The Atlantic Meridional Transect programme (1995–2016)

Accepted Manuscript Preface The Atlantic Meridional Transect Programme (1995 – 2016) Andrew P. Rees, Philip D. Nightingale, Alex J. Poulton, Tim J. Smyth, Glen A. Tarran, Gavin H. Tilstone PII: DOI: Reference:

S0079-6611(17)30163-5 http://dx.doi.org/10.1016/j.pocean.2017.05.004 PROOCE 1802

To appear in:

Progress in Oceanography

Please cite this article as: Rees, A.P., Nightingale, P.D., Poulton, A.J., Smyth, T.J., Tarran, G.A., Tilstone, G.H., The Atlantic Meridional Transect Programme (1995 – 2016), Progress in Oceanography (2017), doi: http:// dx.doi.org/10.1016/j.pocean.2017.05.004

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The Atlantic Meridional Transect Programme (1995 – 2016) Andrew P. Rees*,1, Philip D. Nightingale1, Alex J. Poulton2, Tim J. Smyth1, Glen A. Tarran1, Gavin H. Tilstone1.

* [email protected]; +44 1752 633410 1

Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, PL1 3DH, UK.

2

National Oceanography Centre, Waterfront Campus, Southampton, SO14 3ZH, UK.

Introduction Since 1995 the Atlantic Meridional Transect program (AMT - www.amt-uk.org) has undertaken extensive measurements of oceanographic and atmospheric variables on a passage between the UK and destinations in the South Atlantic (Falkland Islands, Chile, Uruguay and South Africa). This program, which spans more than 100° of latitude, crosses a range of ecosystems from sub-polar to tropical, from eutrophic shelf seas and upwelling systems, to oligotrophic mid-ocean gyres (Fig. 1). The AMT was originally conceived to utilise the biannual passage of the RRS James Clark Ross (JCR) between its home-base in the UK and its field-base in the Falklands. This initial phase, from 1995 to 2000 was largely funded by the Plymouth Marine Laboratory (PML) and the Natural Environment Research Council (NERC) with additional support from NASA in order to test and ground-truth satellite algorithms of ocean colour (Aiken & Bale, 2000). The opportunities offered by this initiative meant that this series of repeated bi-annual cruises rapidly developed into a coordinated study of ocean biodiversity, biogeochemistry and ocean/atmosphere interactions. 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, Robinson et al., 2006) 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. In the third phase, from 2008 to present, funding from NERC was directed through the OCEANS2025 program and latterly through UK National Capability. During this phase, cruises switched from bi-annual to annual, taking place during the boreal autumn (austral spring). A summary of the annual and seasonal coverage of AMT cruises is given in Figure 2.

Throughout the lifetime of the AMT program the objectives have evolved to address topical research questions whilst enabling the maintenance of a continuous set of observations relevant to global issues that are raised throughout the most recent IPCC assessment (Rhein et al., 2013) and UK environmental strategy. The current objectives of the programme are to: (1)

quantify the nature and causes of ecological and biogeochemical variability in

planktonic ecosystems; (2)

quantify the effects of this variability on nutrient cycling, on biogenic export

and on air-sea exchange of climate active gases; (3)

construct a multi-decadal, multidisciplinary ocean time-series which is

integrated within a wider “pole-to-pole” observatory concept; (4)

provide essential sea-truth validation for current and next generation satellite

missions; (5)

provide essential data for global ecosystem model development and

validation; (6)

provide a valuable, highly sought after training arena for the next generation

of UK and International oceanographers. AMT is unique in its ability to acquire repeat data of core parameters on long northsouth transects of the Atlantic Ocean and to make these observations across basin scales (Fig 2). This spatially extensive decadal dataset is made available to the wider community through the British Oceanographic Data Centre (http://www.bodc.ac.uk/projects/uk/amt/). AMT also provides a platform for multi-disciplinary oceanographic research and is open to participation by the international scientific community. Thus far (as of March 2017) the 26 research cruises have involved 256 sea-going scientists from 66 institutes representing 22 countries, resulting in 313 refereed papers (http://www.amt-uk.org/Publications). During the early years of AMT several significant advances were made which included the use of ship measurements to relate remotely sensed observations to biological productivity and in providing the means to define the extent and boundaries of Atlantic Ocean oceanographic provinces (Aiken et al., 2000, Hooker et al., 2000). Preliminary indications of the variability in source and sink regions for CO2 were identified (Lefevre & Moore, 2000) and the first Atlantic Ocean basin-scale measurements of Prochlorococcus and Synechococcus abundance were made (Zubkov et al., 2000), which further assisted in the definition of ocean provinces. Subsequently the AMT programme has enabled extensive investigation of biodiversity and biogeochemical processes and has provided the first descriptions on ocean basin scales of plankton net community production (Serret et al.,

