotic zone plankton production (P) and respiration (R) were determined from the in vitro flux of dissolved oxygen during six latitudinal transects of the Atlantic Ocean, as part of the Atlantic Meridional Transect (AMT) programme. The transects traversed the North and South Atlantic Subtropical Gyres (N gyre, 18–38°N; S gyre, 11–35°S) in April–June and September–November 2003–2005. The route and timing of the cruises enabled the assessment of the seasonal variability of P, R and P/R in the N and S gyres, and the comparison of the previously unsampled N gyre centre with the more frequently sampled eastern edge of the gyre. Mean euphotic zone integrated rates (±SE) were P=63±23 (n=31), R=69±22 (n=30) mmol O2 m−2 d−1 in the N gyre; and P=58±26 (n=30), R=62±24 (n=30) mmol O2 m−2 d−1 in the S gyre. Overall, the N gyre was heterotrophic (R>P) and it was more heterotrophic than the S gyre, but the metabolic balance of both gyres changed with season. Both gyres were net heterotrophic in autumn, and balanced in spring. This seasonal contrast was most pronounced for the S gyre, because it was more autotrophic than the N gyre during spring. This may have arisen from differences in nitrate availability, because spring sampling in the S gyre coincided with periods of deep mixing to the nitracline, more frequently than spring sampling within the N gyre. Our results indicate that the N gyre is less heterotrophic than previous estimates suggested, and that there is an apparent decrease in R from the eastern edge to the centre of the N gyre, possibly indicative of an allochthonous organic carbon source to the east of the gyre.
We present, test and implement two contrasting models to predict euphotic zone net community production (NCP), which are based on 14C primary production (PO14CP) to NCP relationships over two latitudinal (ca. 30°S–45°N) transects traversing highly productive and oligotrophic provinces of the Atlantic Ocean (NADR, CNRY, BENG, NAST-E, ETRA and SATL, Longhurst et al., 1995 [An estimation of global primary production in the ocean from satellite radiometer data. Journal of Plankton Research 17, 1245–1271]). The two models include similar ranges of PO14CP and community structure, but differ in the relative influence of allochthonous organic matter in the oligotrophic provinces. Both models were used to predict NCP from PO14CP measurements obtained during 11 local and three seasonal studies in the Atlantic, Pacific and Indian Oceans, and from satellite-derived estimates of PO14CP. Comparison of these NCP predictions with concurrent in situ measurements and geochemical estimates of NCP showed that geographic and annual patterns of NCP can only be predicted when the relative trophic importance of local vs. distant processes is similar in both modeled and predicted ecosystems. The system-dependent ability of our models to predict NCP seasonality suggests that trophic-level dynamics are stronger than differences in hydrodynamic regime, taxonomic composition and phytoplankton growth. The regional differences in the predictive power of both models confirm the existence of biogeographic differences in the scale of trophic dynamics, which impede the use of a single generalized equation to estimate global marine plankton NCP.This paper shows the potential of a systematic empirical approach to predict plankton NCP from local and satellite-derived P estimates.
Mesozooplankton and 63–200 μm net-collected microzooplankton grazing on phytoplankton and protozoans was evaluated by 24-h incubations on a latitudinal transect in the Atlantic Ocean, from 35°N to 38°S (AMT-15; September–October 2004). The sampling area comprised contrasting ecosystems, including upwelling zones and oligotrophic subtropical gyres. Grazing impacts of mesozooplankton and 63–200 μm microzooplankton on total chlorophyll a (Chl a), >5 μm Chl a, ciliates, and dinoflagellates were low for both zooplankton size fractions, always removing<1.5% of the standing stocks of these groups. Grazing had a slightly greater impact upon primary production (up to 10% of primary production consumed daily), although on most occasions grazing removed<1% of primary production per day. To account for the reduction of micrograzers by predators in the experimental bottles and the consequent reduction of grazing pressure, the data were corrected with knowledge on the decrease of microzooplankton during incubations and global estimates of microzooplankton grazing. The corrected grazing rates for mesozooplankton ranged from 4% to 28% of the primary production consumed daily, and from 1% to 2% of the standing stock of Chl a removed every day. The 63–200 μm microzooplankton corrected grazing impact was always<5% of the primary production and standing stock consumed per day. The corrected grazing activity of 63–200 μm microzooplankton and mesozooplankton rendered daily rations ranging from 3% to 38% of the body carbon consumed daily, not sufficient for basal metabolism in most of the areas studied. Finally, the data on mesozooplankton grazing on primary production confirm the recent hypothesis of a decline of the relative importance of mesozooplankton grazing on primary producers with increasing primary production [Calbet, A., 2001. Mesozooplankton grazing effect on primary production: a global comparative analysis in marine ecosystems. Limnology and Oceanography 46, 1824–1830].
