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Rapid warming and salinity changes mask acidification in Gulf of Maine waters

Posted by mmaheigan

· Wednesday, February 20th, 2019

Why don’t we see ocean acidification in over a decade of high-frequency observations in the Gulf of Maine? The answer lies in a recent decade of changes that raised sea surface temperature and salinity, and in turn dampened the expected acidification signal and caused the saturation states of calcite minerals to increase. From 2004 to 2014, sea surface temperatures in the Gulf of Maine were higher than any observations recorded in the region over the past 150 years. This greatly impacted both CO₂ solubility and the sea surface carbonate system, as detailed in a recent paper in Biogeochemistry.

Over the 34 years of the time-series, the recent event is extreme, but interannual and decadal salinity and temperature variability also influenced carbonate system parameters, which makes it difficult to isolate and quantify an anthropogenic ocean acidification signal, especially if relying on shorter-term observations (Figure 1).

Figure 1: Modeled Ω_Aragonite (top panel) and pH (bottom panel) anomalies relative to monthly 2004 data. The red lines show trends prior to and after 2004, after which warming accelerated.

For those with a stake in profiting from or managing extractive resources that are susceptible to ocean acidification such as commercially important lobster and bivalves, understanding how ecosystems will be affected is critical. These analyses clearly demonstrate how physical processes can either accelerate or mitigate ocean carbonate system changes, thus confounding the detection of ocean acidification that is expected from increasing atmospheric carbon dioxide. To assess whether an ecosystem or species is at risk or aided by such processes, it is important to observe, understand, and be able to model all sources of carbonate system variability.

Authors:
Joe Salisbury and Bror Jönsson (Both at Ocean Processes Analysis Laboratory, University of New Hampshire)

Autonomous measurement of N-loss in the Eastern Tropical North Pacific ODZ: An Invitation for Collaboration

Posted by mmaheigan

· Thursday, January 10th, 2019

By Mark A. Altabet (SMAST/U. Mass. Dartmouth), Craig McNeil, and Eric D’Asaro (both at APL / U. Washington)

Oxygen deficient zones (ODZs) constitute a small fraction of total oceanic volume yet play an important role in regulating global ocean carbon and nitrogen cycles. They are critical for regulating the ocean’s nitrogen budget, as loss of biologically available nitrogen to N₂ gas (N-loss) within ODZs is estimated to be 30 to 50% of the global total. However, temporal and spatial variability in N-loss rates have been undersampled by ship-based process studies leaving substantial uncertainty in overall rates. While local and short-term regulation of N-loss by O₂ and organic matter availability is well documented, little is known about the larger scale temporal/spatial variability in N-loss that may result from physical forcings such as remote ventilation, seasonal variability in vertical exchange with the near-surface layer, and mesoscale eddies. Understanding the impact of larger scale physical forcings on N-loss as mediated through O₂ and organic flux is needed to fully understand the causes and consequences of any future ODZ expansion. To achieve this, we need sustained observations by a distributed array capable of detecting synoptic variability.

To address these issues, a new NSF-funded project will carry out a multiyear, autonomous float-based observational program to answer the following questions:

How does biogenic N₂ production in ODZs vary over weekly to annual time scales and space scales of 10s to 1000s km?
What are the major scales of variability and their associated oceanographic phenomena and how do they relate to control by organic matter flux and O₂ concentration?
How does this variability influence regionally integrated N-loss?

Figure 1. The ETNP ODZ roughly defined by O₂ <1.5 μmol/kg (orange, World Ocean Atlas 2013). Our new NSF-funded project will sample across these patterns of spatial and temporal variability for 2 years with 10 subsurface ODZ floats (red/blue) each measuring profiles of T, S, O₂ (50 nmol/kg LOD) and N₂ (0.1 μmol/kg precision), and the in situ rate of N₂ change. Four Argo floats with O₂ sensors and BioOptical floats provided by collaborators will supplement this array. Bright bar symbols are the planned deployment positions; dimmed bar symbols suggest possible displacements after 2 years. Ship-based measurements (yellow stars) along the deployment cruise track (magenta) will be used for float sensor calibration and identification of ETNP source water properties. The 2-year track of our prototype GasFloat is also shown (black line).

