phytoplankton :: Ocean Carbon & Biogeochemistry

Studying marine ecosystems and biogeochemical cycles in the face of environmental change

Archive for phytoplankton

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)

A close-up view of biomass controls in Southern Ocean eddies

Posted by mmaheigan

· Thursday, August 20th, 2020

Southern Ocean biological productivity is instrumental in regulating the global carbon cycle. Previous correlative studies associated widespread mesoscale activity with anomalous chlorophyll levels. However, eddies simultaneously modify both the physical and biogeochemical environments via several competing pathways, making it difficult to discern which mechanisms are responsible for the observed biological anomalies within them. Two recently published papers track Southern Ocean eddies in a global, eddy-resolving, 3-D ocean simulation. By closely examining eddy-induced perturbations to phytoplankton populations, the authors are able to explicitly link eddies to co-located biological anomalies through an underlying mechanistic framework.

Figure caption: Simulated Southern Ocean eddies modify phytoplankton division rates in different directions of depending on the polarity of the eddy and background seasonal conditions. During summer anticyclones (top right panel) deliver extra iron from depth via eddy-induced Ekman pumping and fuel faster phytoplankton division rates. During winter (bottom right panel) the extra iron supply is eclipsed by deeper mixed layer depths and elevated light limitation resulting in slower division rates. The opposite occurs in cyclones.

In the first paper, the authors observe that eddies primarily affect phytoplankton division rates by modifying the supply of iron via eddy-induced Ekman pumping. This results in elevated iron and faster phytoplankton division rates in anticyclones throughout most of the year. However, during deep mixing winter periods, exacerbated light stress driven by anomalously deep mixing in anticyclones can dominate elevated iron and drive division rates down. The opposite response occurs in cyclones.

The second paper tracks how eddy-modified division rates combine with eddy-modified loss rates and physical transport to produce anomalous biomass accumulation. The biomass anomaly is highly variable, but can exhibit an intense seasonal cycle, in which cyclones and anticyclones consistently modify biomass in different directions. This cycle is most apparent in the South Pacific sector of the Antarctic Circumpolar Current, a deep mixing region where the largest biomass anomalies are driven by biological mechanisms rather than lateral transport mechanisms such as eddy stirring or propagation.

It is important to remember that the correlation between chlorophyll and eddy activity observable from space can result from a variety of physical and biological mechanisms. Understanding the nuances of how these mechanisms change regionally and seasonally is integral in both scaling up local observations and parameterizing coarser, non-eddy resolving general circulation models with embedded biogeochemistry.

Authors:
Tyler Rohr (Australian Antarctic Partnership Program, previously at MIT/WHOI)
Cheryl Harrison (University of Texas Rio Grande Valley)
Matthew Long (National Center for Atmospheric Research)
Peter Gaube (University of Washington)
Scott Doney (University of Virginia)

Multiyear predictions of ocean acidification in the California Current System

Posted by mmaheigan

· Thursday, August 20th, 2020

The California Current System is a highly productive coastal upwelling region that supports commercial fisheries valued at $6 billion/year. These fisheries are supported by upwelled waters, which are rich in nutrients and serve as a natural fertilizer for phytoplankton. Due to remineralization of organic matter at depth, these upwelled waters also contain large amounts of dissolved inorganic carbon, causing local conditions to be more acidic than the open ocean. This natural acidity, compounded by the dissolution of anthropogenic CO₂ into coastal waters, creates corrosive conditions for shell-forming organisms, including commercial fishery species.

A recent study in Nature Communications showcases the potential for climate models to skillfully predict variations in surface pH—thus ocean acidification—in the California Current System. The authors evaluate retrospective predictions of ocean acidity made by a global Earth System Model set up similarly to a weather forecasting system. The forecasting system can already predict variations in observed surface pH fourteen months in advance, but has the potential to predict surface pH up to five years in advance with better initializations of dissolved inorganic carbon (Figure 1). Skillful predictions are mostly driven by the model’s initialization and subsequent transport of dissolved inorganic carbon throughout the North Pacific basin.

Figure 1. Forecast of annual surface pH anomalies in the California Current Large Marine Ecosystem for 2020. Red colors denote anomalously basic conditions for the given location and blue colors indicate anomalously acidic conditions.

These results demonstrate, for the first time, the feasibility of using climate models to make multiyear predictions of surface pH in the California Current. Output from this global prediction system could serve as boundary conditions for high-resolution models of the California Current to improve prediction time scale and ultimately help inform management decisions for vulnerable and valuable shellfisheries.

