Archive for bgc argo

An expanding understanding of Southern Ocean productivity and export

Posted by mmaheigan 
· Monday, February 13th, 2023 

Biology in the Southern Ocean is known to help regulate Earth’s climate by capturing and eventually sequestering carbon from its surface. Unfortunately, accurate estimates of the magnitude of the Southern Ocean (SO) biological carbon sink are limited and subject to ongoing debate. However, a recently published study used the expanding Southern Ocean BGC-Argo fleet to provide new estimates of SO Annual Net Community Production (ANCP) and export production.

Over long enough time and space scales (>1000 km and seasons), ANCP is equal to the amount of carbon fixed during photosynthesis that is not remineralized in the surface layer. What remains is available to be exported to depth. As this organic matter sinks out of the surface ocean, most of it is eventually remineralized by bacteria, leaving behind a signature of depleted oxygen. With enough floats, basin-scale ANCP can be estimated from the seasonal oxygen drawdown measured across their profiles. While similar studies have been carried out on single floats, here, the authors construct a composite of all available profiles and include a greater depth range than previously considered.

Figure 1. All available BGC-ARGO float profiles (25,512) were used to create an A) ensemble seasonal cycle in surface chlorophyll and sub-surface oxygen. B) Annual Net Community Production (ANCP) was then estimated by computing the depth-integrated oxygen depletion during the productive period. C) ANCP was estimated across 12 major regions, separated by the Indian, Pacific and Atlantic basins and Subantarctic (SAZ), Polar (PFZ), Antarctic (AZ), and Southern (S) frontal zones. Each region used 100s-1000s of individual float profiles (color-coded scatter points).

Results from this novel approach estimate SO ANCP (and ~export) at 3.89 GT C year-1, with basin-scale regional estimates as much as a factor 2.8 larger than previous studies. Moreover, nearly 30% of remineralization was measured at depths not typically considered, with 14% below 500 m and another 15% immediately below the euphotic depth but above 100 m. These values suggest a more critical role for the Southern Ocean in regulating oceanic carbon storage, atmospheric CO2 exchange, and climate than previously thought.

 

Authors:
Jiaoyang Su (University of Tasmania, Australia)
Christina Schallenberg (University of Tasmania, and Australian Antarctic Program Partnership)
Tyler Rohr (Australian Antarctic Program Partnership)
Peter G. Strutton (University of Tasmania, Australia)
Helen E. Phillips (University of Tasmania, and Australian Antarctic Program Partnership)

Integrated analysis of carbon dioxide and oxygen concentrations as a quality control of ocean float data

Posted by mmaheigan 
· Friday, August 26th, 2022 

A recent study in Communications Earth & Environment, examined spatiotemporal patterns of the two dissolved gases CO2 and O2 in the surface ocean, using the high-quality global dataset GLODAPv2.2020. We used surface ocean data from GLODAP to make plots of carbon dioxide and oxygen relative to saturation (CORS plots). These plots of CO2 deviations from saturation (ΔCO2) against oxygen deviations from saturation (ΔO2) (Figure 1) provide detailed insight into the identity and intensity of biogeochemical processes operating in different basins.

Figure 1: Relationships between ΔCO2 and ΔO2 in the global ocean basins based on surface data in the GLODAPv2.2020 database. The black dashed lines are the least-squares best-fit lines to the data; unc denotes the uncertainty in the y-intercept value with 95% confidence; r is the associated Pearson correlation coefficient; n is the number of data points.

In addition, data in all basins and all seasons shares some common behaviors: (1) negative slopes of best fit lines to the data, and (2) near-zero y-intercepts of those lines. We utilized these findings to compare patterns in CORS plots from GLODAP with those from BGC-Argo float data from the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program. Given that the float O2 data is likely to be more accurate than the float pH data (from which the float CO2 is calculated), CORS plots are useful for detecting questionable float CO2 data, by comparing trends in float CORS plots (e.g. Figure 2) to trends in GLODAP CORS plots (Figure 1). As well as the immediately detected erroneous data, we discovered significant discrepancies in ΔCO2-ΔO2 y-intercepts compared to the global reference (i.e., GLODAPv2.2020 y-intercepts, Figure 1). The y-intercepts of 48 floats with QCed O2 and CO2 data (at regions south of 55°S) were on average greater by 0.36 μmol kg−1 than the GLODAP-derived ones, implying the overestimations of float-based CO2 release in the Southern Ocean.