This project will exploit our ability to make in situ, ultra-high precision measurement of N₂ concentration (~0.1 umol/kg) and use commercially available O₂ sensors to measure O₂ in the 10s of nM range. Our study area is the Eastern Tropical North Pacific (ETNP), the largest ODZ and the region of our successful pilot deployments (Figure 1). Over a multi-year period, our study will determine in situ nM-level O₂ and biogenic N₂ on float profiles distributed throughout the ETNP and encompassing geographic gradients in O₂ and surface productivity. For the first time, our study will also determine in situ N-loss rates from changes in N₂ concentration during one- to two-week Lagrangian float deployments drifting along a constant density surface (Figure 2). A pilot two-year float (‘GasFloat, Figure 1) deployment in the ETNP has documented our ability to do so. Critically, our float-based approach more closely matches the frequency and distribution of observations to the expected variability in biogenic N₂ production, as compared to prior work. This study will also dramatically increase the data density in this region by acquiring >500 profiles/drifts for N₂ and >1000 profiles for nM O₂.

Figure 2. (a) Schematic of float system to be deployed (b) Example of float mission including 2-week isopycnal drift.

We anticipate float deployment in summer 2020 via a UNOLS vessel. Investigators interested in collaborative participation through contribution of autonomous instrumentation and/or making shipboard measurements are encouraged to contact the lead PI Mark Altabet at maltabet@umassd.edu. Similarly, students interested in graduate research opportunities through this project should contact the lead PI.

Artificial light from sampling platforms changes zooplankton behavior

Posted by mmaheigan

· Monday, November 26th, 2018

When designing sampling we make generally accepted assumptions that what we collect is representative of what is “normal” or naturally occurring at the place, time, and depth of collection. However, a recent study in Science Advances revealed that this might not be true. During round-the-clock shipboard sampling, lights used at night can actually be a form of pollution that disrupts the diel cycle of zooplankton vertical migration.

Effect of light pollution on krill from a ship (left), diel vertical migration in natural dark conditions (middle) and effect of moonlight (right). Figure by Malin Daase (UiT).

Using a Autonomous Surface Vehicle the authors documented zooplankton behavioral patterns of light avoidance never previously seen. The study compared results from high Arctic polar night (unpolluted light environment for an extended time), to near ship samples. During months of near constant darkness in the Arctic, there was still a diel vertical migration of zooplankton limited to the upper 30 m of the water column and centered around the local sun noon. Contrasting the results from light-polluted and unpolluted areas, the authors observed that the vast majority of the pelagic community exhibit a strong light-escape response in the presence of artificial light (both ship light and even headlamps from researchers in open boats). This effect was observed down to 100 m depth and 190 m from the ship. These results suggest that artificial light from traditional sampling platforms may bias studies of zooplankton abundance and diel migration within the upper 100 m. These findings underscore the need for alternative sampling methods such as autonomous platforms, particularly in dim-light conditions, to collect more accurate and representative physical and biological data for ecological studies. In addition to research cruises and sampling, anthropogenic light pollution from predicted increases in shipping, oil and gas exploration, and light-fishing are anticipated to impact the diel rhythms of zooplankton behavior all around the globe.

