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Archive for anthropogenic carbon

Air-sea gas disequilibrium drove deoxygenation of the deep ice-age ocean

Posted by mmaheigan

· Thursday, March 18th, 2021

During the Last Glacial Maximum (~20,000 years ago, LGM) sediment data show that the deep ocean had lower dissolved oxygen (O₂) concentrations than the preindustrial ocean, despite cooler temperatures of this period increasing O₂ solubility in sea water.

Figure 1. a) Whole ocean inventory of the O₂ components in the preindustrial control (PIC): total O₂ (O₂); the preformed components equilibrium O₂ (O₂ equilibrium), physical disequilibrium O₂ (O₂ diseq phys) and biologically-mediated disequilibrium (O₂ diseq bio); and O₂ respired from soft-tissue (O₂ soft). b) The difference in whole ocean inventory of O₂ components between the LGM and PIC simulations.

In a study published in Nature Geoscience, the authors provide one of the first explanations for glacial deoxygenation. The authors combined a data-constrained model of the preindustrial (PIC) and LGM ocean with a novel decomposition of O₂ to assess the processes affecting the oceanic distribution of oxygen. The decomposition allowed for the preformed disequilibrium O₂—the amount of oxygen that deviates from its solubility equilibrium value when at the surface—to be tracked, along with other contributions such as the O₂ consumed by bacterial respiration of organic matter. In the preindustrial ocean, a third of the subsurface oxygen deficit was a result of disequilibrium rather than oxygen consumed by bacteria. This contradicts previous assumptions (Figure 1a). Nearly 80% of the disequilibrium resulted from upwelling waters, depleted in O₂ due to respiration, not fully equilibrating before re-subduction into the ocean interior. This effect was even greater during the LGM (Figure 1b). The authors attributed this largely to the widespread presence of sea ice—which acts as a cap on the surface preventing the water from gaining oxygen from the atmosphere—in the ocean around Antarctica, with a smaller contribution from iron fertilization.

This study provides one of the first mechanistic explanations for LGM deep ocean deoxygenation. As the ocean is currently losing oxygen due to warming, the effect of other processes, including sea ice changes, could prove important for understanding long-term ocean oxygenation changes.

Authors
Ellen Cliff (University of Oxford)
Samar Khatiwala (University of Oxford)
Andreas Schmittner (Oregon State University)

Joint highlight with GEOTRACES International Project Office

Wildfire impacts on coastal ocean phytoplankton

Posted by mmaheigan

· Wednesday, February 24th, 2021

Wildfire frequency, size, and destructiveness has increased over the last two decades, particularly in coastal regions such as Australia, Brazil, and the western United States. While the impact of fire on land, plants, and people is well documented, very few studies have been able to evaluate the impact of fires on ocean ecosystems. A serendipitously planned research cruise one week after the Thomas Fire broke out in California in December 2017 allowed the authors of this study and their colleagues to sample the adjacent Santa Barbara Channel during this devastating extreme fire event.

In a recent paper published in Journal of Geophysical Research: Oceans, the authors describe the phytoplankton community in the Santa Barbara Channel during the Thomas Fire. Phytoplankton community composition was described using a combination of images of phytoplankton from the Imaging FlowCytobot (McLane Labs) and phytoplankton pigments. Dinoflagellates were the dominant phytoplankton group in the surface ocean during the Thomas Fire, according to both methods (Figure 1).

Figure 1. (A) The fraction of total particle volume imaged by the Imaging FlowCytobot (IFCB) comprised of phytoplankton (green) and detritus (brown). Example IFCB images of ash (counted as part of detritus) particles are outlined in brown. (B) The phytoplankton fraction is then further divided by taxonomy, showing the abundance of nano-sized phytoplankton and especially dinoflagellates during the week of sampling. Example IFCB images of Gonyaulax (outlined in dark green), Prorocentrum (outlined in light green), and Umbilicosphaera (outlined in purple) cells are also shown.

