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Updates and Plans for the First EXPORTS Field Campaign

Posted by mmaheigan

· Thursday, February 1st, 2018

Contacts: David Siegel (UCSB; EXPORTS Science Lead) & Ivona Cetinić (NASA GSFC/USRA; EXPORTS Project Scientist)

EXPORTS in a Nutshell

Ocean ecosystems constitute a significant fraction of the world’s primary production, fixing CO₂ and creating oxygen while playing critical roles in sequestering CO₂ from the atmosphere. An improved understanding of the cycling and fate of oceanic organic carbon will not only allow for better prediction of how these processes may change in the future, but it will help underpin the societal value of these ocean ecosystem services. The EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) field campaign aims to provide answers to these questions.

The goal of EXPORTS is to develop a predictive understanding of the export and fate of global ocean net primary production (NPP) and its implications for the Earth’s carbon cycle in present and future climates (oceanexports.org). To develop this quantitative understanding, EXPORTS will measure and model the export pathways that remove fixed organic carbon from the upper ocean and drive the attenuation of these vertical fluxes within the ocean interior. EXPORTS datasets will be used to develop and test numerical predictive and satellite-data diagnostic models of NPP fates and their carbon cycle impacts. EXPORTS builds on decades of NASA-funded research on developing and validating satellite data-driven models of regional to global NPP and hence, EXPORTS will contribute to NASA’s upcoming Plankton, Aerosol, Cloud and ocean Ecosystem (PACE) mission.

A Brief History of EXPORTS

The NASA EXPORTS field campaign is the result of an initial open competition in 2012 by the NASA Ocean Biology and Biogeochemistry (OBB) Program to identify scoping workshops for future field campaigns. This was followed by many years of committee-based planning, community vetting of science and implementation plans, and final peer review. The NASA EXPORTS Science and Implementation Plans were made publicly available by the NASA OBB program. In February 2016, the National Science Foundation held the Biology of the Biological Pump (BoBP) workshop aimed in part to leverage NASA’s planned investment in the EXPORTS field program. In August 2016, NASA announced it would support data mining and observational system simulation experiment (OSSE) projects to help with planning the NASA EXPORTS field campaign and five projects were funded under this pre-EXPORTS call.

In early 2017, NASA released a call for proposals for the EXPORTS field program and the competition for inclusion on the NASA EXPORTS Science Team and its leadership. The call also included the implementation approach for the EXPORTS field program, with two major cruises to collect in situ data, followed by a synthesis and analysis phase to be competed in the future. At the same time, NSF released a Dear Colleague Letter (DCL) stating they would consider proposals that leveraged the NASA investment with objectives that supported the BoBP plan. From the NASA competition, 11 projects were selected for support (Table 1). Three NSF proposals have been recommended for support (at the time of this writing, the awarding of these grants is not yet official), bringing the count to a total of 41 PIs and co-PIs that are supported by NASA and NSF on EXPORTS/BoBP. This level of investment likely makes EXPORTS the largest coordinated U.S.-funded biogeochemical field program since the Joint Global Ocean Flux Study (JGOFS) nearly 2 decades ago. Table 1 lists the funded projects, PI, and co-PIs, project titles, and links to two page descriptions for each project.

Any implementation of the EXPORTS field program must result in the quantification of the major export pathways that remove fixed organic carbon from the upper ocean and sequester it at depth. NASA is uniquely poised, given the global vantage point of space-based observations, to use Earth observing satellite data to meet this objective, while also understanding observational requirements for future advanced Earth Observing missions. Quantification of major carbon export pathways requires the simultaneous measurement of 1) sinking particle fluxes (and their composition), 2) the export of organic carbon to depth via vertically migrating zooplankton, and 3) the vertical transport of dissolved and suspended particulate organic carbon to depth, where it is remineralized by different microbial communities. To develop predictive links to satellite ocean color-retrievable parameters, the quantification of export pathways must be augmented by research programs focused on, but not limited to, the elucidation of plankton community structure, rates of NPP and grazing, and optical oceanography. Complicating this further is the stochastic nature of export flux determinations that necessitates a fully four-dimensional sampling design while maintaining a long-term perspective. This reasoning led to the Agency selection of projects listed in Table 1.