2001), respiration (Robinson et al., 2002), nitrification (Clark et al., 2008), calcite and opal production (Poulton et al., 2006) and export (Thomalla et al., 2008), mixotrophy (Hartmann et al., 2012) and photoheterotrophy (Evans et al., 2015). AMT research has contributed to the quantitative understanding of key interactions and feedbacks between the ocean and the atmosphere. Isotopic measurements provided evidence for the existence of a marine ammonia source and offered clarification over the relative contribution of marine and terrestrial ammonium to aerosols in remote areas and their role in climate regulation (Jickells et al., 2003). Dimethylsulphide (DMS) and dimethylsulphoniopropionate (DMSP) were found to show strong correlations with primary production and some photo-protective pigments, providing a first step for global prediction and model-validation of these climate-relevant compounds (Bell et al., 2006). Following AMT cruises 12 and 13 it was estimated that the Atlantic Ocean contributed 6-15% and 413% of the global marine sources of atmospheric N2O and CH4 respectively (Forster et al., 2009). Methanol is the second most abundant organic gas in the troposphere and in the surface ocean represents a supply of energy and carbon for marine microbes. Yang et al., (2013) determined the air to sea flux of methanol using the eddy covariance method and showed a consistent influx of methanol from the atmosphere throughout the transect and helped to constrain the molecular diffusional resistance above the ocean surface, an important term for improving air-sea gas exchange models. 2015 saw two milestones in the history of the AMT: the achievement of twenty years of this unique ocean-going program and the departure of the 25th cruise on the 15th of September. Both of these events were celebrated in June 2015 with an open science conference hosted by PML. A number of the presentations made during this meeting form the basis of papers presented in this special issue of Progress in Oceanography. This collection of papers provides the state of the art with regards to Atlantic oceanography and includes research on organism diversity or biogeochemical processes derived from one or a number of transects, as well as syntheses covering over two decades of observations.

Review of this issue A running theme throughout the special issue is the biogeography of the regions sampled via the AMT programme, ranging from the identification and validation of hydrographic provinces (Fig. 4) to their biogeochemical functioning (Aiken et al., 2017), and impact on biological processes (Tilstone et al., 2017a) and community structure (Poulton et al., 2017). Smyth et al. (2017) combine in situ hydrographic data (temperature, salinity,

oxygen, nutrients), Earth Observations (satellites) and biogeochemical modelling to determine both the characteristics and variability of the dominant oceanic provinces across the AMT (Fig. 4). Variability in the characteristics and boundaries of the provinces identified are linked to variability in surface currents, net heat flux and subsequent large-scale biological responses (e.g. primary production). This paper by Smyth et al. (2017) highlights the strengths and challenges of an open-ocean biogeochemical time-series such as AMT: its established longevity (>20 yrs) and uniqueness in sampling a wide range of biomes and biogeochemical parameters; versus the scientific impacts of variability in cruise tracks and seasonal aliasing of the sampling, as well as ensuring that the variables measured, and its methodology, remain both state-of-the-art and consistent in data quality. Baker and Jickells (2017) present both a review of previous atmospheric deposition measurements made during AMT and new data that include soluble concentrations of a range of trace metals, including titanium, zinc, vanadium, nickel and copper, in aerosol form. Earlier AMT studies focused on iron, manganese and aluminium in aerosols for which mineral dust represents the major source (Baker et al., 2013). In this new study, the authors found a strong gradient in the soluble concentrations, and in their deposition, of trace metals between the North and South Atlantic (Fig. 5) as might be expected for species with anthropogenic and crustal sources. Concentrations of iron and aluminium in surface seawater generally reflect their atmospheric inputs, but Baker and Jickells (2017) caution that this does not necessarily follow for all soluble metals. For example, deposition fluxes of manganese, zinc, nickel and vanadium are not reflected in the distribution of their surface seawater concentrations, most likely due to biological and chemical processing – as indeed has been observed for cobalt (Shelley et al., 2017). Baker and Jickells (2017) also suggest that emissions from shipping might now represent an important source of nickel and vanadium to the Atlantic Ocean and presumably other ocean basins. Shelley et al. (2017) present measurements of dissolved iron and cobalt concentrations along an AMT transect during the autumn of 2009. Both elements are important micro-nutrients for phytoplankton growth (Morel et al., 1994); indeed iron availability has been found to limit productivity in regions of the world’s oceans (Coale et al., 1996). Interestingly, large differences were observed in the distributions of the dissolved phase of the two metals in the Atlantic Ocean (Fig. 6). Iron concentrations were relatively high in the North Atlantic, most likely due to dust deposition (e.g. Baker and Jickells, 2017) and then declined in the South Atlantic. Perhaps surprisingly, surface cobalt concentrations

