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New evidence suggests that tiny zooplankton might be the biggest problem with carbon cycling in IPCC climate models

Posted by mmaheigan

· Friday, December 1st, 2023

The ocean is the most important sink of anthropogenic emissions and is being considered as a medium to manipulate to draw down even more. Essential in the ocean’s role as a natural carbon-sponge is the net production of organic matter by phytoplankton, some of which sinks and is stored for 100s-1000s of years. Successfully simulating this biological carbon pump is essential for projecting any climate scenario, but it appears that massive uncertainties in the way zooplankton consume phytoplankton are compromising predictions of future climate and our assessment of some strategies to deliberately engineer it.

Figure caption. Grazing pressure is largest source of uncertainty for marine carbon cycling in CMIP6 models a) The global and zonal median winter grazing pressure is shown for all models. b) the coefficient of variation across models (std/mean) is largest for grazing pressure compared 14 major terms in the marine carbon cycle.

A new publication in Communications Earth and Environment explains how our poor understanding of zooplankton biases our best projections of marine carbon sequestration. We compared 11 IPCC climate models and found zooplankton grazing is largest source uncertainty in marine carbon cycling. This uncertainty is over three times larger than that of net primary production and is driven by large differences in different models assumptions about the rate at which zooplankton can consume phytoplankton. Yet, very small changes in zooplankton grazing dynamics (roughly only 5% of the full range used across IPCC models) can increase carbon sequestrations by 2 PgC/yr, which is double the maximum theoretical potential of Southern Ocean Iron Fertilization! Moving forward, to move beyond merely treating zooplankton as a closure term, modelers must look towards novel observational constraints on grazing pressure.

Authors
Tyler Rohr, Anthony J. Richardson, Andrew Lenton, Matthew A. Chamberlain, and Elizabeth H. Shadwick

Unveiling the Past and Future of Ocean Acidification: A Novel Data Product covering 10 Global Surface OA Indicators

Posted by mmaheigan

· Wednesday, August 23rd, 2023

Accurately predicting future ocean acidification (OA) conditions is crucial for advancing research at regional and global scales, and guiding society’s mitigation and adaptation efforts.

As an update to Jiang et al. 2019, this new model-data fusion product:
1. Utilizes an ensemble of 14 distinct Earth System Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) along with three recent observational ocean carbon data products –>instead of relying on just one model (i.e., the GFDL-ESM2M) this approach reduces potential projection biases in OA indicators.
2. Eliminates model biases using observational data, and model drift with pre-Industrial controls.
3. Covers 10 OA indicators, an expansion from the usual pH, acidity, and buffer capacity.
4. Incorporates the new Shared Socioeconomic Pathways (SSPs).

The use of the most recent observational datasets and a large Earth System Model ensemble is a major step forward in the projection of future surface ocean OA indicators and provides critical information to guide OA mitigation and adaptation efforts.

Figure X. Temporal changes of global average surface ocean OA indicators as reconstructed and projected from 14 CMIP6 Earth System Models after applying adjustments with observational data: (a) fugacity of carbon dioxide (fCO₂), (b) total hydrogen ion content ([H⁺]_total), (c) carbonate ion content ([CO₃^2-]), (d) total dissolved inorganic carbon content (DIC), (e) pH on total scale (pH_T), (f) aragonite saturation state (Ω_arag), (g) total alkalinity content (TA), (h) Revelle Factor (RF), and (i) calcite saturation state (Ω_calc). The asterisk signs on the left-side y-axes show the values in 1750. The numbers along right-side y-axes, i.e., 1-1.9, 1-2.6, 2-4.5, 3-7.0, and 5-8.5, indicate the shared socioeconomic pathway SSP1-1.9, SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5, respectively. These are missing from panel g because the trajectories were more dependent on the model than the SSP.

