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Archive for ocean carbon uptake and storage – Page 3

pH: the secrets that you keep

Posted by mmaheigan

· Monday, September 20th, 2021

The Intergovernmental Panel on Climate Change (IPCC) defines ocean acidification as “a reduction in pH of the ocean over an extended period, typically decades or longer, caused primarily by the uptake of carbon dioxide (CO₂) from the atmosphere” (Rhein et al., 2013, p. 295). Does this mean that a greater change in pH at the ocean surface relative to the subsurface, or at one location relative to another, always indicates greater acidification? Based on this IPCC definition of ocean acidification, the answer is yes. But does that make sense?

Seawater pH is the negative base 10 logarithm of the seawater’s hydrogen ion concentration ([H⁺]) and is a useful way to display a wide range of [H⁺] in a compact form. A change in pH reflects a relative change in [H⁺]. Thus, anytime we speak of pH changes, we are really referring to a relative change in the chemical species of interest ([H⁺]). On the other hand, changes in all the other carbonate system variables that we measure are usually absolute. This characteristic of pH can lead to ambiguity in the interpretation and presentation of rates and patterns of change. Improved understanding comes from also studying changes in [H⁺], which can reveal aspects that studying changes in pH alone may conceal or overemphasize.

A recent Biogeosciences article reviewed the history leading to this unintuitive relationship between changes in pH and changes in [H⁺]. The article provides three real-world examples to display how examining pH changes alone can hide the ocean acidification signals of interest (Figure 1). These examples highlight potential challenges associated with comparing surface and subsurface pH changes across ocean domains without accounting for differences in the initial pH values. The authors recommend reporting both pH and [H⁺] in studies that assess changes in ocean chemistry to improve the clarity of ocean acidification research.

Figure Caption: Data used in this figure come from the GFDL ESM2M model for the combined historical and RCP8.5 experiments. Top: the 1950s surface ocean (left) pH and (right) [H+]. Bottom: the 1950s to 2090s change (Δ) in surface ocean (left) pH and (right) [H+]. The color bar for ΔpH is reversed to ease comparison with patterns of Δ[H+]

Authors:
Andrea J. Fassbender (NOAA Pacific Marine Environmental Laboratory)
Andrew G. Dickson (Scripps Institution of Oceanography, University of California, San Diego)
James C. Orr (LSCE/IPSL, Laboratoire des Sciences du Climat et de l’Environnement)

Exploiting phytoplankton as a biosensor for nutrient limitation

Posted by mmaheigan

· Wednesday, September 15th, 2021

In the surface ocean, phytoplankton growth is often limited by a scarcity of key nutrients such as nitrogen, phosphorus, and iron. While this is important, there are methodological and conceptual difficulties in characterizing these nutrient limitations.

A recent paper published in Science Magazine leveraged a global metagenomic dataset from Bio-GO-SHIP to address these challenges. The authors characterized the abundance of genes that confer adaptations to nutrient limitation within the picocyanobacteria Prochlorococcus. Using the relative abundance of these genes as an indicator of nutrient limitation allowed the authors to capture expected regions of nutrient limitation, and novel regions that had not previously been studied. This gene-derived indicator of nutrient limitation matched previous methods of assessing nutrient limitation, such as bottle incubation experiments.

These findings have important implications for the global ocean. Characterizing the impact of nutrient limitation on primary production is especially critical in light of future stratification driven by climate change. In addition, this novel methodological approach allows scientists to use microbial communities as an eco-genomic biosensor of adaptation to changing nutrient regimes. For instance, future studies of coastal microbes or other ecosystems may help communities and environmental managers better understand how local microbial populations are adapting to climate change.