Authors:
Jørgen Berge (Norwegian University of Technology and Science; UiT The Arctic University of Norway)
Martin Ludvigsen (Norwegian University of Technology and Science; University Centre in Svalbard)
Maxime Geoffroy (UiT The Arctic University of Norway, Memorial University of Newfoundland)
Jonathan H. Cohen (University of Delaware)
Pedro R. De La Torre (Norwegian University of Technology and Science)
Stein M. Nornes (Norwegian University of Technology and Science)
Hanumant Singh (Northeastern University)
Asgeir J. Sørensen (Norwegian University of Technology and Science)
Malin Daase (Norwegian University of Technology and Science)
Geir Johnsen (Norwegian University of Technology and Science; Norwegian University of Technology and Science)

Investigating variability and change in subpolar Southern Ocean pCO2 via time-series and float data

Posted by mmaheigan

· Tuesday, November 6th, 2018

The Southern Ocean dominates the mean global ocean sink for anthropogenic carbon, but its sparse sampling relative to other basins limits our capacity to quantify carbon uptake and accompanying seasonal to interannual variability, which is critical to predicting future ocean carbon uptake and storage. Since 2002, underway pCO₂measurements collected as part of the Drake Passage Time-series (DPT) Program have informed our understanding of seasonally varying air-sea pCO₂gradients and by inference, the carbon fluxes in this region. Understanding whether Drake Passage air-sea fluxes are representative of the broader subpolar Southern Ocean was the focus of a recent study in Biogeosciences.

Top left panel: Mean surface ocean seasonal pCO2 cycle estimate for datasets from the Surface Ocean CO2 Atlas (SOCAT) in the subpolar Southern Ocean: black- SOCAT within the Drake Passage (DP) region; green- SOCAT outside the DP region; blue- all SOCAT in Southern Ocean Subpolar Seasonally Stratified (SPSS) biome; red- Self Organizing Map Feed-forward Network (SOM-FFN) product. Shading represents 1 standard error for biome-scale monthly means driven by interannual variability. Bar plot indicates the number of years containing observations in a given month (maximum of 15 years).
Top right panel: Mean surface ocean pCO2 seasonal cycle estimate for black: underway Drake Passage Time-series data for years 2002–2016; purple: DPT for years 2016–2017 to match years covered by the floats; and orange: SOCCOM floats. Seasonal cycles are shown on an 18-month cycle, calculated from a monthly mean time series with the atmospheric correction to year 2017. Shading represents 1 standard error accounting for the spatial and temporal heterogeneity of the sample and the measurement error (2.7 % or ±11 µatm at a pCO2 of 400 µatm for floats; ±2 µatm for DPT data) combined using the square root of the sum of squares.

An analysis of available Southern Ocean pCO₂ data from inside vs. outside the Drake Passage showed agreement in the timing and amplitude of seasonal pCO₂ variations, suggesting that the seasonality so carefully recorded by DPT is in fact representative of the broader subpolar Southern Ocean. DPT’s high temporal resolution sampling is critical to constraining estimates of the seasonal cycle of surface pCO₂ in this region, as wintertime underway pCO₂ data remain sparse outside the Drake Passage. Comparisons of the DPT data to an emerging dataset of float-estimated pCO₂ from the SOCCOM (Southern Ocean Carbon and Climate Observations and Modeling) project showed that both shipboard and autonomous platforms capture the expected seasonal cycle for the subpolar Southern Ocean, with an austral wintertime peak driven by deep mixing and a summertime low driven by biological uptake. However, the seasonal cycle derived from float-estimated pCO₂ has a larger seasonal amplitude compared to the DPT data due to an earlier and much lower observed summertime minimum.

The Drake Passage Time-series illustrates the large variability of surface ocean pCO₂ in the Southern Ocean and exemplifies the value of sustained observations for understanding changing ocean carbon uptake in this dynamic region. Coordinated monitoring efforts that combine a robust ship-based observational network with a well-calibrated array of autonomous biogeochemical floats will improve and expand our understanding of the Southern Ocean carbon cycle in the future.