Authors
Li-Qing Jiang (University Maryland)
John Dunne (NOAA/Geophysical Fluid Dynamics Laboratory)
Brendan R. Carter (University of Washington)
Jerry F. Tjiputra (NORCE Norwegian Research Centre Bjerknes)
Jens Terhaar (Woods Hole Oceanographic Institution)
Jonathan D. Sharp (University of Washington)
Are Olsen (University of Bergen and Bjerknes Centre for Climate Research)
Simone Alin (NOAA/Pacific Marine Environmental Laboratory)
Dorothee C. E. Bakker (University of East Anglia)
Richard A. Feely (NOAA/Pacific Marine Environmental Laboratory)
Jean-Pierre Gattuso (Sorbonne Université)
Patrick Hogan (NOAA/National Centers for Environmental Information)
Tatiana Ilyina (Max Planck Institute for Meteorology)
Nico Lange (GEOMAR Helmholtz Centre for Ocean Research)
Siv K. Lauvset (NORCE Norwegian Research Centre)
Ernie R. Lewis (Brookhaven National Laboratory)
Tomas Lovato (Fondazione Centro Euro-Mediterraneo sui Cambiamenti Climatici)
Julien Palmieri (National Oceanography Centre)
Yeray Santana-Falcón (Université de Toulouse)
Jörg Schwinger (NORCE Norwegian Research Centre)
Roland Séférian (Université de Toulouse)
Gary Strand (US National Center for Atmospheric Research)
Neil Swart (Canadian Centre for Climate Modelling and Analysis)
Toste Tanhua (GEOMAR Helmholtz Centre for Ocean Research)
Hiroyuki Tsujino (JMA Meteorological Research Institute)
Rik Wanninkhof (NOAA/Atlantic Oceanographic Meteorological Laboratory)
Michio Watanabe (Japan Agency for Marine-Earth Science and Technology)
Akitomo Yamamoto (Japan Agency for Marine-Earth Science and Technology)
Tilo Ziehn (CSIRO Oceans and Atmosphere)

Twitter:
@JiangLiqing, @JensTerhaar, @jpGattuso, @j_d_sharp, @AreOlsen, @SimoneAlin, @Dorothee_Bakker, @RFeely, @ilitat, @sivlauvset, @yeraysf, @TosteTanhua,

Severe warming = 15% increase in bacterial respiration: Southern Ocean most impacted

Posted by mmaheigan

· Thursday, March 30th, 2023

The utilization, respiration, and remineralization of organic matter exported from the ocean surface to its depths are key processes in the ocean carbon cycle. Marine heterotrophic Bacteria play a critical role in these activities. However, most three-dimensional (3-D) coupled physical-biogeochemical models do not explicitly include Bacteria as a state variable. Instead, they rely on parameterization to account for the bacteria’s impact on particle flux attenuation.

A recent study examined how bacteria respond to climate change by employing a 3-D coupled ocean biogeochemical model that incorporates explicit bacterial dynamics. The model (CMCC-ESM2) is a part of the Coupled Model Intercomparison Project Phase 6. The authors first evaluated the reliability of century-scale forecasts (2015-2099) for bacterial stocks and rates in the upper 100 m layer against the compiled measurements from the contemporary period (1988-2011). Next the authors analyzed the predicted trends in bacterial stocks and rates under diverse climate scenarios and explored their association with regional differences in temperature and organic carbon stocks. Three crucial findings were revealed. There is a global-scale decrease in bacterial biomass of 5-10%, with a 3-5% increase in the Southern Ocean (Figure 1). In the Southern Ocean, the rise in semi-labile dissolved organic carbon (DOC) leads to an increase in DOC uptake rates of free-living bacteria; in the northern high and low latitudes, the increase in temperature drives the increase in their DOC uptake rates. Importantly, extreme warming could result in a global increase (up to 15%) and even higher in the Southern Ocean (21% increase) in bacterial respiration (Figure 1), potentially leading to a decline in the biological carbon pump.