Watch an illustrated video overview of this research

Authors:
Lucas J. Ustick, Alyse A. Larkin, Catherine A. Garcia, Nathan S. Garcia, Melissa L. Brock, Jenna A. Lee, Nicola A. Wiseman, J. Keith Moore, Adam C. Martiny
(all University of California, Irvine)

How environmental drivers regulated the long-term evolution of the biological pump

Posted by mmaheigan

· Friday, January 22nd, 2021

The marine biological pump (BP) plays a crucial role in regulating earth’s atmospheric oxygen and carbon dioxide levels by transferring carbon fixed by primary producers into the ocean interior and marine sediments, thereby controlling the habitability of our planet. The rise of multicellular life and eukaryotic algae in the ocean about 700 million years ago would likely have influenced the physical characteristics of oceanic aggregates (e.g., sinking rate), yet the magnitude of the impact this biological innovation had on the efficiency of BP is unknown.

Figure. 1. The impact of biological innovations (left) and environmental factors (atmospheric oxygen level and seawater temperature; right) on the efficiency of marine biological pump (BP). Temperatures are ocean surface temperatures (SST), and atmospheric pO2 is shown relative to the present atmospheric level (PAL). The BP efficiency is calculated as the fraction of carbon exported from the surface ocean that is delivered to the sediment-water interface. The results indicate that evolution of larger sized algae and zooplanktons has little influence on the long-term evolution of biological pump (left panel). The change in the atmospheric oxygen level and seawater surface temperature as environmental factors, on the other hand, have a stronger leverage on the efficiency of biological pump (right panel).

The authors of a recent paper in Nature Geoscience constructed a particle-based stochastic model to explore the change in the efficiency of the BP in response to biological and physical changes in the ocean over geologic time. The model calculates the age of organic particles in each aggregate based on their sinking rates, and considers the impact of primary producer cell size, aggregation, temperature, dust flux, biomineralization, ballasting by mineral phases, oxygen, and the fractal geometry (porosity) of aggregates. The model results demonstrate that while the rise of larger-sized eukaryotes led to an increase in the average sinking rate of oceanic aggregates, its impact on BP efficiency was minor. The evolution of zooplankton (with daily vertical migration in the water column) had a larger impact on the carbon transfer into the ocean interior. But results suggest that environmental factors most strongly affected the marine carbon pump efficiency. Specifically, increased ocean temperatures and greater atmospheric oxygen abundance led to a significant decrease in the efficiency of the BP. Cumulatively, these results suggest that while major biological innovations influenced the efficiency of BP, the long-term evolution of the marine carbon pump was primarily controlled by environmental drivers such as climate cooling and warming. By enhancing the rate of heterotrophic microbial degradation, our results suggest that the anthropogenically-driven global warming can result in a less efficient BP with reduced power of marine ecosystem in sequestering carbon from the atmosphere.

Authors:
Mojtaba Fakhraee (Yale University, Georgia Tech, and NASA Astrobiology Institute)
Noah J. Planavsky (Yale University, and NASA Astrobiology Institute)
Christopher T. Reinhard (Georgia Tech, and NASA Astrobiology Institute)

Sea ice loss amplifies CO2 increase in the Arctic

Posted by mmaheigan

· Thursday, January 7th, 2021

Warming and sea ice loss over the past few decades have caused major changes in sea surface partial pressure of CO₂ (pCO₂) of the western Arctic Ocean, but detailed temporal variations and trends during this period of rapid climate-driven changes are not well known.

Based on an analysis of an international Arctic pCO₂ synthesis data set collected between 1994-2017, the authors of a recent paper published in Nature Climate Change observed that summer sea surface pCO₂ in the Canada Basin is increasing at twice the rate of atmospheric CO₂ rise. Warming, ice loss and subsequent CO₂ uptake in the Basin are amplifying seasonal pCO₂ changes, resulting in a rapid long-term increase. Consequently, the summer air-sea CO₂ gradient has decreased sharply and may approach zero by the 2030s, which is reducing the basin’s capacity to remove CO₂from the atmosphere. In stark contrast, sea surface pCO₂ on the Chukchi Shelf remains low and relatively constant during this time frame, which the authors attribute to increasingly strong biological production in response to higher intrusion of nutrient-rich Pacific Ocean water onto the shelf as a result of increased Bering Strait throughflow. These trends suggest that, unlike the Canada Basin, the Chukchi Shelf will become a larger carbon sink in the future, with implications for the deep ocean carbon cycle and ecosystem.