While this study was not able to demonstrate a causal relationship between the Thomas Fire and the presence of dinoflagellates, this result is quite different from previous winters in the Santa Barbara Channel, when picophytoplankton and diatoms typically dominate the winter community. The incidence of dinoflagellates in the Santa Barbara Channel in December 2017 was correlated with the warmer-than-average water temperature during this study, which matched observations from other areas along the Central California coast that winter.

At the time this study was conducted, the Thomas Fire was the largest wildfire in California history. Since then, California fires have increased in danger, destruction, and human mortality; the Mendocino Fire complex (summer 2018) and five separate wildfires in summer 2020 exceeded the impacts of the Thomas Fire. With wildfire severity and frequency increasing not only in California but in coastal regions worldwide, this study gives an important first look at the impact of wildfire smoke and ash on oceanic primary productivity and community composition.

Authors:
Sasha Kramer (University of California Santa Barbara)
Kelsey Bisson (Oregon State University)
Alexis Fischer (University of California Santa Cruz)

Does ocean acidification make marine fish grow differently? What about sex-specific effects?

Posted by mmaheigan

· Monday, February 8th, 2021

The question of whether ocean acidification (OA) will impact the growth of marine fish remains surprisingly uncertain. The bulk of available evidence in the form of laboratory experiments suggests that most fish are not impacted by OA-relevant CO₂ levels, but many studies suffer from the inherent methodical constraints of rearing marine fish in captivity. For example, most experiments cover a small fraction of a species’ lifespan and do not employ restricted feeding regimes which may enable fish to increase feeding to offset metabolic deficits associated with high-CO₂ acclimation.

To address these methodological shortcomings, authors of a recent publication in PLOS One synthesized three years of multiple long-term, food-controlled experiments that reared large populations of the model forage fish Menidia menidia (Atlantic silverside) from fertilization to about a third of their lifespan. Results showed modest but consistent negative, temperature-dependent growth effects, in which silversides from high-CO₂ treatments were shorter (-3% to -9%) and weighed less (-6% to -18%) than ambient-CO₂ conspecifics. However, sometimes it takes more than just looking at means and standard deviations to elucidate these effects. Hence, the authors employed powerful shift functions to analyze how the size distributions of experimental populations shifted to smaller quantiles under future CO₂ conditions.

Figure caption: The length of juvenile Atlantic silversides reared from fertilization under control (blue dots) and high-CO₂ treatments (red dots). Exposure to OA conditions imposed a universal shift to a smaller body size across the size frequency distribution. Black vertical bars overlaying each distribution indicate the .1, .25, .5, .75, and .9 quantiles and quantile shifts are indicated by connecting lines.

It took over 100 days of continuous high-CO₂ exposure until size differences were detectable. This means that long-term CO₂effects could exist in other tested species but are missed in relatively short experiments. Furthermore, the authors sexed several thousand fish to enable a rare sex-specific analysis of CO₂ effects. Both sexes were similarly affected by high CO₂, and the hormonal pathways that mediate environmental sex determination in this species are not impacted by CO₂ level. Our results confirm that Atlantic silversides are relatively tolerant of future OA conditions. But even in this robust estuarine species, high CO₂ can reduce growth. This could have cascading effects on population dynamics by impacting size-dependent traits like reproductive success and over-wintering survival of this widespread and ecologically important prey species.

Authors
Christopher S. Murray (University of Washington)
Hannes Baumann (University of Connecticut)

Warming counteracts acidification in temperate crustose coralline algae communities

Posted by mmaheigan

· Friday, November 6th, 2020

Seawater carbonate chemistry has been altered by dramatic increases in anthropogenic CO₂ release and global temperatures, leading to significant changes in rocky shore habitats and the metabolism of most marine organisms. There has been recent interest in how these anthropogenic stresses affect crustose coralline algae (CCA) communities because CCA photosynthesis and calcification are directly influenced by seawater carbonate chemistry. CCA is a foundation species in temperate macroalgal communities, where species succession and rocky shore community structure are particularly susceptible to anthropogenic disturbance. In particular, the disappearance of turf and foliose macroalgae caused by climate change and herbivore pressure results in the dominance of CCA (Figure 1a).

