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Inorganic carbon outwelling as important blue carbon sink

Posted by mmaheigan

· Wednesday, May 29th, 2024

Blue carbon ecosystems—mangroves, saltmarshes, and seagrass meadows—carbon sequestration powerhouses that can help us mitigate climate change. For many years, our community has focused on studying and quantifying organic carbon storage in the soils of these ecosystems and crediting it as Blue Carbon in carbon markets.

A new paper in Nature Communications reveals that much of that carbon sequestered by mangroves and saltmarshes is actually exported as inorganic carbon to the ocean. Inorganic carbon export dominates blue carbon budgets and rivals or even surpasses carbon stored in soils. Inorganic carbon exports had an alkalinity: dissolved inorganic carbon ratio of 0.8 ± 0.2, impacting the carbonate system and carbon cycling along the coast. Most of the inorganic carbon is exported as bicarbonate which stays permanently dissolved in the ocean and is therefore a permanent atmospheric carbon sink. When we ignore inorganic carbon export, we highly underestimate the potential of mangroves and saltmarshes to mitigate climate change. Consequently, inorganic carbon export should be integrated into blue carbon frameworks to adequately inform carbon markets, which encourage landowners to restore and preserve mangrove and saltmarsh ecosystems.

Authors
Gloria M. S. Reithmaier (University of Gothenburg) Twitter: @GReithmaier@Barefoot_Lab
Alex Cabral (University of Gothenburg)
Anirban Akhand (Hong Kong University of Science and Technology)
Matthew J. Bogard (University of Lethbridge)
Alberto V. Borges (University of Liège)
Steven Bouillon (KU Leuven)
David J. Burdige (Old Dominion University)
Mitchel Call (Southern Cross University)
Nengwang Chen (Xiamen University)
Xiaogang Chen (Westlake University)
Luiz C. Cotovicz Jr (Leibniz Institute for Baltic Sea Research)
Meagan J. Eagle (U.S. Geological Survey)
Erik Kristensen (University of Southern Denmark)
Kevin D. Kroeger (U.S. Geological Survey)
Zeyang Lu (Xiamen University)
Damien T. Maher (Southern Cross University)
Lucas J. Pérez-Lloréns (University of Cádiz)
Raghab Ray (University of Tokyo)
Pierre Taillardat (National University of Singapore)
Joseph J. Tamborski (Old Dominion University)
Rob C. Upstill-Goddard (Newcastle University)
Faming Wang (Chinese Academy of Sciences)
Zhaohui Aleck Wang (Woods Hole Oceanographic Institution)
Kai Xiao (Southern University of Science and Technology)
Yvonne Y. Y. Yau (University of Gothenburg)
Isaac R. Santos (University of Gothenburg)

Tiny parasites, big impact: Species networks and carbon recycling in an oligotrophic ocean

Posted by mmaheigan

· Tuesday, March 12th, 2024

Parasites are everywhere in the ocean. Including the microbial realm where a diverse, widespread group of protist parasites (Syndiniales) infect and kill a range of hosts, such as dinoflagellates, radiolarians, and even larger zooplankton. A complete Syndiniales infection cycle is only 2-3 days. First, the parasite is a free-living spore. Once inside a host, the parasite consumes the host’s carbon and becomes a larger multicellular organism (a trophont) eventually causing the host to burst open and release hundreds of new spores.

Like viruses, parasite lysis is expected to reroute organic carbon to the microbial loop, potentially decreasing the amount of carbon available for export to the deep sea. Yet, the role of Syndiniales in carbon cycling has been hard to define, as depth-specific infection dynamics and links to carbon export remain poorly understood.

Parasites are everywhere in the ocean. Including the microbial realm where a diverse, widespread group of protist parasites (Syndiniales) infect and kill a range of hosts, such as dinoflagellates, radiolarians, and even larger zooplankton. A complete Syndiniales infection cycle is only 2-3 days. First, the parasite is a free-living spore. Once inside a host, the parasite consumes the host’s carbon and becomes a larger multicellular organism (a trophont) eventually causing the host to burst open and release hundreds of new spores.

Figure 1. The mean relative abundance of Syndiniales (purple) in the photic zone (<140 m) is negatively correlated with particulate organic carbon (POC) flux at 150 m (p-value < 0.001). Similar correlations are not significant (p-values > 0.05) for other major 18S taxonomic groups, like Dinophyceae (red) and Arthropoda (green).

In a recent study published in ISME Communications, authors analyzed an 18S rRNA gene metabarcoding dataset from the Bermuda Atlantic Time-series Study (BATS) site that included 4 years (2016-2019) and twelve depths (1-1000 m). Syndiniales were the most dominant 18S group at BATS, present throughout the photic and aphotic zones. These parasites were prominent in species networks constructed with 18S sequence data, with significant associations with dinoflagellates and copepods in the surface, and with radiolarians in the aphotic zone. In addition, Syndiniales were the only major 18S group to be significantly (and negatively) correlated to particulate carbon flux (at 150 m), which was estimated from sediment trap data collected concurrently at BATS (Figure 1). This is in situ evidence of flux attenuation among Syndiniales, as they recycle host carbon that would otherwise transfer up to larger organisms (e.g., via grazing). Lastly, authors found 19% of the Syndiniales community is linked between photic and aphotic zones, indicating that parasites are sinking on particles and/or are recirculated via diel vertical migration. Overall, these findings elevate the role of Syndiniales in microbial food webs and further emphasize the importance in quantifying parasite-host dynamics to inform ocean carbon models.

