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Building ocean biogeochemistry observing capacity, one float at a time: An update on the Biogeochemical-Argo Program

Posted by mmaheigan

· Thursday, July 5th, 2018

By Ken Johnson (MBARI)

The Biogeochemical-Argo (BGC-Argo) Program is an international effort to develop a global network of biogeochemical sensors on Argo profiling floats that has emerged from over a decade of community discussion and planning. While there is no formal funding for this global program, it is being implemented via a series of international research projects that harness the unique capabilities provided by BGC profiling floats. The U.S. Ocean Carbon and Biogeochemistry (OCB) Program maintains and supports a U.S. BGC-Argo subcommittee as a focal point for U.S. community input on the implementation of the global biogeochemical float array and associated science program development.

Figure 1. Steve Riser deploying a SOCCOM float from the R/V Palmer

About BGC-Argo Floats
BGC-Argo floats can carry a suite of chemical and bio-optical sensors (Figure 1 – Float Schematic). They have enough energy to make about 250 to 300 vertical profiles from 2000 m to the surface. At a cycle time of 10 days, that corresponds to a lifetime near 7 years. The long endurance allows the floats to resolve seasonal to interannual variations in carbon and nutrient cycling throughout the water column. These time scales are difficult to study from ships and ocean interior processes are hard to resolve from satellites. BGC profiling floats extend the capabilities of these traditional observing systems in significant ways.

Figure 2. Images of Navis and APEX floats used in the SOCCOM program. These floats carry oxygen, nitrate, pH, and bio-optical (chlorophyll fluorescence and backscatter) sensors.

All of the data from profiling floats operating as part of the Argo program must be available in real-time with no restrictions on access. The Argo Global Data Assembly centers in France and the USA both provide complete listings of all BGC profiles (argo_bio-profile_index.txt) and access to the data. Extensive documentation of the data processing protocols is available from the Argo Data Management Team. Individual research programs, such as SOCCOM (see below), may also provide direct data access to the observed data along with value added products such as best estimates of pCO₂ derived from pH sensor data.

Regional Deployments
In 2018, it is projected that 127 profiling floats with biogeochemical sensors are will be deployed, including ~40 floats by U.S. projects. Most of the U.S. deployments (30+) will be carried out by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project (Figure 2 – Float Deployment). These floats will carry oxygen, nitrate, pH, chlorophyll fluorescence, and backscatter sensors. As part of the NOAA Tropical Pacific Observing System (TPOS) program, Steve Riser’s group (Univ. Washington) will deploy 3 BGC-Argo floats per year in the equatorial Pacific over the next 4 years. These floats will be equipped with oxygen, pH, bio-optical sensors and Passive Acoustic Listener (PAL) sensors, which provide wind speed estimates at 15-minute intervals while the floats are parked at 1000 m. Wind speed is derived from the noise spectrum of breaking waves. Steve Emerson (Univ. Washington), with NSF support, is also deploying floats equipped with oxygen, nitrate and pH sensors in the equatorial Pacific. With funding from NSF, Andrea Fassbender (MBARI) will deploy two floats at Ocean Station Papa in the northeast Pacific in collaboration with the EXPORTS program. These floats will also carry oxygen, nitrate, pH, and bio-optical sensors.

Nearly 90 BGC floats will be deployed in 2018 by other nations in multiple ocean basins. Much of this effort will focus on the North Pacific and North Atlantic. The sensor load on these floats is somewhat variable. Some will be deployed with only oxygen sensors or bio-optical sensors for chlorophyll fluorescence and particle abundance. Others will carry the full suite of six sensors (oxygen, nitrate, pH, chlorophyll fluorescence, backscatter, and irradiance) that are outlined in the BGC-Argo Implementation Plan (BGC-Argo, 2016). These floats will contribute to the existing array of 305 biogeochemical floats (Figure 3 BGC Argo Map).

Community Activities
In response to the tremendous interest in the scientific community in the capabilities of profiling floats, OCB is sponsoring a Biogeochemical Float Workshop at the University of Washington in Seattle from July 9-13, 2018 to begin the process of transferring this expertise to the broader oceanographic community, bringing together potential users of this technology to discuss biogeochemical profiling float technology, sensors, and data management and begin the process of the intelligent design of future scientific experiments. The workshop will provide participating scientists direct access to the facilities of the Float Laboratory operated by Riser. This workshop builds on a previous OCB workshop Observing Biogeochemical Cycles at Global Scales with Profiling Floats and Gliders (Johnson et al., 2009). BGC-Argo will also have a prominent presence at the 6th Argo Science Workshop (October 22-24, 2018, Tokyo, Japan) and OceanObs19 (September 16-20, 2019, Honolulu, HI).

Figure 3. May 2018 map of the location of BGC-Argo floats that have reported in the previous month and sensor types on these floats. From jcommops.org.

BGC-Argo Publications
Several resources now highlight the capabilities of profiling floats to accomplish scientific observing goals. A web-based bibliography of biogeochemical float papers hosted on the Biogeochemical-Argo website currently includes >100 publications and continues to grow. A special issue of Journal of Geophysical Research: Oceans focused on the SOCCOM program is in progress with 11 papers now available and a dozen more forthcoming. These papers include summaries of the technical capabilities of floats and the biogeochemical sensors, comparisons of float bio-optical data with satellite remote sensing observations, seasonal and interannual assessments of air-sea oxygen flux, under-ice biogeochemistry, carbon export, comparisons of pCO₂ estimated from floats with pH vs. time-series data, and net community production. The connection of float observations with numerical models is a special focus of the program and this is highlighted in several papers, including a description of the Biogeochemical Southern Ocean State Estimate (SOSE), which is a data assimilating BGC model. Results from Observing System Simulation Experiments (OSSEs) used to assess the number of floats needed for large-scale observations are also reported. The BGC-Argo steering committee is developing a community white paper for the OceanObs19 conference in September 2019. BGC-Argo also develops and distributes a community newsletter.