Authors:
Riley X. Brady (University of Colorado Boulder)
Nicole S. Lovenduski (University of Colorado Boulder)
Stephen G. Yeager (National Center for Atmospheric Research)
Matthew C. Long (National Center for Atmospheric Research)
Keith Lindsay (National Center for Atmospheric Research)

Turning a spotlight on grazing

Posted by mmaheigan

· Thursday, July 23rd, 2020

Microscopic plankton in the surface ocean make planet Earth habitable by generating oxygen and forming the basis of marine food webs, yielding harvestable protein. For over 100 years, oceanographers have tried to ascertain the physical, chemical, and biological processes governing phytoplankton blooms. Zooplankton grazing of phytoplankton is the single largest loss process for primary production, but empirical grazing data are sparse and thus poorly constrained in modeling frameworks, including assessments of global elemental cycles, cross-ecosystem comparisons, and predictive efforts anticipating future ocean ecosystem function. As sunlight decays exponentially with depth, upper-ocean mixing creates dynamic light environments with predictable effects on phytoplankton growth but unknown consequences for grazing.

Figure caption: Rates (d⁻¹) of phytoplankton growth (μ), grazing mortality (g), and biomass accumulation (r) under four mixed layer scenarios simulated using light as a proxy of (a) sustained deep mixing, (b) rapid shoaling, (c) sustained shallow mixing, and (d) rapid mixed layer deepening. Error bars represent one standard deviation of the mean of duplicate experiments. Grazing was measured but not detected in the sustained deep mixing and rapid shoaling conditions, denoted with x.

Using data from a spring cruise in the North Atlantic, authors of a recent study published in Limnology & Oceanography compared the influences of microzooplankton predation and fluctuations in light availability—representative of a mixing water column—on phytoplankton standing stock. Data from at-sea incubations and light manipulation experiments provide evidence that phytoplankton’s instantaneous and zooplankton’s delayed responses to light fluctuations are key modulators of the balance between phytoplankton growth and grazing rates (Figure 1). These results suggest that light is a potential, remotely retrievable predictor of when and where in the ocean zooplankton grazing may represent an important loss term of phytoplankton production. If broadly verified, this approach could be used to systematically assess sparsely measured grazing across spatial and temporal gradients in representative regions of the ocean. Such data will be essential for enhancing our predictive capacity of ocean food web function, global biogeochemical cycles and the many derived processes, including fisheries production and the flow of carbon through the oceans.

Authors:
Françoise Morison (University of Rhode Island)
Gayantonia Franzè (University of Rhode Island, currently Institute of Marine Research, Norway)
Elizabeth Harvey (University of Georgia, currently University of New Hampshire)
Susanne Menden-Deuer (University of Rhode Island)

Unexpected patterns of carbon export in the Southern Ocean

Posted by mmaheigan

· Tuesday, July 7th, 2020

The Southern Ocean is a major player in driving global distributions of heat, carbon dioxide, and nutrients, making it key to ocean chemistry and the earth’s climate system. In the ocean, biological production and export of organic carbon are commonly linked to places with high nutrient availability. A recent paper, published in Global Biogeochemical Cycles, highlighting new observations from robotic profiling floats demonstrates that areas of high carbon export in the Southern Ocean are actually associated with very low concentrations of iron, an important micronutrient for supporting phytoplankton growth. This suggests a decoupling between the production and export of organic carbon in this region.

Figure caption: (A) Meridional pattern of Annual Net Community Production (ANCP) (equivalent to carbon export) (± standard deviation) in the Southern Ocean (blue line with circles and shaded area), carbon export estimates from previous satellite-based analyses (blue dashed line), and silicate to nitrate (Si:NO₃) ratio of the surface water (black continuous line). Grey dotted line shows a Si:NO₃ = 1 mol mol⁻¹, characteristic of nutrient-replete diatoms. (B) Meridional pattern of Southern Ocean nutrient concentrations, including dissolved iron (Fe) concentration (black line), nitrate (red line), and silicate (blue line). (C) Mean 2014–2015 annual zonally averaged air-sea flux of CO₂ computed using neural network interpolation method. STF = Subtropical Front, PF = Antarctic Polar Front, SIF = Seasonal Ice Front.

Using observations of nutrient and oxygen drawdown from a regional network of profiling Biogeochemical-Argo floats deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM), the authors calculated estimates of Southern Ocean carbon export. A meridional pattern in biological carbon export emerged, showing peak export near the Antarctic Polar Front (PF) associated with minima in surface iron concentrations and dissolved silicate to nitrate ratios. Previous studies have shown that under iron-limiting conditions, diatoms increase their uptake ratio of silicate with respect to other nutrients (e.g., nitrogen), resulting in silicification. Here, the authors hypothesize that iron limitation promotes silicification in Southern Ocean diatoms, as evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic Polar Front. High diatom silicification increases ballasting of particulate organic carbon and hence overall carbon export in this region. The resulting meridional pattern of organic carbon export is similar to that of the air-sea flux of carbon dioxide in the Southern Ocean, underscoring the importance of the biological carbon pump in controlling the spatial pattern of oceanic carbon uptake in this region.