Authors:
Amanda R. Fay (Lamont Doherty Earth Observatory)
Nicole S. Lovenduski (University of Colorado)
Galen A. McKinley (Lamont Doherty Earth Observatory)
David R. Munro (University of Colorado)
Colm Sweeney (University of Colorado, NOAA Earth System Research Laboratory)
Alison R. Gray (University of Washington)
Peter Landschützer (Max Planck Institute for Meteorology, Germany)
Britton B. Stephens (National Center for Atmospheric Research)
Taro Takahashi (Lamont Doherty Earth Observatory)
Nancy Williams (Oregon State University)

Dramatic Increase in Chlorophyll-a Concentrations in Response to Spring Asian Dust Events in the Western North Pacific

Posted by mmaheigan

· Tuesday, October 23rd, 2018

According to Martin’s iron hypothesis, input of aeolian dust into the ocean environment temporarily relieves iron limitation that suppresses primary productivity. Asian dust events that originate in the Taklimakan and Gobi Deserts occur primarily in the spring and represent the second largest global source of dust to the oceans. The western North Pacific, where productivity is co-limited by nitrogen and iron, is located directly downwind of these source regions and is therefore an ideal location for determining the response of open water primary productivity to these dust input events.

Figure 1. Daily aerosol index values (black squares) and chlorophyll-a concentrations (mg m-3, circles) during the spring (a) 2010 (weak dust event), (b) 1998 (strong dust event) in the western North Pacific. Color scale represents difference between mixed layer depth (MLD) and isolume depth (Z0.054) that indicates conditions for typical spring blooms; water column structures of MLD and isolume were identical in the spring of 1998 and 2010. Dramatic increases in chlorophyll-a (pink shading, maximum of 5.3 mg m-3) occurred in spring 1998 with a lag time of ~10 days after the strong dust event (aerosol index >2.5) on approximately April 20 compared to constant chlorophyll-a values (<2 mg m-3) in the spring of 2010.

A recent study in Geophysical Research Letters included an analysis of the spatial dynamics of spring Asian dust events, from the source regions to the western North Pacific, and their impacts on ocean primary productivity from 1998 to 2014 (except for 2002–2004) using long-term satellite observations (daily aerosol index data and chlorophyll-a). Geographical aerosol index distributions revealed three different transport pathways supported by the westerly wind system: 1) Dust moving predominantly over the Siberian continent (>50°N); 2) Dust passing across the northern East/Japan Sea (40°N‒50°N); and 3) Dust moving over the entire East/Japan Sea (35°N‒55°N). The authors observed that strong dust events could increase ocean primary productivity by more than 70% (>2-fold increase in chlorophyll-a concentrations, Figure 1) compared to weak/non-dust conditions. This result suggests that spring Asian dust events, though episodic, may play a significant role in driving the biological pump, thus sequestering atmospheric CO₂ in the western North Pacific.

Another recent study reported that anthropogenic nitrogen deposition in the western North Pacific has significantly increased over the last three decades (i.e. relieving nitrogen limitation), whereas this study indicated a recent decreasing trend in the frequency of spring Asian dust events (i.e. enhancing iron limitation). Further investigation is required to fully understand the effects of contrasting behavior of iron (i.e., decreasing trend) and nitrogen (i.e., increasing trend) inputs on the ocean primary productivity in the western North Pacific, paying attention on how the marine ecosystem and biogeochemistry will respond to the changes.

Authors:
Joo-Eun Yoon (Incheon National University)
Il-Nam Kim (Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)

Shipboard LiDAR: A powerful tool for measuring the distribution and composition of particles in the ocean

Posted by mmaheigan

· Tuesday, October 23rd, 2018

Despite major advances in ocean observing capabilities, characterizing the vertical distribution of materials in the ocean with high spatial resolution remains challenging. Light Detection and Ranging (LiDAR), a technique that relies on measurement of the “time-of-flight” of a backscattered laser pulse to determine the range to a scattering object, could potentially fill this critical gap in our sampling capabilities by providing remote estimates of the vertical distribution of optical properties and suspended particles in the ocean.