The planning of the EXPORTS field campaign is well underway. The first field deployment is planned to take place in the summer of 2018 in the Northeast Pacific, while the tentative second cruise will be in the North Atlantic Ocean in the spring of 2020. NASA has formed a project office staffed of Agency and EXPORTS PIs to direct EXPORTS’ progress. The EXPORTS Science Team, which comprises the funded PIs, is participating on near-weekly teleconferences, and co-chief scientists have been selected. An initial EXPORTS kickoff meeting was held in September 2017 in the Washington, DC area. There, the PIs organized themselves into working groups focused on creating short methodological descriptions for each measurement to be made. This documentation will be critical for the metadata, the project data management, and for ensuring legacy of the program through a set of NASA Technical Memoranda. This has also proven to be an excellent way to foster cross-project collaborations. A second PI meeting is scheduled for mid-February 2018, leveraging the upcoming Ocean Sciences Meeting.

EXPORTS First Field Deployment

The first EXPORTS field deployment will be to the Northeast Pacific Ocean in late summer 2018. Two ships, the R/V Roger Revelle and the R/V Sally Ride, will be deployed for 27 days of coordinated sampling around Station P (50°N 145°W), while EXPORTS’ autonomous component will ensure a longer-term presence. The choice of Station P as an anchor point for the field campaign was made based on results from the data mining and OSSE projects and the availability of a long-term data set for this site, as well as the many sampling partnerships afforded by ongoing programs. Canada’s Line P long-term hydrographic/biogeochemistry program has been running since 1949, and they currently conduct three annual transect cruises from British Columbia to Station P. Other useful partnerships include NOAA Pacific Marine Environmental Laboratory’s (PMEL) air-sea interaction buoy and the NSF’s Ocean Observatories Initiative’s (OOI) global node at Station P.

Figure 1: Cartoon depicting many of the individual elements to be deployed during the 2018 EXPORTS sampling program in the North Pacific.

The EXPORTS 2018 field deployment will comprise four basic components (depicted in Figure 1 above). First, several autonomous vehicles will be deployed before the ship observations. An instrumented Lagrangian float will be deployed at depth and used to set the spatial center of the sampling program, while an instrumented Seaglider will be used to provide vertical and some horizontal spatial information around the Lagrangian float’s drift. In addition, and if approved by the OOI Facility Board, instrumented gliders deployed at the Station P OOI global node will be used to supplement the autonomous vehicle data streams.

Second, the R/V Roger Revelle will be the Process Ship, and will follow the Lagrangian float. The Process Ship will focus on rates (NPP, sinking particle fluxes, grazing, net community production, zooplankton respiration and fecal particle production, aggregate formation, etc.) and vertical information (microbial community structure and particle size spectra) in the water mass surrounding the float. Rate measurements will be made using water sampled with a trace metal-clean rosette system, and sinking particle fluxes from neutrally buoyant sediment traps (NBSTs) and sediment trap array. In particular, microbial community structure will be measured using a variety of techniques, including high-throughput microscopic imaging systems, meta-community genomic sequencing, isolation and experimentation on individual marine snow aggregates, and gel trap-collected sinking particles. The Process Ship will also conduct a complete optical oceanographic sampling program ensuring links to remotely sensed parameters. Drs. Deborah Steinberg (VIMS) and Jason Graff (OSU) have volunteered to be co-chief scientists for the R/V Revelle.