For more information, visit the BGC-Argo website or reach out to the U.S. BGC-Argo Subcommittee.

Unexpected acidification of deep waters in the Sea of Japan due to global warming

Posted by mmaheigan

· Tuesday, May 22nd, 2018

Oceans worldwide are warming up, and thermohaline circulation is expected to slow down. At the same time, ocean acidity is increasing due to the influx of anthropogenic carbon dioxide (CO₂) from the atmosphere, a phenomenon called ocean acidification that has primarily been documented in shallow waters. In general, deeper waters contain less anthropogenic CO₂, but predicted reductions in ventilation of deep waters may impact deep ocean chemistry, as described in a recent study in Nature Climate Change.

Figure caption: Secular trend of total scale pH at in-situ temperature and pressure at various depths between 1965 and 2015 in the Sea of Japan.

The Sea of Japan is a marginal sea with its own deep- and bottom-water formation that maintains relatively elevated oxygen levels. However, time-series data from 1965-2015 (the longest time-series available) reveal that oxygen concentrations in these deep waters are declining, indicating a reduction in ventilation that increases their residence time. As organic matter decomposition in these waters continues to accumulate more CO₂, the pH decreases. As a result, the acidification rate near the bottom of the Sea of Japan is 27% higher than at the surface. As a miniature ocean with its own deep- and bottom-water formation, the Sea of Japan provides insight into how future warming might alter deep-ocean ventilation and chemistry.

Authors:
Chen-Tung Arthur Chen (National SunYat-sen University, Taiwan and Second Institute of Oceanography, China)
Hon-Kit Lui (National SunYat-sen University and Taiwan Research Institute)
Chia-Han Hsieh (National SunYat-sen University, Taiwan)
Tetsuo Yanagi (International Environmental Management of Enclosed Coastal Seas Center, Japan)
Naohiro Kosugi (Japan Meterological Agency)
Masao Ishii (Japan Meterological Agency)
Gwo-Ching Gong (National Taiwan Ocean University)

Sensitivity of future ocean acidification to carbon-climate feedbacks

Posted by mmaheigan

· Thursday, May 10th, 2018

There are vast unknowns about the future oceans, from what species or habitats may be most under threat to the continuity of earth system processes that maintain global climate. Modeling can be used to predict future states and explore the impacts of climate change, but several key uncertainties such as carbon-climate feedbacks hamper our predictive power.

Authors of a recent study in Biogeosciences (Matear and Lenton 2018) used a global earth system model to explore the effects of carbon-climate feedbacks on future ocean acidification. Ocean acidification can have wide-ranging impacts on keystone species from reef-building corals to pteropods, a major food web species in the Southern Ocean. The study included four representative scenarios (from IPCC) comparing concentration pathway simulations to emission pathway simulations (RCP2.6, RCP 4.5, RCP6, RCP8.5) to determine carbon-climate feedbacks. The high emission scenarios (RCP8.5 and RCP6) showed surface water undersaturation a decade or more earlier than expected. Surprisingly, the medium (RCP4.5) scenario carbon-climate feedbacks showed the greatest acidification response, doubling the extent of undersaturation and subsequently halving the area that could sustain coral reefs by 2100. The low emissions scenario also showed significant declines in saturation state.

Surface ocean aragonite saturation state for the 2090s for RCP2.6 and RCP 8.5 concentration and emission pathways. The contour line delineates a saturation state of 3 (coral reef threshold), the white line a saturation state of 1, when aragonite becomes unstable and corals dissolve.

The extra atmospheric CO₂ from the carbon-climate feedback resulted in accelerated ocean acidification in all emission scenarios. These feedbacks may also affect global warming and deoxygenation. This is particularly important, given that many policymakers are aiming for low emission commitments, but may still be severely underestimating the extent and timing of ocean acidification. There is a great need to improve our ability to predict carbon-climate feedbacks so we do not underestimate projected ocean acidification and its impacts on both sensitive ecosystems and the human communities that rely on them for food, coastal protection and other ecosystem services.

Authors:
Richard Matear (CSIRO Oceans and Atmosphere, Australia)
Andrew Lenton (Antarctic Climate and Ecosystems CRC, Australia)

Volcanic carbon dioxide drove ancient global warming event

Posted by mmaheigan

· Thursday, March 29th, 2018

A study recently published in Nature suggests that an extreme global warming event 56 million years ago known as the Palaeocene-Eocene Thermal Maximum (PETM) was driven by massive CO₂ emissions from volcanoes during the formation of the North Atlantic Ocean. Using a combination of new geochemical measurements and novel global climate modelling, the study revealed that atmospheric CO₂ more than doubled in less than 25,000 years during the PETM.

The PETM lasted ~150,000 years and is the most rapid and extreme natural global warming event of the last 66 million years. During the PETM, global temperatures increased by at least 5^°C, comparable to temperatures projected in the next century and beyond. While it has long been suggested that the PETM event was caused by the injection of carbon into the ocean and atmosphere, the source and total amount of carbon, as well as the underlying mechanism have thus far remained elusive. The PETM roughly coincided with the formation of massive flood basalts resulting from of a series of eruptions that occurred as Greenland and North America started separating from Europe, thereby creating the North Atlantic Ocean. What was missing is evidence linking the volcanic activity to the carbon release and warming that marks the PETM.

To identify the source of carbon, the authors measured changes in the balance of isotopes of the element boron in ancient sediment-bound marine fossils called foraminifera to generate a new record of ocean pH throughout the PETM. Ocean pH tells us about the amount of carbon absorbed by ancient seawater, but we can get even more information by also considering changes in the isotopes of carbon, which provide information about the carbon source. When forced with these ocean pH and carbon isotope data, a numerical global climate model implicates large-scale volcanism associated with the opening of the North Atlantic as the primary driver of the PETM.