A recent article in Remote Sensing of Environment details the development of a portable shipboard LiDAR and its capabilities for extending high-frequency measurements of scattering particles into the vertical dimension. The authors deployed the experimental system (shown in Figure 1a) during research cruises off the coast of Virginia and during a passenger ferry crossing of the Gulf of Maine (associated with the Gulf of Maine North Atlantic Time Series program-GNATS). Remote measurements of LiDAR signal attenuation corresponded well with simultaneous in situ measurements of water column optical properties and proxies for the concentration of suspended particles. Interestingly, the researchers also observed that the extent to which the return signal was depolarized (also known as the LiDAR depolarization ratio) may provide information regarding the composition of particles within the scattering volume. This is evidenced by the strong relationship between the depolarization ratio and the backscattering ratio, an indicator of the bulk composition (mineral vs. organic) of the particles within a scattering medium (Figure 1b).

Figure 1. a) LiDAR system deployed to look through a chock at the bow of the M/V Nova Star. b) Relationship between the LiDAR linear depolarization ratio (ρ) and coincident measurements of the particulate backscattering ratio (b_bp/b_p). The black line represents a least-squares exponential fit to the data.

As LiDAR technology becomes increasingly rugged, compact, and inexpensive, the regular deployment of oceanographic LiDAR on a variety of sampling platforms will become an increasingly practical method for characterizing the vertical and horizontal distribution of particles in the ocean. This has the potential to greatly improve our ability to investigate the role of particles in physical and biogeochemical oceanographic processes, especially when sampling constraints limit observations to the surface ocean.

Authors:
Brian L. Collister (Old Dominion University)
Richard C. Zimmerman (Old Dominion University)
Charles I. Sukenik (Old Dominion University
Victoria J. Hill (Old Dominion University)
William M. Balch (Bigelow Laboratory for Ocean Sciences)

Shelf-wide pCO2 increase across the South Atlantic Bight

Posted by mmaheigan

· Thursday, August 2nd, 2018

Relative to their surface area, coastal regions represent some of the largest carbon fluxes in the global ocean, driven by numerous physical, chemical and biological processes. Coastal systems also experience human impacts that affect carbon cycling, which has large socioeconomic implications. The highly dynamic nature of these systems necessitates observing approaches and numerical methods that can both capture high-frequency variability and delineate long-term trends.

Figure 1: The South Atlantic Bight (SAB) was divided into four sections using isobaths: the coastal zone (0 to 15 m), the inner shelf (15 to 30 m), the middle shelf (30 to 60 m), and the outer shelf (60 m and beyond). The X’s indicate the locations of the Gray’s Reef mooring (southern X) and the Edisto mooring (northern X).

In two recent studies using mooring- and ship-based ocean CO₂ system data, authors observed that pCO₂ is increasing from the coastal zone to the outer shelf of the South Atlantic Bight at rates greater than the global average oceanic and atmospheric increase (~1.8 µatm y^-1). In recent publications in Continental Shelf Research and JGR-Oceans, the authors analyzed pCO₂ data from 46 cruises (1991-2016) using a novel linear regression technique to remove the seasonal signal, revealing an increase in pCO₂ of 3.0-3.7 µatm y^-1on the outer and inner shelf, respectively. Using a Generalized Additive Mixed Model (GAMM) approach for trend analysis, authors observed that the rates of increase were slightly higher than the deseasonalization technique, yielding pCO₂ increases of 3.3 to 4.5 µatm y^-1 on the outer and inner shelf, respectively. The reported pCO₂ increases result in potential pH decreases of -0.003 to -0.004 units y^-1.

Figure 2: The time series of fCO2 in the four regions of the SAB (cruise observations) and from the Gray’s Reef mooring on the inner shelf indicate an increase across the shelf. These data are the observed values, however, the trend lines for each time series are calculated using deseasonalized values using the reference year method.