This analysis is an unprecedented and early effort to understand the climate-induced changes in bacterial dynamics on a global scale in a systematic manner. This study takes us one step closer to comprehending how bacteria influence the functioning of the biological carbon pump and the distribution of organic carbon pools between surface and deep layers, especially their response to climate change.

Figure 1. Global projections of bacterial carbon stocks and rates during the baseline period (1990-2013) and their changes as anomalies under the most-severe climate change scenario (i.e., SSP5-8.5) relative to the baseline period (2076-2099). The stocks and rates during the baseline period (a, b, c, g, h, i) and their changes as anomalies under the most-severe climate change scenario (d, e, f, j, k, l). All variables are depth-integrated in the upper 100 m. Solid-line contours as standard deviation from averaging over 1990-2013. Anomalies are 2076-2099 average values relative to 1990-2013 average values. Global bacterial biomass has decreased by 5-10%, with a 3-5% increase in the Southern Ocean. However, extreme warming may increase bacterial respiration worldwide, thereby reducing the efficiency of the biological carbon pump. This study provides an early attempt to understand the response of bacteria to climate change and their impact on the distribution of organic carbon in the ocean.

Author
Heather Kim, Woods Hole Oceanographic Institution

Drivers of recent Chesapeake Bay warming

Posted by mmaheigan

· Friday, August 26th, 2022

Coastal water temperatures have been increasing globally with more frequent marine heat waves threatening marine life and nearshore communities reliant upon these ecosystems. Often, this warming is assumed to be uniform in space and time; however, this is not the case in the Chesapeake Bay, where warming waters play a major role in exacerbating low oxygen levels and indirectly limiting the efficacy of nutrient reduction efforts on land.

New research published in the Journal of the American Water Resources Association combined long-term observations and a hydrodynamic model to quantify the temporal and spatial variability in warming Chesapeake Bay waters, and identify the contributions of different mechanisms driving these historical temperature changes. While winter temperatures have warmed by less than a half a degree over the past 30 years, summer temperatures have warmed by nearly 1.5 °C, with similar increases at the surface and bottom. In cooler months, the atmosphere was the dominant driver of warming throughout the majority of the Bay, but oceanic warming explained more than half of the increased summer temperatures in the southern Bay nearest the Atlantic.

Figure 1: Relative contribution of different factors to warm-month Chesapeake Bay temperature change over the period 1985-2015. Percentages correspond to average main channel contributions for each component.

Warming temperatures have potentially significant implications for the future size of the Chesapeake Bay dead zone, and the marine species directly affected by these low oxygen conditions. Better quantifying warming contributions from the atmosphere, ocean, sea level, and rivers will also help constrain regional temperature projections throughout the estuary. More accurate projections of future Bay temperatures can help coastal managers better understand the potential for invasive species expansion and endemic species loss, impacts to fisheries and aquaculture, and how changes to ecosystem processes may impact coastal communities dependent on a healthy Bay.

Authors:
Kyle E. Hinson (Virginia Institute of Marine Science, William & Mary)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science, William & Mary)
Pierre St-Laurent (Virginia Institute of Marine Science, William & Mary)
Fei Da (Virginia Institute of Marine Science, William & Mary)
Raymond G. Najjar (The Pennsylvania State University)

Nutrient management improves hypoxia in the Chesapeake Bay despite record-breaking precipitation and warming

Posted by mmaheigan

· Friday, August 26th, 2022

Hypoxia is currently one of the greatest threats to coastal and estuarine ecosystems around the world, and this threat is projected to get worse as waters warm and human populations continue to increase. Over the past 35-years, a massive effort has been underway to decrease hypoxia in the Chesapeake Bay by reducing nutrient input from land. Despite this effort, record-breaking precipitation in 2018-2019 fueled particularly large hypoxic volumes in the Bay, calling into question the efficacy of management actions.

