deep ocean :: Ocean Carbon & Biogeochemistry

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

Archive for deep ocean – Page 2

Little big exporters

Posted by mmaheigan

· Wednesday, April 8th, 2020

In the Southern Ocean, coccolithophores are thought to account for a major fraction of marine carbonate production and export to the deep sea. Despite their importance in the ocean carbon cycle, we lack fundamental information about Southern Ocean coccolithophore abundance, species composition, and contribution to carbonate export.

Figure caption: Heliscosphaera carteri (left), Coccolithus pelagicus (right) and Emiliania huxleyi (bottom right, partially behind C. pelagicus) coccospheres retrieved from the subantarctic waters south of Tasmania. Image Ruth Eriksen, courtesy AAD EMU.

A recent study in Biogeosciences has generated annual observations of coccolithophore species composition and contribution to calcium carbonate fluxes at two sites that are representative of a large portion of the Subantarctic zone. Coccolithophores account for roughly half of the annual calcium carbonate exported to the deep sea. Notably, it is not the most abundant species (Emiliania huxleyi), but rather the less abundant and larger species (e.g. Calcidiscus leptoporus, Helicosphaera carteri and Coccolithus pelagicus) that make the greatest contribution to carbonate export to the deep sea. Since these larger species exhibit substantially different ecological traits from the opportunistic E. huxleyi, predictions of future response of Southern Ocean coccolithophore communities should not be based on the physiological results from experiments with E. huxleyi. Rather, new physiological response experiments of those less abundant, larger coccolithophore species are urgently needed to constrain responses of these important carbonate exporters to environmental change in the Southern Ocean. This study underscores the importance of phytoplankton ecological traits on the regulation of the marine carbon cycle and emphasizes the need for more species-specific studies to improve predictions of marine ecosystem response to ongoing climate change.

Authors
Andrés S. Rigual Hernández (Universidad de Salamanca)
Thomas W. Trull (CSIRO and ACE CRC)
Scott D. Nodder (NIWA)
José A. Flores (Universidad de Salamanca)
Helen Bostock (University of Queensland,)
Fátima Abrantes (Portuguese Institute for Sea and Atmosphere and CCMAR)
Ruth S. Eriksen (CSIRO and IMAS)
Francisco J. Sierro (Universidad de Salamanca)
Diana M. Davies (CSIRO and ACE CRC)
Anne-Marie Ballegeer (Universidad de Salamanca)
Miguel A. Fuertes (Universidad de Salamanca)
Lisa C. Northcote (NIWA)

Tiny, but effective: Gelatinous zooplankton and the ocean biological carbon pump

Posted by mmaheigan

· Wednesday, March 25th, 2020

Barely visible to the naked eye, gelatinous zooplankton play important roles in marine food webs. Cnidaria, Ctenophora, and Urochordata are omnipresent and provide important food sources for many more highly developed marine organisms. These small, nearly transparent organisms also transport large quantities of “jelly-carbon” from the upper ocean to depth. A recent study in Global Biogeochemical Cycles focused on quantifying the gelatinous zooplankton contribution to the ocean carbon cycle.

Figure 1. Processes and pathways or gelatinous carbon transfer to the deep ocean.

Using >90,000 data points (1934 to 2011) from the Jellyfish Database Initiative (JeDI), the authors compiled global estimates of jellyfish biomass, production, vertical migration, and jelly carbon transfer efficiency. Despite their small biomass relative to the total mass of organisms living in the upper ocean, their rapid, highly efficient sinking makes them a globally significant source of organic carbon for deep-ocean ecosystems, with 43-48% of their upper ocean production reaching 2000 m, which translates into 0.016 Pg C yr^-1.

Figure 2. Mass deposition event of jellyfish at 3500 m in the Arabian Sea (Billett et al. 2006).

Sediment trap data have suggested that carbon transport associated with large, episodic gelatinous blooms in localized open ocean and continental shelf regions could often exceed phytodetrital sources, in particular instances. These mass deposition events and their contributions to deep carbon export must be taken into account in models to better characterize marine ecosystems and reduce uncertainties in our understanding of the ocean’s role in the global carbon cycle.

