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Archive for POC

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)

Small particles contribute significantly to the biological carbon pump in the subpolar North Atlantic

Posted by mmaheigan

· Monday, February 13th, 2023

The ocean’s biological carbon pump (BCP) is a collection of processes that transport organic carbon from the surface to the deep ocean where the carbon is sequestered for decades to millennia. Variations in the strength of the BCP can substantially change atmospheric CO₂ levels and affect the global climate. It is important to accurately estimate this carbon flux, but direct measurement is difficult so this remains a challenge.

Figure 1. (a) A schematic illustrating the downward transport of small and large POC into the deep ocean and the subsequent remineralization and fragmentation which breaks large POC into small POC. (b) Trajectories of BGC-Argo float segments. (c) Relative contributions to the annually averaged vertical carbon flux show the dominant role of gravitational sinking flux of large POC as well as the significant contributions from small POC at 100 m due to different mechanisms and at 600 m due to fragmentation.

A recent paper published in Limnology and Oceanography performed a novel mass budget analysis using observations of dissolved oxygen and particulate organic carbon (POC) from BGC-Argo floats in the subpolar North Atlantic. The authors assessed relative importance of different mechanisms contributing to the BCP and related processes, the sinking velocity and remineralization rate of different particle size classes as well as the rate of fragmentation which breaks large particles into smaller ones. Results suggest that on annual timescales, the gravitational settling of large POC is the dominant mechanism. Small POC supplements the vertical carbon flux at 100 m significantly, through various mechanisms, and contributes to carbon sequestration below 600 m due to fragmentation of large POC. In addition, sensitivity experiments highlight the importance of considering remineralization and fragmentation when estimating the vertical carbon flux of small POC.

This novel method provides additional independent constraints on current estimates and improves our mechanistic understanding of the BCP. In addition, it demonstrates the great potential of BGC-Argo float data for studying the biological carbon pump.

Authors:
Bin Wang (Dalhousie University)
Katja Fennel (Dalhousie University)

How do coccolithophores survive the darkness?

Posted by mmaheigan

· Friday, April 1st, 2022

Coccolithophores have survived several major extinction events over geologic time. The most significant was the asteroid impact at the K/T boundary, followed by months of darkness. Additionally, coccolithophores regularly reside in the twilight zone, just beyond the reach of sunlight. A paper recently published in the New Phytologist addresses how these photosynthetic algae can persist and grow, albeit slowly, in darkness using osmotrophy.

The authors discovered that the osmotrophic uptake of certain types of dissolved organic carbon (DOC) can support survival in low light. They completed a 30-day darkness experiment to determine how the concentration of several DOC compounds affects growth. The coccolithophore species Cruciplacolithus neohelis growth rate increased with the increasing concentration of dissolved organic compounds. They also examined the kinetics of short-term uptake of radiolabeled DOC compounds and found that the uptake rate generally showed Michaelis-Menten-like saturation kinetics. All radiolabeled DOC compounds were incorporated into the POC fraction, but surprisingly also into the particulate inorganic carbon (PIC) fraction (i.e., calcite coccoliths).

These results suggest that osmotrophic uptake in coccolithophores may be significant enough to be included in carbon cycle models, especially if they can simultaneously take up a wide range of organic compounds. Surprisingly, we detected ¹⁴C-DOC in the PIC fraction after only 24 hours. This remarkably rapid incorporation is most likely due to the respiration of radiolabeled DOC into dissolved inorganic carbon (DIC), subsequently used by coccolithophores for calcification. These results have implications for the biological carbon pump and alkalinity pump paradigms, as we confirmed that both POC and PIC originate from DOC on short time scales.

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)

Partitioning carbon export into particulate and dissolved pools from biogeochemical profiling float observations

Posted by mmaheigan

· Thursday, December 17th, 2020

Carbon export from the surface into the deep ocean via the biological pump is a significant sink for atmospheric carbon dioxide. The relative contributions of sinking particles—particulate organic carbon (POC) and dissolved organic carbon (DOC)—to the total export affect the efficiency of carbon export.