Figure 1. The number of days of additional hypoxia (O2 < 3 mg L^-1) that would have occurred in the Chesapeake Bay if the 35 years of nutrient reductions never occurred, as calculated by differences between a realistic numerical model simulation and one with 1985 nitrogen levels. This management effort has had the greatest impact at the northern and southern edges of the hypoxia in the Bay, where there would have been an additional 60-90 days of O2 < 3 mg L^-1 if nutrient reductions never occurred.

In a recent paper published in Science of the Total Environment, researchers used empirical and numerical modeling to quantify the impact of nutrient management efforts on hypoxia in the Chesapeake Bay. Results suggest that if the nutrient reduction efforts beginning in 1985 had not taken place, hypoxia would have been ~50–120% greater during the average discharge years of 2016–2017 and ~20–50% greater during the wet years of 2018–2019. The management impact was most pronounced in regions of the Bay where the hypoxia season would have been 60-90 days longer if nutrient reductions did not occur (Figure 1).

Although these results suggest that management has reduced hypoxic conditions in the Bay, additional analysis revealed that warming temperatures have already offset 6-34% of this improvement. This highlights the importance of factoring in climate change when setting future management goals.

Figure 2. The number of days of additional hypoxia (O2 < 3 mg L^-1) that would have occurred in the Chesapeake Bay if the 35 years of nutrient reductions never occurred, as calculated by differences between a realistic numerical model simulation and one with 1985 nitrogen levels. This management effort has had the greatest impact at the northern and southern edges of the hypoxia in the Bay, where there would have been an additional 60-90 days of O2 < 3 mg L^-1 if nutrient reductions never occurred.

Authors:
Luke T. Frankel (Virginia Institute of Marine Science, William & Mary)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science, William & Mary)
Pierre St-Laurent (Virginia Institute of Marine Science, William & Mary)
Aaron J. Bever (Anchor QEA)
Romuald N. Lipcius (Virginia Institute of Marine Science, William & Mary)
Gopal Bhatt (Pennsylvania State University; Chesapeake Bay Program)
Gary W. Shenk (USGS; Chesapeake Bay Program)

Why are sand lance embryos so sensitive to future high CO2-oceans?

Posted by mmaheigan

· Friday, August 26th, 2022

Two decades of ocean acidification experiments have shown that elevated CO₂ can affect many traits in fish early life stages. Only few species, however, show direct CO₂-induced survival reductions. This may partly reflect a bias in our current empirical record, which is dominated by species from nearshore tropical-to-temperate environments. There, these organisms already experience highly variable CO₂ conditions. In contrast, fishes from more offshore habitats, especially at higher latitudes are adapted to more CO₂-stable conditions, which could make them more CO₂-sensitive. This group of fishes is still underrepresented in the literature, despite its enormous commercial and ecological importance.

To help address this gap, we conducted new experimental work on northern sand lance Ammodytes dubius, a key forage fish on offshore Northwest Atlantic sand banks with trophic links to more than 70 different predator species of fish, squid, seabirds, and marine mammals. On Stellwagen Bank in the southern Gulf of Maine, sand lance are the ‘backbone’ of the eponymous National Marine Sanctuary.

We followed up on the intriguing findings of a pilot study a few years ago. Over two years and two trials, we again produced embryos from wild, Stellwagen Bank spawners and reared them at several pCO₂ levels (~400−2000 μatm) in combination with static and dynamic temperatures. Again, we observed consistently large CO₂-induced reductions in hatching success (-23% at 1,000 µatm, -61% at ~2,000 µatm), but this time the effects were temperature-independent.

Intriguingly, we again saw that many sand lance embryos at high CO₂ treatments did not merely arrest in their development (indicative of acidosis), but appeared to develop fully to hatch but were somehow incapable of doing so. We show several lines of evidence supporting the hypothesis that CO₂ directly impairs hatching in this species. Most fish rely on hatching enzymes that help embryos break the chorion (egg shell), but these ubiquitous enzymes may work less efficiently under high CO₂, low pH conditions.