North Atlantic microfossil-derived isotope records from extinct planktonic foraminiferal species M. subbotinae relative to the onset of the PETM carbon isotope excursion (CIE). The negative trend in carbon isotope composition (A) during the carbon emission phase is accompanied by decreasing pH (decreasing δ¹¹B, panel B) and increasing temperature (decreasing δ¹⁸O, panel C). Panels D and E zoom in on the PETM CIE, showing microfossil δ¹³C (D) and δ¹¹B-based pH (E) reconstructions. Also included in E are data from Penman et al. (2014) on their original age model, with recalculated (lab-based) pH values.

These new results suggest that the PETM was associated with a total input of >12,000 petagrams of carbon from a predominantly volcanic source. This is a vast amount of carbon—30 times larger than all of the fossil fuels burned to date and equivalent to all current conventional and unconventional fossil fuel reserves. In the following Earth System Model simulations, it resulted in the concentration of atmospheric CO₂ increasing from ~850 parts per million to >2000 ppm. The Earth’s mantle contains more than enough carbon to explain this dramatic rise, and it would have been released as magma poured from volcanic rifts at the Earth’s surface.

How the ancient Earth system responded to this carbon injection at the PETM can tell us a great deal about how it might respond in the future to man-made climate change. Earth’s warming at the PETM was about what we would expect given the CO₂ emitted and what we know about the sensitivity of the climate system based on Intergovernmental Panel on Climate Change (IPCC) reports. However, the rate of carbon addition during the PETM was about twenty times slower than today’s human-made carbon emissions.

In the model outputs, carbon cycle feedbacks such as methane release from gas hydrates—once the favoured explanation of the PETM—did not play a major role in driving the event. Additionally, one unexpected result was that enhanced organic matter burial was important in ultimately drawing down the released carbon out of the atmosphere and ocean and thereby accelerating the recovery of the Earth system.

Authors:
Marcus Gutjahr (National Oceanography Centre Southamption, GEOMAR)
Andy Ridgwell (Bristol University, University of California Riverside)
Philip F. Sexton (The Open University, UK)
Eleni Anagnostou (National Oceanography Centre Southamption)
Paul N. Pearson (Cardiff University)
Heiko Pälike (University of Bremen)
Richard D. Norris (Scripps Institution of Oceanography)
Ellen Thomas (Yale University, Wesleyan University)
Gavin L. Foster (National Oceanography Centre Southamption)

NASA Arctic-COLORS scoping study report

Posted by mmaheigan

· Monday, March 5th, 2018

Arctic-COLORS (Arctic-COastal Land Ocean inteRactionS): A Science Plan for a NASA Field Campaign in the Coastal Arctic

View or download the PDF

Executive Summary
Over the past century, average Arctic surface air temperatures have increased at almost twice the global average rate and this rapid warming trend is expected to continue over the next century. Consequently, Arctic ecosystems have become an area of intense research focus, with specific emphasis on identifying compounding and exacerbating factors that can be critically important to our understanding and modeling of key biogeochemical processes. Short-term climate forcings and feedbacks that potentially accelerate local warming and environmental change in the Arctic are also increasingly affecting society in a variety of ways: from erosion of Arctic coastlines, to modifications of wildlife habitat and ecosystems that affect subsistence opportunities, to changes in transportation infrastructure, mineral development, and other ecological, socio-cultural, and economic uses of coastal ecosystem services. Arctic climate change has global implications, contributing to global sea level rise and affecting heat-flux changes, atmospheric circulation, and ocean circulation and dynamics beyond the Arctic region. The realization that changes within the Arctic have profound impacts on ecosystems and human populations across the globe has motivated greater attention by researchers, funding agencies, governmental policy makers, and non-governmental organizations. Recognizing the challenges associated with climate change and the emergence of a new Arctic environment, the White House released The National Strategy for the Arctic Region in May 2013. Included among the main principles highlighted in this strategy is: “Making decisions using the best available information by promptly sharing – nationally and internationally – the most current understanding and forecasts based on up-to-date science and traditional knowledge”. Yet major gaps remain in our understanding of the feedbacks, response, and resilience of coastal Arctic ecosystems, communities, and natural resources to current and future pressures. The biogeochemistry of the Arctic nearshore coastal zone, a vulnerable and complex contiguous landscape of lakes, streams, wetlands, permafrost, rivers, lagoons, estuaries and coastal seas, all modified by snow and ice, remains poorly understood.

To improve our mechanistic understanding and prediction capabilities of land-ocean interactions in the rapidly changing Arctic coastal zone, our team proposed a Field Campaign Scoping Study called Arctic-COLORS (Arctic-COastal Land Ocean inteRactionS) to NASA’s Ocean Biology and Biogeochemistry (OBB) Program. The goal of the project was to develop a scoping study report for NASA that describes and justifies the science imperative and design of an integrative, interdisciplinary oceanographic field campaign program that addresses high priority science questions related to land-ocean interactions in the Arctic. During the preparation of the scoping study report, our team consulted with the community to refine the high priority science questions for Arctic-COLORS, determine the study domain and research phases for the field campaign, and explore opportunities for linking to other field activities in the Arctic. Addressing the campaign’s objectives will require multidisciplinary expertise, a coordinated engagement of regional authorities and local communities, and a combination of field studies, remotely sensed observations from various platforms (shipboard, buoys, gliders, ground-based, airborne, satellite, for example), process studies, and numerical modeling. This scoping study report does not describe a comprehensive field campaign activity in detail, but rather sketches out key aspects of a field campaign program including the study region, sampling approaches, critical measurements, remote sensing assets, and modeling activities necessary to address the science objectives.