Figure 1: (a) Examples of crustose coralline algae (CCA)-dominated seaweed bed in the East Sea of Korea showing barren ground dominated by CCA (bright white and pink color on the rock; see arrows) on a rocky subtidal zone grazed by sea urchins. (b) Specific growth rate of marginal encrusting area under future climate conditions.

In a recent study published in Marine Pollution Bulletin, the authors conducted a mesocosm experiment to investigate the sensitivity of temperate CCA Chamberlainium sp. to future climate stressors, as simulated by three experimental treatments: 1) Acidification: doubled CO₂; 2) Warming: +5ºC; and 3) Greenhouse: doubled CO₂ and +5ºC. After a 47-day acclimation period, when compared with present-day (control: 490 μatm and 20ºC) conditions, the Acidification treatment showed decreased photosynthesis rates of Chamberlainium sp, whereas the Warming treatment showed increased photosynthesis. The Acidification treatment also showed reduced encrusting growth rates relative to the Control, but when acidification was combined with warming in the Greenhouse treatment, encrusting growth rates increased substantially (Figure 1b). Taken together, these results suggest that the negative ecophysiological responses of Chamberlainium sp to acidification are ameliorated by elevated temperatures in a greenhouse world. In other words, if the foliose macroalgal community responses negatively in the greenhouse environment, the dominance of CCA will increase further, and the biodiversity of the algae community will be reduced.

Authors:
Ju-Hyoung Kim (Faculty of Marine Applied Biosciences, Kunsan National University)
Il-Nam Kim (Department of Marine Science, Incheon National University)

Marine heatwave implications for future phytoplankton blooms

Posted by mmaheigan

· Thursday, October 15th, 2020

Ocean temperature extreme events such as marine heatwaves are expected to intensify in coming decades due to anthropogenic warming. Although the effects of marine heatwaves on large plants and animals are becoming well documented, little is known about how these warming events will impact microbes that regulate key biogeochemical processes such as ocean carbon uptake and export, which represent important feedbacks on the global carbon cycle and climate.

Figure caption: Relationship between phytoplankton bloom response to marine heatwaves and background nitrate concentration in the 23 study regions. X-axis denotes the annual-mean sea-surface nitrate concentration based on the model simulation (1992-2014; OFAM3, blue) and the in situ climatology (WOA13, orange). Y-axis denotes the mean standardised anomalies (see Equation 1 of the paper) of simulated sea-surface phytoplankton nitrogen biomass (1992-2014; OFAM3, blue) and observed sea-surface chlorophyll a concentration (2002-2018; MODIS, orange) during the co-occurrence of phytoplankton blooms and marine heatwaves.

In a recent study published in Global Change Biology, authors combined model simulations and satellite observations in tropical and temperate oceanographic regions over recent decades to characterize marine heatwave impacts on phytoplankton blooms. The results reveal regionally‐coherent anomalies depicted by shallower surface mixed layers and lower surface nitrate concentrations during marine heatwaves, which counteract known light and nutrient limitation effects on phytoplankton growth, respectively (Figure 1). Consequently, phytoplankton bloom responses are mixed, but derive from the background nutrient conditions of a study region such that blooms are weaker (stronger) during marine heatwaves in nutrient-poor (nutrient-rich) waters.

Given the projected expansion of nutrient-poor waters in the 21st century ocean, the coming decades are likely to see an increased occurrence of weaker blooms during marine heatwaves, with implications for higher trophic levels and biogeochemical cycling of key elements.