Measurements of the climate-cooling trace gas dimethylsulphide (DMS) and other ancillary data, including pigments, nutrient concentrations and the depth of the mixed layer, were made over a wide latitude range during the UK Atlantic Meridional Transect (AMT) programme. The data were used to test algorithms from the recent literature for their effectiveness at predicting surface DMS concentrations. Dividing the data by research cruise (i.e. year and season) and into biogeochemical provinces aided data interpretation. For this new data set, many of the available algorithms over-predicted the measured DMS concentration. The best fit was found with a dilution model based on the depth of the mixed layer [Aranami, K., Tsunogai, S., 2004. Seasonal and regional comparison of oceanic and atmospheric dimethylsulfide in the northern North Pacific: dilution effects on its concentration during winter. Journal of Geophysical Research 109D (12), art. no. D12303], but there are still parts of the data that cannot be explained. We suggest that in order to test, improve and refine current predictive models, further data for DMS and related compounds are needed for remote oligotrophic regions.
The atmosphere can be an important source of nutrients to remote ocean waters, e.g., in supplying iron to nitrogen fixers in the tropical North Atlantic. We use results obtained from aerosol collection during four meridional transect cruises of the Atlantic Ocean in an attempt to identify sources of soluble nutrient species (Fe, N, P and Si) to the Atlantic atmosphere. The Sahara desert was the dominant source of soluble aerosol Fe and Si and also a significant source of aerosol P. Biomass burning and another source, possibly primary particles derived from vegetation, also contribute to the aerosol P loading. Industrial (NOx) and agricultural (ammonium) sources contribute to high nitrogen concentrations in the northern hemisphere, while biomass burning appears to be a significant seasonal source of N to the southern hemisphere. This work is an initial step in our aim to describe the atmospheric nutrient inputs to the Atlantic basin using a much larger set of aerosol and rain samples that will be obtained during the Atlantic Meridional Transect (AMT) project and other field campaigns.
Subtropical ocean gyres are believed to be characterized by low carbon export from the surface into the deep ocean. However, due to their large areas, even relatively small average export could be of significance for the global carbon cycle. To better constrain carbon export from the surface ocean in such regions, radioactive disequilibria between the particle-reactive, short-lived radionuclide 234Th (half-life 24.1 d) and its parent 238U were used to estimate fluxes of 234Th and particulate organic carbon (POC) from surface waters of the North and South Atlantic subtropical gyres and their fringes. Samples were collected between ∼50°S and ∼50°N as part of the Atlantic Meridional Transect (AMT) programme during April/May 2004 (AMT14). Application of a steady-state model to the 234Th data revealed particle export from the surface (234Th deficit) and, in one instance, some evidence for shallow particle remineralisation at depth (234Th excess). Export fluxes of POC were calculated from water column 234Th /238U disequilibria and the POC to 234Th ratios on large rapidly sinking particles (>50 μm). Based on latitudinal distributions of selected hydrographic and biological parameters within the topmost 300 m of the water column, the transect was divided into six regions: ‘temperate’ (35°–50°N and 35°–50°S), ‘oligotrophic’ (20°–35°N and 5°–35°S), ‘equatorial’ (5°S–5°N), and ‘upwelling’ (5°–20°N). The lowest 234Th-derived POC export fluxes were found in the oligotrophic gyres and ranged from 0 in the northern to 6 mmol C m−2 d−1 in the southern oligotrophic, indicating a tightly coupled food web. Enhanced POC export was associated with the equatorial region (25 mmol C m−2 d−1) and the upwelling region north of the equator (15 mmol C m−2 d−1). POC export in the temperate regions ranged from 7 mmol C m−2 d−1 to a maximum of 41 mmol C m−2 d−1. High fluxes at the poleward edges of the oligotrophic gyres probably result from episodic nutrient-loading processes associated with submesoscale features. Estimates of instantaneous primary production (PP) were compared with 234Th-derived POC export, the latter bearing information from the past few weeks. Most export efficiencies that were calculated based on this comparison were high (10 s%), suggesting uncoupling of PP and export estimates due to the different time scales of the approaches. Moreover, this uncoupling points to the occurrence of pulsed high-export events that could be easily missed by instantaneous sampling but traced by temporally quasi-integrating tracers such as 234Th. Results from this study suggest that although carbon export in the oligotrophic centres of the gyres may be low, carbon sequestration in the temperate fringes of the gyres as well as in the equatorial and upwelling regions can be substantial, and that spatio–temporal variability in these areas of the world's oceans needs to be considered more fully in the context of global oceanic carbon sequestration.
