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Global change impacts soil carbon storage in blue carbon ecosystems

Posted by mmaheigan

· Wednesday, May 20th, 2020

Vegetated coastal “blue carbon” ecosystems, including sea grasses, mangroves, and salt marshes, provide valuable ecosystem services such as carbon sequestration, storm protection, critical habitat, etc.. Many of these services are supported by the ability of blue carbon ecosystems to accumulate soil organic carbon over thousands of years. Rapidly changing climate and environmental conditions will impact decomposition and thus the global reservoir of organic carbon in coastal soils. A recent Perspective article published in Nature Geoscience focused on the biogeochemical factors affecting decomposition in coastal soils, such as mineral protection, redox zonation, water content and movement, and plant-microbe interactions. The authors explored the spatial and temporal scales of these decomposition mechanisms and developed a conceptual framework to characterize how they may respond to environmental disturbances such as land-use change, nutrient loading, warming, and sea-level rise.

Figure caption: Temperate salt marshes (MA, USA). Healthy salt marshes have lush stands of grasses (top). Storms can expose peat deposits that have been buried for thousands of years (bottom). The fate of this soil carbon is unknown, but some fraction will be respired by microbes and returned to the atmosphere as CO₂.

Improved estimates of soil organic carbon in blue carbon systems will require better characterization of these processes from sustained data sets. Furthermore, incorporation of these decomposition mechanisms into ecosystem evolution models will improve our capacity to quantify and predict changes in these soil carbon reservoirs, which could facilitate their inclusion in global budgets and management tools.

Temperate salt marshes (MA, USA). Healthy salt marshes have lush stands of grasses (left/top). Storms can expose peat deposits that have been buried for thousands of years (right/bottom). The fate of this soil carbon is unknown, but some fraction will be respired by microbes and returned to the atmosphere as CO₂.

Authors:
Amanda C Spivak (University of Georgia)
Jon Sanderman (Woods Hole Research Center)
Jennifer Bowen (Northeastern University)
Elizabeth A. Canuel (Virginia Institute of Marine Science)
Charles S Hopkinson (University of Georgia)

Light matters for biological pump assessments

Posted by mmaheigan

· Thursday, May 7th, 2020

Organic carbon produced during photosynthesis in the sunlit euphotic zone is transported to the deep ocean via the ocean’s biological carbon pump (BCP). Even small changes in the BCP efficiency changes the carbon dioxide gradient across the ocean‐atmosphere interface, thus influencing global climate. A recent study in PNAS demonstrate that prior studies that estimate BCP efficiencies at a fixed depth fail because they do not consider the varying depth of light penetration, which ultimately controls production of sinking organic carbon and varies by location and season. Subsequently, the fixed depth approach introduces regional biases and underestimates global estimates of BCP efficiency by two-fold (Figure 1). These new findings make the case for using euphotic zone‐based metrics rather than applying a fixed depth to compare BCP efficiencies between sites. Improved estimates of BCP efficiency will lead to a better understanding of the mechanisms that control ocean carbon fluxes and its feedbacks on climate.

Figure 1: Carbon loss from the surface ocean shows more variability and is twice as high if measured at the depth where sunlight penetrates (left) vs. 150 meters (about 500 feet; right) where it is commonly measured. One Pg is 10¹⁵ grams with close to 6 Pg of carbon being transported to depth per year in left panel. In comparison, about 10 Pg C/yr is released to the atmosphere as a result of human activity.

Authors:
Ken Buesseler (WHOI)
Philip Boyd (IMAS Univ. Tasmania)
Erin Black (Dalhousie University)
David Siegel (University of California, Santa Barbara)

Also see: Tiny plankton drive processes in the ocean that capture twice as much carbon as scientists thought on The Conversation.

Featured on the cover of the PNAS May 5, 2020 issue:

An Important Biogeochemical Link between Organic and Inorganic Carbon Cycling: Contributions of Organic Alkalinity

Posted by mmaheigan

· Wednesday, April 8th, 2020

As a part of dissolved organic carbon (DOC), organic acid charge groups can contribute significantly to total alkalinity (TA) in natural waters. Such a contribution is termed as organic alkalinity (OrgAlk). Beyond being part of TA, OrgAlk represents an important biogeochemical linkage between organic and inorganic carbon cycling. In other words, the biogeochemical cycling of organic acid charge groups – i.e. their sources, sinks, and biogeochemical behaviors – directly impacts pH and carbonate speciation, which may ultimately influence air-water CO₂ exchange and inorganic carbon fluxes. However, the effects of OrgAlk is often ignored or treated as a calculation uncertainty in many aquatic CO₂ studies. How we treat and study OrgAlk may need a new paradigm under biogeochemical cycles.

