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Archive for phytoplankton

A role for tropical nitrogen fixers in glacial CO2 drawdown

Posted by mmaheigan

· Wednesday, December 4th, 2019

Iron fertilization of marine phytoplankton by Aeolian dust is a well-established mechanism for atmospheric carbon dioxide (CO₂) drawdown by the ocean. When atmospheric CO₂ decreased by 90-100 ppm during previous ice ages, fertilization of iron-limited phytoplankton in the high latitudes was thought to have contributed up to ¹/₃ (30 ppm) of the total CO₂ drawdown. Unfortunately, recent modeling studies suggest that substantially less CO₂ (only 2-10 ppm) is sequestered by the ocean in response to high latitude fertilization.

The limited capacity for high latitude CO₂sequestration in response to iron enrichment motivated the authors of a new study published in Nature Communications to address how lower latitude phytoplankton could contribute to CO₂ drawdown. The authors used an ocean model to show that in response to Aeolian iron fertilization, dinitrogen (N₂) fixers, specialized phytoplankton that introduce bioavailable nitrogen to tropical surface waters, drive the sequestration of an additional 7-16 ppm of CO₂ by the ocean.

Figure 1: Scenarios of Fe supply to the tropical Pacific. In the low iron scenario, analogous to the modern climate, N₂ fixation (yellow zone and dots) is concentrated in the Northwest and Southwest subtropical Pacific where aeolian dust deposition is greatest. Non-limiting PO₄ concentrations (green zone and dots) exist within the tropics and spread laterally from the area of upwelling near the Americas and at the equator (blue zone). In the high Fe scenario, analogous to the glacial climate, N₂ fixation couples to the upwelling zones in the east Pacific, enabling strong utilisation of PO₄, the vertical expansion of suboxic zones (grey bubbles) and a deeper injection of carbon-enriched organic matter (downward squiggly arrows).

These results provide evidence of a tropical ocean CO₂ sequestration pathway, the mere existence of which is hotly debated. Importantly, the study describes an additional mechanism of CO₂ drawdown that is complementary to the high latitude mechanism. When combined, their contributions elevate iron-driven CO₂ drawdown towards the expected 30 ppm, making iron fertilization a driver of a stronger biological pump on a global scale.

Authors:
Pearse Buchanan (University of Liverpool, University of Tasmania, CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate System Science)
Zanna Chase (University of Tasmania)
Richard Matear (CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate Extremes)
Steven Phipps (University of Tasmania)
Nathaniel Bindoff (University of Tasmania, CSIRO Oceans and Atmosphere, ARC Centre of Excellence in Climate Extremes, Antarctic Climate and Ecosystems Cooperative Research Centre)

A new tidal non-photochemical quenching model reveals obscured variability in coastal chlorophyll fluorescence

Posted by mmaheigan

· Tuesday, October 15th, 2019

Although chlorophyll fluorescence is widely-used as a proxy for chlorophyll concentration, sunlight exposure makes fluorescence measurements inaccurate through a process called non-photochemical quenching, limiting its proxy accuracy during daylight hours. In the open ocean, where time and space scales are large relative to variability in phytoplankton concentration, daytime chlorophyll fluorescence—necessary for satellite algorithm validation and for understanding diurnal variability in phytoplankton abundance—can be estimated by averaging across successive nighttime, unquenched values. In coastal waters, where semidiurnal tidal advection drives small scale patchiness and short temporal variability, successive nighttime observations do not accurately represent the intervening daytime. Thus, it is necessary to apply a non-photochemical quenching correction that accounts for the additional effect of tidal advection.

In a recent study in L&O Methods, authors developed a model that uses measurements of tidal velocity to correct daytime chlorophyll fluorescence for non-photochemical quenching and tidal advection. The model identifies high tide and low tide endmember populations of phytoplankton from tidal velocity, and estimates daytime chlorophyll fluorescence as a conservative interpolation between endmember fluorescence at those tidal maxima and minima (Figure 1). Rather than removing nearly 12 hours’ worth of hourly chlorophyll fluorescence observations (i.e., all of the daytime observations) as was previously necessary, this model recovers them. The model output performs more accurately as a proxy for chlorophyll concentration than raw daytime chlorophyll fluorescence measurements by a factor of two, and enables tracking of phytoplankton populations with chlorophyll fluorescence in a Lagrangian sense from Eulerian measurements. Finally, because the model assumes conservation, periods of non-conservative variability are revealed by comparison between model and measurements, helping to elucidate controls on variability in phytoplankton abundance in coastal waters.