Sunlight-initiated photolysis of chromophoric dissolved organic matter (CDOM) is the dominant source of carbon monoxide (CO) in the open-ocean. A modelling study was conducted to constrain this source. Spectral solar irradiance was obtained from two models (GCSOLAR and SMARTS2). Water-column CDOM and total light absorption were modelled using spectra collected along a Meridional transect of the Atlantic ocean using a 200-cm pathlength liquid waveguide UV-visible spectrophotometer. Apparent quantum yields for the production of CO (AQYCO) from CDOM were obtained from a parameterisation describing the relationship between CDOM light absorption coefficient and AQYCO and the CDOM spectra collected. The sensitivity of predicted rates to variations in model parameters (solar irradiance, cloud cover, surface-water reflectance, CDOM and whole water light absorbance, and AQYCO) was assessed. The model's best estimate of open-ocean CO photoproduction was 47±7 Tg CO-C yr−1, with lower and upper limits of 38 and 84 Tg CO-C yr−1, as indicated by sensitivity analysis considering variations in AQYs, CDOM absorbance, and spectral irradiance. These results represent significant constraint of open-ocean CO photoproduction at the lower limit of previous estimates. Based on these results, and their extrapolation to total photochemical organic carbon mineralisation, we recommend a downsizing of the role of photochemistry in the open-ocean carbon cycle.
Carbon monoxide (CO) atmospheric mixing ratios and surface-water concentrations were determined during Atlantic Meridional Transect cruise number 10, April–May 2000. Atmospheric CO increased from south (mean=74±9 ppbv) to north (mean=151±19 ppbv) with a steep increase around the intertropical convergence zone. Surface-water CO (0.2–2.6 nmol L−1) showed pronounced diurnal variations with afternoon maxima exceeding pre-dawn minima 5–7 fold. Modest regional variations, as indicated by maximum daily CO concentrations, were also observed. Highest CO maxima occurred at ∼11.5°N, where high solar irradiance was combined with elevated coloured dissolved organic matter (CDOM) levels and modest winds, while lowest CO maxima occurred during periods of high winds and lowest solar irradiance near the western European margin at 45°N. Atlantic Ocean CO emissions were estimated to be 1.5±1.1 Tg CO-C yr−1 based on near-instantaneous atmospheric CO, sea-surface CO and windspeeds from the cruise. However, as spatial and temporal variability in both terms was considered to be unique to the timing and path of the cruise, the mean Atlantic diel cycle of sea-surface CO concentration was estimated by pooling all cruise data into 1-h sections, yielding a mean of 0.94 nmol L−1; and diurnal variations from 0.4 to 1.6 nmol L−1. Using the mean diurnal cycle, the Atlantic and global open-ocean sources of CO to the atmosphere were estimated to be 0.9±0.6 and 3.7±2.6 Tg CO-C yr−1, respectively. Therefore it is our contention that IPCC-2001 (Prather, M., Ehhalt, D., Dentener, F., Derwent, R., Dlugokencky, E., Holland, E., Isaksen, I., Katima, J., Kirchhoff, V., Matson, P., Midgley, P., Wang, M., 2001. Chapter 4: Atmospheric chemistry and greenhouse gases. In: Houghton, J.T., Ding, Y., Griggs, D.J., Noguer, M., van der Linden, P.J., Dai, X., Maskell, K., Johnson, C.A. (Eds.), Climate Change 2001: The Scientific Basis. Contribution of working group 1 to the third assessment report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, pp. 239–287; 21 Tg CO-C yr−1) overestimates the source of atmospheric CO from the global ocean by around 5 fold.
