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

Archive for New OCB Research – Page 17

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)

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

Posted by mmaheigan

· Tuesday, October 15th, 2019

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

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

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

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

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

A new roadmap of climate change driven ocean changes

Posted by mmaheigan

· Wednesday, October 2nd, 2019

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

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

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

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

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

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

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

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

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

Posted by mmaheigan

· Wednesday, October 2nd, 2019

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

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

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

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

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

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

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

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

Where the primary production goes determines whether you catch tuna or cod

Posted by mmaheigan

· Friday, September 6th, 2019

Fishes are incredibly diverse, fill various roles in their ecosystems, and are an important resource—economically, socially, and nutritionally. The relationship between primary productivity and fish catches is not straightforward; fisheries oceanographers and managers have long struggled to predict abundances and fully understand the controls of cross-ecosystem differences in fish abundances and assemblages. A recent study in Progress in Oceanography modeled the relationships between fish abundances and assemblages and ecosystem factors such as physical properties and plankton productivity.

The mechanistic model simulated feeding, growth, reproduction, and mortality of small pelagic forage fish, large pelagic fish, and demersal (bottom-dwelling) fish in the global ocean using plankton food web estimates and ocean conditions from a high-resolution earth system model of the 1990s. Modeled fish assemblages were more related to the separation of secondary production into pelagic zooplankton or benthic fauna secondary production than to primary productivity. Specifically, the ratio of pelagic to benthic production drove spatial differences in dominance by large pelagic fish or by demersal fish. Similarly, demersal fish abundance was highly sensitive to the efficiency of energy transfer from exported surface production to benthic fauna.

The model results offer a systematic understanding of how marine fish communities are structured by spatially varying environmental conditions. With global climate change, the expected decrease in exported primary production would lead to fewer demersal fish around the world. This model provides a framework for testing the effect of changing conditions on fish communities at a global scale, which can also help inform managers of potential impacts on economic, social, and nutritional resources worldwide.

Figure 1: (A) Sample food web with three fish types, two habitats, two prey categories, and feeding interactions (arrows). Dashed arrow denotes feeding only occurs in shelf regions with depth <200 m. (B) Fraction of large pelagic vs. demersal fishes (LP/(LP+D)) as a function of the ratio of zooplankton production lost to higher predation (Zoop) to detritus flux to the seafloor (Bent) averaged over large marine ecosystems. Solid line: predicted linear model response, dashed lines: standard error. (Lower panels) Circles=mean biomasses (g m^-2) and lines=fluxes of biomass (g m^-2 d^-1) through the pelagic (top 100m) and benthic components of the food webs at two test locations, (C) Peruvian Upwelling (PUP) ecosystem and (D) Eastern Bering Sea (EBS) shelf ecosystem. Circles and lines scale with the modeled biomasses and fluxes. Circle color key: Gray=net primary productivity (NPP); yellow=medium and large zooplankton; red=forage fish; blue=large pelagic fish; brown=benthos; green=demersal fish.

Authors:
Colleen M. Petrik (Princeton University, Texas A&M University)
Charles A. Stock (NOAA Geophysical Fluid Dynamics Laboratory)
Ken H. Andersen (Technical University of Denmark)
P. Daniël van Denderen (Technical University of Denmark)
James R. Watson (Oregon State University)

Predicting marine ecosystem futures

Posted by mmaheigan

· Wednesday, September 4th, 2019

Earth System Models (ESMs) are powerful and effective tools for exploring and predicting marine ecosystem response to environmental change, including biogeochemical processes that underlie threats to ocean health such as ocean acidification, deoxygenation, and changes in productivity. Seasonal to interannual marine biogeochemical predictions with ESMs hold great promise for exploring links between climate and marine resources such as fisheries but have thus far been challenged by limitations associated with observational initialization, model structure, and computational availability. In a recent study published in Science, authors integrated the Geophysical Fluid Dynamics Laboratory’s (GFDL) COBALT (Carbon, Ocean Biogeochemistry and Lower Trophics) marine biogeochemical model with seasonal to multi-annual climate predictions from GFDL’s CM2.1 climate model to examine marine ecosystem futures on these shorter time scales. The global biogeochemical forecasts were initialized on the first of each month between 1991 and 2017 with 12 ensemble members in each prediction, creating a database of nearly 4000 forecasts and 8000 simulation years. The model skillfully predicted seasonal to multi-annual chlorophyll fluctuations in many ocean regions (Figure 1).