Figure 2. CORS plots from data collected by SOCCOM floats F9096 and F9099 in the high-latitude Southern Ocean. Circles with solid edges denote data flagged as ‘good’, whereas crosses denote data flagged as ‘questionable’.

Our study demonstrates CORS plots’ ability to identify questionable data (data shown to be questionable by other QC methods) and to reveal issues with supposed ‘good’ data (i.e., quality issues not picked up by other QC methods). CORS plots use only surface data, hence this QC method complements existing methods based on analysis of deep data. As the oceanographic community becomes increasingly reliant on data collected from autonomous platforms, techniques like CORS will help diagnose data quality, and immediately detect questionable data.

 

Authors:
Yingxu Wu (Polar and Marine Research Institute, Jimei University, Xiamen, China; University of Southampton)
Dorothee C.E. Bakker (University of East Anglia)
Eric P. Achterberg (GEOMAR Helmholtz Centre for Ocean Research Kiel)
Amavi N. Silva (University of Southampton)
Daisy D. Pickup (University of Southampton)
Xiang Li (George Washington University)
Sue Hartman (National Oceanography Centre, Southampton)
David Stappard (University of Southampton)
Di Qi (Polar and Marine Research Institute, Jimei University, Xiamen, China)
Toby Tyrrell (University of Southampton)

Using BGC-Argo to obtain depth-resolved net primary production

Posted by mmaheigan 
· Friday, July 23rd, 2021 

Net primary production (NPP)—the organic carbon produced by the phytoplankton minus the organic carbon respired by phytoplankton themselves—serves as a major energy source of the marine ecosystem. Traditional methods for measuring NPP rely on ship-based discrete sampling and bottle incubations (e.g., 14C incubation), which introduce potential artifacts and limit the spatial and temporal data coverage of the global ocean. The global distribution of NPP has been estimated using satellite observations, but the satellite remote sensing approach cannot provide direct information at depth.

Figure 1. Panel A. Trajectories of 5 BGC-Argo and 1 SOS-Argo with the initial float deployment locations denoted by filled symbols. The dash-line at 47° N divided the research area into the northern (temperate) and southern (subtropical) regions. Stars indicate ship stations where 14C NPP values were measured during NAAMES cruises and compared with NPP from nearby Argo floats. Panels B and C. Monthly climatologies of net primary production (NPP, mmol m-3 d-1) profiles in the northern and southern regions of the research area, derived from BGC-Argo measurements using the PPM model. The shadings indicate one standard deviation. The red dotted line indicates mixed layer depth (MLD, m), and the yellow dashed line shows euphotic depth (Z1%, m).

To fill this niche, a recent study in Journal of Geophysical Research: Biogeosciences, applied bio-optical measurements from Argo profiling floats to study the year-round depth-resolved NPP of the western North Atlantic Ocean (39° N to 54° N). The authors calculated NPP with two bio-optical models (Carbon-based Productivity Model, CbPM; and Photoacclimation Productivity Model, PPM). A comparison with NPP profiles from 14C incubation measurements showed advantages and limitations of both models. CbPM reproduced the magnitude of NPP in most cases, but had artifacts in the summer (a large NPP peak in the subsurface) due to the subsurface chlorophyll maximum caused by photoacclimation. PPM avoided the artifacts in the summer from photoacclimation, but the magnitude of PPM-derived NPP was smaller than the 14C result. Latitudinally varying NPP were observed, including higher winter NPP/lower summer NPP in the south, timing differences in NPP seasonal phenology, and different NPP depth distribution patterns in the summer months. With a 6-month record of concurrent oxygen and bio-optical measurements from two Argo floats, the authors also demonstrated the ability of Argo profiling floats to obtain estimates of the net community production (NCP) to NPP ratio (f-ratio), ranging from 0.3 in July to -1.0 in December 2016.