We report over 600 absorption spectra (250–800 nm) of unfiltered surface waters, <250 m depth, collected at daily stations during three Atlantic Meridional Transect cruises (AMT9, 10, 11) between the UK and Uruguay. AMT cruises 9 and 11 were southbound (15 September 1999–13 October 1999 and 12 September 2000–11 October 2000, respectively), while AMT10 was northbound (12 April 2000–8 May 2000). Absorption coefficients at 300 nm, a300, ranged from 0.13 to 1.32 m−1, showed insignificant differences between filtered and unfiltered samples, and were therefore attributed to chromophoric dissolved organic matter (CDOM). A non-linear single exponential regression provided the best fit to our CDOM absorbance spectra and was used to parameterise the spectral slope, S, of the monotonic absorbance decrease with increasing wavelength over the wavelength ranges 290–350 nm (S290–350) and 250–650 nm (S250–650). We observed distinct patterns in the latitudinal and depth distribution of CDOM absorbance characteristics. Distinct subsurface a300 maxima, characterised by lowest spectral slope values (S290–350=0.010 nm−1 and S250–650=0.014 nm−1) were observed in the vicinity of the deep chlorophyll maximum (DCM) in open ocean and upwelling regions, and indicated in-situ production of CDOM. Converserly, the surface a300 minima and surface S290–350 and S250–650 maxima in these regions were attributed to CDOM photo-oxidation. In order to assess the nature of the observed CDOM variability along our AMT transects, we grouped our data into 12 individual, oceanographic provinces, and further into two seasons (spring: April–May, and Autumn: September–October) and three depth zones (surface mixed layer, pycnocline, and below pycnocline). Comparisons between individual provinces, seasons and depth zones indicated that CDOM variability was dominated by regional factors and depth distribution patterns, while seasonal variability was generally less important in our data. Based on depth distribution patterns together with analyses of inter-relations between a300, and S290–350, S250–650 we propose that our data reflect the presence of two CDOM end-members, each characterised by distinct spectral slope factors, S290–350 and S250–650. The first CDOM end-member (S290–350=0.010 and S250–650=0.014 nm−1) was situated in the vicinity of the DCM, and was attributed to CDOM production from phytoplankton-derived organic matter via planktonic foodweb interactions. The second end-member (S290–350=0.028 nm−1 and S250–650=0.029 nm−1) was attributed to microbial CDOM production. We propose that the CDOM distribution along the AMT cruise track is controlled by autochthonous production near the DCM and subsequent photo-oxidation in surface waters.
Temporal changes of prokaryoplankton in three different provinces of the Atlantic Ocean were examined between 1996 and 2004. The abundance and integrated biomass of three prokaryote groups (Prochlorococcus spp., Synechococcus spp. and other prokaryoplankton) were used to detect standing stock changes in the northern and southern oligotrophic gyres and in the equatorial region. Mean cell concentrations (±standard error of the mean) of Prochlorococcus spp., Synechococcus spp. and other prokaryoplankton above the nitracline in the northern oligotrophic gyre were 1.2×105 (±0.08), 5.0×103 (±1.22) and 0.9×106 (±0.03) cells mL−1, respectively. Similar concentrations of 1.2×105 (±0.06) Prochlorococcus mL−1, 1.9×103 (±0.29) Synechococcus mL−1 and 0.7×106 (±0.03) other prokaryoplankton mL−1 were measured in the southern oligotrophic gyre, with higher concentrations of all prokaryote groups in equatorial waters. Integrated biomass (±standard error of the mean) of Prochlorococcus spp. above the nitracline was 173 (±21) mg C m−2 in the northern oligotrophic gyre, 190 (±14) mg C m−2 in the southern oligotrophic gyre and 141 (±15) mg C m−2 in the equatorial region. Synechococcus spp. biomass was lower in each of the three provinces (18 (±2), 17 (±4) and 32 (±5) mg C m−2, respectively). The data showed no statistically significant inter-annual variability in Prochlorococcus or Synechococcus abundance or integrated biomass above the nitracline in any of the provinces. The abundance and biomass of the remaining prokaryoplankton were variable, but these variations could not be ascribed to seasonal differences and did not follow a clear inter-annual trend. In light of results presented here, recommendations on the frequency and spatial resolution of sampling needed to characterise province-scale temporal variability of prokaryoplankton communities have been suggested.