For additional context, we also derived long-term, seasonal pCO₂ projections specifically for Stellwagen Bank, which together with the experimental data suggested that increasing CO₂ levels alone could reduce sand lance hatching success to 71% of contemporary levels by the year 2100.

We believe that the importance of sand lances as forage fishes across most northern hemisphere shelf ecosystem warrants a strategic effort of OA researchers to begin testing other sand lance species or populations to understand the magnitude of the problem and its underlying mechanisms.

Authors:
Hannes Baumann (University of Connecticut)
Lucas Jones (University of Connecticut)
Christopher Murray (University of Washington)
Samantha Siedlecki (University of Connecticut)
Michael Alexander (NOAA Physical Sciences Laboratory)
Emma Cross (Southern Connecticut State University)

What can algae tell us about translating laboratory science to nature?

Posted by mmaheigan

· Thursday, June 9th, 2022

Ocean acidification research has grown over the past few decades. Much of recent research documents negative impacts of changing carbonate chemistry on calcifying marine organisms in laboratory experiments. At the 2018 Ocean Acidification PI Meeting, a group of us asked “Can these laboratory responses to ocean acidification be scaled up to accurately predict the responses of marine ecosystems?” To answer this research question, we developed a semi-quantitative synthesis of benthic calcifying algae responses to ocean acidification, recently published in the ICES Journal of Marine Science.

Figure 1. Comparing directional responses of individuals and communities to acidification in laboratory and field settings highlights mismatches. Specifically, field studies document higher proportion of negative responses compared to laboratory experiments. We provide a series of recommendations for future research to better bridge this gap of understanding in responses to ocean acidification. Figure modified from Page et al. 2022.

We detail in the paper how the proportion of positive, neutral, and negative responses in laboratory experiments often didn’t match field observations. Additionally, laboratory experiments mainly report short-term responses (days to weeks) across tropical and temperate locations. In contrast, field studies emphasize long-term responses (months to years) from fewer global locations. Using our synthesis, we developed nine recommendations that will enhance our ability to translate laboratory experiment results into actual responses of marine taxa to ongoing and future acidification in the natural environment. These future research directions are applicable not only to ocean acidification studies but can be directly applied to the broader field of climate change. We hope these recommendations will lead to greater confidence in our projections of climate change impacts at different ecological scales, and better inform the conservation and management of our valuable marine ecosystems.

Backstory

Initially, we set out to answer this research question through a meta-analysis comparing the effect size of the impacts of ocean acidification on benthic calcifying macroalgae in laboratory and field settings. We quickly realized this approach was not going to work because of the much smaller number of responses recorded in field settings, the different methods used, and response parameters measured between the laboratory and field; these differences made calculating and comparing effect sizes impossible. Therefore, we landed on the approach of conducting a semi-quantitative synthesis to compare directional responses in laboratory and field settings. The results of this synthesis and the process of developing a robust research approach to answer our question inspired us to discuss and develop the recommendations for future research presented in the paper.

Authors (affils and Twitter handles)
Heather N. Page (Sea Education Association) @heathernicopage
Keisha D. Bahr (Texas A&M University – Corpus Christi) @thebahrlab
Tyler Cyronak (Nova Southeastern University) @tcyronak
Elizabeth B. Jewett (National Oceanic and Atmospheric Administration) @LibbyJewett
Maggie D. Johnson (King Abdullah University of Science and Technology) @MaggieDJohnson
Sophie J. McCoy (University of North Carolina at Chapel Hill) @MarEcology