Third, the R/V Sally Ride will be the Survey Ship making spatial patterns about the Process Ship on scales from roughly 1 km to nearly 100 km. The focus of the Survey Ship will be collecting horizontal spatial information on particle export (²³⁴Th disequilibrium), net community production (O₂/Ar), organic carbon stocks, phytoplankton composition, and inherent and apparent optical properties. The Survey Ship will also deploy a suite of instrumentation to characterize the particle size spectrum from 20 nm to nearly a cm. It will also be responsible for validating the calibration of the autonomous vehicles’ bio-optical instrumentation and the development of the biogeochemical proxies. Norm Nelson (UCSB) and Mary Jane Perry (self-affiliated) have agreed to be the co-chief scientists on the R/V Sally Ride.

Last, EXPORTS needs a long-term sampling presence to tie the ship-based observations to climatically relevant time and space scales. The Lagrangian float and Seaglider will sample for ~6 months, bracketing EXPORTS’ intensive ship observations, and thus providing some long-term perspectives to the ship sampling. Partnering programs like Line P and the OOI Global Node will allow for some additional in situ sampling opportunities and broader temporal context. Further, the PMEL mooring and a profiling float project recommended for funding by NSF will extend the long-term biogeochemical observations.

The integration of the observations will generate a data set that will not only be invaluable for building new algorithms for retrievals of new and refined data products from NASA’s current fleet of Earth Observing Satellites, but also will be critical in the development of new sets of requirements for future satellite observations of our Earth system. As described in the EXPORTS Implementation Plan, the likelihood of the EXPORTS achieving its predictive goals will increase as the number and variety of observations available to develop and test novel algorithms increases. Hence, the EXPORTS program is particularly motivated to collaborate with international partners who would be interested to share their data sets to address these important issues.

An Amazing Opportunity for Ocean Science

EXPORTS is the first large-scale, coordinated opportunity aimed at understanding the ocean’s biological pump since the JGOFS program. Hence, the EXPORTS team is planning to create a long-term legacy for these one-of-a-kind datasets. NASA is supporting a full-time data manager to make sure that all of the information collected is easily accessible to all; as per NASA policies, all data will be freely available. Efforts are being made to ensure the intercalibration and interoperability of measurements made across different platforms, thus ensuring continuity of the datasets. EXPORTS also plans to over-collect whole water, filtered particulate, and trap-collected samples that can be used for many purposes, both now by collaborators, and in the future as analytical methodologies become more powerful.

The timing for EXPORTS could not be better. Our understanding of the biological pump and in particular, the fate of ocean NPP has rapidly advanced over the past decade. We now know that the biological pump is four-dimensional, which complicates our observational approaches, and that food web and aggregate dynamics, microbial community composition and function, individual organism physiology and behavior, and submesoscale turbulent transport are all components that need to be quantified. Further, our observing tools and capabilities have witnessed giant leaps over just the past couple of years. Novel imaging instruments can now measure particle and aggregate size distributions and identify and quantify plankton abundances. Genomic approaches enable the characterization of plankton communities and their physiology. Novel hyperspectral optical measurements of ocean reflectance as well as component inherent optical properties provide strong links to present and future satellite ocean color missions. High-resolution numerical models now enable the elucidation of submesoscale (100s m to ~10 km) processes that include food webs and biogeochemistry, while autonomous vehicles provide persistent and spatially distributed observations that complement the shipboard sampling. It seems the time for EXPORTS is now.