Based on direct titration data of OrgAlk, the authors of a recent study conducted a comprehensive assessment of OrgAlk variability, sources, and characteristics in a sub-estuary of Waquoit Bay (Massachusetts). The sub-estuary is influenced by a salt marsh, groundwater input, and offshore water. Both the salt marsh and groundwater OrgAlk contributed up to 4.3% of the TA across all sampled seasons. Estuarine OrgAlk:DOC ratios varied across space and time, which suggests that their abundances are controlled by different biogeochemical processes. In addition, the study demonstrates the insufficiency of using a fixed proportion of DOC to account for OrgAlk, as well as the challenge of using measured pH, TA, and dissolved inorganic carbon (DIC) to estimate OrgAlk. The effects of OrgAlk in these waters are equivalent to a pH change of ~ 0.03 – 0.26, or a pCO₂ change of ~30–1600 matm. If extrapolating OrgAlk results to other coastal systems ranging from estuaries to continental shelves, OrgAlk would exert a strong control on both carbonate speciation and, ultimately, air-sea CO₂ fluxes. This study provides a new conceptual framework for cycling of OrgAlk species and associated links between DOC and DIC pools in coastal systems (Figure 1).

Figure caption: A conceptual model of organic alkalinity cycling in coastal systems. BioP and ChemP represent in-situ biological production and chemical production of organic acid charge groups, respectively. Alk denotes total alkalinity. Arrows with dashed lines indicate processes that were not studied in the present study. The values in the boxes of pH, pCO₂, and buffer capacity represent the magnitude of OrgAlk effects on pH, pCO₂, and buffer capacity in the range of OrgAlk% in TA observed in this study (0.9 – 4.3%).

Authors
Shuzhen Song (East China Normal University)
Zhaohui Aleck Wang (Woods Hole Oceanographic Institution)
Meagan Eagle Gonneea (U. S. Geological Survey)
Kevin D. Kroeger (U. S. Geological Survey)

The past, present, and future of the ocean carbon cycle: A global data product with regional insights

Posted by mmaheigan

· Tuesday, January 21st, 2020

A new study published in Scientific Reports debuts a global data product of ocean acidification (OA) and buffer capacity from the beginning of the Industrial Revolution to the end of the century (1750-2100 C.E.). To develop this product, the authors linked one of the richest observational carbon dioxide (CO₂) data products (6^th version of the Surface Ocean CO₂ Atlas, 1991-2018, ~23 million observations) with temporal trends modeled at individual locations in the global ocean. By linking the modeled pH trends to the observed modern pH distribution, the climatology benefits from recent improvements in both model design and observational data coverage, and is likely to provide more accurate regional OA trajectories than the model output alone. The authors also show that air-sea CO₂ disequilibrium is the dominant mode of spatial variability for surface pH, and discuss why pH and calcium carbonate mineral saturation states (Omega), two important metrics for OA, show contrasting spatial variability. They discover that sea surface temperature (SST) imposes two large but cancelling effects on surface ocean pH and Omega, i.e., the effects of SST on (a) chemical speciation of the carbonic system; and (b) air-sea exchange of CO₂ and the subsequent DIC/TA ratio of the seawater. These two processes act in concert for Omega but oppose each other for pH. As a result, while Omega is markedly lower in the colder polar regions than in the warmer subtropical and tropical regions, surface ocean pH shows little latitudinal variation.

Figure 1. Spatial distribution of global surface ocean pH_T (total hydrogen scale, annually averaged) in past (1770), present (2000) and future (2100) under the IPCC RCP8.5 scenario.

This data product, which extends from the pre-Industrial era (1750 C.E.) to the end of this century under historical atmospheric CO₂ concentrations (pre-2005) and the Representative Concentrations Pathways (post-2005) of the Intergovernmental Panel on Climate Change (IPCC)’s 5^th Assessment Report, may be helpful to policy-makers and managers who are developing regional adaptation strategies for ocean acidification.

