mmaheigan :: Ocean Carbon & Biogeochemistry

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

Author Archive for mmaheigan – Page 22

Zooplankton-fueled carbon export is changing in the North Atlantic Ocean

Posted by mmaheigan

· Monday, June 10th, 2019

Zooplankton-mediated carbon export is an important, but variable and relatively unconstrained part of the biological carbon pump—the processes that fix atmospheric carbon dioxide in organic material and transport it from the upper sunlit ocean to depth. Changes in the biological pump impact the climate system, but are challenging to quantify because such analyses require spatially and temporally explicit information about biological, chemical, and physical properties of the ocean, where empirical observations are in short supply.

A recent study in Nature, Ecology and Evolution focused on copepods in the northern half of the North Atlantic Ocean, where the Continuous Plankton Recorder (CPR) time series program has documented surface plankton abundance and taxonomic composition for nearly six decades. Copepods transport carbon passively by producing sinking fecal pellets while feeding near the sea surface, and actively via daily and seasonal migrations to deeper waters where carbon is released through respiration, defecation, and mortality. Using allometry, metabolic theory, and an optimal behavior model, the authors examined patterns of passive and active carbon transport from 1960 to 2014 and sensitivity of carbon export to different model inputs.

Figure caption: Spatial distribution and change, from 1960 to 2014, of modeled copepod-mediated carbon flux: top left – mean passive carbon flux (sinking fecal pellets), bottom left – change in passive carbon flux, top right – mean active carbon flux (respiration plus fecal pellets produced during diel vertical migration), bottom right – change in active carbon flux.

The authors observed that from southern Iceland to the Gulf of Maine, copepod-mediated carbon transport has increased over the last six decades, with the highest rates around 30 mgC m^-2 y^-1 each decade for passive flux, and 4 mgC m^-2 y^-1 each decade for active flux. Meanwhile, it has decreased across much of the more temperate central northern North Atlantic with highest rates around 69 mgC m^-2 y^-1 each decade for passive flux and 8 mgC m^-2 y^-1 each decade for active flux. This pattern is largely driven by changes in copepod population distributions and community structure, specifically the distributions of large and abundant species (e.g. Calanus spp.). These results suggest that shifts in species distributions driven by a changing global climate are already impacting ecosystem function across the northern North Atlantic Ocean. These shifts are not latitudinally uniform, thus highlighting the complexity of marine ecosystems. This study demonstrates the importance of these sustained plankton measurements and how plankton-mediated carbon fluxes can be mechanistically implemented in next-generation biogeochemical models.

Authors:
Philipp Brun (Technical University of Denmark and Swiss Federal Research Institute)
Karen Stamieszkin (University of Maine, Bigelow Lab and Virginia Institute of Marine Science)
Andre W. Visser (Technical University of Denmark)
Priscilla Licandro (Sir Alister Hardy Foundation for Ocean Science, Plymouth Marine Laboratory, and Stazione Zoologica Anton Dohrn, Italy)
Mark R. Payne (Technical University of Denmark)
Thomas Kiørboe (Technical University of Denmark)

Also see OCB2019 plenary session: The effect of size on ocean processes (allometry) and implications for export (Thursday, June 27, 2019)

Ocean microbes drive fluctuating nutrient loss

Posted by mmaheigan

· Tuesday, May 28th, 2019

The removal of bioavailable nitrogen (N) by anaerobic microbes in the ocean’s oxygen deficient zones (ODZs) is thought to vary over time primarily as a result of climate impacts on ocean circulation and primary production. However, a recent study in PNAS using a data-constrained model of the microbial ecosystem in the world’s largest ODZ revealed that internal species oscillations cause local- to basin-scale fluctuations in the rate of N loss, even in a completely stable physical environment. Such ecosystem oscillations have been hypothesized for nearly a century in idealized models, but are rarely shown to persist in a three-dimensional ocean circulation model.

Figure caption. Ecological variability in the basin-scale rate of nitrogen loss over time (left) and in the local-scale contribution of autotrophic anammox to total N loss (right) in a model with unchanging ocean circulation. In the left panel, colors represent model simulations with different biological parameters. In the right panel, colors represent distinct locations within the ODZ in the standard model simulation.

These emergent ecosystem dynamics arise at the oxic-anoxic interface from O₂-dependent resource competition between aerobic and anaerobic microbes, and leave a unique geochemical fingerprint: infrequent spikes in ammonium that are observable in nutrient measurements from the ODZ. Non-equilibrium ecosystem behavior driven by competition among aerobic nitrifiers, anaerobic denitrifiers, and anammox bacteria also generates fluctuations in the balance of autotrophic versus heterotrophic N loss pathways that help reconcile conflicting field observations.