Click for PDF of article and table

Table 1: EXPORTS Science Team

Lead PI	Co-PIs	Project Title
Michael Behrenfeld (OSU) – NASA	Emmanuel Boss (UMaine), Jason Graff (OSU), Lionel Guidi (LOV), Kim Halsey (OSU), & Lee Karp-Boss (UMaine)	First Step – Linking Remotely-Detectable Optical Signals, Photic Layer Plankton Properties, and Export Flux
Ken Buesseler (WHOI) – NASA	Claudia Benitez-Nelson (USC) & Laure Resplandy (Princeton)	Elucidating Spatial and Temporal Variability in the Export and Attenuation of Ocean Primary Production using Thorium-234
Craig Carlson (UCSB) – NASA	Dennis Hansell (RSMAS)	Evaluating the Controls of Dissolved Organic Matter Accumulation, its Availability to Bacterioplankton, its Subsequent Diagenetic Alteration and Contribution to Export Flux
Meg Estapa (Skidmore) -NASA	Ken Buesseler (WHOI), Colleen Durkin (MLML) & Melissa Omand (URI)	Linking Sinking Particle Chemistry and Biology with Changes in the Magnitude and Efficiency of Carbon Export into the Deep Ocean
Craig Lee (UW) – NASA	Eric D’Asaro (UW), David Nicholson (WHOI), Melissa Omand (URI), Mary Jane Perry (self-affiliated) & Andrew Thompson (CalTech)	Autonomous Investigation of Export Pathways from Hours to Seasons
Adrian Marchetti (UNC) – NASA	Nicolas Cassar (Duke) & Scott Gifford (UNC)	Quantifying the Carbon Export Potential of the Marine Microbial Community: Coupling of Biogenic Rates and Fluxes with Genomics at the Ocean Surface
Susanne Menden-Deuer (URI) – NASA	Tatiana Rynearson (URI)	Quantifying Plankton Predation Rates, and Effects on Primary Production, Phytoplankton Community Composition, Size Spectra and Potential for Export
Collin Roesler (Bowdoin) – NASA	Heidi Sosik (WHOI)	Phytoplankton community structure, carbon stock, carbon export and carbon flux: What role do diatoms play in the North Pacific and North Atlantic Oceans?
David Siegel (UCSB) – NASA	Adrian Burd (UGA), Andrew McDonnell (UAF), Norm Nelson (UCSB) & Uta Passow (UCSB)	Synthesizing Optically and Carbon Export-Relevant Particle Size Distributions for the EXPORTS Field Campaign
Deborah Steinberg (VIMS) – NASA	Amy Maas (BIOS)	Zooplankton-Mediated Export Pathways: Quantifying Fecal Pellet Export and Active Transport by Diel and Ontogenetic Vertical Migration in the North Pacific and Atlantic Oceans
Xiaodong Zhang (UND) – NASA	Deric Gray (NRL), Lionel Guidi (LOV) & Yannick Huot (Sherbrooke)	Optically Resolving Size and Composition Distributions of Particles in the Dissolved-Particulate Continuum from 20 nm to 20 mm to Improve the Estimate of Carbon Flux
Bethany Jenkins (URI) – NSF^*	Mark Brzezinski (UCSB) & Kristen Buck (USF)	Collaborative Research: Diatoms, Food Webs and Carbon Export – Leveraging NASA EXPORTS to Test the Role of Diatom Physiology in the Biological Carbon Pump
Ben Van Mooy (WHOI) -NSF^*		Environmental Lipidomics of Suspended and Sinking Particles in the Upper Ocean
Andrea Fassbender (MBARI) – NSF^*		Constraining Upper-Ocean Carbon Export with Biogeochemical Profiling Floats

^*Project recommended for funding by NSF, but not officially funded as of this publication.

WBC Series: Fine-scale biophysical controls on nutrient supply, phytoplankton community structure, and carbon export in western boundary current regions

Posted by mmaheigan

· Friday, November 10th, 2017

Sophie Clayton¹, Peter Gaube¹, Takeyoshi Nagai², Melissa M. Omand³, Makio Honda⁴

1. University of Washington
2. Tokyo University of Marine Science and Technology, Japan
3. University of Rhode Island
4. Japan Agency for Marine-Earth Science and Technology, Japan