were greatest in the South Atlantic, particularly in the subtropical gyres. Shelley et al. (2017) argue that, although it might have been expected that cobalt levels would have been highest in the North Atlantic gyres due to dust deposition, concentrations were actually reduced by enhanced biological uptake. The authors propose that this uptake is by marine cyanobacteria, particularly Prochlorococcus and/or Trichodesmium species, which are observed in high numbers in these waters (e.g. Zubkov et al., 2000; Moore et al., 2009). Sustained observations are a key component of our ability to monitor and detect longterm changes in the ocean, driven by both natural variability and anthropogenic change. Kitidis et al. (2017) quantify changes in surface ocean CO2 and the carbonate system throughout the AMT period (1995 to 2013). Observations from AMT over the last ~20 years’ show an increase in seawater CO2 concurrent with increases in atmospheric CO2, and a decline in pH of ~0.0013 (± 0.0009) units per year (Fig. 7). Both trends are consistent with patterns seen in the Atlantic and more globally (e.g. Bates et al., 2014, Lauvset & Gruber, 2014, Lauvset et al., 2015). Scaling the pH decline from Kitidis et al. (2017) forwards in time implies a further decline of ~0.11 units to the end of the present century, though important regional and seasonal trends also need to be considered. Kitidis et al. (2017) also note that despite ~20 years of observations, there is a continuing need for further observations to fully resolve the impact of such regional, seasonal and long-term climatic variability, as well as to provide essential data for the predictive modelling of future climate. Decreases in ocean pH (termed ocean acidification), such as those documented in the Atlantic (Kitidis et al., 2017), may have significant impacts on oceanic biogeochemical cycles, for example carbon consumption and nitrogen recycling (e.g. Riebesell et al., 2007; Kitidis et al., 2011). Many of these wider impacts are tied to the ecological effect of declining pH on cellular physiological processes, such as calcification (Bach et al., 2015) and photosynthesis (Tilstone et al., 2017b). Using a bioassay approach, (Tilstone et al., 2017b) investigated how lower pH (higher pCO2) influences community structure, primary production and photosynthesis of plankton in the North Atlantic Ocean. Such experiments are among relatively few that have been done in warm oligotrophic waters (Lomas et al., 2012), rather than more temperate or cooler eutrophic waters (Engel et al., 2013). The results of Tilstone et al. (2017b) highlight the positive effect CO2 enrichment can have at the physiological level on carbon fixation (Fig. 8), as well as the need for greater attention to how climate change will impact ecosystems and biogeochemical cycles in the extensive subtropical gyres.

During two cruises in 2005, AMT16 (May/June) and AMT17 (October/November), Hale et al. (2017) performed a series of bioassay type experiments at 13 oceanographic stations per cruise. They use these experiments to describe seasonal and spatial differences in the limitation or co-limitation of bacterial growth by inorganic nutrients and organic carbon. Dominance of nitrogen as the limiting nutrient for bacteria during May/June and the colimitation of nitrogen with organic carbon during October/November largely reflect the delivery of atmospheric nutrients and the quality of dissolved organic material (Hale et al., 2017). Their conclusion that bacteria directly compete with autotrophs for inorganic nutrients further informs the current discussion over the heterotrophic balance of the oligotrophic ocean (e.g. Del Giorgio & Duarte, 2002; Robinson et al., 2002), and has implications for the modelling of carbon export through the biological and microbial pumps (Ducklow, 2001; Legendre et al., 2015). García-Martín et al. (2017) estimate bacterial and total community plankton respiration in the 0.2 to 0.8 μm size-fraction along two Atlantic Meridional transects in 2010 and 2011. Two different methodologies were compared: changes in dissolved O2 concentration from standard 24 hr dark bottle incubations; and measurements of in vivo reduction of 2-(ρ- iodophenyl)-3- (ρ-nitrophenyl)-5phenyl tetrazolium salt (INT). This comparison showed reasonable statistical correlation between the rates of community respiration estimated by the two methods. They also found that depth-integrated community respiration varied three-fold along the two transects, with the highest rates on the European and Patagonian shelves and the lowest rates in the North and South oligotrophic gyres. Garíca-Martín et al. (2017) found that the proportion of total community respiration attributed to bacteria is similar between different Atlantic provinces (Fig. 9). Tilstone et al. (2017a) evaluate the contribution of the mico-phytoplankton (>20 µm) community to total primary production in the Atlantic Ocean by using a combination of in situ data and satellite models. The in situ data, gathered on the AMT amounted to 940 measurements at 258 sites on 23 separate cruises between 1995 and 2013. They found that the highest contribution (38%) to total primary production was in the South Subtropical Convergence province and the lowest (18%) in the North Atlantic Gyre province. They also determined the micro-phytoplankton photosynthetic rate parameters in each of the Atlantic provinces, with the highest rates encountered in the North Atlantic gyre and the lowest in the North Atlantic Drift and South Atlantic gyre regions. These values were then used to parameterise a size-fractionated production model for use on satellite data to enable estimates of primary and export production for each of the Atlantic provinces which were determined