Authors:
Hakase Hayashida (University of Tasmania)
Richard Matear (CSIRO)
Pete Strutton (University of Tasmania)

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)

Nitrate enrichment may threaten coastal wetland carbon storage

Posted by mmaheigan

· Thursday, February 27th, 2020

With their high primary productivity and slow decomposition in anoxic soils, salt marshes and other coastal wetlands can store carbon more efficiently than terrestrial uplands. These wetlands also provide critical ecosystem services such as interception of land-derived nutrients before they can enter the coastal ocean. Therefore, it is important to understand how anthropogenic supplies of nitrate (NO₃^–) affect marsh sustainability and carbon storage.

In marsh sediment studies, the most common form of experimental nitrogen enrichment uses pelletized fertilizer composed of ammonium, urea, or other organic based fertilizers. Authors of a recent study published in Global Change Biology hypothesized that when nutrients were instead added in the form of nitrate (NO₃^–), the most common form of nitrogen enrichment in coastal waters, it would stimulate microbial decomposition of organic matter by serving as an electron acceptor for microbial respiration in anoxic salt marsh sediments. Furthermore, decomposition would vary with sediment depth, with decreased decomposition at greater depths, where less biologically available organic matter accumulated over time.

Figure 1: DIC production as a proxy for microbial respiration in salt marsh sediments from three distinct depth horizons (shallow 0-5cm, mid 10-15cm, deep 20-25cm) that span a range of biological availability. The addition of NO3- (green) stimulated DIC production relative to unenriched sediments, regardless of sediment depth. All samples were run under anoxic conditions (without the presence of oxygen), closely matching that of normal salt marsh sediments.

Surprisingly, NO₃^–addition stimulated decomposition of organic matter at all depths, with the highest decomposition rates in the surface sediments. This suggests that there is a pool of “NO₃^–-labile” organic matter in marsh sediments that microbes can decompose under high-NO₃^– conditions that would otherwise remain stable. As human activities continue to enrich our coastal waters with NO₃^–through agricultural runoff, septic systems, and other pathways, it could inadvertently decrease coastal wetlands’ carbon storage capacity, with negative consequences for both blue carbon offsets and marsh sustainability in the face of sea level rise.

Authors:
Jennifer Bowen (Northeastern University)
Ashley Bulseco (MBL/WHOI)
Anne Giblin (MBL)

Krillin’ it with poop: Highlighting the importance of Antarctic krill in ocean carbon and nutrient cycling

Posted by mmaheigan

· Tuesday, February 4th, 2020

Scientists have long known the role of Antarctic krill (Euphausia superba) in Southern Ocean ecosystems. Evidence is gathering about krill’s biogeochemical importance through releasing millions of faecal pellets in swarms and stimulating primary production through nutrient excretion. Here, we explore and synthesise the known impacts that this highly abundant and rather large species has on the environment. Krill exemplify how metazoa can play a dominant role in shaping ocean biogeochemistry, thus providing additional motivation for protecting certain harvested species.

Figure 1: The ecological roles of krill in Southern Ocean biogeochemical cycles, including releasing faecal pellets, excreting nutrients whilst grazing, and larval krill migrating throughout the water column, shedding exoskeletons, and feeding on the seabed.

A review published in Nature Communications uncovers at least 13 possible pathways by which Antarctic krill either influence the carbon sink or release fertilizing nutrients (Figure 1). Their large size (up to 7 cm) and swarming nature (millions of krill aggregate) enable krill to strongly impact ocean biogeochemistry. Swarms release large numbers of faecal pellets, overwhelming detritivores and resulting in a large sink of faecal carbon. Krill may physically mix nutrients from the deep ocean and become a decades-long carbon store in whale biomass. Antarctic krill larvae, which live near the sea-ice, undergo deeper diel vertical migrations compared to adult Antarctic krill (400 m vs. 200 m), so any carbon respired or faecal pellets released by larvae could remain in the deep ocean longer than those released by adult krill at a shallower depth; the larval krill contribution to carbon export has not been quantified. Furthermore, it is currently unknown how many krill larvae are removed from the Antarctic krill fishery as by-catch. Perhaps the biggest challenge in constraining the role of krill (adult and larvae) in biogeochemical cycles is our limited capacity to quantify the abundance and biomass of Antarctic krill, since shipboard sampling methods (nets or acoustics) have limited spatial and temporal coverage. Ultimately, the Southern Ocean is an important physical AND biological sink of carbon, and we must consider the role krill and other animals have in this cycle.