Analysis of the pCO₂ time-series from the Gray’s Reef mooring (using a NOAA Moored Autonomous pCO₂ system from July 2006 -July 2015) yielded a rate of increase (3.5 ± 0.9 µatm y^-1) that was comparable to the cruise data on the inner shelf (3.7 ± 2.2 and 4.5 ± 0.6 µatm y^-1, linear and GAMM methods, respectively). Validation data collected at the mooring suggest that underway data from cruises and the moored data are comparable. Neither thermal processes nor atmospheric dissolution (the primary driver of oceanic acidification) can explain the observed pCO₂ increase and concurrent pH decrease across the shelf. Unlike the middle and outer shelves, where an increase in SST could account for up to 1.1 µatm y^-1 of the observed pCO₂ trend, there is no thermal influence in the coastal zone and inner shelf. While 1.8 µatm y^-1 could be attributed to the global average atmospheric increase, the remainder is likely due to transport from coastal marshes and in situ biological processes. As the authors have shown, the increasing coastal and oceanic trend in pCO₂ can lead to a decrease in pH, especially if there is no increase in buffering capacity. More acidic waters can have a long term affect on coastal ecosystem services and biota.

Also see Eos Editor’s Vox on this research by Peter Brewer https://eos.org/editors-vox/coastal-ocean-warming-adds-to-co2-burden

Authors:

Multidecadal fCO2 Increase Along the United States Southeast Coastal Margin (JGR-Oceans)
Janet J. Reimer (University of Delaware)
Hongjie Wang (Texas A &M University – Corpus Christi)
Rodrigo Vargas (University of Delaware)
Wei-Jun Cai (University of Delaware)

And

Time series pCO2 at a coastal mooring: Internal consistency, seasonal cycles, and interannual variability (Continental Shelf Research)
Janet J. Reimer (University of Delaware)
Wei-Jun Cai (University of Delaware; University of Georgia)
Liang Xue (University of Delaware; First Institute of Oceanography, China)
Rodrigo Vargas (University of Delaware)
Scott Noakes (University of Georgia)
Xinping Hu (Texas A &M University – Corpus Christi)
Sergio R. Signorini (Science Applications International Corporation)
Jeremy T. Mathis (NOAA Arctic Research Program)
Richard A. Feely (NOAA Pacific Marine Environmental Laboratory)
Adrienne J. Sutton (NOAA Pacific Marine Environmental Laboratory; University of Washington)
Christopher Sabine (University of Hawaii Manoa)
Sylvia Musielewicz (NOAA Pacific Marine Environmental Laboratory; University of Washington)
Baoshan Chen (University of Delaware; University of Georgia)
Rik Wanninkhof (NOAA Atlantic Oceanographic and Meteorological Laboratory)

Marine Snowfall at the Equator

Posted by mmaheigan

· Thursday, July 19th, 2018

The continual flow of organic particles such as dead organisms and fecal material towards the deep sea is called “marine snow,” and it plays an important role in the ocean carbon cycle and climate-related processes. This snowfall is most intense where high primary production can be observed near the surface. This is the case along the equator in the Pacific and Atlantic Oceans. However, it is not well known how particles are distributed at depth and which processes influence this distribution. A recent study published in Nature Geoscience involved the use of high-resolution particle density data using the Underwater Vision Profiler (UVP) from the equatorial Atlantic and Pacific Oceans down to a depth of 5,000 meters, revealing that several previously accepted ideas on the downward flux of particles into the deep sea should be revisited.

Figure 1. The Underwater Vision Profiler (UVP) during a trial in the Kiel Fjord. The UVP provided crucial data for the new study. Photo: Rainer Kiko, GEOMAR

It is typically assumed that the largest particle density can be found close to the surface and that density attenuates continuously with depth. However, high-resolution particle data show that density increases again in the 300-600-meter depth range. The authors attribute this observation to the daily migratory behavior of organisms such as zooplankton that retreat to these depths during the day, contributing to the particle load via defecation and mortality.

Another surprising result is the observation of many small particles below 1,000 meters depth that contribute a large fraction of the bathypelagic particle flux. This observation counters the general assumption, especially in many biogeochemical models, that particle flux at depth comprises fast sinking particles such as fecal pellets. Diminished remineralization rates of small particles or increased disaggregation of larger particles may contribute to the elevated small particle fluxes at this depth.