over the period 1998 to 2010 (Fig 10) and showed that that micro-phytoplankton primary production remained constant in the NADR, NATL, Canary Current Coastal upwelling (CNRY), ETRA, WTRA and SATL, but showed a gradual increase in the Benguela Upwelling zone (BENG) and SSTC. Brewin et al. (2017) have taken measurements of primary production made during previous AMT cruises and used them to validate remote-sensing techniques capable of producing basin-scale estimates of primary production. In order to progress the development of remote sensing algorithms in-line with many marine biogeochemistry models (e.g. Marinov et al., 2010; Ward, 2015) they have taken an approach which improves upon the estimation of size-fractionated primary production. Previously, models used to estimate sizefractionated primary production from satellite data were developed using measurements of phytoplankton pigments rather than direct measurements of phytoplankton size structure. Brewin et al., (2017) introduce an approach to refine a remote-sensing primary production model to estimate production in three size fractions of phytoplankton (<2 µm, 2–10 µm and >10 µm). The model was parameterised using measurements of total chlorophyll, euphotic zone depth, size-fractionated chlorophyll and size-fractionated photosynthesis-irradiance parameters and then evaluated against independent estimates of size-fractionated primary production determined using 14C incubation experiments. Comparison of modelled output with ship-based observations gave confidence in the application of the model to satellite data which, following Monte Carlo simulations was constrained with an average model error of between 0.27 and 0.63 for log10-transformed size-fractionated production. Application of the model to monthly satellite data from 2007 indicated that cells <2 µm and >2 µm contribute equally to total primary production in the Atlantic Ocean (Fig. 11). In Poulton et al. (2017) coccolithophore species composition was determined in 199 samples collected from the upper 300 m of the Atlantic Ocean, spanning temperate, tropical and subtropical waters in both hemispheres during four Atlantic Meridional Transect (AMT) cruises over the period 2003 to 2005 (Fig. 12). Of the 171 taxa observed, 140 consistently represented <5% of total cell numbers, and were classed as rare. They found sharper gradients in species composition vertically over depth ranges of tens of metres than horizontally over the entire transect. Throughout the AMT, from subtropical to equatorial waters, high levels of species richness and low levels of species dominance were observed. The communities could be classified into three floral groups: the upper euphotic zone, where Umbellosphaera spp. and holococcolithophores are abundant; the lower euphotic zone, where Emiliania huxleyi and Gephyrocapsa ericsonii are abundant; and the sub-euphotic zone