As Arctic sea ice melting accelerates, more fresh, low-buffer capacity, high-CO₂ water will enter the upper layer of the Canada Basin, which may rapidly acidify the surface water, endanger marine calcifying organisms, and disrupt ecosystem function.

Figure. 1: TOP) Sea surface pCO₂ trend in the Canada Basin and Chukchi Shelf. The grey dots represent the raw observations of pCO₂, black dots are the monthly mean of pCO₂ at in situ SST, and red dots are the monthly means of pCO₂ normalized to the long-term means of SST. The arrows indicate the statistically significant change in ∆pCO₂. BOTTOM) Sea ice-loss amplifying surface water pCO₂ in the Canada Basin. Black dots represent the initial condition for pCO₂ and DIC at -1.6 ℃. The arrows indicate the processes of warming (red), CO₂ uptake from the atmosphere (green), dilution by ice meltwater (blue). The yellow shaded areas indicate the possible seasonal variations of pCO₂, which are amplified by the synergistic effect of ice melt, warming and CO₂ uptake.

Authors:
Zhangxian Ouyang (University of Delaware, USA),
Di Qi (Third Institute of Oceanography, China),
Liqi Chen (Third Institute of Oceanography, China),
Taro Takahashi† (Columbia University, USA),
Wenli Zhong (Ocean University of China, China),
Michael D. DeGrandpre (University of Montana, USA),
Baoshan Chen (University of Delaware, USA),
Zhongyong Gao (Third Institute of Oceanography, China),
Shigeto Nishino (Japan Agency for Marine-Earth Science and Technology, Japan),
Akihiko Murata (Japan Agency for Marine-Earth Science and Technology, Japan),
Heng Sun (Third Institute of Oceanography, China),
Lisa L. Robbins (University of South Florida, USA),
Meibing Jin (International Arctic Research Center, USA),
Wei-Jun Cai* (University of Delaware, USA)

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)

How zooplankton control carbon export in the Southern Ocean

Posted by mmaheigan

· Thursday, December 3rd, 2020

The Southern Ocean exhibits an inverse relationship between surface primary production and export flux out of the euphotic zone. The causes of this production-export decoupling are still under debate. A recently published mini review in Frontiers in Marine Science focused on zooplankton, an important component of Southern Ocean food webs and the biological pump. The authors compared carbon export regimes from the naturally iron-fertilised Kerguelen Plateau (high surface production, but generally low export) with the iron-limited and less productive high nutrient, low chlorophyll (HNLC) waters south of Australia, where carbon export is relatively high.

Figure 1: The role of zooplankton in establishing the characteristic export regimes at two sites in the Southern Ocean, (a) the highly productive northern Kerguelen Plateau, which exhibits low export, and (b) the iron-limited waters south of Australia with low production, but relatively high carbon export.

Size structure and zooplankton grazing pressure are found to shape carbon export at both sites. On the Kerguelen Plateau, a large size spectrum of zooplankton acts as “gate-keeper” to the mesopelagic by significantly reducing the sinking flux of phytoaggregates, which establishes the characteristic low export regime. In the HNLC waters, however, the zooplankton community is low in biomass and grazes predominantly on smaller particles, which leaves the larger particles for export and leads to relatively high export flux.

Gaps in knowledge related to insufficient seasonal data coverage, understudied carbon flux pathways, and associated mesopelagic processes limit our current understanding of carbon transfer through the water column and export. More integrated data collection efforts, including the use of autonomous profiling floats (e.g., BGC-Argo), stationary moorings, etc., will improve seasonal carbon flux data coverage, thus enabling more reliable estimation of carbon export and storage in the Southern Ocean and improved projection of future changes in carbon uptake and atmospheric carbon dioxide levels.