Links:

Jellyfish Database Initiative http://jedi.nceas.ucsb.edu, http://jedi.nceas.ucsb.edu-dmo.org/dataset/526852 )

Authors:
Mario Lebrato (Christian‐Albrechts‐University Kiel and Bazaruto Center for Scientific Studies, Mozambique)
Markus Pahlow (GEOMAR)
Jessica R. Frost (South Florida Water Management District)
Marie Küter (Christian‐Albrechts‐University Kiel)
Pedro de Jesus Mendes (Marine and Environmental Scientific and Technological Solutions, Germany)
Juan‐Carlos Molinero (GEOMAR)
Andreas Oschlies (GEOMAR)

Hurricane-driven surge of labile carbon into the deep North Atlantic Ocean

Posted by mmaheigan

· Thursday, February 27th, 2020

Tropical cyclones (hurricanes and typhoons) are the most extreme episodic weather event affecting subtropical and temperate oceans. Hurricanes generate intense surface cooling and vertical mixing in the upper ocean, resulting in nutrient upwelling into the photic zone and episodic phytoplankton blooms. However, their influence on the deep ocean is unknown.

Figure 1. (a) Particulate organic carbon (POC) flux and percentage of the total mass flux (yellow) (top panel); fluxes (middle panel) and POC-normalized concentrations (bottom panel) of diagnostic lipid biomarkers for phytoplankton-derived and labile material, zooplankton, bacteria, and other (see legend); (b) Lipid concentrations (left panel) and POC-normalized concentrations (right panel) of diagnostic lipid biomarkers for the same sources as in (a) (see legend) measured two weeks after Nicole’s passage (25-29 Oct. 2016). Shown for reference are total lipid concentration profiles in April 2015 (dark gray, typical post spring bloom conditions) and Nov 2015 (light gray, typical minimum production period).

In October 2016, Category 3 Hurricane Nicole passed over the Bermuda time-series site (Oceanic Flux Program (OFP) and Bermuda Atlantic Time-Series site (BATS)) in the oligotrophic NW Atlantic Ocean. In a recent study published in Geophysical Research Letters, authors synthesized multidisciplinary data from hydrographic and phytoplankton measurements and lipid composition of sinking and suspended particles collected from OFP and BATS, respectively, after Hurricane Nicole in 2016. After the hurricane passed, particulate fluxes of lipids diagnostic of fresh phytodetritus, zooplankton, and microbial biomass increased by 30-300% at 1500 m depth and 30-800% at 3200 m depth (Figure 1a). In addition, mesopelagic suspended particles were enriched in phytodetrital material, as well as zooplankton- and bacteria-sourced lipids (Figure 1b), indicating particle disaggregation and a deep-water ecosystem response.

These results suggest that carbon export and biogeochemical cycles may be impacted by climate-induced changes in hurricane frequency, intensity, and tracks, and, underscore the sensitivity of deep ocean ecosystems to climate perturbations.

Authors:
Rut Pedrosa-Pamies (Marine Biological Laboratory)
Maureen H. Conte (Bermuda Institute of Ocean Science and Marine Biological Laboratory)
JC Weber (Marine Biological Laboratory)
Rodney Johnson (Bermuda Institute of Ocean Science)

Unexpected DOC additions in the deep Atlantic

Posted by mmaheigan

· Tuesday, January 7th, 2020

Oceanic dissolved organic carbon (DOC) ultimately exchanges with atmospheric CO₂ and thus represents an important carbon source/sink with consequence for climate. Most of the DOC is recalcitrant to microbial degradation, with some fractions surviving for thousands of years. Therefore, DOC in the deep ocean was thought to be stable or to decrease slowly over decades to centuries due to biotic and abiotic sinks. However, a study published in Global Biogeochemical Cycles shows that there are some zones of the deep Atlantic Ocean where recalcitrant DOC experiences net production. Using data from oceanographic cruises across the Atlantic Ocean, the authors first identified the major water masses in the basin and the percentage of each in every sample taken for DOC analysis. The study revealed net additions of 27 million tons of dissolved organic carbon per year in the deep South Atlantic. On the other hand, the North Atlantic serves as a net sink, removing 298 million tons of carbon annually. DOC production observed in the deep Atlantic is probably due to the sinking particles that solubilize into DOC, since DOC enrichment was most evident at latitudes characterized as elevated productivity divergence zones.