This work highlights the utility of float bio-optical profiles in comparison to traditional measurements and indicates that environmental conditions (e.g. light availability, nutrient supply) are major factors controlling the seasonality and spatial (horizontal and vertical) distributions of NPP in the western North Atlantic Ocean.

 

Authors:
Bo Yang (University of Virginia, UM CIMAS/NOAA AOML)
James Fox (Oregon State University)
Michael J. Behrenfeld (Oregon State University)
Emmanuel S. Boss (University of Maine)
Nils Haëntjens (University of Maine)
Kimberly H. Halsey (Oregon State University)
Steven R. Emerson (University of Washington)
Scott C. Doney (University of Virginia)

How zooplankton control carbon export in the Southern Ocean

Posted by mmaheigan 
· Thursday, December 3rd, 2020 

The Southern Ocean exhibits an inverse relationship between surface primary production and export flux out of the euphotic zone. The causes of this production-export decoupling are still under debate. A recently published mini review in Frontiers in Marine Science focused on zooplankton, an important component of Southern Ocean food webs and the biological pump. The authors compared carbon export regimes from the naturally iron-fertilised Kerguelen Plateau (high surface production, but generally low export) with the iron-limited and less productive high nutrient, low chlorophyll (HNLC) waters south of Australia, where carbon export is relatively high.

Figure 1: The role of zooplankton in establishing the characteristic export regimes at two sites in the Southern Ocean, (a) the highly productive northern Kerguelen Plateau, which exhibits low export, and (b) the iron-limited waters south of Australia with low production, but relatively high carbon export.

Size structure and zooplankton grazing pressure are found to shape carbon export at both sites. On the Kerguelen Plateau, a large size spectrum of zooplankton acts as “gate-keeper” to the mesopelagic by significantly reducing the sinking flux of phytoaggregates, which establishes the characteristic low export regime. In the HNLC waters, however, the zooplankton community is low in biomass and grazes predominantly on smaller particles, which leaves the larger particles for export and leads to relatively high export flux.

Gaps in knowledge related to insufficient seasonal data coverage, understudied carbon flux pathways, and associated mesopelagic processes limit our current understanding of carbon transfer through the water column and export. More integrated data collection efforts, including the use of autonomous profiling floats (e.g., BGC-Argo), stationary moorings, etc., will improve seasonal carbon flux data coverage, thus enabling more reliable estimation of carbon export and storage in the Southern Ocean and improved projection of future changes in carbon uptake and atmospheric carbon dioxide levels.

 

Authors:
Svenja Halfter (University of Tasmania)
Emma Cavan (Imperial College London)
Ruth Eriksen (CSIRO)
Kerrie Swadling (University of Tasmania)
Philip Boyd (University of Tasmania)

Profiling floats reveal fate of Southern Ocean phytoplankton stocks

Posted by mmaheigan 
· Tuesday, September 1st, 2020 

More observations are needed to constrain the relative roles of physical (advection), biogeochemical (downward export), and ecological (grazing and biological losses) processes in driving the fate of phytoplankton blooms in Southern Ocean waters. In a recent paper published in Nature Communications, authors used seven Biogeochemical Argo (BGC-Argo) floats that vertically profiled the upper ocean every ten days as they drifted for three years across the remote Sea Ice Zone of the Southern Ocean. Using the floats’ biogeochemical sensors (chlorophyll, nitrate, and backscattering) and regional ratios of nitrate consumption:chlorophyll synthesis, the authors developed a new approach to remotely estimate the fate of the phytoplankton stocks, enabling calculations of herbivory and of downward carbon export. The study revealed that the major fate of phytoplankton biomass in this region is grazing, which consumes ~90% of stocks. The remaining 10% is exported to depth. This pattern was consistent throughout the entire sea ice zone where the floats drifted, from 60°-69° South.