Predators Set Range for the Ocean’s Most Abundant Phytoplankton

Posted by mmaheigan

· Friday, April 1st, 2022

Prochlorococcus is the world’s smallest phytoplankton (microscopic plant-like organisms) and the most numerous, with more than ten septillion individuals. This tiny plankton lives ubiquitously in warm, blue, tropical waters but is conspicuously absent in more polar regions. The prevailing theory was the cold: Prochlorococcus doesn’t grow at low temperatures. In a recent paper, the authors argue ecological control, in particular, predation by zooplankton. Cold polar waters are greener because they contain more nutrients, leading to more life and more organic matter production. This production feeds more and larger heterotrophic bacteria, who then feed larger predators—specifically the same zooplankton that consume Prochlorococcus. If the shared zooplankton increases enough, it will consume Prochlorococus faster than it can grow, causing the species to collapse at higher latitudes. These results show that an understanding of both ecology and temperature is required to predict how these ecosystems will shift in a warming ocean.

Figure 1: Surface populations of Prochlorococcus collapse (dashed lines) moving northward from Hawaii as seen in transects (transect line shown in red on map, lower left) from cruises in April 2016 (black dots) and September 2017 (green triangles). This collapse of the Prochlorococcus emerges in dynamical computer models (lower right, color indicates Prochlorococcus biomass in mgC/m3) when heterotrophic bacteria and Prochlorococcus share a grazer (top schematic). Increased organic production heading poleward first increases the heterotrophic bacterial population, increasing the shared zooplankton population which eventually consumes Prochlorococcus faster than it can grow (dashed contour).

Authors
Christopher L. Follett (MIT)
Stephanie Dutkiewicz (MIT)
François Ribalet (UW)
Emily Zakem (USC)
David Caron (USC)
E. Virginia Armbrust (UW)
Michael J. Follows (MIT)

Zooplankton evolutionary rescue is limited by warming and acidification interactions

Posted by mmaheigan

· Friday, November 19th, 2021

A key issue facing ocean global change scientists is predicting the fate of biota under the combined effects of ocean warming and acidification (OWA). In addition to the constraints of studying multifactor drivers, predictions are hampered by few evolutionary studies, especially for animal populations. Evolutionary studies are essential to assess the possibility of evolutionary rescue under OWA– the recovery of fitness that prevents population extirpation in the face of environmental change.

Figure 1. Population fitness of the copepod Acartia tonsa vs generation under ambient, AM (18^oC, 400 µat pCO₂), ocean warming, OW (22^oC, 400 µat pCO₂), ocean acidification, ocean acidification (18^oC, 2000 µat pCO₂), and ocean warming and acidification ( 22^oC, 2000 µat pCO₂). Shown are means and 95% confidence intervals around the mean. The purple line shows that while fitness decreased after the 12^th generation, it was still considerably higher than at generation zero. Treatment lines are offset for clarity. N_o and N_τ (Y-axis legend) represent population size at the beginning and end of a generation (τ), and their ratio is the population fitness. Adapted from Dam et al. (2021).

A paper by Dam et al. published in Nature Climate Change examined the response of a ubiquitous copepod (zooplankter) to OWA for 25 generations to test for evolutionary rescue (Fig. 1). Using a suite of life-history traits, the researchers determined population fitness (the net reproductive rate per generation) under ambient, ocean warming, ocean acidification and OWA conditions. While population fitness decreased drastically under OWA conditions, it recovered in a few generations. However, after 12 generations under OWA, in contrast to OW or OA, fitness started to decrease again, suggesting incomplete evolutionary rescue driven by antagonistic interactions between warming and acidification. Such interactions add complexity to predictions of the fate of the oceanic biota under climate change.

Limited copepod evolutionary rescue would mean lower fisheries yields under OWA conditions as copepods are a main food source for forage fish. Copepods are also important vectors of the sequestration of CO₂ to deeper waters of the ocean. Limited copepod adaptation under OWA could weaken the efficiency of the biological carbon pump.