Figure 1. Water masses along GO-SHIP line A16 (colored dots) and recalcitrant DOC variations due to biogeochemical processes (black dots within each water mass) in the deep Atlantic Ocean. Water mass domains are defined as the set of samples with the corresponding water mass proportion ≥50%. Recalcitrant DOC latitudinal variations per water stratum due to biogeochemical processes (ΔDOC) is in μmol kg^-1. Numbers on the plots are DOC values for the corresponding dots. Scales (not shown) are the same for all the plots, from -4 to 6 μmol kg^-1. Positive (negative) ΔDOC indicates values higher (lower) than the average DOC calculated for each water mass using an optimum multiparameter (OMP) analysis. DOC = dissolved organic carbon. AAIW = Antarctic Intermediate Water; UNADW = upper North Atlantic Deep Water; ISOW = Iceland Scotland Overflow Water; CDW = Circumpolar Deep Water; WSDW = Weddell Sea Deep Water. Figure created with Ocean Data View (Schlitzer, 2015).

Considering that the net DOC production over the entire Atlantic basin euphotic zone is 0.70–0.75 Pg C year^-1, the authors estimated that 30–39% of that DOC is consumed in the deep Atlantic subsequent to its export by overturning circulation. The upper North Atlantic Deep Water (UNADW) acts as the primary sink, accounting for 66% of the recalcitrant DOC removal in the North Atlantic. Conversely, the Antarctic Intermediate Water (AAIW) is the primary recipient, with 45% of recalcitrant DOC production in the South Atlantic, closely followed by the old UNADW that gains 44% of the recalcitrant DOC in the southern basin.

The Atlantic works as a mosaic of water masses, where both removal and addition of recalcitrant DOC occurs, with the dominant term dependent on the origin, temperature, age and depth of the water masses. The production of recalcitrant DOC in the deep ocean should be considered in biogeochemical models dealing with the carbon cycle and climate.

Authors:
C. Romera-Castillo and J. L. Pelegrí (Instituto de Ciencias del Mar, CSIC, Spain)
M. Álvarez (Instituto Español de Oceanografía, Spain)
D. A. Hansell (University of Miami, USA)
X. A. Álvarez-Salgado (Instituto de Investigaciones Marinas, CSIC, Spain)

What really controls deep-seafloor calcite dissolution?

Posted by mmaheigan

· Monday, December 16th, 2019

On time scales of tens to millions of years, seawater acidity is primarily controlled by biogenic calcite (CaCO₃) dissolution on the seafloor. Our quantitative understanding of future oceanic pH and carbonate system chemistry requires knowledge of what controls this dissolution. Past experiments on the dissolution rate of suspended calcite grains have consistently suggested a high-order, nonlinear dependence on undersaturation that is independent of fluid flow rate. This form of kinetics has been extensively adopted in models of deep-sea calcite dissolution and pH of benthic sediments. However, stirred-chamber and rotating-disc dissolution experiments have consistently demonstrated linear kinetics of dissolution and a strong dependence on fluid flow velocity. This experimental discrepancy surrounding the kinetic control of seafloor calcite dissolution precludes robust predictions of oceanic response to anthropogenic acidification.

In a recent study published in Geochimica et Cosmochimica Acta, authors have reconciled these divergent experimental results through an equation for the mass balance of the carbonate ion at the sediment-water interface (SWI), which equates the rate of production of that ion via dissolution and its diffusion in sediment porewaters to the transport across the diffusive sublayer (DBL) at the SWI. If the rate constant derived from suspended-grain experiments is inserted into this balance equation, the rate of carbonate ion supply to the SWI from the sediment (sediment-side control) is much greater in the oceans than the rate of transfer across the DBL (water-side control). Thus, calcite dissolution at the seafloor, while technically under mixed control, is strongly water-side dominated. Consequently, a model that neglects boundary-layer transport (sediment-side control alone) invariably predicts CaCO₃-versus-depth profiles that are too shallow compared to available data (Figure 1). These new findings will inform future attempts to model the ocean’s response to acidification.

Figure 1: Plots of the calcite (CaCO₃) content of deep-sea sediments as a function of oceanic depth. Left panel: data from the Northwestern Atlantic Ocean. Right panel: data from the Southwest Pacific Ocean. The blue line represents predicted CaCO₃ content assuming no boundary-layer effects (pure sediment-side control). The red line is the prediction that includes both sediment and water effects (mixed control), and the green line is the prediction with pure water-side control. The agreement between the red and green lines signifies that calcite dissolution is essentially water-side controlled at the seafloor. These results are duplicated for all tested regions of the oceans.