Authors:
Lionel A. Arteaga (Princeton University)
Markus Pahlow (Helmholtz Centre for Ocean Research Kiel, GEOMAR)
Seth M. Bushinsky (University of Hawaii)
Jorge L. Sarmiento (Princeton University)

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 C_phyto (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 (C_phyto) estimates. Latitudinally varying phytoplankton dynamics were observed, with a higher (and later) C_phyto 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)

Can phytoplankton help us determine ocean iron bioavailability?

Posted by mmaheigan

· Wednesday, March 11th, 2020

Iron (Fe) is a key element to sustaining life, but it is present at extremely low concentrations in seawater. This scarcity limits phytoplankton growth in large swaths of the global ocean, with implications for marine food webs and carbon cycling. The acquisition of Fe by phytoplankton is an important process that mediates the movement of carbon to the deep ocean and across trophic levels. It is a challenge to evaluate the ability of marine phytoplankton to obtain Fe from seawater since it is bound by a variety of poorly defined organic complexes.

Figure 1: Schematic representation of the reactions governing dissolved Fe (dFe) bioavailability to phytoplankton (a) Bioavailability of dFe in seawater collected from various basins and depth and probed with different iron-limited phytoplankton species under dim laboratory light and sunlight (b) (See paper for further details on samples and species)

A recent study in The ISME Journal proposes a new approach for evaluating seawater dissolved Fe (dFe) bioavailability based on its uptake rate constant by Fe-limited cultured phytoplankton. The authors collected samples from distinct regions across the global ocean, measured the properties of organic complexation, loaded these complexes with a radioactive Fe isotope, and then tracked the internalization rates from these forms to a diverse set of Fe-limited phytoplankton species. Regardless of origin, all of the phytoplankton acquired natural organic complexes at similar rates (accounting for cell surface area). This confirms that multiple Fe-limited phytoplankton species can be used to probe dFe bioavailability in seawater. Among water types, dFe bioavailability varied by ~4-fold and did not clearly correlate with Fe concentrations or any of the measured Fe speciation parameters. This new approach provides a novel way to determine Fe bioavailability in samples from across the oceans and enables modeling of in situ Fe uptake rates by phytoplankton based simply on measured Fe concentrations.

Authors:
Yeala Shaked (Hebrew University of Jerusalem)
Kristen N. Buck (University of South Florida)
Travis Mellett (University of South Florida)
Maria. T. Maldonado (University of British Columbia)

Hurricane-driven surge of labile carbon into the deep North Atlantic Ocean

Posted by mmaheigan

· Thursday, February 27th, 2020

Tropical cyclones (hurricanes and typhoons) are the most extreme episodic weather event affecting subtropical and temperate oceans. Hurricanes generate intense surface cooling and vertical mixing in the upper ocean, resulting in nutrient upwelling into the photic zone and episodic phytoplankton blooms. However, their influence on the deep ocean is unknown.

Figure 1. (a) Particulate organic carbon (POC) flux and percentage of the total mass flux (yellow) (top panel); fluxes (middle panel) and POC-normalized concentrations (bottom panel) of diagnostic lipid biomarkers for phytoplankton-derived and labile material, zooplankton, bacteria, and other (see legend); (b) Lipid concentrations (left panel) and POC-normalized concentrations (right panel) of diagnostic lipid biomarkers for the same sources as in (a) (see legend) measured two weeks after Nicole’s passage (25-29 Oct. 2016). Shown for reference are total lipid concentration profiles in April 2015 (dark gray, typical post spring bloom conditions) and Nov 2015 (light gray, typical minimum production period).

In October 2016, Category 3 Hurricane Nicole passed over the Bermuda time-series site (Oceanic Flux Program (OFP) and Bermuda Atlantic Time-Series site (BATS)) in the oligotrophic NW Atlantic Ocean. In a recent study published in Geophysical Research Letters, authors synthesized multidisciplinary data from hydrographic and phytoplankton measurements and lipid composition of sinking and suspended particles collected from OFP and BATS, respectively, after Hurricane Nicole in 2016. After the hurricane passed, particulate fluxes of lipids diagnostic of fresh phytodetritus, zooplankton, and microbial biomass increased by 30-300% at 1500 m depth and 30-800% at 3200 m depth (Figure 1a). In addition, mesopelagic suspended particles were enriched in phytodetrital material, as well as zooplankton- and bacteria-sourced lipids (Figure 1b), indicating particle disaggregation and a deep-water ecosystem response.

These results suggest that carbon export and biogeochemical cycles may be impacted by climate-induced changes in hurricane frequency, intensity, and tracks, and, underscore the sensitivity of deep ocean ecosystems to climate perturbations.