Western boundary current (WBC) regions are largely thought to be hotspots of productivity, biodiversity, and carbon export. The distinct biogeographical characteristics of the biomes bordering WBC fronts change abruptly from stable, subtropical waters to highly seasonal subpolar gyres. The large-scale convergence of these distinct water masses brings different ecosystems into close proximity allowing for cross-frontal exchange. Although the strong horizontal density gradient maintains environmental gradients, instabilities lead to the formation of meanders, filaments, and rings that mediate the exchange of physical, chemical, and ecological properties across the front. WBC systems also act as large-scale conduits, transporting tracers over thousands of kilometers. The combination of these local perturbations and the short advective timescale for water parcels passing through the system is likely the driver of the enhanced local productivity, biodiversity, and carbon export observed in these regions. Our understanding of biophysical interactions in the WBCs, however, is limited by the paucity of in situ observations, which concurrently resolve chemical, biological, and physical properties at fine spatial and temporal scales (1-10 km, days). Here, we review the current state of knowledge of fine-scale biophysical interactions in WBC systems, focusing on their impacts on nutrient supply, phytoplankton community structure, and carbon export. We identify knowledge gaps and discuss how advances in observational platforms, sensors, and models will help to improve our understanding of physical-biological-ecological interactions across scales in WBCs.

Mechanisms of nutrient supply

Nutrient supply to the euphotic zone occurs over a range of scales in WBC systems. The Gulf Stream and the Kuroshio have been shown to act as large-scale subsurface nutrient streams, supporting large lateral transports of nutrients within the upper thermocline (Pelegrí and Csanady 1991; Pelegrí et al. 1996; Guo et al. 2012; Guo et al. 2013). The WBCs are effective in transporting nutrients in part because of their strong volume transports, but also because they support anomalously high subsurface nutrient concentrations compared to adjacent waters along the same isopycnals (Pelegrí and Csanady 1991; Nagai and Clayton 2017; Komatsu and Hiroe pers. comm.). It is likely that the Gulf Stream and Kuroshio nutrient streams originate near the southern boundary of the subtropical gyres (Nagai et al. 2015a). Recent studies have suggested that nutrients in the Gulf Stream originate even farther south in the Southern Ocean (Williams et al. 2006; Sarmiento et al. 2004). These subsurface nutrients can then be supplied to the surface through a range of vertical supply mechanisms, fueling productivity in the WBC regions.

We currently lack a mechanistic understanding of how elevated nutrient levels in these “nutrient streams” are maintained, since mesoscale stirring should act to homogenize them. While it is well understood that the deepening of the mixed layer toward subpolar regions (along nutrient stream pathways) can drive a large-scale induction of nutrients to the surface layer (Williams et al., 2006), the detailed mechanisms driving the vertical supply of these nutrients to the surface layer at synoptic time and space scales remain unclear. Recent studies focusing on the oceanic (sub)mesoscale (spatial scales of 1-100 km) are starting to reveal mechanisms driving intermittent vertical exchange of nutrients and organisms in and out of the euphotic zone.

Recent surveys that resolved micro-scale mixing processes in the Kuroshio Extension and the Gulf Stream have reported elevated turbulence in the thermocline, likely a result of near-inertial internal waves (Nagai et al. 2009, 2012, 2015b; Kaneko et al. 2012, Inoue et al. 2010). In the Tokara Strait, upstream of the Kuroshio Extension, where the geostrophic flow passes shallow topography, pronounced turbulent mixing oriented along coherent banded layers below the thermocline was observed and linked to high-vertical wavenumber near-inertial internal waves (Nagai et al. 2017; Tsutsumi et al. 2017). Within the Kuroshio Extension, measurements made by autonomous microstructure floats have revealed vigorous microscale temperature dissipation within and below the Kuroshio thermocline over at least 300 km following the main stream, which was attributed to active double-diffusive convection (Nagai et al. 2015c). Within the surface mixed layer, recent studies have shown that downfront winds over the Kuroshio Extension generate strong turbulent mixing (D’Asaro et al. 2011; Nagai et al. 2012). The influence of fine-scale vertical mixing on nutrient supply was observed during a high-spatial resolution biogeochemical survey across the Kuroshio Extension front, revealing fine-scale “tongues” of elevated nitrate arranged along isopycnals (Figure 1, Clayton et al. 2014). Subsequent modeling work has shown that these nutrient tongues are ubiquitous features along the southern flank of the Kuroshio Extension front, formed by submesoscale surface mixed layer fronts (Nagai and Clayton 2017).