Figure 1: Model (light blue line) is a tidal interpolation between high tide (blue points) and low tide (red points) phytoplankton endmembers. The model represents nighttime, unquenched chlorophyll fluorescence measurements well (black points), while daytime, quenched measurements are visibly reduced (gray points).

This result is a critical achievement, as it enables the use of daytime chlorophyll fluorescence, which increases the temporal resolution of coastal chlorophyll fluorescence measurements, and also provides a mechanism for satellite validation of the ocean color chlorophyll data product in coastal waters. The model’s capacity to accurately simulate the pervasive effect of non-photochemical quenching makes it a vital tool for any researcher or coastal water manager measuring chlorophyll fluorescence. This model will help to provide new insights on the movement of and controls on phytoplankton populations across the land-ocean continuum.

Authors:
Luke Carberry (University of California, Santa Barbara)
Collin Roesler (Bowdoin College)
Susan Drapeau (Bowdoin College)

A new roadmap of climate change driven ocean changes

Posted by mmaheigan

· Wednesday, October 2nd, 2019

When will we see significant changes in the ocean due to climate change? A new study in Nature Climate Change confirms that outcomes tied directly to the escalation of atmospheric carbon dioxide have already emerged in the existing 30-year observational record. These include sea surface warming, acidification, and increases in the rate at which the ocean removes carbon dioxide from the atmosphere.

In contrast, processes tied indirectly to the ramp-up of atmospheric carbon dioxide through the gradual modification of climate and ocean circulation will take longer, from three decades to more than a century. These include changes in upper-ocean mixing, nutrient supply, and the cycling of carbon through marine plants and animals.

The researchers performed model simulations of potential future climate states that could result from a combination of human-made climate change and random chance (figure 1). These experiments were performed with an Earth System Model, a climate model that has an interactive carbon cycle such that changes in the climate and carbon cycle can be considered in tandem.

Figure 1: Percentage of ocean with emergent anthropogenic trends in ocean biogeochemical and physical variables. A time series of the percentage of the global ocean area with locally emergent anthropogenic trends illustrates the disparity of emergence timescales for anthropogenic changes in the ocean carbon cycle. Emergence is defined as the point in time when the LE’s signal-to-noise ratio for a linear trend referenced to the year 1990 first exceeds a magnitude of two, which represents a 95% confidence in the identification of an anthropogenic trend in the LE Ω applies to the saturation state of both the aragonite and calcite forms of calcium carbonate (CaCO₃), for which the emergence times are approximately equivalent. The CaCO₃ and soft-tissue pumps were calculated as the export flux at 100 m depth of CaCO₃ and particulate organic carbon, respectively. The heat content was calculated as an integral over 0–700 m, whereas the oxygen (O₂) inventories consider the integral 200–600 m, and chlorophyll inventories were considered over 0–500 m. NPP represents an integral over 0–100 m. All the other variables represent sea surface properties.

The finding of a 30- to 100-year delay in the emergence of effects suggests that ocean observation programs should be maintained for many decades into the future to effectively monitor the changes occurring in the ocean. The study also indicates that the detectability of some changes in the ocean would benefit from improvements to the current observational sampling strategy. These include looking deeper into the ocean for changes in phytoplankton and capturing changes in both summer and winter ocean-atmosphere exchange of carbon dioxide rather than just the annual mean.

Figure 2. Venn Diagram schematic of sources of uncertainty in simulation (using Earth-System Modeling approach) and observation of changes in the Earth system. For emergence, detection or attribution of an observed or simulated signal to occur, the signal must overcome the sources of uncertainty in their respective brackets.

Many types of observational efforts, including time-series or permanent locations of continuous measurement, as well as regional sampling programs and global remote sensing platforms are critical for understanding our changing planet and improving our capacity to detect change.

Authors:
Sarah Schlunegger (Princeton University)
Keith B. Rodgers (Institute for Basic Science and Busan National University, South Korea)
Jorge L. Sarmiento (Princeton University)
Thomas L. Frölicher (University of Bern)
John P. Dunne (NOAA Geophysical Fluid Dynamics Laboratory)
Masao Ishii (Japan Meteorological Agency)
Richard Slater (Princeton University)

Industrial era climate forcing drives multi-century decline in North Atlantic productivity

Posted by mmaheigan

· Wednesday, October 2nd, 2019

Phytoplankton respond directly to climate forcing, and due to their central role in global oxygen production and atmospheric carbon sequestration, they are critical components of the Earth’s climate system. There are however few observations detailing past variability in marine primary productivity, particularly over multi-decadal to centennial timescales. This limits our understanding of the long-term impact of climatic forcing on both past and future marine productivity.