Authors:
Rut Pedrosa-Pamies (Marine Biological Laboratory)
Maureen H. Conte (Bermuda Institute of Ocean Science and Marine Biological Laboratory)
JC Weber (Marine Biological Laboratory)
Rodney Johnson (Bermuda Institute of Ocean Science)

Diatoms commit iron piracy with stolen bacterial gene

Posted by mmaheigan

· Tuesday, February 4th, 2020

Since diatoms carry out much of the primary production in iron-limited marine environments, constraining the details of how these phytoplankton acquire the iron they need is paramount to our understanding of biogeochemical cycles of iron-depleted high-nutrient low-chlorophyll (HNLC) regions. The proteins involved in this process are largely unknown, but McQuaid et al. (2018) scientists described a carbonate-dependent uptake protein that enables diatoms to access inorganic iron dissolved in seawater. As increasing atmospheric CO₂ results in decreased seawater carbonate ion concentrations, this iron uptake strategy may have an uncertain future. In a recent study published in PNAS, authors used CRISPR technology to characterize a parallel uptake system that requires no carbonate and is therefore not impacted by ocean acidification.

This system targets an organically complexed form of iron (siderophores, molecules that bind and transport iron in microorganisms) that is only produced by co-occurring microbes. Two genes are required to convert siderophores from a potent toxicant to an essential nutrient. One of these (FBP1) is a receptor that was horizontally acquired from siderophore-producing bacteria. The other (FRE2) is a eukaryotic reductase that facilitates the dissociation of Fe-siderophore complexes.

Figure caption: (A) Growth curves of diatom cultures ( • = WT, ◇ = ΔFBP1, ☐ = ΔFRE2) in low iron media. (B) Growth in same media with siderophores added. (C) Diatoms under 1000x magnification, brightfield. (D) mCherry-FBP1. (E) Plastid autofluorescence. (F) YFP-FRE2. (G) Phylogenetic tree of FBP1 and related homologs.

Are diatoms really stealing siderophores from hapless bacteria? The true nature of this interaction is unknown and may at times be mutualistic. For example, when iron availability limits the carbon supply to a microbial community, heterotrophic bacteria may benefit from using siderophores to divert iron to diatom companions. Further work is needed to understand the true ecological basis for this interaction, but these results suggest that as long as diatoms and bacteria co-occur, iron limitation in marine ecosystems will not be exacerbated by ocean acidification.

Authors:
Tyler Coale (Scripps Institution of Oceanography, J.Craig Venter Institute)
Mark Moosburner (Scripps Institution of Oceanography, J.Craig Venter Institute)
Aleš Horák (Biology Centre CAS, Institute of Parasitology, University of South Bohemia)
Miroslav Oborník (Biology Centre CAS, Institute of Parasitology, University of South Bohemia)
Katherine Barbeau (Scripps Institution of Oceanography)
Andrew Allen (Scripps Institution of Oceanography, J.Craig Venter Institute)

Also see joint post on the GEOTRACES website

The ocean’s smallest phytoplankton may be bigger than we thought

Posted by mmaheigan

· Tuesday, December 17th, 2019

Flow cytometry can sort hundreds of thousands of phytoplankton cells in minutes, a tool that has been exploited for over thirty years in marine science. However, skilled analysts are still needed for manual interpretation of these cells into different types and then further into size distributions and optical properties.

In a recent study published in Applied Optics, the authors developed and implemented an automated scheme on the large Atlantic Meridional Transect flow cytometric database, which contains around 10⁴ samples and 10⁹ cells (the entire AMT flowcytometric dataset which spans a decade of transects (AMT18 – AMT27). This unique, well-calibrated dataset spans 100° of latitude between the UK and the Falklands, with multiple samples between 0-200m. The results clearly show that Prochlorococcus, very small marine cyanobacteria, are consistently larger than previously thought (>0.65 µm), and their size distribution reveals a distinctive double peak (0.75 µm and 1.75 µm) that varies strongly with depth. This is coupled with changes in Prochlorococcus optical properties, a term we have coined “opto-types.” By contrast, Synechococcus are typically 1.5 µm in diameter and more homogeneously dispersed.

Figure 1: North to South transect (bottom left) of the Atlantic Ocean showing the variability in the abundance (top left), size (top right) and refractive index (bottom right) of Prochlorococcus

This work has uncovered new information regarding the size distribution of the ocean’s smallest phytoplankton, which has implications for how energy is transferred between different biological organisms.

Authors:
Tim Smyth (Plymouth Marine Laboratory)
Glen Tarran (Plymouth Marine Laboratory)
Shubha Sathyendranath (Plymouth Marine Laboratory)

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.

Archive for phytoplankton

Filter by Keyword