Authors
Sean Anderson (University of New Hampshire / Woods Hole Oceanographic Institution)
Leocadio Blanco-Bercial (Bermuda Institute of Ocean Sciences / Arizona State University)
Craig Carlson (University of California, Santa Barbara)
Elizabeth Harvey (University of New Hampshire)

Size does matter: larger krill leads to more POC export in the West Antarctic Peninsula

Posted by mmaheigan

· Friday, December 1st, 2023

Despite the importance of particulate organic carbon (POC) export on carbon sequestration and marine ecology, there have been few multi-decade studies in the world’s oceans. A new analysis published in Nature analyzed two decades of POC export data in the West Antarctic Peninsula and found that export oscillates on a 5-year cycle.

Figure caption: A) Particulate organic carbon (POC) export oscillates on a 5-year timescale in sync with the oscillation in the body size of the krill Euphausia superba on the West Antarctic Peninsula. B) POC export is significantly correlated with krill body size (p = 0.01).

Using a unique combination of krill data from penguin diet samples and net tows over two decades, Trinh et al. found that the cycle of POC export is intimately tied to the Antarctic krill (Euphausia superba) life cycle, as the bulk of the POC in their sediment traps was krill fecal pellets. Surprisingly, more krill did not lead to more POC export. Instead, when the krill population size was smaller but dominated by larger, older adults, POC export increased.

E. superba is the longest-lived (5-6 years) and largest krill species. They exhibit continuous annual growth throughout their life cycle. After about five years a krill population reaches its end stage and the population size is at a minimum. This end-stage population is composed of large, 50-60 mm long individuals that produce large, fast-sinking fecal pellets, leading to increased POC export. Increasing temperatures and deterioration of sea ice cover during the winter season due to climate change will likely impact the recruitment of new cohorts of krill and their success in replenishing aging populations. It is unclear how changes in the krill population and life cycle will impact long-term carbon sequestration on the West Antarctic Peninsula and nutrients exported to the benthic ecosystem

Authors:
Rebecca Trinh (Lamont Doherty Earth Observatory, Columbia University)
Hugh Ducklow (Lamont Doherty Earth Observatory, Columbia University)
Deborah Steinberg (Virginia Institute of Marine Science, College of William and Mary)
William Fraser (Polar Oceans Research Group)

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

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)

How atmospheric and oceanographic forcing impact the carbon sequestration in an ultra-oligotrophic marine system

Posted by mmaheigan

· Wednesday, August 11th, 2021

Sinking particles are a critical conduit for the export of material from the surface to the deep ocean. Despite their importance in oceanic carbon cycling, little is known about the composition and seasonal variability of sinking particles which reach abyssal depths. Oligotrophic waters cover ~75% of the ocean surface and contribute over 30% of the global marine carbon fixation. Understanding the processes that control carbon export to the deep oligotrophic areas is crucial to better characterize the strength and efficiency of the biological pump as well as to project the response of these systems to climate fluctuations and anthropogenic perturbations.

In a recent study published in Frontiers in Earth Science, authors synthesized data from atmospheric and oceanographic parameters, together with main mass components, and stable isotope and source-specific lipid biomarker composition of sinking particles collected in the deep Eastern Mediterranean Sea (4285m, Ierapetra Basin) for a three-year period (June 2010-June 2013). In addition, this study compared the sinking particulate flux data with previously reported deep-sea surface sediments from the study area to shed light on the benthic–pelagic coupling.

Figure Caption: a) Biplot of net primary productivity vs export efficiency (top and bottom horizontal dashed lines indicate threshold for high and low export efficiency regimes). b) Biplot of POC-normalized concentrations of terrestrial vs. phytoplankton-derived lipid biomarkers of the sinking particles collected in the deep Eastern Mediterranean Sea (Ierapetra Basin, NW Levantine Basin) from June 2010–June 2013, and surface sediments collected from January 2007 to June 2012 in the study area.

Both seasonal and episodic pulses are crucial for POC export to the deep Eastern Mediterranean Sea. POC fluxes peaked in spring April–May 2012 (12.2 mg m⁻² d⁻¹) related with extreme atmospheric forcing. Overall, summer particle export fuels more efficient carbon sequestration than the other seasons. The results of this study highlight that the combination of extreme weather events and aerosol deposition can trigger an influx of both marine labile carbon and anthropogenic compounds to the deep. Finally, the comparison of the sinking particles flux data with surface sediments revealed an isotopic discrimination, as well as a preferential degradation of labile organic matter during deposition and burial, along with higher preservation of land-derived POC in the underlying sediments. This study provides key knowledge to better understand the export, fate and preservation vs. degradation of organic carbon, and for modeling the organic carbon burial rates in the Mediterranean Sea.

Authors:
Rut Pedrosa-Pamies (The Ecosystems Center, Marine Biological Laboratory, US; Research Group in Marine Geosciences, University of Barcelona, Spain)
Constantine Parinos (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)
Anna Sanchez-Vidal (Group in Marine Geosciences, University of Barcelona, Spain)
Antoni Calafat (Group in Marine Geosciences, University of Barcelona, Spain)
Miquel Canals (Group in Marine Geosciences, University of Barcelona, Spain)
Dimitris Velaoras (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)
Nikolaos Mihalopoulos (Environmental Chemical Processes Laboratory, University of Crete; National Observatory of Athens, Greece)
Maria Kanakidou (Environmental Chemical Processes Laboratory, University of Crete Greece)
Nikolaos Lampadariou (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)
Alexandra Gogou (Institute of Oceanography, Hellenic Centre for Marine Research, Greece)

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)

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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