Multi-century decline of subarctic Atlantic productivity. From top: standardized (z-score units relative to ad 1958-2016) indices of Continuous Plankton Recorder (CPR)-based diatom, dinoflagellate and coccolithophore relative-abundances; North Atlantic [chlorophyll-α] reconstruction from Boyce et al. (2010, Nature); ice core-based [MSA] PC1 productivity index. The “Industrial Onset” range shows the estimated initiation of declining subarctic Atlantic productivity; reconstructed (Rahmstorf et al., 2015, Nat. Clim. Change) and observed sea-surface temperature-based Atlantic Meridional Overturning Circulation (i.e., AMOC) index, alongside 5-year averaged subarctic Atlantic freshwater storage anomalies (relative to A.D. 1955) from Curry and Mauritzen (2005; Science).

Authors of a new study published in Nature used a high-resolution signal of marine biogenic aerosol emissions (methanesulfonic acid, or “MSA”) preserved within twelve Greenland ice cores to reconstruct a ~250-year record of marine productivity variations across the subarctic Atlantic basin, one of the most biologically productive and climatically sensitive regions on Earth. These results provide the most continuous proxy-based reconstruction of basin-scale productivity to date in this region, illuminating the following major findings: (1) subarctic Atlantic marine productivity has declined over the industrial era by as much as 10 ± 7%; (2) the early 19th century onset of declining productivity coincides with the regional onset of industrial-era surface warming, and also strongly covaries with regional sea surface temperatures and basin-scale gyre circulation strength; (3) there is strong decadal- to centennial-scale coherence between northern Atlantic productivity variability and declining Atlantic Meridional Overturning Circulation (AMOC) strength, as predicted by prior model-based studies.

Future atmospheric warming is predicted to contribute to accelerating Greenland Ice Sheet runoff, ocean-surface freshening, and AMOC slowdown, suggesting the potential for continued declines in productivity across this dynamic and climatically important region. Such declines will, in turn, have important implications for future maritime economies, global food security, and drawdown of atmospheric carbon dioxide.

Authors:
Matthew Osman (Massachusetts Institute of Technology)
Sarah Das (Woods Hole Oceanographic Institution)
Luke Trusel (Rowan University)
Matthew Evans (Wheaton College)
Hubertus Fischer (University of Bern)
Mackenzie Griemann (University of California, Irvine)
Sepp Kipfstuhl (Alfred-Wegener-Institute)
Joseph McConnell (Desert Research Institute)
Eric Saltzman (University of California, Irvine)

Figure references:
Boyce, D. G., Lewis, M. R. & Worm, B. (2010) Global phytoplankton decline over the past century. Nature 466, 591–596.

Curry, R. & Mauritzen, C. (2005) Dilution of the northern North Atlantic Ocean in recent decades. Science 308, 1772–1774.

Rahmstorf, S. et al. (2015) Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation. Nat. Clim. Change 5, 475–480.

Can microzooplankton shape the depth distribution of phytoplankton?

Posted by mmaheigan

· Tuesday, July 23rd, 2019

Photosynthetic, single-celled phytoplankton form the base of many marine and lacustrine (lake) food webs. These microscopic algae typically occur in the sunlit surface layer, but in many ecosystems, there are also sub-surface peaks in phytoplankton and chlorophyll-a, their key photosynthetic pigment. Historically, scientists have explained deep chlorophyll maximum (DCM) formation by invoking “bottom-up” processes such as nutrient and light co-limitation, while less attention has been paid to “top-down” controls such as predation.

A recent study in Nature Communications challenges this conventional wisdom by arguing that microzooplankton (top-down control) can cause the formation of DCMs by preferentially consuming phytoplankton near the surface. This can occur when microzooplankton exhibit light-dependent grazing—a known but not well-understood phenomenon in which prey consumption rates increase with increasing light intensity. By incorporating this phenomenon into mathematical models, the authors showed that this can create a “spatial refuge” for phytoplankton in deeper, darker parts of the water column, where there is enough sunlight to photosynthesize, but too little for efficient microzooplankton predation. Furthermore, when light-dependent grazing is incorporated into a global ocean biogeochemistry model (COBALT: Carbon, Ocean Biogeochemistry and Lower Trophics – planktonic ecosystem model), DCMs that are already present due to bottom-up controls deepen, improving agreement between model predictions, satellite data, and in situ observations.