Figure 1: Prediction skill in reproducing observed variations of monthly chlorophyll anomaly. (Top) Correlation coefficient between predicted and observed chlorophyll at 1-3 month lead time during the period 1997-2017. Stippled areas indicate that the correlation is significantly greater than 0 with 95% confidence. Areas with less than 80% satellite chlorophyll coverage are masked in grey. (Lower panels) Correlation coefficient between predicted and observed chlorophyll as a function of forecast initialization month (x-axis) and lead time (y-axis) in tropical Pacific, Indian, North Atlantic, North Pacific, and South Pacific oceans. In all panels, the darker the red, the higher the correlation up to a perfect correlation of 1.0. White indicates no correlation, while blue indicates negative correlation.

These results suggest that annual fish catches in selected large marine ecosystems can be predicted from chlorophyll and sea surface temperature anomalies up to 2-3 years in advance. Given that fisheries predictions sometimes failed to the point of commercial stock collapse in the past, this prediction capacity could be vital for fisheries managers. Biogeochemical prediction systems can extend beyond sea surface temperature and chlorophyll to include other potential drivers (e.g., oxygen, acidity, net primary production, zooplankton, etc.) as highly valuable tools for marine resource managers of dynamic and changing ecosystems.

Authors:
Jong-Yeon Park (Princeton Univ, NOAA GDFL, Chonbuk National Univ., Korea)
Charles A. Stock, John P. Dunne, Xiaosong Yang, and Anthony Rosati (NOAA GFDL)

Upwelling and solubility drive global surface dissolved inorganic carbon (DIC) distribution

Posted by mmaheigan

· Tuesday, August 20th, 2019

What drives the latitudinal gradient in open-ocean surface DIC concentration? Understanding the processes that drive the distribution of carbon in the surface ocean is essential to the study of the ocean carbon cycle and future predictions of ocean acidification and the ocean carbon sink.

Authors of a recent study in Biogeosciences investigated causes of the observed latitudinal trend in DIC and salinity-normalized DIC (nDIC) (Figure 1). The latitudinal trend in nDIC is not driven solely by the latitudinal gradient in temperature (through its effects on solubility), as is commonly assumed. Careful analysis using the Global Ocean Data Analysis Project version 2 (GLODAPv2) database revealed that physical supply from below (upwelling, entrainment in winter) at high latitudes is another major driver of the latitudinal pattern. The contribution of physical exchange explains an otherwise puzzling observation: Surface waters are lower in nDIC in the high-latitude North Atlantic than in other basins. This cannot be accounted for by temperature difference but rather is explained by a difference in the carbon content of deeper waters (lower in the subarctic North Atlantic than in the subarctic North Pacific or Southern Ocean) that are mixed up into the surface during winter months.

Figure caption: (Top) spatial distributions of surface ocean DIC and (bottom) salinity-normalised (nDIC). Both, most notably nDIC, increase towards the poles. Values are normalised to year 2005 to remove bias from changing levels of atmospheric CO2 in some observations before and after 2005. Data are from GLODAPv2.

These results also suggest that the upwelling/entrainment of water that is high in alkalinity generates a large and long-lasting effect on DIC, one that persists beyond the timescale of CO₂ gas exchange equilibration with the . That is to say, the impact of changes in upwelling on the ocean’s carbon source-sink strength depends not only on the DIC content of the upwelled water but also on its TA content.

Authors:
Yingxu Wu (University of Southampton)
Mathis Hain (University of California, Santa Cruz)
Matthew Humphreys (University of East Anglia and University of Southampton)
Sue Hartman (National Oceanography Centre, Southampton)
Toby Tyrrell (University of Southampton)

Air-sea gas exchange estimates biased by multi-day surface trapping

Posted by mmaheigan

· Tuesday, August 20th, 2019

Routine measurements of air-sea gas exchange assume a homogeneous gas concentration across the upper few meters of the ocean. But is this assumption valid? A recent study in Biogeosciences revealed substantial systematic gradients of nitrous oxide (N₂O) in the top few meters of the Peruvian upwelling regime. These gradients lead to a 30% overestimate of integrated N₂O emissions across the entire region, with local emissions overestimated by as much as 800%.

Figure caption: Air-sea gas exchange estimates can be biased by gas concentration gradients within the upper few meters of the ocean; in particular, surface trapping over several days’ duration can generate substantial gradients.

The N₂O gradients off Peru form during multi-day events of surface trapping, in which near-surface stratification dampens turbulent mixing. Until now, surface trapping was assumed to be a diurnal (driven by solar warming) process without memory, whereby only weak gradients would form during the hours of trapping and then dissipate. It is likely that multi-day surface trapping occurs in other ocean regions as well. The total impact on emission estimates of different greenhouse gases is yet to be quantified, but given the findings in the Peruvian upwelling system, could be significant globally.