Figure Caption: Southern Ocean Chlorophyll a climatology and floats’ trajectories (top panel). Total losses of Chlorophyll a (including grazing and phytodetritus export, left panel). Phytodetritus export (right panel).

 

This study region comprises two of the three major krill growth and development areas—the eastern Weddell and King Haakon VII Seas and Prydz Bay and the Kerguelen Plateau—so the observed grazing was probably due to Antarctic krill, underscoring their pivotal importance in this ecosystem. Building upon the greater understanding of ocean ecosystems via satellite ocean colour development in the 1990s, BGC-Argo floats and this new approach will allow remote monitoring of the different fates of phytoplankton stocks and insights into the status of the ecosystem.

 

Authors:
Sebastien Moreau (Norwegian Polar Institute, Tromsø, Norway)
Philip Boyd (Institute for Marine and Antarctic Studies, Hobart, Australia)
Peter Strutton (Institute for Marine and Antarctic Studies, Hobart, Australia)

Autonomous platforms yield new insights on North Atlantic bloom phenology

Posted by mmaheigan 
· Wednesday, April 22nd, 2020 

Phytoplankton produces organic carbon, which serves as a major energy source in marine food webs and plays an important role in the global carbon cycle. Studies of phytoplankton seasonal timing (phenology) have been a major focus in oceanography, especially in the subpolar North Atlantic region, where massive increases in phytoplankton biomass (blooms) occur during the winter-spring transition.

Figure 1. Panel a: Each line represents the trajectory of a profiling Argo float deployed during the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES) expeditions (12 total); the initial float deployment location is denoted by a filled circle. The bar chart (inset right bottom) indicates float deployment durations. Panel b: Seasonal climatologies of Cphyto (green), µ (blue), l (red), and r (grey) from Argo floats for all 4 regions (D1-D4 as indicated on map in Panel a).

Many hypotheses based on data from shipboard discrete sampling or satellite remote sensing have been proposed to explain drivers of phytoplankton bloom formation and dynamics. However, discrete shipboard sampling limits both spatial and temporal coverage, and satellite approaches cannot provide direct information at depth. To address this gap in spatiotemporal coverage, a recent study in Frontiers in Marine Science, applied bio-optical measurements from 12 Argo profiling floats to study the year-round phytoplankton phenology in a north-south section of the western North Atlantic Ocean (40° N to 60° N). The authors calculated phytoplankton division rate (µ), loss rate (l), and carbon accumulation rate (r) using the Argo-based Chlorophyll-a (Chl) and phytoplankton carbon (Cphyto) estimates. Latitudinally varying phytoplankton dynamics were observed, with a higher (and later) Cphyto peak in the north, and stronger μ–r decoupling and increased proportion of winter to total annual production in the south (Figure 1). Seasonal phenology patterns arise from interactions between “bottom-up” (e.g., resources for growth) and “top-down” (e.g., grazing, mortality) factors that involve both biological and physical drivers. The Argo float data are consistent with the disturbance recovery hypothesis (DRH) over a full annual cycle. Float-based mixed layer phytoplankton phenology observations were comparable to satellite remote sensing observations. In a data-model comparison, outputs from an eddy-resolving ocean simulation only reproduced some of the observed phytoplankton phenology, indicating possible biases in the simulated physical forcing, turbulent dynamics, and biophysical interactions.

In addition to seasonal patterns in the mixed layer, float-based measurements provide information on the vertical distribution of physical and biogeochemical quantities and therefore are complementary to the satellite measurements. This powerful combination of observing assets enhances spatiotemporal coverage, thus enabling us to better observe, compare, model, and predict seasonal phytoplankton dynamics in the subpolar North Atlantic.

 

Authors:
Bo Yang (University of Virginia)
Emmanuel S. Boss (University of Maine)
Nils Haëntjens (University of Maine)
Matthew C. Long (National Center for Atmospheric Research)
Michael J. Behrenfeld (Oregon State University)
Rachel Eveleth (Oberlin College)
Scott C. Doney (University of Virginia)

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