Figure legend: Global comparison of annual mean deep chlorophyll maxima (DCM) depths (A) predicted by the unmodified COBALT model, (B) predicted by the COBALT model modified to include light-dependent microzooplankton grazing, and (C) estimated based on satellite data. Incorporating light-dependent grazing deepens the DCM, especially in oligotrophic gyres, and improves agreement with observational data.

These findings highlight the importance of higher trophic levels in regulating aquatic primary productivity. The model predictions suggest that not only can microzooplankton suppress primary production near the surface, but by shifting phytoplankton abundances deeper, they may increase carbon export via the biological pump. Future field tests of this hypothesis—i.e. detailed grazing measurements in stratified water columns with DCMs—can elucidate the extent to which light-dependent grazing shapes phytoplankton distribution in real biological systems.

Authors:
Holly Moeller (University of California Santa Barbara)
Charlotte Laufkötter (University of Bern and Princeton University)
Edward Sweeney (Sea Education Association and Santa Barbara Museum of Natural History)
Matthew Johnson (Woods Hole Oceanographic Institution)

Upwelled hydrothermal Fe stimulates massive phytoplankton blooms in the Southern Ocean

Posted by mmaheigan

· Tuesday, July 9th, 2019

Joint feature with GEOTRACES

Figure 1a: Southern Ocean phytoplankton blooms showing distribution, biomass (circle size) and type (color key).

In a recent study, Ardyna et al combined observations of profiling floats with historical trace element data and satellite altimetry and ocean color data from the Southern Ocean to reveal that dissolved iron of hydrothermal origin can be upwelled to the surface. Furthermore, the activity of deep hydrothermal sources can influence upper ocean biogeochemical cycles of the Southern Ocean, and in particular stimulate the biological carbon pump.

Authors:
Mathieu Ardyna
Léo Lacour
Sara Sergi
Francesco d’Ovidio
Jean-Baptiste Sallée
Mathieu Rembauville
Stéphane Blain
Alessandro Tagliabue
Reiner Schlitzer
Catherine Jeandel
Kevin Robert Arrigo
Hervé Claustre

Ocean color offers early warning signal of climate change’s impact on marine phytoplankton

Posted by mmaheigan

· Monday, April 15th, 2019

Marine phytoplankton form the foundation of the marine food web and play a crucial role in the earth’s carbon cycle. Typically, satellite-derived Chlorophyll a (Chl a) is used to evaluate trends in phytoplankton. However, it may be many decades (or longer) before we see a statistically significant signature of climate change in Chl a due to its inherently large natural variability. In a recent study in Nature Communications, authors explored how other metrics, in particular the color of the ocean, may show earlier and stronger signals of climate change at the base of the marine food web.

Figure 1. Computer model results indicating the year in which the signature of climate change impact is larger than the natural variability for (a) Chl a, and (b) remotely sensed reflectance in the blue-green waveband. White areas indicate where there is not a statistically significant change by 2100, or for regions that are currently ice-covered.

In this study, the authors use a unique marine physical-biogeochemical and ecosystem model that also captures how light penetrates the ocean and is reflected upward. The model shows that over the course of the 21^st century, remote sensing reflectance (R_RS, the ratio of upwelling radiance to the downwelling irradiance at the ocean’s surface) in the blue-green portions of the light spectrum is likely to have an earlier, more spatially extensive climate change-driven signal than Chl a (Figure 1). This is because R_RS integrates not only changes to Chl a, but also alterations in other optically important water constituents. In particular, R_RS also captures changes in phytoplankton community structure, which strongly affects ocean optics and is likely to be altered over the 21^st century. Monitoring the response of marine phytoplankton to climate change is important for predicting changes at higher trophic levels, including commercial fisheries. Our study emphasizes the importance of 1) maintaining ocean color sensor compatibility and long-term stability, particularly in the blue-green wavebands; 2) maintaining long-term in situ time-series of plankton communities – e.g., the Continuous Plankton Recorder survey and repeat stations (e.g., HOT, BATS); and 3) reducing uncertainties in satellite-derived phytoplankton community structure estimates.