Microscale turbulence, double-diffusive convection, and submesoscale stirring are all processes associated with meso- and submesoscale fronts. The results from the studies mentioned above support the hypothesis that WBCs are an efficient conduit for transporting nutrients, not only over large scales but also more locally on fine scales, as isopycnal transporters, lateral stirrers, and diapycnal suppliers. It is the sum of these transport processes that ultimately fuels the elevated primary production observed in these regions.

Figure 1. Vertical sections of nitrate (μM) observed across the Kuroshio Extension in October 2009. The panels are organized such that they line up with respect to the density structure of the Kuroshio Extension Front. Cyan contour lines show the mixed layer depth (taken from Nagai and Clayton 2017).

Phytoplankton biomass, community structure, and dynamics

WBCs separate regions with markedly different biogeochemical and ecological characteristics. Subpolar gyres are productive, highly seasonal, tend to support ecosystems with higher phytoplankton biomass, and can be dominated by large phytoplankton and zooplankton taxa. Conversely, subtropical gyres are mostly oligotrophic, support lower photoautotrophic biomass, and are not characterized by a strong seasonal cycle. In turn, these subtropical regions tend to support ecosystems that comprise smaller cells and a tightly coupled microbial loop. As boundaries to these diverse regions, WBCs are the main conduit linking the equatorial and polar oceans and their resident plankton communities. Within the frontal zones, mesoscale dynamics act to stir water masses together and can transport ecosystems across the WBC into regions of markedly different physical and biological characteristics. Furthermore, mesoscale eddies can modulate vertical fluxes via the displacement of ispycnals during eddy intensification or eddy-induced Ekman pumping, or generating submesoscale patches of vertical exchange. At these smaller scales, vigorous vertical circulations ¾ with magnitudes reaching 100 m/day ¾ can fertilize the euphotic zone or transport phytoplankton out of the surface layer.

Numerous studies have hypothesized that the combination of large-scale transport, mesoscale stirring and transport, and submesoscale nutrient input leads to both high biodiversity and high population densities. Using remote sensing data, D’Ovidio et al. (2010) showed that mesoscale stirring in the Brazil-Malvinas Confluence Zone brings together communities from very different source regions, driving locally enhanced biodiversity. In a numerical model, in which physical and biological processes can be explicitly separated and quantified, Clayton et al. (2013) showed that high modeled biodiversity in the WBCs was due to a combination of transport and local nutrient enhancements. And finally, in situ taxonomic surveys crossing the Brazil-Malvinas Confluence (Cermeno et al. 2008) and the Kuroshio Extension (Honjo and Okada 1974; Clayton et al, 2017) showed both enhanced biomass and biodiversity associated with the WBC fronts. Beyond these local enhancements, WBCs might play a larger role in setting regional biogeography. Sugie and Suzuki (2017) found a mixture of temperate and subpolar diatom species in the Kuroshio Extension, suggesting that the boundary current might play a key role in setting downstream diatom diversity.

However tantalizing these results are, they remain relatively inconclusive, in part because of their relatively small temporal and spatial scales. Extending existing approaches for assessing phytoplankton community structure, leveraging emerging ‘omics and continuous sampling techniques, larger regions might be surveyed at high taxonomic and spatial resolution. Combining genomic and transcriptomic observations would provide measures of both organism abundance and activity (Hunt et al. 2013), as well as the potential to better define the relative roles of growth and loss processes. With genetically resolved data and appropriate survey strategies, it will be possible to conclusively determine the presence of these biodiversity hotspots. A better characterization and deeper understanding of these regions will provide insight into the long-term and large-scale biodiversity, stability, and function of the global planktonic ecosystem.