These internally driven fluctuations in microbial community structure partially obscure a direct correspondence between the chemical environment and microbial rates, a universal assumption in biogeochemical models. Because of the fundamental nature of the underlying mechanism, similar dynamics are hypothesized to occur across wide-ranging microbial communities in diverse habitats.

Authors:
Justin L. Penn (University of Washington)
Thomas Weber (University of Rochester),
Bonnie X. Chang (University of Washington, NOAA)
Curtis Deutsch (University of Washington)

See also the OCB2019 plenary session: Anthropogenic changes in ocean oxygen: Coastal and open ocean perspectives (Monday, June 24, 2019)

Suddenly shallow: A new aragonite saturation horizon will soon emerge in the Southern Ocean

Posted by mmaheigan

· Monday, May 27th, 2019

Earth System Models (ESMs) project that by the end of this century, the aragonite saturation horizon (the boundary between shallower, saturated waters and deeper, undersaturated waters that are corrosive to aragonitic shells) will shoal all the way to the surface in the Southern Ocean, yet the temporal evolution of the horizon has not been studied in much detail. Rather than a gradual shoaling, a new shallow aragonite saturation horizon emerges suddenly across many locations in the Southern Ocean between now and the end of the century (Figure 1, left), as detailed in a new study published in Nature Climate Change.

Figure 1: Maximum step-change in the depth of the aragonite saturation horizon (left), timing of the step-change (center), and cause of the change (right). Xs on the time axis (center) indicate when the shallow horizon emerges in each ensemble member. (click image to enlarge)

The emergence of the shallow aragonite saturation horizon is apparent in each member of an ensemble of climate projections from an ESM, but the step change occurs during different years (Figure 1, center). The shoaling is driven by the gradual accumulation of anthropogenic CO₂ in the Southern Ocean thermocline, where the carbonate ion concentration exhibits a local minimum and approaches undersaturation (Figure 1, right).

The abrupt shoaling of the Southern Ocean aragonite saturation horizon occurs under both business-as-usual and emission-stabilizing scenarios, indicating an inevitable and sudden decrease in the volume of suitable habitat for aragonitic organisms such as shelled pteropods, foraminifers, cold-water corals, sea urchins, molluscs, and coralline algae. Widespread reductions in these habitats may have far-reaching consequences for fisheries, economies, and livelihoods.

Authors:
Gabriela Negrete-García (Scripps Institution of Oceanography)
Nicole Lovenduski (University of Colorado Boulder)

See also OCB2019 plenary session: Carbon cycle feedbacks from the seafloor (Wednesday, June 26, 2019)

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

South Pacific particulate organic carbon fate challenges Martin’s Law

Posted by mmaheigan

· Tuesday, May 14th, 2019

Joint science highlight with GEOTRACES

Carbon storage in the ocean is sensitive to the depths at which particulate organic carbon (POC) is respired back to CO₂ within the twilight zone (100-1000m). For decades, it has been an oceanographic priority to determine the depth scale of this regeneration process. To investigate this, GEOTRACES scientists are deploying new isotopic tools that provide a high-resolution snapshot of POC flux and regeneration across steep biogeochemical gradients in the South Pacific Ocean.

A recent paper in PNAS reported on particulate organic carbon (POC) fluxes throughout the water column (focusing on the upper 1000 m) along the GP16 GEOTRACES section between Peru and Tahiti (Figure 1A). POC fluxes (Figure 1B) were derived by normalizing concentrations of POC to ²³⁰Th following analysis of samples collected by in situ filtration. This work builds on a research theme initiated at the GEOTRACES-OCB synthesis workshop held at Lamont-Doherty Earth Observatory in 2016.

Figure caption: Site map and POC flux characteristics from GEOTRACES GP16 section. Plot A) shows the GP16 station locations as white circles, with nearby sediment trap deployments as black stars, with 2013 MODIS satellite-derived net primary productivity in the background. Plot B) shows POC fluxes from particulate 230Th-normalization from selected stations spanning the zonal extent of the GP16 section. Plot C) shows power law exponent b values for each GP16 station (blue), compared to estimates from bottom-moored sediment traps in the South Pacific (black and red dashed lines), a compilation of sediment traps in the North Pacific (green dashed line), and neutrally buoyant sediment traps in the subtropical North Pacific (yellow shaded band). GP16 regeneration length scales from 230Th-normalization agree most closely with the estimates from neutrally buoyant sediment traps.