Authors:
Stephanie Dutkiewicz, Oliver Jahn (Massachusetts Institute of Technology)
Anna E. Hickman (University of Southampton)
Stephanie Henson (National Oceanography Centre Southampton)
Claudie Beaulieu (University of California, Santa Cruz)
Erwan Monier (University of California, Davis)

A half century perspective: Seasonal productivity and particulates in the Ross Sea

Posted by mmaheigan

· Tuesday, April 2nd, 2019

Studies of cruise observations in the Ross Sea are typically biased to a single or a few year(s), and long-term trends have predominantly come from satellites. Consequently, the in situ climatological patterns of nutrients and particulate matter have remained vague and unclear. What are the typical patterns of nutrients and particulate matter concentrations in the Ross Sea in spring and summer? How do these concentrations affect annual productivity estimates?

Patterns of nutrient and particulate matter in the Ross Sea can play a wide-ranging role in a productive region like the Ross Sea. Smith and Kaufman (2018) recently synthesized austral spring and summer (November to February) observations from 42 Ross Sea research cruises (1967-2016) to analyze broad biogeochemical patterns. The resulting climatologies revealed interesting seasonal patterns of nutrient uptake and particulate organic carbon (POC) to chlorophyll (chl) ratios (POC:chl). Temporal patterns in the nitrate and phosphate climatologies confirm the role of early spring haptophyte (Phaeocystis antarctica) growth, followed by limited nitrogen and phosphorus removal in summer. However, a notable increase in POC occurred later in summer that was largely independent of chlorophyll changes, resulting in a dramatic increase in POC:chl. A gradual decline in silicic acid concentrations throughout the summer, along with an increased occurrence of biogenic silica during this time suggest that diatoms may be responsible for this later POC spike. Revised estimates of primary productivity based on these observed climatological POC:chl ratios suggests that summer blooms may be a significant contributor to seasonal productivity, and that estimates of productivity based on satellite pigments underestimate annual production by at least 70% (Figure 1).

Figure 1. Bio-optical estimates of mean productivity using a constant POC:chl ratio (black dots and lines) and modified estimates of productivity using the monthly climatological POC:chl ratios (red dots and lines), in a) the Ross Sea polynya region and b) the western Ross Sea region.

By clarifying typical seasonal patterns of nutrient uptake and POC:chl, these climatologies underscore the biogeochemical importance of both spring haptophyte growth and previously underestimated summer diatom growth in the Ross Sea. Further investigation of the causes and consequences of elevated summer ratios is needed, as assessments of regional food webs and biogeochemical cycles depend on more accurate understanding of primary productivity patterns. Likewise, these results highlight the need for continued efforts to constrain satellite productivity estimates in the Ross Sea using in situ constituent ratios.

For other relevant work on seasonal biogeochemical patterns in the Ross Sea, please see https://doi.org/10.1016/j.dsr2.2003.07.010. And for intra-seasonal estimates of particulate organic carbon to chlorophyll using gliders, please see: https://doi.org/10.1016/j.dsr.2014.06.011.

Authors:
Walker O. Smith Jr. (VIMS, College of William and Mary)
Daniel E. Kaufman (VIMS, College of William and Mary; now at Chesapeake Research Consortium)

Dust-borne iron in the Southern Ocean was more bioavailable during glacial periods

Posted by mmaheigan

· Wednesday, January 23rd, 2019

The Southern Ocean is iron (Fe)-limited, and increased fluxes of dust-borne Fe to the Southern Ocean during the Last Glacial Maximum (LGM) have been associated with phytoplankton growth and CO₂ drawdown. Dust contains different mixes of Fe-bearing minerals, depending on the source region. Fe(II) silicate minerals from physical weathering are more bioavailable than Fe(III) oxyhydroxide minerals from chemical weathering. The Fe(II) silicates are dominant in dust sources that have been weathered from bedrock by glaciers in Patagonia, but the impact of glacial activity on dust-borne Fe speciation (Fe oxidation state and mineral composition) and bioavailability over the last glacial cycle has not previously been quantified.

Figure 1. The fraction of Fe(II) in dust (Fe(II)/Fetotal, dominated by Fe(II) silicates, shown as blue dots connected with dotted lines on blue axes) in marine sediment cores from (A) the South Atlantic and (B) the South Pacific plotted with the total dust flux (grey lines on grey axes).

A recent study in PNAS reconstructs the speciation of dust-borne Fe over the last glacial cycle in South Atlantic and South Pacific marine sediment cores using Fe K-edge X-ray absorption spectroscopy. The authors observed that the highly bioavailable Fe(II) silicate content of dust-borne Fe is higher in both regions during cold glacial periods, suggesting that a given flux of Fe is more bioavailable in glacial versus interglacial periods (Figure 1). Therefore, all Fe cannot be considered equal in biogeochemical models working on glacial-interglacial timescales. The bioavailability of a given flux of Fe at the LGM was likely a dominant driver of phytoplankton growth, with more bioavailable Fe driving increased phytoplankton activity and associated atmospheric CO₂ drawdown and subsequent cooling. The observed association between glacial periods and increased Fe bioavailability in the Southern Ocean may indicate an important positive feedback mechanism between glacial activity and cold glacial temperatures through Fe speciation and the efficiency of the biological pump.