Authors:
Svenja Halfter (University of Tasmania)
Emma Cavan (Imperial College London)
Ruth Eriksen (CSIRO)
Kerrie Swadling (University of Tasmania)
Philip Boyd (University of Tasmania)

Austral summer vertical migration patterns in Antarctic zooplankton

Posted by mmaheigan

· Thursday, October 15th, 2020

Sunrise and sunset are the main cues driving zooplankton diel vertical migration (DVM) throughout the world’s oceans. These marine animals balance the trade-off between feeding in surface waters at night and avoiding predation during the day at depth. Near-constant daylight during polar summer was assumed to dampen these daily migrations. In a recent paper published in Deep-Sea Research I, authors assessed austral summer DVM patterns for 15 taxa over a 9-year period. Despite up to 22 hours of sunlight, a diverse array of zooplankton – including copepods, krill, pteropods, and salps – continued DVM.

Figure caption: Mean day (orange) and night (blue) abundance of (A) the salp Salpa thompsoni, (B) the krill species Thysanoessa macrura, (C) the pteropod Limacina helicina, and (D) chaetognaths sampled at discrete depth intervals from 0-500m. Horizontal dashed lines indicate weighted mean depth (WMD). N:D is the night to day abundance ratio for 0-150 m. Error bars indicate one standard error. Sample size n = 12 to 22. Photos by Larry Madin, Miram Gleiber, and Kharis Schrage.

The Palmer Antarctica Long-Term Ecological Research (LTER) Program conducted this study using a MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) to collect depth-stratified samples west of the Antarctic Peninsula. The depth range of migrations during austral summer varied across taxa and with daylength and phytoplankton biomass and distribution. While most taxa continued some form of DVM, others (e.g., carnivores and detritivores) remained most abundant in the mesopelagic zone, regardless of photoperiod, which likely impacted the attenuation of vertical carbon flux. Given the observed differences in vertical distribution and migration behavior across taxa, ongoing changes in Antarctic zooplankton assemblages will likely impact carbon export pathways. More regional, taxon-specific studies such as this are needed to inform efforts to model zooplankton contributions to the biological carbon pump.

Authors:
John Conroy (VIMS, William & Mary)
Deborah Steinberg (VIMS, William & Mary)
Patricia Thibodeau (VIMS, William & Mary; currently University of Rhode Island)
Oscar Schofield (Rutgers University)

Will global change “stress out” ocean DOC cycling?

Posted by mmaheigan

· Tuesday, September 29th, 2020

The dissolved organic carbon (DOC) pool is vital for the functioning of marine ecosystems. DOC fuels marine food webs and is a cornerstone of the earth’s carbon cycle. As one of the largest pools of organic matter on the planet, disruptions to marine DOC cycling driven by climate and environmental global changes can impact air-sea CO2 exchange, with the added potential for feedbacks on Earth’s climate system.

Figure 1. Simplified view of major dissolved organic carbon (DOC) sources (black text) and sinks (yellow text) in the ocean.

Since DOC cycling involves multiple processes acting concurrently over a range of time and space scales, it is especially challenging to characterize and quantify the influence of global change. In a recent review paper published in Frontiers in Marine Science, the authors synthesize impacts of global change-related stressors on DOC cycling such as ocean warming, stratification, acidification, deoxygenation, glacial and sea ice melting, inflow from rivers, ocean circulation and upwelling, and atmospheric deposition. While ocean warming and acidification are projected to stimulate DOC production and degradation, in most regions, the outcomes for other key climate stressors are less clear, with much more regional variation. This synthesis helps advance our understanding of how global change will affect the DOC pool in the future ocean, but also highlights important research gaps that need to be explored. These gaps include for example a need for studies that allow to understand the adaptation of degradation/production pathways to global change stressors, and their cumulative impacts (e.g. temperature with acidification).