The study results show that POC regeneration depth is shallower than anticipated, especially in warm stratified waters of the subtropical gyre. Regeneration depth—expressed in terms of the Martin-curve power-law exponent “b” (Figure 1C)—is shown to be greater than previous estimates (horizontal dashed lines), but similar to values obtained using neutrally buoyant sediment traps at the Hawaii Ocean Time-series Station Aloha. In contrast to the rapid regeneration of POC in warm stratified waters, POC regeneration within the ODZ is below our detection limits. Models have shown that shallower regeneration of POC leads to less efficient carbon storage in the ocean, making the authors speculate that global warming, yielding expanded and more stratified gyres, may induce a reduction of the ocean’s efficacy for carbon storage via the biological pump.

Authors:
Frank J. Pavia, Robert F. Anderson, Sebastian M. Vivancos, Martin Q. Fleisher (Columbia University)
Phoebe J. Lam (University of California Santa Cruz)
B.B. Cael (now at University of Hawai’i Manoa, formerly at MIT)
Yanbin Lu, Pu Zhang, R. Lawrence Edwards (University of Minnesota)
Hai Cheng (University of Minnesota and Xi’an Jiaotong University)

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)

A synthesis of North American coastal carbon fluxes

Posted by mmaheigan

· Tuesday, April 30th, 2019

Carbon fluxes in the coastal ocean and across its boundaries with the atmosphere, land, and the open ocean are an important but poorly constrained component of the global carbon budget. By synthesizing available observations and model simulations, a recent study aims to answer 1) whether the coastal ocean of North America takes up atmospheric CO₂ and exports carbon to the open ocean; and 2) if so, how much? The authors estimate a net carbon sink of 160±80 Tg C yr⁻¹ in the North American Exclusive Economic Zone (EEZ) with the Arctic, sub-Arctic and mid-latitude Atlantic EEZ regions as the major contributors.

Portion of EEZ	Tg C yr⁻¹	% of the total area
Arctic and sub-Arctic	104	51%
Mid-latitude Atlantic	62	25%
Mid-latitude Pacific	-3.7	24%

Table 1: Regional breakdown of estimated carbon sink in the North Atlantic EEZ (negative values imply a carbon source).

Combining the net uptake with an estimate of carbon input from land of minus estimates of burial and accumulation of dissolved carbon in EEZ waters as follows implies a carbon export of 151±105 Tg C yr⁻¹ to the open ocean.

160±80

Tg C yr⁻¹

106±30

Tg C yr⁻¹

–

65±55

Tg C yr⁻¹

–

50±25

Tg C yr⁻¹

151±105

Tg C yr⁻¹

Net uptake

Carbon input from land

Estimated burial

Estimated accumulation DOC in EEZ waters

Carbon export to open ocean (estimated C export to open ocean)

The estimated uptake of atmospheric carbon in the North American EEZ amounts to 6.4% of the global ocean uptake of atmospheric CO₂ (est. 2,500 Tg C yr⁻¹). The North American EEZ only represents ~4% of the global ocean surface area, thus the CO₂ uptake is about 50% more efficient in the North American EEZ than the global average. Given the importance of coastal margins, both in contributing to carbon budgets and in the societal benefits they provide, further efforts to improve assessments of the carbon cycle in these regions are paramount. It is critical to maintain and expand existing coastal observing programs, continue national and international coordination and integration of observations, modeling capabilities, and stakeholder needs.

Figure: Area-specific carbon fluxes for North American coastal regions (a, b and d) and total fluxes for a decomposition of the EEZ (c, e).

Authors:
Katja Fennel, Timothée Bourgeois (Dalhousie University, Canada)
Simone Alin, Richard A. Feely, Adrienne Sutton (NOAA Pacific Marine Environmental Laboratory)
Leticia Barbero (NOAA Atlantic Oceanographic and Meteorological Laboratory)
Wiley Evans (Hakai Institute, Canada)
Sarah Cooley (Ocean Conservancy)
John Dunne (NOAA Geophysical Fluid Dynamics Laboratory)
Jose Martin Hernandez-Ayon (Autonomous University of Baja California, Mexico)
Xinping Hu (Texas A&M University)
Steven Lohrenz (University of Massachusetts, Dartmouth)
Frank Muller-Karger, Lisa Robbins (University of South Florida)
Raymond Najjar (Pennsylvania State University)
Elizabeth Shadwick (CSIRO, Australia)
Samantha Siedlecki, Penny Vlahos (University of Connecticut)
Nadja Steiner (Department of Fisheries and Oceans Canada)
Daniela Turk (Lamont-Doherty Earth Observatory)
Zhaohui Aleck Wang (Woods Hole Oceanographic Institution)