In a recent study published in Global Biogeochemical Cycles, the authors used measurements from biogeochemical profiling floats in the Northeast Pacific from 2009 to 2017 to estimate net community production (NCP), an analog for carbon export. In order to close three tracer budgets (nitrate, dissolved inorganic carbon, and total alkalinity), the authors combined these float measurements with data from the Ocean Station Papa mooring and recently developed algorithms for carbonate system parameters. By constraining end-member nutrient ratios of the POC and DOC produced, this multi-tracer approach was used to estimate regional NCP across multiple depth horizons throughout the annual cycle, partition NCP into the POC and DOC contributions, and calculate particulate inorganic carbon (PIC) production, a known ballast material for sinking particles (Figure 1). The authors also estimated POC attenuation with depth, POC export across deeper horizons, and in situ export efficiency via a particle backscatter-based approach.

With the advent of “fully-loaded” biogeochemical profiling floats equipped with nitrate, oxygen, pH and bio-optical sensors, this approach may be used to assess the magnitude and efficiency of carbon export in other ocean regions from a single platform, which will greatly reduce the risks and costs associated with traditional ship-based measurements, while broadening the spatiotemporal scales of observation.

Figure caption: Climatological mean NCP (blue line) over the entire study period (2009-2017); the POC portion of NCP (filled blue area), the DOC portion (white space) and PIC production rate (red line), in the mixed layer (left), and the euphotic zone (right). The numbers in parentheses are the integrated annual NCP rates for each curve and uncertainty reported was determined using a Monte Carlo approach.

Authors:
William Haskell (MBARI, now Mote Marine Laboratory)
Andrea Fassbender (MBARI, now PMEL)
Jacki Long (MBARI)
Joshua Plant (MBARI)

Estuarine sediment resuspension drives non-local impacts on biogeochemistry

Posted by mmaheigan

· Friday, September 18th, 2020

Sediment processes, including resuspension and transport, affect water quality in estuaries by altering light attenuation, primary productivity, and organic matter remineralization, which then influence oxygen and nitrogen dynamics. In a recent paper published in Estuaries and Coasts, the authors quantified the degree to which sediment resuspension and transport affected estuarine biogeochemistry by implementing a coupled hydrodynamic-sediment transport-biogeochemical model of the Chesapeake Bay. By comparing summertime model runs that either included or neglected seabed resuspension, the study revealed that resuspension increased light attenuation, especially in the northernmost portion of the Bay, which subsequently shifted primary production downstream (Figure 1). Resuspension also increased remineralization in the central Bay, which experienced higher organic matter concentrations due to the downstream shift in primary productivity. When combined with estuarine circulation, these resuspension-induced shifts caused oxygen to increase and ammonium to increase throughout the Bay in the bottom portion of the water column. Averaged over the channel, resuspension decreased oxygen by ~25% and increased ammonium by ~50% for the bottom water column. Changes due to resuspension were of the same order of magnitude as, and generally exceeded, short-term variations within individual summers, as well as interannual variability between wet and dry years. This work highlights the importance of a localized process like sediment resuspension and its capacity to drive biogeochemical variations on larger spatial scales. Documenting the spatiotemporal footprint of these processes is critical for understanding and predicting the response of estuarine and coastal systems to environmental changes, and for informing management efforts.

Figure 1: Schematic of how resuspension affects biogeochemical processes based on HydroBioSed model estimates for Chesapeake Bay.

Authors:
Julia M. Moriarty (University of Colorado Boulder)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science)
Courtney K. Harris (Virginia Institute of Marine Science)

Also see the Geobites piece “Muddy waters lead to decreased oxygen in Chesapeake Bay” on this publication, by Hadley McIntosh Marcek

Estimating the large-scale biological pump: Do eddies matter?

Posted by mmaheigan

· Wednesday, December 4th, 2019

One factor that limits our capacity to quantify the ocean biological carbon pump is uncertainty associated with the physical injection of particulate (POC) and dissolved (DOC) organic carbon to the ocean interior. It is challenging to integrate the effects of these pumps, which operate at small spatial (<100 km) and temporal (<1 month) scales. Previous observational and fine-scale modeling studies have thus far been unable to quantify these small-scale effects. In a recent study published in Global Biogeochemical Cycles, authors explored the influence of these physical carbon pumps relative to sinking (gravity-driven) particles on annual and regional scales using a high-resolution (2 km) biophysical model of the North Atlantic that simulates intense eddy-driven subduction hotspots that are consistent with observations.