where Florisphaera profunda and Gladiolithus spp. occur. Statistical differences were found in the species composition between the different cruises which related to cruise timing (May/October) rather than interannual variability. Measurements were made by Fileman et al. (2017) to investigate the impact of stressors on planktonic communities in the near-surface ocean. In particular they focussed on the response of planktonic communities to the penetration of ultra-violet radiation (UV) through their expression of protective UV-absorbing compounds called mycosporine-like amino acids (MAAs). UV has been shown to be detrimental to phytoplankton and zooplankton and may inhibit photosynthesis and algal growth, and particularly in copepods can increase mortality, decrease egg production and hatching success, and lead to a higher incidence of deformed nauplii. During AMT 20 (2010), Fileman et al. (2017) found that MAAs were found in all seston samples with total MAA concentrations ranging from 0.01 to 1.4 μg L-1 of seawater (Fig. 13a). When normalised to MAA values per unit of chlorophyll-a (Fig. 13b), the north Atlantic gyre and northern equatorial regions proved dominant, coincident with an increase in UV transparency of the water-column. MAAs were detected in a third of the zooplankton tested and these taxa varied greatly both in the amount and diversity of the MAAs that they contained, with copepods in temperate regions containing the highest concentration of MAAs. In contrast, juvenile copepods were found not to contain any MAAS. The lack of any measurable MAA compounds in nauplii across the whole transect was associated with the severe (3 to 6-fold) reduction in nauplii densities in the near-surface layer, as compared to the underlying water column. Fileman et al. (2017) observed that the UV stress on marine life near the surface, particularly in the warmer, oligotrophic and brightly-lit low latitudes, imposes radically different pressures on zooplankton communities compared to the rest of the epipelagic and that some zooplankton use photo-protective compounds as a defence mechanism enabling them to inhabit waters with high UV. This issue includes some fascinating research into the biodiversity, distribution and abundance of different components of zooplankton communities in the Atlantic Ocean, as sampled by the AMT programme. This includes Crustacea, in the form of copepods (Goetze et al., 2017, Peralba et al., 2017), gastropod molluscs, in the form of pteropods and heteropods (Burridge et al., 2017a) and hyperiid amphipods (Burridge et al., 2017b). Peralba et al.’s (2017) work from AMT 15 (2004) focussed on the copepod genus Clausocalanus and the biogeography of 11 species within the genus to identify the different species and also discriminate between males, females and juveniles. The sex ratio of the genus as a whole was skewed towards females, males tending to make up less than 10% of

adults. Juveniles were abundant and often made up more than 50% of total individuals (Fig. 14). Well-defined ecological niches were found for some species, with C. furcatus dominating in the North Atlantic Gyre and down to 11o S, to be replaced by C. paululus through the South Atlantic Gyre and into the Southern Subtropical Zone (Fig. 14d). These two species, along with C. pergens, whilst being the smallest species recorded, accounted for 85% of total Clausocalanus adult abundance along the AMT. Goetze et al. (2017) took a genetic approach on AMT 22 (2012) to evaluate the role of potential ecological barriers to the dispersal of planktonic species in the Atlantic Ocean by studying mitochondrial sequence data from the dominant migratory copepod Pleuromamma xiphians. These authors discovered that P. xiphians were genetically subdivided across subtropical and tropical waters, with a major genetic break being observed in the equatorial Atlantic (Fig. 15). They suggested that the Atlantic equatorial province is probably an important region of evolutionary novelty for the holoplankton in general. The realisation that the oceans are becoming more acidic has led to increasing research into calcifying organisms and the potential effects of decreasing pH on their ability to calcify and survive. Pteropods and heteropods are important groups of planktonic shelled gastropod molluscs which may act as good indicators of the effects of ocean acidification. Burridge et al. (2017a) studied these molluscs on AMT 24 (2014), looking at their latitudinal diversity and abundance from 46o N to 46o S by microscopy. Pteropods were the most abundant of the two groups along the transect (Fig. 16). A maximum abundance of pteropods of just over 4000 individuals per 1000 m3 was recorded, just south of 40o S, whereas heteropod maximum abundance was found at approx. 18o S in the middle of the South Atlantic Gyre. Species richness of pteropods was low at both ends of the transect and high over a 60o latitudinal stretch from 30o N to 30o S. Heteropod species richness was not maintained over such a long latitudinal range, with highest species richness being limited to the equatorial upwelling region, approximately 4o either side of the equator. The authors concluded that, due to a wide variation in distribution patterns among pteropod and heteropod species, there are likely to be differential species responses to ocean acidification. Burridge et al. (2017b) were able to study the latitudinal diversity and distribution of hyperiid amphipods, which are commensal and parasitic within gelatinous zooplankton from samples collected during AMT 22 (2012). 70 species were identified from 17 families and their relative abundance and diversity were mapped between 39o N and 45o S (Fig. 17). Maximum species richness was observed in the equatorial upwelling region from 7o N to 8o S. Species richness was positively correlated with surface temperature and distinct hyperiid