The published paper is available here: https://www.nature.com/articles/s41598-019-55039-4

The data product is available here: https://www.nodc.noaa.gov/oads/data/0206289.xml

Authors:
Li-Qing Jiang (University of Maryland and NOAA NCEI)
Brendan Carter (NOAA PMEL and University of Washington JISAO)
Richard Feely (NOAA PMEL)
Siv Lauvset, Are Olsen (University of Bergen and Bjerknes Centre for Climate Research, Norway)

Pumped up by the cold: Increased elemental density in polar diatoms

Posted by mmaheigan

· Monday, October 28th, 2019

Large diatoms are common in polar phytoplankton blooms, contributing significantly to food webs and carbon export, but relatively little is known about their elemental biogeochemistry. A recent study in Frontiers in Marine Science showed that the size-dependent increase in cell nutrient content for polar diatoms was similar to published values for temperate diatoms, whereas the elemental density (mass per unit volume) of polar diatoms was substantially greater for all elements measured (carbon, nitrogen, silicon and phosphorus). Furthermore, at near freezing culture temperatures, there was a positive relationship between diatom size and realized growth rates near their theoretical maximum (Figure 1). Because of the differences in elemental density between carbon and silica, these diatoms exhibited particulate C:Si ratios that are commonly interpreted as a sign of iron limitation; yet these cultures were trace metal-replete. The observed elemental composition differences suggest that it may be important for polar biogeochemical models to include different representations of diatom biogeochemistry by accounting for the functions of size and near freezing temperature.

Figure 1. Left: Cellular carbon content for polar diatoms across four orders of magnitude in biovolume compared to the same relationship for a wide range of non-polar diatoms (MD&L = Menden-Deuer & Lessard, 2000). The y-intercept is the estimate of the baseline carbon density in these polar diatoms, and is significantly higher than the literature values reviewed in MD&L (2000). Right: Growth rate of the same polar diatoms expressed as a percent of their calculated maximum growth rate at 2°C. Error bars represent the range of values observed in the experiments. Maximum growth rate was estimated by 1) applying the growth rate/biovolume relationships published by Chisholm (1992) and Edwards et al. (2012) to the observed biovolume for each culture, and 2) scaling this growth rate to 2°C growth temperature using the relationship of Eppley (1972).

Authors:
Michael Lomas (Bigelow Laboratory for Ocean Sciences)
Steven Baer (Maine Maritime Academy)
Sydney Acton (Dauphin Island Sea Lab)
Jeffrey Krause (Dauphin Island Sea Lab and University of South Alabama)

The ecology of the biological carbon pump

Posted by mmaheigan

· Tuesday, October 15th, 2019

Plankton in the surface ocean convert CO₂ into organic biomass thereby fueling marine food webs. Part of this organic biomass sinks down into the deep ocean, where the surface-derived organic carbon, or respired CO₂, is locked in for decades to millennia. Without the biological carbon pump, atmospheric CO₂ would be ~200 ppm higher than it is today. We know that ecological processes in the surface ocean plankton communities have a paramount importance on the efficiency of the biological carbon pump. Unfortunately, however, the mechanisms how ecology determines sinking fluxes are poorly understood.

A recent study in Global Biogeochemical Cycles used large-scale in situ mesocosms to explore how the ecological interplay within plankton communities affects the downward flux of organic material. Organic biomass tends to sink faster when produced by smaller organisms because the sinking material they generate forms dense aggregates. Conversely, larger organisms produce relatively porous particles that sink more slowly.

Figure: Flow chart illustrating how plankton community structure affects the properties of sinking organic particles and ultimately the strength and efficiency of the biological carbon pump. The thick arrows at the bottom indicate that flux attenuation depends on the properties of particulate matter formed in the surface ocean. For example, slow-sinking porous aggregates containing large amounts of easily degradable organic substances will decay faster (right side) than dense aggregates of more refractory organic matter (left side).

The key finding of this study was the unexpectedly large influence that plankton community composition has on the degradation rate of sinking organic biomass. In fact, degradation rates changed maximally 15-fold over the course of the study while sinking speed changed only 3-fold. Degradation rate of sinking material, measured in oxygen consumption assays, was quite variable and tended to be higher for more easily degradable fresh organic matter. The rate was lower during harmful algal blooms, which produce toxic substances that inhibit organisms that feed on aggregates thereby reducing degradation rates. These findings are an important step forward as they show that our predictive understanding of the biological carbon pump could be improved substantially when linking degradation rates of sinking material with ecological processes in surface ocean plankton communities.

Authors:
L. T. Bach (University of Tasmania)
P. Stange, J. Taucher, E. P. Achterberg, M. Esposito, U. Riebesell (GEOMAR)
M. Algueró‐Muñiz (Alfred-Wegener-Institut Helmholtz)
H. Horn (NIOZ and Utrecht University)

Microbes: Gatekeepers of earth’s deep carbon?