Paper link: https://doi.org/10.1073/pnas.1809755115

Authors:
Elizabeth M. Shoenfelt (Lamont-Doherty Earth Observatory, Columbia University)
Gisela Winckler (Lamont-Doherty Earth Observatory, Columbia University)
Frank Lamy (Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research)
Robert F. Anderson (Lamont-Doherty Earth Observatory, Columbia University)
Benjamin C. Bostick (Lamont-Doherty Earth Observatory, Columbia University)

The past, present, and future of artificial ocean iron fertilization experiments

Posted by mmaheigan

· Wednesday, January 23rd, 2019

Since the beginning of the industrial revolution, human activities have greatly increased atmospheric CO₂ concentrations, leading to global warming and indicating an urgent need to reduce global greenhouse gas emissions. The Martin (or iron) hypothesis suggests that ocean iron fertilization (OIF) could be a low-cost effective method for reducing atmospheric CO₂ levels by stimulating carbon sequestration via the biological pump in iron-limited, high-nutrient, low-chlorophyll (HNLC) ocean regions. Given increasing global political, social, and economic concerns associated with climate change, it is necessary to examine the validity and usefulness of artificial OIF (aOIF) experimentation as a geoengineering solution.

Figure 1. (a) Global annual distribution of surface chlorophyll concentrations (mg m^-3) with locations of 13 aOIF experiments. Maximum and initial values in (b) maximum quantum yield of photosynthesis (Fv/Fm ratios) and (c) chlorophyll-a concentrations (mg m^-3) during aOIF experiments. (d) Changes in primary productivity (ΔPP = [PP]_{post-fertilization (postf)} ‒ [PP]_{pre-fertilization (pref)}; mg C m^-2 d^-1). (e) Distributions of chlorophyll-a concentrations (mg m^-3) on day 24 after iron addition in the Southern Ocean iron experiment-north (SOFeX-N) from MODIS Terra Level-2 daily image and on day 20 in the SOFeX-south (SOFeX-S) from SeaWiFS Level-2 daily image (white dotted box indicates phytoplankton bloom during aOIF experiments). (f) Changes in nitrate concentrations (ΔNO₃^– = [NO₃^–]_postf ‒ [NO₃^–]_pref; μM). (g) Changes in partial pressure of CO₂ (ΔpCO₂ = [pCO₂]_postf ‒ [pCO₂]_pref; μatm). The color bar indicates changes in dissolved inorganic carbon (ΔDIC = [DIC]_postf ‒ [DIC]_pref; μM). The numbers on the X axis indicate the order of aOIF experiments as given in Figure 1a and are grouped according to ocean basins; Equatorial Pacific (EP) (yellow bar), Southern Ocean (SO) (blue bar), subarctic North Pacific (NP) (red bar), and subtropical North Atlantic (NA) (green bar).

A review paper published in Biogeosciences on aOIF experiments provides a thorough overview of 13 scientific artificial OIF experiments conducted in HNLC regions over the last 25 years. These aOIF experiments have demonstrated that iron addition stimulates substantial increases in phytoplankton biomass and primary production, resulting in drawdown of macro-nutrients and dissolved inorganic carbon (Figure 1). Many of the aOIF experiments have also precipitated community shifts from smaller (pico- and nano-) to larger (micro) phytoplankton. However, the impact on the net transfer of CO₂ from the atmosphere to below the winter mixed layer via the biological pump is not yet fully understood or quantified and appears to vary with environmental conditions, export flux measurement techniques, and other unknown factors. These results, including possible side effects, have been debated among those who support and oppose aOIF experimentation, and many questions remain about the effectiveness of scientific aOIF, possible side effects, and international aOIF law frameworks. Therefore, it is important to continue undertaking small-scale, scientifically controlled studies to better understand natural processes in the HNLC regions, assess the associated risks, and lay the groundwork for evaluating the potential effectiveness and impacts of large-scale aOIF as a geoengineering solution to anthropogenic climate change. Additionally, this paper suggests considerations for the design of future aOIF experiments to maximize the effectiveness of the technique and begin to answer open questions under international aOIF regulations.

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

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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