Organic carbon export via physical and biological processes

Export, the removal of fixed carbon from the surface ocean, is driven by gravitational particle sinking, active transport, and (sub)mesoscale processes such as eddy-driven subduction. While evidence suggests that WBCs are likely hot spots of biological (Siegel et al. 2014; Honda et al. 2017a) and physical (Omand et al. 2015) export fluxes out of the euphotic zone, only a small handful of studies have explored this. Recent results from sediment trap studies at the Kuroshio Extension Observatory (KEO) mooring, located just south of the Kuroshio Extension, suggest that there is a link between the passage of mesoscale eddies and carbon export (Honda et al. 2017b). They observed that high export events at 5000 m lagged behind the passage of negative (cyclonic) sea surface height anomalies (SSHA) at the mooring by one to two months (Figure 2). In other regions, underway measurements (Stanley et al. 2010) and optical sensors on autonomous platforms (Briggs et al. 2011; Estapa et al. 2013; Estapa et al. 2015; Bishop et al. 2016) have revealed large episodicity in export proxies over timescales of hours to days and spatial scales of 1-10 km.

Figure 2. Time series of ocean temperature in the upper ~550 m (less than 550 dbar) at station KEO between July 2014 and June 2016. The daily data shown in the figure are available on the KEO database. White contour lines show the temporal variability in the daily satellite-based sea surface height anomaly (SSHA). White open bars show the total mass flux (TMF) observed by the time series sediment trap at 5000 m (based on a figure in Honda et al. 2017b).

Another avenue of carbon export from the surface ocean results from grazing and vertical migration. Vertically migrating zooplankton feed near the surface in the dark and evade predation at depth during the day. Fronts generated by WBCs produce gradients in zooplankton communities, both in terms of grazer biomass and species compositions (e.g., Wiebe and Flierl, 1983), and influence the extent and magnitude of diel vertical migrations. Submesoscale variability in zooplankton abundance can be observed readily in echograms collected by active acoustic sensors, but submesoscale variability in zooplankton community structure and dynamics remains difficult to measure. Thus, the nature of this variability remains largely unknown.

Future research directions

Building a better understanding of how physical and biogeochemical dynamics in WBC regions interact relies on observing these systems at the appropriate scales. This is particularly challenging because of the range of scales at play in these systems and the limitation of existing in situ and remote observing platforms and techniques. As has been outlined above, the ecological and biogeochemical environment of WBCs is the result of long range transport from the flanking subtropical and subpolar gyres, as well as local modification by meso- and submesocale physical dynamics in these frontal systems.

Another challenge in disentangling the relationships between physical and biogeochemical processes in WBCs is the difficulty in measuring rates rather than standing stocks. In such dynamic systems, lags in biological responses mean that the changes in standing stocks may not be collocated with the physical process forcing them. Small-scale lateral stirring spatially and temporally decouples net community production and export while secondary circulations contribute to vertical transport. As much as possible, future process studies should include approaches that can explicitly quantify biological rates and physical transport pathways. New platforms are beginning to fill these observational gaps: BGC-Argo floats, autonomous platforms (e.g., Saildrone), high-frequency underway measurements, and continuous cytometers (including imaging cytometers) are all capable of generating high-spatial resolution datasets of biological and chemical properties over large regions. Gliders and profiling platforms (e.g., WireWalker) are making it possible to measure vertical profiles of biogeochemical properties at high frequency. Operating within a Lagrangian framework, while resolving lateral gradients of physical and biogeochemical tracers with ships or autonomous vehicles, may someday allow us to quantitatively partition the observed small-scale variability in biogeochemical tracers between that attributable to biological or physical processes.

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Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.