Posted by mmaheigan

· Tuesday, May 14th, 2019

In 2017, an interdisciplinary group of early career scientists, the Biology Meets Subduction team, visited Costa Rica’s subduction zone, where the ocean floor sinks beneath the continent, to find out if subterranean microbes affect geological processes that move carbon from Earth’s surface into the deep interior.

Using carbon and helium isotope measurements of water and nearby sediments from geothermal springs in northern and central Costa Rica, the study published recently in Nature demonstrated that microbes consume and trap a small but measurable amount of the carbon sinking into the trench off Costa Rica’s Pacific coast. The microbes may also be involved in chemical processes that pull out even more carbon, leaving cement-like veins of calcite in the crust.

Figure 1: Schematic of deep carbon cycle subduction at the forearc region and into the mantle.

The team discovered that low temperatures in the forearc support microbial life and water-rock interactions that divert the down-going carbon from the subducting slab and trap it in the crust. The study estimates that about 94 percent of that redirected carbon transforms into calcite minerals and microbial biomass.

Figure 2: Biofilm in a natural seep in Costa Rica. Credit: Peter Barry.

These unexpected findings have important implications for how much carbon moves from Earth’s surface into the interior, especially over geological timescales. If these biological and geochemical processes occur worldwide, they would translate to 19% less carbon entering the deep mantle than previously estimated.

Authors:
PH Barry
JM de Moor
D Giovannelli
M Schrenk
D Hummer
T Lopez
CA Pratt
Y Alpízar Segura
A Battaglia
P Beaudry
G Bini
M Cascante
G d’Errico

M di Carl
D Fattorini
K Fullerton
E Gazel
G González
SA Halldórsson
K Iacovino
JT Kulongoski
E Manini
M Martínez
H Miller
M Nakagawa
S Ono

S Patwardhan
CJ Ramírez
F Regoli
F Smedile
S Turner
C Vetriani
M Yücel
CJ Ballentine
TP Fischer
DR Hilton
KG Lloyd

Nutrient and carbon limitation drive broad-scale patterns of mixotrophy in the ocean

Posted by mmaheigan

· Tuesday, May 14th, 2019

In the ocean, unicellular eukaryotes are often mixotrophic, which means they photosynthesize and also consume prey. In recent decades, it has become clear that mixotrophs are ubiquitous in sunlit ocean habitats. Additionally, models predict that mixotrophs have important impacts on productivity, nutrient cycling, carbon export, and food web structure. However, there is little understanding of the environmental conditions that select for a mixotrophic lifestyle, and it is unclear how mixotrophs succeed in competition with autotrophic and heterotrophic specialists. A recent study in PNAS that synthesized measurements of mixotrophic nanoflagellates showed that mixotrophs are more abundant in stratified, well-lit, low latitude environments (Figure 1A). They are also more abundant, relative to pure heterotrophs, in productive coastal environments (Figure 1B). A trait-based model analysis revealed that the success of mixotrophs depends on the fact that they are less nutrient-limited than autotrophs (due to prey-derived nutrients) and less carbon-limited than heterotrophs (due to photosynthesis). This synergy requires sufficient light, leading to success in low latitude environments. Similarly, a greater supply of dissolved nutrients relative to prey, as commonly observed in coastal environments, favors mixotrophs relative to heterotrophs. One implication of these results is that carbon fixation at lower latitudes may be enhanced by mixotrophy, while limiting nutrients may be more efficiently transferred to higher trophic levels.

Figure 1. Estimated abundance of autotrophic, mixotrophic, and heterotrophic nanoflagellates across environmental gradients in the ocean.

Author:
Kyle Edwards (Univ. Hawaii at Manoa)

Ocean’s heat cycle shows that atmospheric carbon may be headed elsewhere

Posted by hbenway

· Thursday, August 16th, 2018

Studies over the past 25 years have supported the existence of a large net land biosphere CO₂ sink of 0.5–2 PgC yr^-1. Significant uncertainties remain, however, regarding the long-term partitioning between northern, tropical, and southern land sinks, in part connected to the uncertain ocean carbon sink. These uncertainties limit our capacity to predict earth system response to anthropogenic changes and design effective mitigation strategies.

Land sinks from atmospheric inversion (1990-2010 average) with two different ocean/river fluxes: (top) previous ocean inversion-based carbon fluxes; and (bottom) updated pCO2-based air-sea flux with a scaled-up river flux of 0.78 PgC /yr.