Authors:
Hans G. Dam (University of Connecticut)
James de Mayo (University of Connecticut)
Gihong Park (University of Connecticut)
Lydia Norton (University of Connecticut)
Xuejia He (Jinan University, China)
Michael B. Finiguerra (University of Connecticut)
Hannes Baumann (University of Connecticut)
Reid S. Brennn (University of Vermont)
Melissa H. Pespeni (University of Vermont)

Contrasting N2O fluxes of source vs. sink in western Arctic Ocean during summer 2017

Posted by mmaheigan

· Wednesday, October 20th, 2021

During the western Arctic summer season both physical and biogeochemical features differ with latitude between the Bering Strait and Chukchi Borderland. The southern region (Bering Strait to the Chukchi Shelf) is relatively warm, saline, and eutrophic, due to the intrusion of Pacific waters that bring heat and nutrients in to the western Arctic Ocean (WAO). Because of the Pacific influence, the WAO is one of the most productive stretches of ocean in the world. In contrast, the northern region (Chukchi Borderland to the Canada Basin) is primarily influenced by freshwater originating from sea ice melt and rivers, and is relatively cold, fresh, and oligotrophic. A frontal zone exists between the southern region and northern region (~73°N) due to the distinct physicochemical contrast between mixing Pacific waters and freshwater. These regions support distinct bacterial communities also, making the environmental variations drivers extremely relevant to nitrous oxide (N₂O) dynamics.

A recent study published in Scientific Reports examined the role of the WAO as a source and a sink of atmospheric N₂O. There are obvious differences in N₂O fluxes between southern Chukchi Sea (SC) and northern Chukchi Sea (NC). In the SC (Pacific water characteristics dominate) N₂O emissions act as a net source to the atmosphere (Figure 1a). In the NC (freshwater dominant) absorption of atmospheric N₂O into the water column suggests that this region acts as a net sink (Figure 1a). The positive fluxes of SC occurred with relatively high sea surface temperature (SST), sea surface salinity (SSS), and biogeochemically-derived N₂O production, whereas the negative fluxes of NC were associated with relatively low SST, SSS, and little N₂O production. These linear relationships between N₂O fluxes and environmental variables suggest that summer WAO N₂O fluxes are remarkably sensitive to environmental changes.

Figure 1. (a) Map of the sampling stations using the Ice Breaking R/V Araon during August 2017. The sampling locations were coloured with N2O fluxes (blue to red gradient, see color bar; sink, air → sea (−), and source, sea → air (+). The southern Chukchi Sea (SC) extends from Bering Strait to Chukchi Shelf and the northern Chukchi Sea (NC) extends from Chukchi Borderland and Canada Basin. The frontal zone arises between SC and NC (black dotted line). (b) Illustration showing future changes in the distribution of the WAO N2O flux constrained by the positive feedback scenario of increasing inflow of Pacific waters and rapidly declining sea-ice extent under accelerating Arctic warming.

This study suggests a potential scenario for future WAO changes in terms of accelerating Arctic change. Increasing inflow of the Pacific waters and rapidly declining sea-ice extent are critical. The increasing inflow of warm nutrient-enriched Pacific waters will likely extend the SC N₂O source region northward, increasing productivity, and thereby intensifying nitrification. All of which would lead to a strengthening of the WAO’s role as an N₂O source. A rapid loss of the sea ice extent could ultimately lead to a sea-ice-free NC, and again, a northward shift, which would result in a diminished role of the NC as an N₂O sink (Figure 1b). While improving our understanding of WAO N₂O dynamics, this study suggests both a direction for future work and a clear need for a longer-term study to answer questions about both seasonal variations in these dynamics and possible interannual to climatological trends.

Authors:
Jang-Mu Heo (Department of Marine Science, Incheon National University)
Sang-Min Eom (Department of Marine Science, Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)
Hyo-Ryeon Kim (Department of Marine Science, Incheon National University)
Joo-Eun Yoon (Department of Marine Science, Incheon National University)
Il-Nam Kim (Department of Marine Science, Incheon National University)

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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