species assemblages were in broad agreement with Longhurst’s (1995) biogeochemical provinces. The North and South Atlantic Sub-Tropical Gyres (STGs) represent about 10% of the Earth’s surface area and, unlike much of the ocean, are permanently stratified on seasonal and annual timescales and are regions of low macro-nutrient concentrations and low surface plankton biomass. Using a powerful combination of in-situ physical and biogeochemical measurements, a suite of data derived from Earth Observation techniques and a coupled 1-D ecosystem model, Aiken et al. (2017) investigate seasonal changes in the biogeography of these gyres and test whether there are any long-term trends in their characteristics. Statistically significant increases in chlorophyll and sea surface temperature are reported for the northern STG, together with evidence that its surface area has also been increasing (Fig. 18). In contrast, whilst chlorophyll also increased in the southern STG, there was no overall change in sea surface temperature or areal extent (Fig. 19). These long term trends are superimposed on largescale seasonal changes. Although AMT has been running for more than 20 years, the authors caution that this is still a relatively short time-series with which to draw conclusions on how climate change might be influencing these ocean gyres. Aiken et al. (2017) therefore argue that the AMT program needs to be sustained into the future and indeed augmented by further measurements derived from satellites and autonomous platforms. They conclude by arguing that a higher priority ought to be given to acquiring data along the AMT transect during the key months of January and July in order to better observe seasonal cycles in the STGs and understand the impact of environmental drivers on these regions.

Future directions for AMT Funding mechanisms which have supported AMT over the past two decades have proven variable and this will continue into the future. The NERC who provided the majority of funding to AMT are currently implementing changes to the UK National Capability which will no doubt play some part in shaping the delivery of the future AMT program. In preparation for managing change, a “Task Team” comprising leaders of the UK oceanography and atmospheric communities were asked to reflect upon the historical role and outputs of AMT with a view to directing future aspirations. This committee advised that AMT was unique in: •

observing the entire range of biogeographical biomes contained within ~100° of latitude in the Atlantic Ocean;

•

using the NERC British Antarctic Survey vessels, effectively as Ships of Opportunity, as they make passage between the UK and the South Atlantic, providing excellent value for money;

•

providing a globally unique platform for hands-on training of next generation scientists;

•

making repeat visits to the most remote areas of the Atlantic Ocean which facilitates the regular servicing of moorings and deployment of profiling Argo floats.

As the underpinning themes for future research activities AMT should provide: •

the backbone of a pole-to-pole in situ observing system, linking the north and south Atlantic;

•

a managed environment where ship based and autonomous (vehicle) based observation can be used in parallel to advance the potential and scope of autonomous instrumentation;

•

a crucial platform with which to enable the calibration and validation of the next generation of Europe’s environmental satellite platforms;

•

continuation of collaborative links between the UK research institutes, universities and Government sectors, nationally and internationally;

•

extension of the multi-decadal biogeochemical time-series with a focus on validating and improving Earth System Models;

The AMT program provides an incredibly unique facility and data rich library of observations to the international communities of oceanographers and atmospheric scientists. With continuation into future decades AMT will advance understanding of the Earth System as a whole, and the role that the oceans play within it, to enable us to quantify the nature and scale of global environmental change and associated implications for mitigation and adaption.

Acknowledgements The authors wish to acknowledge the pioneering outlook of the AMT originators Jim Aiken, Patrick Holligan, Roger Harris, Dave Robbins, Chuck McClain and Chuck Trees, and to recognise Carol Robinson and Tim Jickells who joined with this team and steered AMT towards its current manifestation. The delivery of AMT has been accentuated through the involvement of NEODAAS and particularly through the input of Ben Taylor and has been

made available to the wider world through the data management of Rob Thomas and colleagues at BODC. We would like to thank all of the officers and crews on RRS James Clark Ross, RRS Discovery and RRS James Cook. None of this work would have been delivered without the professionalism of the seagoing technicians of NMF and BAS, neither without the inputs of Beth House, Natalie Clark, Jon Short, Randy Sliester, Kath Nicholson and Colin Day. This study is a contribution to the international IMBeR project and was supported by the UK Natural Environment Research Council National Capability funding to Plymouth Marine Laboratory and the National Oceanography Centre, Southampton. This is contribution number 313 of the AMT programme.

Figure legends

Figure 1: Cruise tracks achieved during the three phases of AMT. In a) AMT cruises 1 to 11 between 1995 and 2000; in b) AMT cruises 12 to 17 between 2003 and 2005; in c) AMT cruises 18 to 26 between 2008 and 2016. Composite chlorophyll images from the ESA Climate Change Initiative, provided by NEODAAS.

Figure 2: (From Fig. 2 in Aiken et al., 2017). Annual and seasonal coverage of AMT cruises from AMT-1 through to AMT-25 (1995–2015). Green indicates cruise sector in the northern hemisphere (mostly NAG) and blue indicates cruise sector in the southern hemisphere (mostly SAG).