Authors:
Bernard P. Boudreau (Dalhousie University)
Olivier Sulpis (University of Utrecht)
Alfonso Mucci (McGill University)

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)

Increased temperatures suggest reduced capacity for carbon

Posted by mmaheigan

· Thursday, January 18th, 2018

The ocean’s biological pump works to draw down atmospheric carbon dioxide (CO₂) by exporting carbon from the surface ocean. This process is less efficient at higher temperatures, implying a possible climate feedback. Recent work by Cael et al. provides an explanation of why this feedback occurs and an estimate of its severity.

In a highly simplified view, carbon export depends on the balance between two temperature-dependent processes: 1) The autotrophic production and 2) the heterotrophic respiration of organic carbon. Cael and Follows (Geophysical Research Letters 2016) recently developed a mechanistic model based on established temperature dependencies for photosynthesis and respiration to explore feedbacks between export efficiency and climate. Heterotrophic growth rates increase more so than phototrophic rates with increasing temperature, which suggests that at higher temperatures, community respiration will increase relative to production, thereby decreasing export efficiency. Although simplistic, the model captures the temperature dependence of export efficiency observations.

Figure: Schematic of the mechanism on which the Cael and Follows (2016) model is based. (a) Photosynthesis (dark grey) and respiration (light grey) respond to temperature differently, yielding (b) a decline in export efficiency at higher temperatures.

More recently, Cael, Bisson, and Follows (Limnology and Oceanography 2017) applied this model to sea surface temperature records and estimated a ~1.5% decline in globally-averaged export efficiency over the past three decades of increasing ocean temperatures as a result of this metabolic mechanism. This ~1.5% decline is equivalent to a reduced ocean sequestration of approximately 100 million fewer tons of carbon annually, comparable to the annual carbon emissions of the United Kingdom. The model provides a framework in which to consider the relationship between climate and ocean carbon export that might also elucidate large-scale (e.g., glacial-interglacial) atmospheric CO₂ changes of the past.

Authors:
B. B. Cael (MIT/WHOI)
Kelsey Bisson (UCSB)
Mick Follows (MIT)

Lasers shed light on giant larvacean filtration impact on the ocean’s biological pump

Posted by mmaheigan

· Thursday, January 4th, 2018

To accurately assess the impacts of climate change, we need to understand how atmospheric carbon is transported from surface waters to the deep sea. Grazers and filter feeders drive the ocean’s biological pump as they remove and sequester carbon at various rates. This pump extends down into the midwater realm, the largest habitat on earth. Giant larvaceans are fascinating and enigmatic occupants of the upper 400 m of the water column, where they build complex filtering structures out of mucus that can reach diameters greater than 1 m in longest dimension (Figure 1A). Because of the fragility of these structures, direct measurements of filtration rates require us to study them in situ. We developed DeepPIV, an ROV-deployable instrument (Figure 1B) to directly measure fluid motion and filtration rates in situ (Figure 1C).

Figure 1. (A) Traditional view of a giant larvacean illuminated by white ROV lights. (B) DeepPIV instrument is seen attached to Monterey Bay Aquarium Research Institute’s (MBARI) MiniROV. (C) DeepPIV-illuminated interior view of a giant larvacean house, where particle motion in ambient seawater serves as a proxy for fluid motion. White arrows in (A) and (C) indicate larvacean head/trunk; white arrow in (B) indicates DeepPIV.

The filtration rates we measured for giant larvaceans are far greater than for any other zooplankton filter feeder. When combined with abundance data from a 22-year time series, the grazing impact of giant larvaceans indicates that within 13 days, they can filter the total volume of water within their habitable depth range (~100-300 m; based on maximum abundance and measured filtration rates). Our results reveal that the contribution of giant larvaceans to vertical carbon flux is much greater than previously thought. Small larvaceans, which are present in the water column in even larger quantities than giant larvaceans, may also have a measurable impact on carbon fluxes. New technologies such as DeepPIV are yielding more quantitative observations of midwater filter feeders, which is improving our understanding of the roles that deep-water biota play in the long-term removal of carbon from the atmosphere.

Read the full journal article: http://advances.sciencemag.org/content/3/5/e1602374.full

Authors: (All at MBARI)
Kakani Katija
Rob E. Sherlock
Alana D. Sherman
Bruce H. Robison

Zooplankton play a key and diverse role in the ocean carbon cycle

Posted by mmaheigan

· Thursday, December 7th, 2017

How does the enormous diversity of zooplankton species, life cycles, size, feeding ecology, and physiology affect their role in ocean food webs and cycling of carbon?