Impacts of atmospheric nitrogen deposition and coastal nitrogen fluxes on oxygen concentrations in Chesapeake Bay

Posted by mmaheigan

· Tuesday, April 30th, 2019

How do atmospheric and oceanic nutrients impact oxygen concentrations in the Chesapeake Bay? Generally, researchers focus on how terrestrial nutrients impact hypoxia. The relative importance of river, atmosphere, and ocean inputs have not been quantified, largely because estimates of nitrogen fluxes from the atmosphere and ocean are limited.

A recent study in Journal of Geophysical Research: Oceans quantified the relative impacts of atmospheric and oceanic nitrogen inputs on dissolved oxygen (DO) in the Chesapeake Bay. The authors combined a 3-D biogeochemical model and estimates of atmospheric deposition from the Community Multiscale Air Quality model and interpolations of nitrogen concentrations along the continental shelf from the Ocean Acidification Data Stewardship Project. Atmospheric nitrogen deposition and coastal nitrogen fluxes most impact Chesapeake Bay DO concentrations during the summer when surface waters are depleted in nitrogen. Overall, atmospheric nitrogen deposition has about the same gram-for-gram impact on Chesapeake Bay DO as riverine loading. Although all three nutrient sources vary spatially and temporally, in the central bay, where summer hypoxia is most prevalent, coastal nitrogen fluxes and atmospheric nitrogen fluxes have roughly the same impact on bottom oxygen as a ~10% change in riverine nitrogen loading (Figure 1).

Figure caption: (Left) Four-year (2002–2005) average increase in DO in the summer by removing the atmospheric nitrogen deposition (AtmN), reducing the riverine loading (ΔRiverN) by ~10% (roughly equivalent to turning off the atmospheric deposition), and removing the nitrogen fluxes from the continental shelf (CoastalN). (Right) Relative impacts of the three nitrogen modification scenarios on summertime bottom DO.

These results indicate that two often-neglected sources of nitrogen—direct atmospheric deposition and fluxes of nitrogen from the continental shelf—substantially impact Chesapeake Bay DO, especially in the summer. Future study is needed to investigate the long-term trend of these relative impacts by continued coordination between modeling and observational work, such as applying higher-resolution atmospheric deposition products and integrating more in situ data along the model ocean boundary when they are available. These efforts will improve our understanding of the impacts of different nutrient sources on biogeochemical cycles in coastal water bodies.

Authors:
Fei Da (VIMS, College of William & Mary)
Marjorie A. M. Friedrichs (VIMS, College of William & Mary)
Pierre St-Laurent (VIMS, College of William & Mary)

Northeast Pacific time-series reveals episodic events as major player in carbon export

Posted by mmaheigan

· Tuesday, April 16th, 2019

Temporal fluctuations in the oceanic carbon budget play an important role in the cycling of organic matter from production in surface waters to consumption and sequestration in the deep ocean. A 29-year time-series (1989-2017) of particulate organic carbon (POC) fluxes and seafloor measurements of oxygen consumption in the abyssal northeast Pacific (Sta. M, 4,000 m depth) recently revealed an increasing proportional contribution from episodic events over the past seven years. From 2011 to 2017, 43% of POC flux arrived during high-magnitude (≥ mean + 2 σ) episodic events. Time lags between changes in satellite-estimated export flux (EF), POC flux to the seafloor, and seafloor oxygen consumption varied from 0 to 70 days among six flux events, which could be attributed to variable remineralization rates and/or particle sinking speeds. The Martin equation, a commonly used model to estimate carbon flux, predicted background fluxes well but missed episodic fluxes, subsequently underestimating the measured fluxes by almost 50% (Figure 1). This study reveals the potential importance of episodic POC pulses into the deep sea in the oceanic carbon budget, which has implications for observing infrastructure, model development, and field campaigns focused on quantifying carbon export.

Figure Caption: (A) Station M POC flux measured from sediment traps compared to Martin model estimates, from 1989 to 2017. (B) Model performance for years with >50% sampling coverage: (POC flux_Martin − POC flux_trap)/POC flux_trap 100.

Authors:
Kenneth Smith (MBARI)
Henry Ruhl (MBARI, NOC)
Christine Huffard (MBARI)
Monique Messié (MBARI, Aix Marseille Université)
Mati Kahru (Scripps)

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)

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.

Author Archive for mmaheigan – Page 22

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