Figure 2. Zonal current velocity and Particulate Organic Carbon (POC) content across the equatorial Atlantic at 23˚W as observed in November 2012. From left to right: Zonal current velocity, POC content in small particle fraction and POC content in large particle fraction (adapted from Kiko et al. 2017).

This study highlights the importance of coupled biological and physical processes in understanding and quantifying the biological carbon pump. Further work on this important topic can now also be submitted to the new Frontiers in Marine Science research topic “Zooplankton and Nekton: Gatekeepers of the Biological Pump” (https://www.frontiersin.org/research-topics/8114/zooplankton-and-nekton-gatekeepers-of-the-biological-pump; Co-editors R. Kiko, M. Iversen, A. Maas, H. Hauss and D. Bianchi). The research topic welcomes a broad range of contributions, from individual-based process studies, to local and global field observations, to modeling approaches to better characterize the role of zooplankton and nekton for the biological pump.

Authors:
R. Kiko (GEOMAR)
A. Biastoch (GEOMAR)
P. Brandt (GEOMAR, University of Kiel)
S. Cravatte (LEGOS, University of Toulouse)
H. Hauss (GEOMAR)
R. Hummels (GEOMAR)
I. Kriest (GEOMAR)
F. Marin (LEGOS, University of Toulouse)
A. M. P. McDonnell (University of Alaska Fairbanks)
A. Oschlies (GEOMAR)
M. Picheral (Laboratoire d’Océanographie de Villefranche-sur-Mer, Observatoire Océanologique)
F. U. Schwarzkopf (GEOMAR)
A. M. Thurnherr (Lamont-Doherty Earth Observatory,)
L. Stemmann (Sorbonne Universités, Observatoire Océanologique)

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.

Hotspots of biological production: Submesoscale changes in respiration and production

Posted by mmaheigan

· Thursday, April 26th, 2018

The biological pump is complex and variable. To better understand it, scientists have often focused on variations in biological parameters such as fluorescence and community structure, and have less often observed variations in rates of production. Production rates can be estimated using oxygen as a tracer, since photosynthesis produces oxygen and respiration consumes it. In a recent article in Deep Sea Research Part I, the authors presented high-resolution maps of oxygen in the upper 140 m of the ocean in the subtropical and tropical Atlantic, produced from a towed undulating instrument. This provides a synoptic, high-resolution view of oxygen anomalies in the surface ocean. These data reveal remarkable hotspots of biological production and respiration co-located with areas of elevated fluorescence. These hotspots are often several kilometers wide (horizontal) and ~10 m long (vertical). They are preferentially associated with edges of eddies, but not all edges sampled contained hotspots. Although this study captures only two-dimensional glimpses of these hotspots, precluding formal calculations of production rates, likely estimates of source water suggest that many of these hotspots may actually be areas of enhanced respiration rather than enhanced photosynthesis. The paper describes a conceptual model of nutrients, new production, respiration, fluorescence, and oxygen during the formation and decline of these hotspots. These data raise intriguing questions–if the hotspots do indeed have substantially different rates of production and respiration than surrounding waters, then they could lead to significant changes in estimates of production in the upper ocean. Additionally, understanding the mechanisms that produce these hotspots could be critical for predicting the effects of climate change on the magnitude of the biological pump.

(a) Oxygen concentrations and (b) fluorescence at ~1 km resolution over 300 km from 15.13°N, 57.47°W to 12.30°N, 56.42° W, as measured by sensors attached to the (c) Video Plankton Recorder II. Note that no contouring was used for this plot – every pixel represents an actual data point. Figure modified from Stanley et al., 2017. VPR image photograph by Phil Alatalo.

Authors:
Rachel H. R. Stanley (Wellesley College)
Dennis J. McGillicuddy Jr. (WHOI)
Zoe O. Sandwith (WHOI)
Haley Pleskow (Wellesley College)

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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