What is Arctic-COLORS? Arctic-COLORS is a proposed NASA-funded field campaign designed to quantify the response of the Arctic coastal environment to global change and anthropogenic disturbances – an imperative for developing mitigation and adaptation strategies for the region. The Arctic-COLORS field campaign is unprecedented, as it represents the first attempt to study the nearshore coastal Arctic (from riverine deltas and estuaries out to the coastal sea) as an integrated land-ocean-atmosphere-biosphere system.

The overarching objective of Arctic-COLORS is to determine present and future impacts of terrigenous, atmospheric, and oceanic fluxes on the biogeochemistry, ecology and ecosystem services of the Arctic coastal zone in the context of environmental (short-term) and climate (long-term) change. This focus on land-ocean interactions in the nearshore coastal zone is a unique contribution of Arctic-COLORS compared to other NASA field campaigns in polar regions. The science of our field campaign will focus on five key science questions:

1. How and where are materials from the land, atmosphere, and ocean transformed within the land-ocean continuum of the Arctic coastal zone?
2. How does thawing of Arctic permafrost—either directly through coastal erosion or indirectly through changing freshwater loads from upstream thaw—translate to changes in coastal ecology and biogeochemistry?
3. How do changes in snow/ice conditions and coastal circulation influence Arctic coastal ecology and biogeochemistry?
4. How do changes in fluxes of materials, heat, and buoyancy from the land, atmosphere, and ocean influence Arctic coastal ecology and biogeochemistry?
5. How do changing environmental (short-term) and climate (long-term) conditions alter the Arctic coastal zone’s availability and use of ecosystem services?

Scoping Study Principal Investigators and Primary Authors (alphabetical order):
Antonio Mannino, Carlos Del Castillo, Marjorie Friedrichs, Peter Hernes, Patricia Matrai, Joseph Salisbury, Maria Tzortziou

Collaborators and Co-Authors:
Matthew Alkire, Marcel Babin, Simon Bélanger, Emmanuel Boss, Eddy Carmack, Lee Cooper, Susanne Craig, Jerome Fiechter, Joaquim Goes, Peter Griffith,
Dan Hodkinson, Christopher Hostetler, David Kirchman, Diane Lavoie, Bonnie Light, James McClelland, Donald McLennan, Irina Overeem, Christopher Polashenski, Michael Rawlins, Rick Reynolds, Rachael Sipler, Michael Steele, Robert Striegl, James Syvitski, Suzanne Tank, Muyin Wang, Tom Weingartner

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.

Increased temperatures suggest reduced capacity for carbon

Posted by mmaheigan

· Thursday, January 18th, 2018

The ocean’s biological pump works to draw down atmospheric carbon dioxide (CO₂) by exporting carbon from the surface ocean. This process is less efficient at higher temperatures, implying a possible climate feedback. Recent work by Cael et al. provides an explanation of why this feedback occurs and an estimate of its severity.

In a highly simplified view, carbon export depends on the balance between two temperature-dependent processes: 1) The autotrophic production and 2) the heterotrophic respiration of organic carbon. Cael and Follows (Geophysical Research Letters 2016) recently developed a mechanistic model based on established temperature dependencies for photosynthesis and respiration to explore feedbacks between export efficiency and climate. Heterotrophic growth rates increase more so than phototrophic rates with increasing temperature, which suggests that at higher temperatures, community respiration will increase relative to production, thereby decreasing export efficiency. Although simplistic, the model captures the temperature dependence of export efficiency observations.

Figure: Schematic of the mechanism on which the Cael and Follows (2016) model is based. (a) Photosynthesis (dark grey) and respiration (light grey) respond to temperature differently, yielding (b) a decline in export efficiency at higher temperatures.

More recently, Cael, Bisson, and Follows (Limnology and Oceanography 2017) applied this model to sea surface temperature records and estimated a ~1.5% decline in globally-averaged export efficiency over the past three decades of increasing ocean temperatures as a result of this metabolic mechanism. This ~1.5% decline is equivalent to a reduced ocean sequestration of approximately 100 million fewer tons of carbon annually, comparable to the annual carbon emissions of the United Kingdom. The model provides a framework in which to consider the relationship between climate and ocean carbon export that might also elucidate large-scale (e.g., glacial-interglacial) atmospheric CO₂ changes of the past.

Authors:
B. B. Cael (MIT/WHOI)
Kelsey Bisson (UCSB)
Mick Follows (MIT)

Zooplankton play a key and diverse role in the ocean carbon cycle

Posted by mmaheigan

· Thursday, December 7th, 2017

How does the enormous diversity of zooplankton species, life cycles, size, feeding ecology, and physiology affect their role in ocean food webs and cycling of carbon?

In the 2017 issue of Annual Review of Marine Science, Steinberg and Landry review the fundamental and multifaceted roles that zooplankton play in the cycling and export of carbon in the ocean. Carbon flows through marine pelagic ecosystems are complex due to the diversity of zooplankton consumers and the many trophic levels they occupy in the food web–from single-celled herbivores to large carnivorous jellyfish. Zooplankton also contribute to carbon export processes through a variety of mechanisms (mucous feeding webs, fecal pellets, molts, carcasses, and vertical migrations).

Figure 1. Pathways of cycling and export of carbon by zooplankton in the ocean.

Climate change and other stressors are already affecting zooplankton abundance, distribution, and life cycles, and are predicted to result in widespread changes in zooplankton carbon cycling in the future. These changes will affect both the larger marine food web that depends upon zooplankton for food (fish) or recycled products for growth (primary producers) and the amount of carbon exported into the deep sea–where far from contact with the atmosphere it no longer contributes to global warming.