Figure 3: Key biogeochemical boundary depths determined during AMT cruises between 1995 and 2013. Nitracline (♦) – defined as depth of 1µM NO3- (715 data points from 16 cruises); fluorescence maximum ( - 982 data points from 16 cruises ); and photic depth (∆) – defined as depth of 1% of surface PAR (283 data points from 10 cruises).

Figure 4: (From Fig. 1 in Smyth et al., 2017). Map of the Longhurst provinces under investigation with the various AMT cruise tracks superimposed. NADR – North Atlantic Drift Region; NASE - North Atlantic Sub-tropical Gyre, East; NATR - North Atlantic Tropical Gyre; WTRA – Western Tropical Atlantic; SATL - South Atlantic Gyre; SSTC South Sub-tropical Convergence. Background image courtesy of Google Earth.

Figure 5: (From Fig.3 in Baker & Jickells, 2017). Box and whisker plots showing the variations in the concentrations of (a) NO3-, (b) NH4+, (c) Na+, (d) Br-, (e) oxalate, (f) nssSO42-, (g) nss-K+ and (h) nss-Ca2+ with air transport/source type for the AMT18–21 cruises. Upper and lower limits of boxes represent the interquartile range of data in each category, with the median shown as bars in each box. Whiskers represent the range of the data, except where extremes (values greater than 1.5 times the interquartile range above the upper quartile) were present (crosses). Numbers of samples in each category are given..

Figure 6: (From Fig. 4 in Shelley et al., 2017). The distribution of dCo (pM) overlaid with potential density anomaly (kg m-3; top panel), dFe (nM) overlaid with the TdFe (nM; bottom panel) in the upper 150 m of the Atlantic Ocean during AMT-19, with the approximate depth of the mixed layer marked (MLD) shown as a solid white line. The biogeochemical provinces are displayed above the top panel (refer to Fig. 1 for acronyms).

Figure 7: (From Fig. 6 in Kitidis et al., 2017). Annual rate of change in CO2 fugacity in seawater (fCO2 sea) and annual rate of pH change (bottom) for southbound cruises in boreal autumn.

Figure 8: (From Fig. 11 in Tilstone et al., 2017a). Changes in Primary production (mg C m3 -1 d ) at T0 (black bars), 48 h in control (light grey bars) and 760 ppm CO2 (dark grey bars) for experiments I-II.

Figure 9: (From Fig. 5 in Garcia-Martin et al., 2017). Depth-integrated plankton community respiration (black dots) and respiration measured in the 0.2-0.8 µm size-fraction (white dots) normalized by integrated depth (weighted average rate in the epipelagic zone) measured with the INT reduction method (A, B) and with the dissolved oxygen method (C); and cell-specific INT0.2-0.8 rates (D) along the north-south latitudinal transects. Only data from the 2011 is available for the CRO2. Error bars represent the standard error of the measurement. The approximate boundaries between the different regions are indicated by dotted vertical lines.

Figure 10: (From Fig. 9 in Tilstone et al., 2017b). Anomaly in micro-phytoplankton primary production in (A.) NADR (B.) NATL, (C.) CNRY, (D.) ETRA, (E.) WTRA, (F.) SATL, (G.) BENG, (H.) SSTC. Solid line is regression through the anomalies. Dotted line in (G.) and (H.) is 0 to illustrate trend in regression.

Figure 11: (From Fig. 13 in Brewin et al., 2017). Total primary production (P), and primary production for small (<2 µm, denoted P1), medium (2–10 µm, denoted P2) and large (>10 µlm, denoted P3) cells, for May and October 2007, in the Atlantic Ocean.

Figure 12: (From Fig. 7 in Poulton et al., 2017). Latitudinal trends of Coccolithophore composition in samples from the near-surface (optical depth = 0.6 (55% surface irradiance)) during AMT 14. (a) Chlorophyll a (Chl, mg m-3), cell abundance (cells, mL-1) and the number of coccolithophore taxa (species richness) (S). (b) Pielou’s Evenness (J') and Bray-Curtis Similarity (Sim) referenced to a Northern Gyre station at 22.3°N. (c) Normalised abundance of Umbellosphaera tenuis, U. irregularis and E. huxleyi. (d) Relative abundance of Coccolithophore flora (see Table 4 in Poulton et al., 2017) representative of Temperate waters, Gyre waters (northern and southern), and equatorial waters, as well as the rare group (consistently <5% of total cell numbers). (e) The degree of similarity of species composition from the 14%, 1% and 0.1% surface irradiance depths compared to the 55% sample (see a) expressed as percentage Bray-Curtis Similarity (Sim). Vertical dashed lines in panel (a)

indicate the positions of the major hydrographic provinces (TMP, temperate waters; NG, Northern Gyre waters; EQ, equatorial waters; SG, Southern Gyre waters).