We have made daily measurements of phytoplankton pigments, size-fractionated (<2 and >2-μm) carbon fixation and chlorophyll-a concentration during four Atlantic Meridional Transect (AMT) cruises in 2003–04. Surface rates of carbon fixation ranged from <0.2-mmol C m−3 d−1 in the subtropical gyres to 0.2–0.5-mmol C m−3 d−1 in the tropical equatorial Atlantic. Significant intercruise variability was restricted to the subtropical gyres, with higher chlorophyll-a concentrations and carbon fixation in the subsurface chlorophyll maximum during spring in either hemisphere. In surface waters, although picoplankton (<2-μm) represented the dominant fraction in terms of both carbon fixation (50–70%) and chlorophyll-a (80–90%), nanoplankton (>2-μm) contributions to total carbon fixation (30–50%) were higher than to total chlorophyll-a (10–20%). However, in the subsurface chlorophyll maximum picoplankton dominated both carbon fixation (70–90%) and chlorophyll-a (70–90%). Thus, in surface waters chlorophyll-normalised carbon fixation was 2–3 times higher for nanoplankton and differences in picoplankton and nanoplankton carbon to chlorophyll-a ratios may lead to either higher or similar growth rates. These low chlorophyll-normalised carbon fixation rates for picoplankton may also reflect losses of fixed carbon (cell leakage or respiration), decreases in photosynthetic efficiency, grazing losses during the incubations, or some combination of all these. Comparison of nitrate concentrations in the subsurface chlorophyll maximum with estimates of those required to support the observed rates of carbon fixation (assuming Redfield stoichiometry) indicate that primary production in the chlorophyll maximum may be light rather than nutrient limited.
The phytoplankton pigment composition (chlorophylls and carotenoids) from 17 Atlantic Meridional Transect (AMT) cruises over the period 1995–2005 was analysed to determine the distributions of pigments and plankton in the Atlantic Ocean between 50°N and 50°S. Data were quality assured by statistical methods, including regression of total chlorophyll a (TChla) versus accessory pigments (AP) and comparison of the AMT-TChla with contemporary SeaWiFS-TChla (cruises AMT-05 to -17). Comparisons of province-mean TChla (±SD) for in situ and satellite data showed good agreement for each cruise. ‘Taxa-specific’ pigments were used to define phytoplankton functional types (PFTs) for each of the biogeochemical provinces along the AMT. Pigment ratios (e.g. TChla/AP) were analysed for each cruise and for each province as indices (characteristic properties) of particular PFTs. Mostly robust positive correlations were observed between TChla and pigment ratios for different PFTs, for some provinces and most cruises. These were consistent with previous observations. Generally there were no significant trends of mean TChla or pigment ratios within provinces over the period 1995–2005, although the previously reported perturbation due to the 1997–1998 ENSO was evident.
Photosynthetic electron transport directly generates the energy required for carbon fixation and thus underlies the aerobic metabolism of aquatic systems. We determined photosynthetic electron turnover rates, ETRs, from ca. 100 FRR fluorescence water-column profiles throughout the subtropical and tropical Atlantic during six Atlantic Meridional Transect cruises (AMT 6, May–June 1998, to AMT 11, September–October 2000). Each FRR fluorescence profile yielded a water-column ETR-light response from which the maximum electron turnover rate , effective absorption (σPSII) and light saturation parameter (Ek) specific to the concentration of photosystem II reaction centres (RCIIs) were calculated. and Ek increased whilst σPSII decreased with mixed-layer depth and the daily integrated photosynthetically active photon flux when all provinces were considered together. These trends suggested that variability in maximum ETR can be partly attributed to changes in effective absorption. Independent bio-optical measurements taken during AMT 11 demonstrated that σPSII variability reflects taxonomic and physiological differences in the phytoplankton communities. and Ek, but not σPSII, remained correlated with mixed-layer depth and daily integrated photosynthetically active photon flux when data from each oceanic province were considered separately, indicating a decoupling of electron turnover and carbon fixation rates within each province. Comparison of maximum ETRs with 14C-based measurements of Pmax further suggests that light absorption and C fixation are coupled to differing extents for the various oligotrophic Atlantic provinces. We explore the importance of quantifying RCII concentration for determination of ETRs and interpretation of ETR-C fixation coupling.