Authors:

Deborah K. Steinberg, Virginia Institute of Marine Science, The College of William and Mary
Michael R. Landry, Scripps Institution of Oceanography

WBC Series: Decadal variability of the Kuroshio Extension system and its impact on subtropical mode water formation

Posted by mmaheigan

· Friday, November 10th, 2017

Bo Qiu¹, Eitarou Oka², Stuart P. Bishop³, Shuiming Chen¹, Andrea J. Fassbender⁴

1. University of Hawaii at Manoa
2. The University of Tokyo
3. North Carolina State University
4. Monterey Bay Aquarium Research Institute

After separating from the Japanese coast at 36°N, 141°E, the Kuroshio enters the open basin of the North Pacific, where it is renamed the Kuroshio Extension (KE). Free from the constraint of coastal boundaries, the KE has been observed to be an eastward-flowing inertial jet accompanied by large-amplitude meanders and energetic pinched-off eddies (see Qiu 2002 and Kelly et al. 2010 for comprehensive reviews). Compared to its upstream counterpart south of Japan, the Kuroshio, the KE is accompanied by a stronger southern recirculation gyre that increases the KE’s eastward volume transport to more than twice the maximum Sverdrup transport (~ 60Sv) in the subtropical North Pacific Ocean (Wijffels et al. 1998). This has two important consequences. Dynamically, the increased transport enhances the nonlinearity of the KE jet, rendering the region surrounding the KE jet to have the highest mesoscale activity level in the Pacific basin. Thermodynamically, the enhanced KE jet brings a significant amount of tropical-origin warm water to the mid-latitude ocean to be in direct contact with cold, dry air blowing off the Eurasian continent. This results in significant wintertime heat loss from the ocean to atmosphere surrounding the Kuroshio/KE paths, contributing to the formation of North Pacific subtropical mode water (STMW; see Hanawa and Talley (2001) and Oka and Qiu (2012) for comprehensive reviews).

Figure 1. Yearly paths of the Kuroshio and KE plotted every 14 days using satellite SSH data (updated based on Qiu and Chen 2005). KE was in stable state in 1993–94, 2002–05, and 2010–15, and unstable state in 1995-2001, 2006–09, and 2016, respectively.

Although the ocean is known to be a turbulent medium, variations in both the level of mesoscale eddy activity and the formation rate of STMW in the KE region are by no means random on interannual and longer timescales. One important feature emerging from recent satellite altimeter measurements and eddy-resolving ocean model simulations is that the KE system exhibits clearly defined decadal modulations between a stable and an unstable dynamical state (e.g., Qiu & Chen 2005, 2010; Taguchi et al. 2007; Qiu et al. 2007; Cebollas et al. 2009; Sugimoto and Hanawa 2009; Sasaki et al. 2013; Pierini 2014; Bishop et al. 2015). As shown in Figure 1, the KE paths were relatively stable in 1993–95, 2002–05, and 2010–15. In contrast, spatially convoluted paths prevailed during 1996–2001 and 2006–09. When the KE jet is in a stable dynamical state, satellite altimeter data further reveal that its eastward transport and latitudinal position tend to increase and migrate northward, its southern recirculation gyre tends to strengthen, and the regional eddy kinetic energy level tends to decrease. The reverse is true when the KE jet switches to an unstable dynamical state. In fact, the time-varying dynamical state of the KE system can be well represented by the KE index, defined by the average of the variance-normalized time series of the southern recirculation gyre intensity, the KE jet intensity, its latitudinal position, and the negative of its path length (Qiu et al. 2014). Figure 2a shows the KE index time series in the satellite altimetry period of 1993–present; here, a positive KE index indicates a stable dynamical state and a negative KE index, an unstable dynamical state. From Figure 2a, it is easy to discern the dominance of the decadal oscillations between the two dynamical states of the KE system.

Figure 2. (a) Time series of the KE index from 1993‑present; available at http://www.soest.hawaii.edu/oceanography/bo/KE_index.asc. (b) Year-mean SSH maps when the KE is in stable (2004 and 2011) versus unstable (1997 and 2008) states. (c) SSH anomalies along the zonal band of 32°-34°N from satellite altimetry measurements. (d) Time series of the PDO index from 1989-present; available at http://jisao.washington.edu/pdo/PDO.latest.

Transitions between the KE’s two dynamical states are caused by the basin-scale wind stress curl forcing in the eastern North Pacific related to the Pacific Decadal Oscillation (PDO). Specifically, when the central North Pacific wind stress curl anomalies are positive during the positive PDO phase (see Figure 2d), enhanced Ekman flux divergence generates negative local sea surface height (SSH) anomalies in 170°–150°W along the southern recirculation gyre latitude of 32°–34°N. As these wind-induced negative SSH anomalies propagate westward as baroclinic Rossby waves into the KE region after a delay of 3–4 years (Figure 2c), they weaken the zonal KE jet, leading to an unstable (i.e., negative index) state of the KE system with a reduced recirculation gyre and an active eddy kinetic energy field (Figure 2b). Negative anomalous wind stress curl forcing during the negative PDO phase, on the other hand, generates positive SSH anomalies through the Ekman flux convergence in the eastern North Pacific. After propagating into the KE region in the west, these anomalies stabilize the KE system by increasing the KE transport and by shifting its position northward, leading to a positive index state.

The dynamical state of the KE system exerts a tremendous influence upon the STMW that forms largely along the paths of the Kuroshio/KE jet and inside of its southern recirculation gyre (e.g., Suga et al. 2004; Qiu et al. 2006; Oka 2009). Figure 3a shows the monthly time series of temperature profile, constructed by averaging available Argo and XBT/CTD/XCTD data inside the KE southern recirculation gyre (see Qiu and Chen 2006 for details on the constructing method). The black line in the plot denotes the base of the mixed layer, defined as where the water temperature drops by 0.5°C from the sea surface temperature. Based on the temperature profiles, Figure 3b shows the monthly time series of potential vorticity. STMW in Figure 3b is characterized by water columns with potential vorticity of less than 2.0 x 10^-10 m^-1s^-1 beneath the mixed layer. From Figure 3, it is clear that both the late winter mixed layer depth and the low-potential vorticity STMW layer underwent significant decadal changes over the past 25 years. Specifically, deep mixed layer and pronounced low-potential vorticity STMW were detected in 1993–95, 2001–05, and 2010–15, and these years corresponded roughly to the periods when the KE index was in the positive phase (cf. Figure 2a).