In the 2017 issue of Annual Review of Marine Science, Steinberg and Landry review the fundamental and multifaceted roles that zooplankton play in the cycling and export of carbon in the ocean. Carbon flows through marine pelagic ecosystems are complex due to the diversity of zooplankton consumers and the many trophic levels they occupy in the food web–from single-celled herbivores to large carnivorous jellyfish. Zooplankton also contribute to carbon export processes through a variety of mechanisms (mucous feeding webs, fecal pellets, molts, carcasses, and vertical migrations).

Figure 1. Pathways of cycling and export of carbon by zooplankton in the ocean.

Climate change and other stressors are already affecting zooplankton abundance, distribution, and life cycles, and are predicted to result in widespread changes in zooplankton carbon cycling in the future. These changes will affect both the larger marine food web that depends upon zooplankton for food (fish) or recycled products for growth (primary producers) and the amount of carbon exported into the deep sea–where far from contact with the atmosphere it no longer contributes to global warming.

Authors:

Deborah K. Steinberg, Virginia Institute of Marine Science, The College of William and Mary
Michael R. Landry, Scripps Institution of Oceanography

Tiny marine animals strongly influence the carbon cycle

Posted by mmaheigan

· Thursday, August 31st, 2017

What controls the amount of organic carbon entering the deep ocean? In the sunlit layer of the ocean, phytoplankton transform inorganic carbon to organic carbon via a process called photosynthesis. As these particulate forms of organic carbon stick together, they become dense enough to sink out of the sunlit layer, transferring large quantities of organic carbon to the deep ocean and out of contact with the atmosphere.

However, all is not still in the dark ocean. Microbial organisms such as bacteria, and zooplankton consume the sinking, carbon-rich particles and convert the organic carbon back to its original inorganic form. Depending on how deep this occurs, the carbon can be physically mixed back up into the surface layers for exchange with the atmosphere or repeat consumption by phytoplankton. In a recent study published in Biogeosciences, researchers used field data and an ecosystem model in three very different oceanic regions to show that zooplankton are extremely important in determining how much carbon reaches the deep ocean.

Figure 1. Particle export and transfer efficiency to the deep ocean in the Southern Ocean (SO, blue circles), North Atlantic Porcupine Abyssal Plain site (PAP, red squares) and the Equatorial Tropical North Pacific (ETNP, orange triangles) oxygen minimum zone. a) particle export efficiency of fast sinking particles (Fast PEeff) against primary production on a Log10 scale. b) transfer efficiency of particles to the deep ocean expressed as Martin’s b (high b = low efficiency). Error bars in b) are standard error of the mean for observed particles, error too small in model to be seen on this plot.

In the Southern Ocean (SO), zooplankton graze on phytoplankton and produce rapidly sinking fecal pellets, resulting in an inverse relationship between particle export and primary production (Fig. 1a). In the North Atlantic (NA), the efficiency with which particles are transferred to the deep ocean is comparable to that of the Southern Ocean, suggesting similar processes apply; but in both regions, there is a large discrepancy between the field data and the ecosystem model (Fig. 1b), which poorly represents particle processing by zooplankton. Conversely, much better data-model matches are observed in the equatorial Pacific, where lower oxygen concentrations mean fewer zooplankton; this reduces the potential for zooplankton-particle interactions that reduce particle size and density, resulting in a lower transfer efficiency.

This result suggests that mismatches between the data and model in the SO and NA may be due to the lack of zooplankton-particle parameterizations in the model, highlighting the potential importance of zooplankton in regulating carbon export and storage in the deep ocean. Zooplankton parameterizations in ecosystem models must be enhanced by including zooplankton fragmentation of particles as well as consumption. Large field programs such as EXPORTS could help constrain these parameterisation by collecting data on zooplankton-particle interaction rates. This will improve our model estimates of carbon export and our ability to predict future changes in the biological carbon pump. This is especially important in the face of climate-driven changes in zooplankton populations (e.g. oxygen minimum zone (OMZ) expansion) and associated implications for ocean carbon storage and atmospheric carbon dioxide levels.

Authors:
Emma L. Cavan (University of Tasmania)
Stephanie A. Henson (National Oceanography Centre, Southampton)
Anna Belcher (University of Southampton)
Richard Sanders (National Oceanography Centre, Southampton)

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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