Figure 2: Processes in the biological carbon pump including the sinking of dead phytoplankton aggregates, zooplankton, krill and fish faecal pellets and dead animals. Microbial remineralisation is depicted through the return of particulate organic carbon to dissolved organic carbon (DOC) and eventually carbon dioxide.

Authors:
Emma Cavan (Imperial College London and University of Tasmania)
Anna Belcher (British Antarctic Survey)
Angus Atkinson (Plymouth Marine Laboratory)
Simeon Hill (British Antarctic Survey)
So Kawaguchi (Australian Antarctic Division)
Stacey McCormack (University of Tasmania)
Bettina Meyer (Alfred Wegener Institute for Polar and Marine Research and University of Oldenburg)
Stephen Nicol (University of Tasmania)
Lavenia Ratnarajah (University of Liverpool)
Katrin Schmidt (University of Plymouth)
Deborah Steinberg (Virginia Institute of Marine Science)
Geraint Tarling (British Antarctic Survey)
Philip Boyd (University of Tasmania and Antarctic Climate and Ecosystems Cooperative Research Centre)

The past, present, and future of the ocean carbon cycle: A global data product with regional insights

Posted by mmaheigan

· Tuesday, January 21st, 2020

A new study published in Scientific Reports debuts a global data product of ocean acidification (OA) and buffer capacity from the beginning of the Industrial Revolution to the end of the century (1750-2100 C.E.). To develop this product, the authors linked one of the richest observational carbon dioxide (CO₂) data products (6^th version of the Surface Ocean CO₂ Atlas, 1991-2018, ~23 million observations) with temporal trends modeled at individual locations in the global ocean. By linking the modeled pH trends to the observed modern pH distribution, the climatology benefits from recent improvements in both model design and observational data coverage, and is likely to provide more accurate regional OA trajectories than the model output alone. The authors also show that air-sea CO₂ disequilibrium is the dominant mode of spatial variability for surface pH, and discuss why pH and calcium carbonate mineral saturation states (Omega), two important metrics for OA, show contrasting spatial variability. They discover that sea surface temperature (SST) imposes two large but cancelling effects on surface ocean pH and Omega, i.e., the effects of SST on (a) chemical speciation of the carbonic system; and (b) air-sea exchange of CO₂ and the subsequent DIC/TA ratio of the seawater. These two processes act in concert for Omega but oppose each other for pH. As a result, while Omega is markedly lower in the colder polar regions than in the warmer subtropical and tropical regions, surface ocean pH shows little latitudinal variation.

Figure 1. Spatial distribution of global surface ocean pH_T (total hydrogen scale, annually averaged) in past (1770), present (2000) and future (2100) under the IPCC RCP8.5 scenario.

This data product, which extends from the pre-Industrial era (1750 C.E.) to the end of this century under historical atmospheric CO₂ concentrations (pre-2005) and the Representative Concentrations Pathways (post-2005) of the Intergovernmental Panel on Climate Change (IPCC)’s 5^th Assessment Report, may be helpful to policy-makers and managers who are developing regional adaptation strategies for ocean acidification.

The published paper is available here: https://www.nature.com/articles/s41598-019-55039-4

The data product is available here: https://www.nodc.noaa.gov/oads/data/0206289.xml

Authors:
Li-Qing Jiang (University of Maryland and NOAA NCEI)
Brendan Carter (NOAA PMEL and University of Washington JISAO)
Richard Feely (NOAA PMEL)
Siv Lauvset, Are Olsen (University of Bergen and Bjerknes Centre for Climate Research, Norway)

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)

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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