Figure 3. Monthly time series of (a) temperature (°C) and (b) potential vorticity (10-10 m-1 s-1) averaged in the KE’s southern recirculation gyre. The thick black and white lines in (a) and (b) denote the base of the mixed layer, defined as where the temperature drops by 0.5°C from the surface value. Red pluses (at the top of each panel) indicate the individual temperature profiles used in constructing the monthly T(z, t) profiles. The potential vorticity, Q(z,t) = fα∂T(z,t)/∂z, where f is the Coriolis parameter and α the thermal expansion coefficient.

The close connection between the dynamical state of the KE system and the STMW formation has been detected by many recent studies based on different observational data sources and analysis approaches (Qiu and Chen 2006; Sugimoto and Hanawa 2010; Rainville et al. 2014; Bishop and Watts 2014; Oka et al. 2012; 2015; Cerovecki and Giglio 2016). Physically, this connection can be understood as follows. When the KE is in an unstable state (or a negative KE index phase), high-regional eddy variability infuses high-potential vorticity KE and subarctic-gyre water into the southern recirculation gyre, increasing the upper-ocean stratification and hindering the development of deep winter mixed layer and formation of STMW. A stable KE path with suppressed eddy variability (in the positive KE index phase), on the other hand, favors the maintenance of a weak stratification in the recirculation gyre, leading to the formation of a deep winter mixed layer and thick STMW.

Since the STMW is renewed each winter, due to combined net surface heat flux and wind stress forcing that modulate on interannual timescales, a question arising naturally is the timescale on which the dynamical state change of the KE system is able to alter the upper ocean stratification and potential vorticity inside the recirculation gyre. If the influence of the KE dynamical state acts on interannual timescales, one may expect a stronger control on the STMW variability by the wintertime atmospheric condition (e.g., Suga and Hanawa 1995; Davis et al. 2011). Intensive observations from the Kuroshio Extension System Study (KESS) program, spanning the period from April 2004 to July 2006, captured the 2004–05 transition of the KE system from a stable to an unstable state. The combined measurements by profiling Argo floats, moored current meter, current and pressure inverted echo sounder (CPIES), and the Kuroshio Extension Observatory (KEO) surface mooring revealed that the KE dynamical state change was able to change the STMW properties both significantly in amplitude and effectively in time (Qiu et al. 2007; Bishop 2013; Cronin et al. 2013; Bishop and Watts 2014). Relative to 2004, the low-potential vorticity signal in the core of STMW was diminished by one-half in 2005, and this weakening of STMW’s intensity occurred within a period of less than seven months. These significant and rapid responses of STMW to the KE dynamical state change suggests that the variability in STMW formation is more sensitive to the dynamical state of the KE than to interannual variations in overlying atmospheric conditions over the past 25 years.

The decadal variability of STMW in the KE’s southern recirculation gyre is able to affect the water property distributions in the entire western part of the North Pacific subtropical gyre (Oka et al. 2015). Measurements by Argo profiling floats during 2005–14 revealed that the volume and spatial extent of STMW decreased (increased) in 2006–09 (after 2010) during the unstable (stable) KE period in its formation region north of ~28°N, as well as in the southern, downstream regions with a time lag of 1-2 years. Such decadal subduction variability affects not only physical but also biogeochemical structures in the downstream, interior subtropical gyre. Shipboard observations at 25°N and along the 137°E repeat hydrographic section of the Japan Meteorological Agency exhibited that, after 2010, enhanced subduction of STMW consistently increased dissolved oxygen, pH, and aragonite saturation state and decreased potential vorticity, apparent oxygen utilization, nitrate, and dissolved inorganic carbon. Changes in dissolved inorganic carbon, pH, and aragonite saturation state were opposite their long-term trends.

KE State and the Ocean Carbon Cycle

Western boundary current (WBC) regions display the largest magnitude air-to-sea carbon dioxide (CO₂) fluxes of anywhere in the global ocean. STMW formation processes are thought to account for a majority of the anthropogenic CO₂ sequestration that occurs outside of the polar, deep water formation regions (Sabine et al. 2004; Khatiwala et al. 2009). Once subducted and advected away from the formation region, mode waters often remain out of contact with the atmosphere on timescales of decades to hundreds of years, making them short-term carbon silos relative to the abyssal carbon storage reservoirs. One of the physical impacts on carbon uptake via air-sea CO₂flux is due to the temperature dependence of the solubility of pCO₂ in the surface waters. Cooler surface waters during the wintertime months reduce the oceanic pCO₂and subsequently enhance the CO₂flux into the ocean. This carbon uptake corresponds with the timing of peak STMW formation.

As mentioned above, the formation of STMW is modulated by the dynamic states of the KE, with less STMW forming during unstable states and more during stable states. To complicate matters, more enhanced levels of surface chlorophyll (Chla) have also been observed from satellite ocean color during unstable states (Lin et al. 2014), which points to the potential importance of biophysical interactions on carbon uptake. Elevated levels of Chla can further modify the pCO₂ of surface waters and enhance carbon export at depth from sinking of particulate organic matter following an individual bloom. Given that submesoscale processes result from deep wintertime mixed layers and from the presence of the larger mesoscale lateral shear and strain fields (McWilliams 2016), it is expected that submesoscale processes are also important in STMW formation during unstable states of the KE. An open question in the research community is to what extent do elevated levels of mesoscale and submesoscale eddy activity modulate STMW formation and carbon uptake during unstable states of the KE? With large variations in STMW formation occurring in concert with decadal variability in the mesoscale eddy field, it is possible that submesoscale processes may impact STMW formation through restratification of the mixed layer within density classes encompassing STMW and timing of the spring bloom. These mesoscale and submesoscale processes may then also impact the uptake of CO₂ in the North Pacific on interannual to decadal timescales.