In a recent study published in Nature Geoscience, Resplandy et al. (2018) used models and field observations to demonstrate that the world’s oceans transport heat between the northern and southern hemispheres in the same way that carbon is transported. The transport of heat, however, is easier to observe. By tracking this heat, they showed that the Southern Ocean — while still a substantial carbon sink —may not take up as much carbon as previously thought, and that ocean currents might transport 20 to 100% more carbon from the northern to the southern hemisphere. To maintain this additional transport of carbon, they showed that the amount of carbon entering the ocean from rivers may be as much as 70% higher than estimated in previous global carbon budget studies. These changes in the ocean and river carbon transport imply that up to 40% of the world’s atmospheric carbon absorbed by land ecosystems needs to be reallocated from existing estimates.

Authors
L. Resplandy, Princeton University
Ralph Keeling, Scripps Institution of Oceanography/ UCSD
Christian Rödenbeck, Max Planck Institute
Briton Stephens, NCAR
Matthew Long, NCAR
Samar Khatiwala, University of Oxford
Keith Rodgers, Princeton University
Laurent Bopp, ENS Paris
Pieter Tans, NOAA’s ESRL

Marine Snowfall at the Equator

Posted by mmaheigan

· Thursday, July 19th, 2018

The continual flow of organic particles such as dead organisms and fecal material towards the deep sea is called “marine snow,” and it plays an important role in the ocean carbon cycle and climate-related processes. This snowfall is most intense where high primary production can be observed near the surface. This is the case along the equator in the Pacific and Atlantic Oceans. However, it is not well known how particles are distributed at depth and which processes influence this distribution. A recent study published in Nature Geoscience involved the use of high-resolution particle density data using the Underwater Vision Profiler (UVP) from the equatorial Atlantic and Pacific Oceans down to a depth of 5,000 meters, revealing that several previously accepted ideas on the downward flux of particles into the deep sea should be revisited.

Figure 1. The Underwater Vision Profiler (UVP) during a trial in the Kiel Fjord. The UVP provided crucial data for the new study. Photo: Rainer Kiko, GEOMAR

It is typically assumed that the largest particle density can be found close to the surface and that density attenuates continuously with depth. However, high-resolution particle data show that density increases again in the 300-600-meter depth range. The authors attribute this observation to the daily migratory behavior of organisms such as zooplankton that retreat to these depths during the day, contributing to the particle load via defecation and mortality.

Another surprising result is the observation of many small particles below 1,000 meters depth that contribute a large fraction of the bathypelagic particle flux. This observation counters the general assumption, especially in many biogeochemical models, that particle flux at depth comprises fast sinking particles such as fecal pellets. Diminished remineralization rates of small particles or increased disaggregation of larger particles may contribute to the elevated small particle fluxes at this depth.

Figure 2. Zonal current velocity and Particulate Organic Carbon (POC) content across the equatorial Atlantic at 23˚W as observed in November 2012. From left to right: Zonal current velocity, POC content in small particle fraction and POC content in large particle fraction (adapted from Kiko et al. 2017).

This study highlights the importance of coupled biological and physical processes in understanding and quantifying the biological carbon pump. Further work on this important topic can now also be submitted to the new Frontiers in Marine Science research topic “Zooplankton and Nekton: Gatekeepers of the Biological Pump” (https://www.frontiersin.org/research-topics/8114/zooplankton-and-nekton-gatekeepers-of-the-biological-pump; Co-editors R. Kiko, M. Iversen, A. Maas, H. Hauss and D. Bianchi). The research topic welcomes a broad range of contributions, from individual-based process studies, to local and global field observations, to modeling approaches to better characterize the role of zooplankton and nekton for the biological pump.

Authors:
R. Kiko (GEOMAR)
A. Biastoch (GEOMAR)
P. Brandt (GEOMAR, University of Kiel)
S. Cravatte (LEGOS, University of Toulouse)
H. Hauss (GEOMAR)
R. Hummels (GEOMAR)
I. Kriest (GEOMAR)
F. Marin (LEGOS, University of Toulouse)
A. M. P. McDonnell (University of Alaska Fairbanks)
A. Oschlies (GEOMAR)
M. Picheral (Laboratoire d’Océanographie de Villefranche-sur-Mer, Observatoire Océanologique)
F. U. Schwarzkopf (GEOMAR)
A. M. Thurnherr (Lamont-Doherty Earth Observatory,)
L. Stemmann (Sorbonne Universités, Observatoire Océanologique)

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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