Monthly chlorophyll-a (Chl-a) concentrations derived from SeaWiFS data for 1997–2005 and chlorophyll measurements from the Atlantic Meridional Transect for 1995–2001 have been analysed to describe seasonal and inter-annual variability of surface Chl-a in the Mauritanian upwelling. There was a moderate to strong correspondence between the seasonal cycles of surface Chl-a and the seasonal cycles of ocean physical and meteorological fields (such as sea-surface temperature, sea-surface height, and prevailing wind), with a noticeable exception in 1998 that corresponded to a strong anomalous Chl-a event (∼250% increase) in the Mauritanian upwelling. Alongshore wind-stress and wind-stress curl were found to be the most significant factors controlling the variability of Chl-a (jointly explaining more than 50% of total variance). The biological response to the alongshore wind-stress was immediate, but it lagged the wind-stress curl by 1–2 months (each explaining more than 40% of the total Chl-a variability). These observations also demonstrate a link, hitherto unreported, between the Pacific El-Niño Southern Oscillation (ENSO) and anomalous Chl-a field in the Mauritanian upwelling. The multivariate ENSO index was shown to account for a significant part of the variability of the autumn–winter Chl-a anomaly (r=−0.52, p<0.01). A cold event, following an intense El Niño in the Pacific during summer, was found to mirror the intensity of wind forcing and phytoplankton concentration in the Mauritanian upwelling a few months later. Therefore, ENSO-related changes in the local atmospheric fields are considered as the preferred candidates for explaining the observed biological changes in the Mauritanian upwelling during 1998–1999.
The AMT data management experience. Deep-Sea Research II, this issue
- C Castellani
- G Moncoiffe
- J Brown
Castellani, C., Moncoiffe, G., Brown, J., 2009. The AMT data management
experience. Deep-Sea Research II, this issue [doi:10.1016/j.dsr2.2008.10.007].
UK E-mail address: pmh1@noc.soton.ac
Plymouth Marine Laboratory, Prospect Place, West Hoe,
Plymouth PL1 3DH, UK
E-mail address: Carol.Robinson@uea.ac.uk
Patrick Holligan
National Oceanography Centre, University of Southampton,
Southampton SO14 3ZN, UK
E-mail address: pmh1@noc.soton.ac.uk
Tim Jickells
University of East Anglia, School of Environmental Sciences,
Norwich NR4 7TJ, UK
E-mail address: T.Jickells@uea.ac.uk
Samantha Lavender
University of Plymouth, School of Earth, Ocean and Environmental
Sciences, Drake Circus, Plymouth PL4 8AA, UK
E-mail address: S.Lavender@plymouth.ac.uk
Available online 5 November 2008
Phytoplankton pigments and functional types in the Atlantic Ocean: a decadal assessment
- J Aiken
- Y Pradhan
- R Barlow
- S Lavender
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Aiken, J., Pradhan, Y., Barlow, R., Lavender, S., Poulton, P., Holligan, P., Hardman-Mountford, N., 2009. Phytoplankton pigments and functional types in the
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- A Calbet
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in the Atlantic Ocean: a latitudinal study. Deep-Sea Research II, this issue
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Seasonal and spatial variability in plankton production and respiration in the Subtropical Gyres of the Atlantic Ocean. Deep-Sea Research II, this issue
- N Gist
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Gist, N., Serret, P., Woodward, E.M.S., Chamberlain, K., Robinson, C., 2009. Seasonal
and spatial variability in plankton production and respiration in the
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[doi:10.1016/j.dsr2.2008.10.035].
The Atlantic Meridional Transect Programme (AMT): a contextual view
- C Robinson
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- A R Baker
- G Forster
- N Gist
- T D Jickells
- G Malin
- R Upstill-Goddard
- R G Williams
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Robinson, C., Poulton, A.J., Holligan, P.M., Baker, A.R., Forster, G., Gist, N., Jickells,
T.D., Malin, G., Upstill-Goddard, R., Williams, R.G., Woodward, E.M.S., Zubkov,
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Predicting plankton net community production in the Atlantic Ocean. Deep-Sea Research II, this issue
- P Serret
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- E Fernandez
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Serret, P., Robinson, C., Fernandez, E., Teira, E., Tilstone, G., Perez, V., 2009.
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Measured and remotely sensed estimates of primary production in the Atlantic Ocean from
- G Tilstone
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Tilstone, G., Smyth, T., Poulton, A., Hutson, R., 2009. Measured and remotely sensed
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