References

Bishop, S. P., 2013: Divergent eddy heat fluxes in the Kuroshio Extension at 143°-149°E. Part II: Spatiotemporal variability. J. Phys. Oceanogr., 43, 2416-2431, doi: 10.1175/JPO-D-13-061.1.

Bishop, S. P., and D. R. Watts, 2014: Rapid eddy-induced modification of subtropical mode water during the Kuroshio Extension System Study. J. Phys. Oceanogr., 44, 1941-1953, doi:10.1175/JPO-D-13-0191.1.

Bishop, S. P., F. O. Bryan, and R. J. Small, 2015: Bjerknes-like compensation in the wintertime north Pacific. J. Phys. Oceanogr., 45, 1339-1355, doi:10.1175/JPO-D-14-0157.1.

Ceballos, L., E. Di Lorenzo, C. D. Hoyos, N. Schneider, and B. Taguchi, 2009: North Pacific Gyre oscillation synchronizes climate variability in the eastern and western boundary current systems. J. Climate, 22, 5163-5174, doi:10.1175/2009JCLI2848.1.

Cerovecki, I., and D. Giglio, 2016: North Pacific subtropical mode water volume decrease in 2006–09 estimated from Argo observations: Influence of surface formation and basin-scale oceanic variability. J. Climate, 29, 2177-2199, doi:10.1175/JCLI-D-15-0179.1.

Cronin, M. F., N. A. Bond, J. T. Farrar, H. Ichikawa, S. R. Jayne, Y. Kawai, M. Konda, B. Qiu, L. Rainville, and H. Tomita, 2013: Formation and erosion of the seasonal thermocline in the Kuroshio Extension Recirculation Gyre. Deep-Sea Res. II, 85, 62-74, doi:10.1016/j.dsr2.2012.07.018.

Davis, X. J., L. M. Rothstein, W. K. Dewar, and D. Menemenlis, 2011: Numerical investigations of seasonal and interannual variability of North Pacific subtropical mode water and its implications for Pacific climate variability. J. Climate, 24, 2648-2665, doi:10.1175/2010JCLI3435.1.

Hanawa, K., and L. D. Talley, 2001: Mode waters. Ocean Circulation and Climate: Observing and Modelling the Global Ocean, G. Siedler, J. Church, and J. Gould, Eds., Academic Press, 373-386.

Khatiwala, S., Primeau, F., and Hall, T., 2009: Reconstruction of the history of anthropogenic CO2 concentrations in the ocean. Nature, 462, 346–349, doi:10.1038/nature08526.

Kelly, K. A., R. J. Small, R. M. Samelson, B. Qiu, T. M. Joyce, Y.-O. Kwon, and M. F. Cronin, 2010: Western boundary currents and frontal air-sea interaction: Gulf Stream and Kuroshio Extension. J. Climate, 23, 5644-5667, doi:10.1175/2010JCLI3346.1.

Lin, P., F. Chai, H. Xue, and P. Xiu, 2014: Modulation of decadal oscillation on surface chlorophyll in the Kuroshio Extension. J. Geophys. Res., 119, 187–199, doi:10.1002/2013JC009359.

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Oka, E., 2009: Seasonal and interannual variation of North Pacific subtropical mode water in 2003–2006. J. Oceanogr., 65, 151-164, doi:10.1007/s10872-009-0015-y.

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Oka, E., B. Qiu, S. Kouketsu, K. Uehara, and T. Suga, 2012: Decadal seesaw of the central and subtropical mode water formation associated with the Kuroshio Extension variability. J. Oceanogr., 68, 355-360, doi: 10.1007/s10872-015-0300-x.

Oka, E., B. Qiu, Y. Takatani, K. Enyo, D. Sasano, N. Kosugi, M. Ishii, T. Nakano, and T. Suga, 2015: Decadal variability of subtropical mode water subduction and its impact on biogeochemistry. J. Oceanogr., 71, 389-400, doi: 10.1007/s10872-015-0300-x.

Pierini, S., 2014: Kuroshio Extension bimodality and the North Pacific Oscillation: A case of intrinsic variability paced by external forcing. J. Climate, 27, 448-454, doi:10.1175/JCLI-D-13-00306.1.

Qiu, B., 2002: The Kuroshio Extension system: Its large-scale variability and role in the midlatitude ocean-atmosphere interaction. J. Oceanogr., 58, 57-75, doi:10.1023/A:1015824717293.

Qiu, B., and S. Chen, 2005: Variability of the Kuroshio Extension jet, recirculation gyre and mesoscale eddies on decadal timescales. J. Phys. Oceanogr., 35, 2090-2103, doi: 10.1175/JPO2807.1.

Qiu, B., and S. Chen, 2006: Decadal variability in the formation of the North Pacific subtropical mode water: Oceanic versus atmospheric control. J. Phys. Oceanogr., 36, 1365-1380, doi: 10.1175/JPO2918.1.

Qiu, B., and S. Chen, 2010: Eddy-mean flow interaction in the decadally-modulating Kuroshio Extension system. Deep-Sea Res. II, 57, 1098-1110, doi:10.1016/j.dsr2.2008.11.036.

Qiu, B., S. Chen, and P. Hacker, 2007: Effect of mesoscale eddies on subtropical mode water variability from the Kuroshio Extension System Study (KESS). J. Phys. Oceanogr., 37, 982-1000, doi:10.1175/JPO3097.1.