Authors:
Tim Fischer, Annette Kock, Damian L. Arévalo-Martínez, Marcus Dengler, Peter Brandt, Hermann W. Bange (GEOMAR)

Regional circulation changes and a growing atmospheric CO2 concentration drive accelerated anthropogenic carbon uptake in the South Pacific

Posted by mmaheigan

· Tuesday, August 6th, 2019

About one tenth of human CO₂ emissions are currently being taken up by the Pacific Ocean, which makes the seawater more corrosive to the calcium carbonate shells and skeletons of the plants and animals that live there. Now, thanks to hard work by international teams of scientists from the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP), there are decades of data, enough to test how much this anthropogenic CO₂ accumulation varies throughout the Pacific Ocean and regionally on the timescales of decades.

Figure caption: Map of the concentration of human-emitted CO₂ along the sections where data were available from more than one decade, estimated for the year 2015.

Using a new take on an old technique, along with a wide variety of repeat biogeochemical measurements, a study in Biogeochemical Cycles revealed that Pacific anthropogenic CO₂ accumulation increased from the 1995-2005 decade to the 2005-2015 decade. While the magnitude of the decadal increase was consistent with increases in human CO₂ emissions over this period for most of the Pacific, the rate of change was greater than expected in the South Pacific subtropical gyre. The authors suggest that recent increases in circulation in the gyre region could have delivered an unexpectedly large amount of anthropogenic CO₂-laden seawater from the surface to the ocean interior. Programs like GO-SHIP will continue to be critical for tracking the fate of human CO₂emissions and associated feedbacks on climate and marine ecosystems.

Authors:
B. R. Carter (Univ. Washington and PMEL)
R. A. Feely, G. C. Johnson, J. L. Bullister (PMEL)
R. Wanninkhof (NOAA AOML)
S. Kouketsu, A. Murata (JAMSTEC
R. E. Sonnerup, S. Mecking (Univ. Washington)
P. C. Pardo (Univ. Tasmania)
C. L. Sabine (Univ. Hawai‘i, Mānoa)
B. M. Sloyan, B. Tilbrook (CSIRO, Australia)
K. Speer (Florida State University
L. D. Talley (Scripps Institution of Oceanography)
F. J. Millero (Univ. Miami)
S. E. Wijffels (CSIRO and WHOI)
A. M. Macdonald (WHOI)
N. Gruber (ETH Zurich)

A new era of observing the ocean carbonate system

Posted by mmaheigan

· Tuesday, August 6th, 2019

Amidst a backdrop of natural variability, the ocean carbonate system is undergoing a massive anthropogenic change. To capture this anthropogenic signal and differentiate it from natural variability, carbonate observations are needed across a range of spatial and temporal scales (Figure 1), many of which are not captured by traditional oceanographic platforms. A new review of autonomous carbonate observations published in Current Climate Change Reports highlights the development and deployment of pH sensors capable of in situ measurements on autonomous platforms, which represents a major step forward in observing the ocean carbonate system. These sensors have been rigorously field-tested via large-scale deployments on profiling floats in the Southern Ocean (Southern Ocean Carbon and Climate Observations and Modeling, SOCCOM), providing an unprecedented wealth of year-round data that have demonstrated the importance of wintertime outgassing of carbon dioxide in the Southern Ocean.

Figure 1: Observational capabilities and carbonate system processes as a function of time and space. Ocean processes that affect the carbonate system (solid color shapes—labeled in the legend) are depicted as a function of the temporal and spatial scales over which they must be observed to capture important variability and/or long-term change.

Most current autonomous platforms routinely measure only a single carbonate parameter, which then requires an algorithm to estimate a second parameter so that the rest of the carbonate system can be calculated. However, the ongoing development of sensors and systems to measure, rather than estimate, other carbonate parameters may greatly reduce uncertainty in constraining the full carbonate system. It is critical that the community continue to develop and adhere to best practices for calibration and data handling as existing sensors are deployed in increasing numbers and new sensors become available. Expanding autonomous carbonate measurements will increase our understanding of how anthropogenic change impacts natural variability and will provide a means to monitor carbon uptake by the ocean in real-time at high spatial and temporal resolution. This will not only help to understand the mechanisms driving changes in the ocean carbonate system, but will allow new insights in the role of mesoscale processes in regional and global biogeochemical cycles.

Authors:
Seth M. Bushinsky (Princeton University/University of Hawai’i Mānoa)
Yuichiro Takeshita (Monterey Bay Aquarium Research Institute)
Nancy L. Williams (Pacific Marine Environmental Laboratory – NOAA / University of South Florida)

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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