Figure 1: North Atlantic idealized double gyre ocean biophysical model. Top: Sea surface temperature, surface chlorophyll and mixed-layer depth during the spring bloom (March 21). Bottom: total export of organic carbon (POC+DOC) at 100 and individual contributions from the gravitational (particle sinking) and subduction (mixing, eddy advection and Ekman pumping) pumps for one day during the spring bloom (March 21) and averaged annually. Physical subduction hotspots visible on the daily export contribute little to the annual export due to strong compensation of upward and downward motions.

The authors showed that eddy dynamics can transport carbon below the mixed-layer (500-1000 m depth), but this mechanism contributes little (<5%) to annual export at the basin scale due to strong compensation between upward and downward fluxes (Figure 1). Additionally, the authors evidenced that small-scale mixing events intermittently export large amounts of suspended DOC and POC.

These results underscore the need to expand the traditional view of the mixed-layer carbon pump (wintertime export of DOC) to include downward mixing of POC associated with short-lived springtime mixing events, as well as eddy-driven subduction, which can contribute to longer-term ocean carbon storage. High-resolution measurements are needed to validate these model results and constrain the magnitude of the compensation between upward and downward carbon transport by small-scale physical processes.

Authors:
Laure Resplandy (Princeton University)
Marina Lévy (Sorbonne Université)
Dennis J. McGillicuddy Jr. (WHOI)

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 half century perspective: Seasonal productivity and particulates in the Ross Sea

Posted by mmaheigan

· Tuesday, April 2nd, 2019

Studies of cruise observations in the Ross Sea are typically biased to a single or a few year(s), and long-term trends have predominantly come from satellites. Consequently, the in situ climatological patterns of nutrients and particulate matter have remained vague and unclear. What are the typical patterns of nutrients and particulate matter concentrations in the Ross Sea in spring and summer? How do these concentrations affect annual productivity estimates?

Patterns of nutrient and particulate matter in the Ross Sea can play a wide-ranging role in a productive region like the Ross Sea. Smith and Kaufman (2018) recently synthesized austral spring and summer (November to February) observations from 42 Ross Sea research cruises (1967-2016) to analyze broad biogeochemical patterns. The resulting climatologies revealed interesting seasonal patterns of nutrient uptake and particulate organic carbon (POC) to chlorophyll (chl) ratios (POC:chl). Temporal patterns in the nitrate and phosphate climatologies confirm the role of early spring haptophyte (Phaeocystis antarctica) growth, followed by limited nitrogen and phosphorus removal in summer. However, a notable increase in POC occurred later in summer that was largely independent of chlorophyll changes, resulting in a dramatic increase in POC:chl. A gradual decline in silicic acid concentrations throughout the summer, along with an increased occurrence of biogenic silica during this time suggest that diatoms may be responsible for this later POC spike. Revised estimates of primary productivity based on these observed climatological POC:chl ratios suggests that summer blooms may be a significant contributor to seasonal productivity, and that estimates of productivity based on satellite pigments underestimate annual production by at least 70% (Figure 1).

Figure 1. Bio-optical estimates of mean productivity using a constant POC:chl ratio (black dots and lines) and modified estimates of productivity using the monthly climatological POC:chl ratios (red dots and lines), in a) the Ross Sea polynya region and b) the western Ross Sea region.

By clarifying typical seasonal patterns of nutrient uptake and POC:chl, these climatologies underscore the biogeochemical importance of both spring haptophyte growth and previously underestimated summer diatom growth in the Ross Sea. Further investigation of the causes and consequences of elevated summer ratios is needed, as assessments of regional food webs and biogeochemical cycles depend on more accurate understanding of primary productivity patterns. Likewise, these results highlight the need for continued efforts to constrain satellite productivity estimates in the Ross Sea using in situ constituent ratios.

For other relevant work on seasonal biogeochemical patterns in the Ross Sea, please see https://doi.org/10.1016/j.dsr2.2003.07.010. And for intra-seasonal estimates of particulate organic carbon to chlorophyll using gliders, please see: https://doi.org/10.1016/j.dsr.2014.06.011.

Authors:
Walker O. Smith Jr. (VIMS, College of William and Mary)
Daniel E. Kaufman (VIMS, College of William and Mary; now at Chesapeake Research Consortium)

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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