Authors:
C. Lønborg (Aarhus University)
C. Carreira (CESAM, Universidade de Aveiro)
Tim Jickells (University of East Anglia)
X.A. Álvarez-Salgado (CSIC, Instituto de Investigacións Mariñas)

Sea ice loss and the changing Arctic carbon cycle

Posted by mmaheigan

· Friday, September 18th, 2020

Loss of Arctic Ocean ice cover is altering the carbon cycle in ways that are not well understood. Effectively “popping the top off” the Arctic Ocean, ice loss exposes the sea surface to warming and exchange of CO₂ with the atmosphere. These processes are expected to increase CO₂ levels in the Arctic Ocean, changing its contribution to the global carbon cycle, but limited data collection in the region has thus far precluded the establishment of a clear relationship between CO₂ and ice cover. In a recent study published in Geophysical Research Letters, authors report on observed partial pressure of CO₂ (pCO₂) trends from several years of data collection in the surface waters of the Canada Basin of the Arctic Ocean. These data show that the pCO₂ is higher during years when ice cover is low. Uptake of atmospheric CO₂ and heating are the primary sources of the CO₂ increase, with only a small counteracting offset from biological production. These processes vary significantly from year to year, masking the likely increase in pCO₂ over time. Based on these results, we can expect that, while the Arctic Ocean has thus far been a significant sink for atmospheric CO₂, if ice loss continues the uptake of CO₂ will diminish in coming years.

Figure caption: Sea surface pCO₂ increases with decreasing ice concentration (left), determined using the mean of spatially gridded data. The sea surface pCO₂ data were collected on five research cruises on the Canadian icebreaker, CCGS Louis S. St-Laurent, from 2012 to 2017 (shown at right for 2017). The pCO₂ levels are indicated by the color along the ship cruise track (right color bar). The dark shading (left color bar) represents sea ice concentration averaged from the daily satellite data collected during the cruise.

Authors:
Michael DeGrandpre (University of Montana-Missoula)
Wiley Evans (Hakai Institute)
Mary-Louise Timmermans (Yale University)
Richard Krishfield (Woods Hole Oceanographic Institution)
Bill Williams (Institute of Ocean Sciences)
Michael Steele (University of Washington)

Modern OMZ copepod dynamics provide analog for future oceans

Posted by mmaheigan

· Thursday, July 23rd, 2020

Global warming increases ocean deoxygenation and expands the oxygen minimum zone (OMZ), which has implications for major zooplankton groups like copepods. Reduced oxygen levels may impact individual copepod species abundance, vertical distribution, and life history strategy, which is likely to perturb intricate oceanic food webs and export processes. In a study recently published in Biogeosciences, authors conducted vertically-stratified day and night MOCNESS tows (0-1000 m) during four cruises (2007-2017) in the Eastern Tropical North Pacific, sampling hydrography and copepod distributions in four locations with different water column oxygen profiles and OMZ intensity (i.e. lowest oxygen concentration and its vertical extent in a profile). Each copepod species exhibited a different vertical distribution strategy and physiology associated with oxygen profile variability. The study identified sets of species that (1) changed their vertical distributions and maximum abundance depth associated with the depth and intensity of the OMZ and its oxycline inflection points, (2) shifted their diapause depth, (3) adjusted their diel vertical migration, especially the nighttime upper depth, or (4) expanded or contracted their depth range within the mixed layer and upper part of the thermocline in association with the thickness of the aerobic epipelagic zone (habitat compression concept) (Figure 1). Distribution depths for some species shifted by 10’s to 100’s of meters in different situations, which also had metabolic (and carbon flow) implications because temperature decreased with depth. This observed present-day variability may provide an important window into how future marine ecosystems will respond to deoxygenation.

Figure caption: Schematic diagram showing how future OMZ expansion may affect zooplankton distributions, based on present-day responses to OMZ variability. The dashed line indicates diel vertical migration (DVM) and highlights the shoaling of the nighttime depth as the aerobic habitat is compressed. The lower oxycline community and the diapause layer for some species, associated with a specific oxygen concentration, may deepen as the OMZ expands.

Authors:
Karen F. Wishner (University of Rhode Island)
Brad Seibel (University of South Florida)
Dawn Outram (University of Rhode Island)

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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