Qiu, B., N. Schneider, and S. Chen, 2007: Coupled decadal variability in the North Pacific: An observationally-constrained idealized model. J. Climate, 20, 3602-3620, doi:10.1175/JCLI4190.1.

Qiu, B., S. Chen, N. Schneider, and B. Taguchi, 2014: A coupled decadal prediction of the dynamic state of the Kuroshio Extension system. J. Climate, 27, 1751-1764, doi:10.1175/JCLI-D-13-00318.1.

Qiu, B., P. Hacker, S. Chen, K. A. Donohue, D. R. Watts, H. Mitsudera, N. G. Hogg and S. R. Jayne, 2006: Observations of the subtropical mode water evolution from the Kuroshio Extension System Study. J. Phys. Oceanogr., 36, 457-473, doi:10.1175/JPO2849.1.

Rainville, L., S. R. Jayne, and M. F. Cronin, 2014: Variations of the North Pacific subtropical mode water from direct observations. J. Climate, 27, 2842-2860, doi:10.1175/JCLI-D-13-00227.1.

Sabine, C. L., Feely, R. A., Gruber, N., Key, R. M., Lee, K., Bullister, J. L., Wanninkhof, R., Wong, C., Wallace, D. W. R., Rilbrook, B., Millero, F. J., Peng, T.-H., Kozyr, A., Ono, T., and Rios, A. F., 2004. The oceanic sink for anthropogenic CO₂. Science, 305, 367–371.

Sasaki, Y. N, S. Minobe, and N. Schneider, 2013: Decadal response of the Kuroshio Extension jet to Rossby waves: Observation and thin-jet theory. J. Phys. Oceanogr., 43, 442-456, doi:10.1175/JPO-D-12-096.1.

Suga, T., and K. Hanawa, 1995: Interannual variations of North Pacific subtropical mode water in the 137°E section. J. Phys. Oceanogr., 25, 1012–1017, doi:10.1175/1520-0485(1995)025<1012:IVONPS>2.0.CO;2.

Suga, T., K. Motoki, Y. Aoki, and A. M. MacDonald, 2004: The North Pacific climatology of winter mixed layer and mode waters. J. Phys. Oceanogr., 34, 3–22, doi:10.1175/1520-0485(2004)034<0003:TNPCOW>2.0.CO;2.

Sugimoto, S., and K. Hanawa, 2009: Decadal and interdecadal variations of the Aleutian Low activity and their relation to upper oceanic variations over the North Pacific. J. Meteor. Soc. Japan, 87, 601-614, doi:10.2151/jmsj.87.601.

Sugimoto, S., and K. Hanawa, 2010: Impact of Aleutian Low activity on the STMW formation in the Kuroshio recirculation gyre region. Geophys. Res. Lett., 37, doi:10.1029/ 2009GL041795.

Taguchi, B., S.-P. Xie, N. Schneider, M. Nonaka, H. Sasaki, and Y. Sasai, 2007: Decadal variability of the Kuroshio Extension. Observations and an eddy-resolving model hindcast. J. Climate, 20, 2357-2377, doi:10.1175/JCLI4142.1.

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Role for iron in controlling microbial phosphorus acquisition in the ocean

Posted by mmaheigan

· Thursday, October 12th, 2017

In the subtropical North Atlantic, dissolved inorganic phosphorus (DIP) concentrations are depleted and might co-limit N₂ fixation and microbial productivity. There are relatively large pools of dissolved organic phosphorus (DOP), but microbes need an enzyme to access this P source. One such alkaline phosphatase (APase) enzyme requires zinc (Zn) as its activating cofactor. This has been known for almost 30 years. However, recent crystallography studies revealed that two other widespread APase enzymes contain Fe. Via this requirement, Fe availability could regulate microbial access to the DOP pool.

As detailed in a recent publication in Nature Communications (Browning et al. 2017), this hypothesis was tested on a cruise across the tropical North Atlantic by adding Fe and Zn to incubated seawater and monitoring changes in bulk APase using a simple fluorescence assay. Adding Fe significantly increased APase activity in seawater samples collected in areas that were far-removed from coastal and aerosol Fe sources. Despite seawater Zn concentrations being much lower than Fe, it appeared not to be limiting.

Iron (Fe) and zinc (Zn) enrichment experiments conducted in the DIP-depleted tropical North Atlantic suggested that Fe, not Zn, could limit alkaline phosphatase activity (APA). DIP*=DIP–DIN/16, and represents excess DIP availability assuming a 16-fold higher microbial N requirement. Results in the bar chart represent a subset of treatments from one experiment (out of eight conducted).

DIP is depleted in surface waters of the tropical North Atlantic because inputs of North African aerosol Fe stimulates N₂ fixation and leads to microbial drawdown of DIP. If the modern ocean is a good analog for the past, the lack of APase stimulation following experimental Zn addition could reflect limited evolutionary selection for Zn-containing APase. In general, DIP is only substantially depleted where there is enhanced Fe input fueling N₂ fixation; it therefore follows that any significant requirement for APases might be restricted to these relatively high-Fe, low-Zn waters.

On a shorter timescale, growing anthropogenic nitrogen input to the ocean relative to phosphorus could result in more prevalent oceanic phosphorus deficiency. Corresponding iron inputs might then serve as an important control on phosphorus availability for microbes in these regions.

Authors:

Tom Browning (GEOMAR Helmholtz Centre for Ocean Research, Kiel, Germany)
Eric Achterberg (GEOMAR)
Jaw Chuen Yong (GEOMAR)
Insa Rapp (GEOMAR)
Caroline Utermann (GEOMAR)
Anja Engel (GEOMAR)
Mark Moore (Ocean and Earth Science, University of Southampton, Southampton, UK)

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.

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