Figure 13: (From Fig. 5 in Fileman et al., 2017). Latitudinal distribution of MAAs in seawater and seston. a) total MAA concentration (μg L-1) b) MAA:chl a ratio and c) phytoplankton biomass specific MAAs (μg μg C-1) . Figure 14: (From Fig. 5 in Peralba et al., 2017). Distribution of Clausocalanus along the AMT-15 transect: (a) Clausocalanus abundance (ind. m_3); (b) Clausocalanus contribution to total copepod abundance (%); (c) Clausocalanus population structure (%); (d) relative contribution of Clausocalanus species to total adult Clausocalanus abundance (%). NECS, North-East Atlantic Continental Shelf; NADR, North Atlantic Drift province; NAST, North Atlantic Subtropical Gyre province; NATR, North Atlantic Tropical Gyre province; CNRY, Canary Current Coastal province (UpW, upwelling zone); WTRA, Western Tropical Atlantic province; SATL, South Atlantic Gyre province (SAG, South Atlantic Gyre zone); SSTC, South Subtropical Convergence province. Figure 15: (From Fig. 4 in Goetze et al., 2017). Distribution of cytochrome c oxidase subunit I (mtCOI) haplotypes across 12 sites in the North and South Atlantic. Each pie represents a sampling site (site number shown), with mtCOI haplotypes in color as indicated by the legend (at right). The thick dashed line marks the location of a major genetic break among populations in the subtropical North and South Atlantic, and thin dashed lines indicate the northern and southern range limits for Clade 2. Haplotypes with an asterisk (*) are within clade 2, in addition to some of the singletons. Only sites included in population-level analyses are shown. Figure 16: (From Fig. 1 in Burridge et al., 2017a). (A) Overview of pteropod and heteropod sampling locations along Atlantic Meridional Transect 24. (B–E) Distribution of (B) pteropods and heteropods, (C) euthecosome, pseudothecosome and gymnosome pteropods, (D) genera of uncoiled euthecosomes and (E) species of coiled euthecosomes along Atlantic Meridional Transect 24. The size of the pie charts is scaled according to the total abundance of the examined groups (size of the maximum abundance is shown in legend for each plot). True sampling locations are indicated with a white (B) or black (C–E) line if pie sizes did not allow placement at true locations.

Figure 17: (From Fig. 1 in Burridge et al., 2017b). Overview of hyperiid diversity observed in open waters of the Atlantic Ocean. (A) Distribution of hyperiid families along Atlantic Meridional Transect 22. Piecharts indicate relative abundances of families at each station >30 specimens. (B) Most commonly found representatives of the 18 families that were sampled along Atlantic Meridional Transect cruise 22. Legend colors are arranged by infraorder and superfamily of hyperiids (following the current taxonomy as presented in the World Register of Marine Species: http://www.marinespecies.org). All scale bars represent 1 mm.

Figure 18: (From Fig. 15 in Aiken et al., 2017). Annual anomalies and trends in the North Atlantic Gyre (NAG) for sea surface temperature (SST), chlorophyll-a (CHL), gyre area (GA) and photosynthetically active radiation (PAR), from 1998 to 2012, along with the Multivariate ENSO Index (MEI). Variables were spatially averaged within the NAG using a 0.15 mg m-3 boundary in CHL. Figure 19: (From Fig. 16 in Aiken et al., 2017). Annual anomalies and trends in the South Atlantic Gyre (SAG) for sea surface temperature (SST), chlorophyll-a (CHL), gyre area (GA) and photosynthetically active radiation (PAR), from 1998 to 2012, along with the Multivariate ENSO Index (MEI). Variables were spatially averaged within the SAG using a 0.15 mg m-3 boundary in CHL.

References

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Figure 3: Key biogeochemical boundary depths determined during AMT cruises between 1995 and 2013. Nitracline (♦) – defined as depth of 1µM NO3- (715 data points from 16 cruises); fluorescence maximum (ᴏ - 982 data points from 16 cruises ); and photic depth (∆) – defined as depth of 1% of surface PAR (283 data points from 10 cruises).

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