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

Archive for New OCB Research – Page 6

Towards using historical oxygen observations to reconstruct the air-sea flux of biological oxygen

Posted by mmaheigan

· Tuesday, December 13th, 2022

Dissolved oxygen (O₂) is a central observation in oceanography with a long history of relatively high precision measurements and increasing coverage over the 21^st century. O₂ is a powerful tracer of physical, chemical and biological processes (e.g., photosynthesis and respiration, wave-induced bubbles, mixing, and air-sea diffusion). A commonly used approach to partition the processes controlling the O₂ signal relies on concurrent measurements of argon (an inert gas), which has solubility properties similar to O₂. However, only a limited fraction of O₂ measurements have paired argon measurements.

Figure 1. (a) The newly developed empirical model to parameterize the physical oxygen saturation anomaly (ΔO_2[phy]) in order to separate the biological contribution from total oxygen, and (b-c) regional, inter-annual, and decadal variability of air-sea gas flux of biological oxygen (F[O2]bio as) reconstructed from the historical dissolved oxygen record.

A recent study published in the Journal of Global Biogeochemical Cycles presents semi-analytical algorithms to separate the biological and physical O₂ oxygen signals from O₂ observations. Among the approaches, a machine-learning algorithm using ship-based measurements and historical records of physical parameters from reanalysis products as predictors shows encouraging performance. The researchers leveraged this new algorithm to reconstruct regional, inter-annual, and decadal variability of the air-sea flux of biological oxygen (from historical O₂ records.

The long-term objective of this proof-of-concept effort is to estimate from historical oxygen records and a rapidly growing number of O₂ measurements on autonomous platforms. In regions where vertical and horizontal mixing is weak, the projected approximates net community production, providing an independent constraint on the strength of the biological carbon pump.

Authors:
Yibin Huang (Duke University)
Rachel Eveleth (Oberlin College)
David (Roo) Nicholson (Woods Hole Oceanographic Institution)
Nicolas Cassar (Duke University)

Linking the calcium carbonate and alkalinity cycles in the North Pacific ocean

Posted by mmaheigan

· Tuesday, December 13th, 2022

The marine carbon and alkalinity cycles are tightly coupled. Seawater stores so much carbon because of its high alkalinity, or buffering capacity, and the main driver of alkalinity cycling is the formation and dissolution of biologically produced calcium carbonate (CaCO3). In a recent publication in GBC, the authors conducted novel carbon-13 tracer experiments to measure the dissolution rates of biologically produced CaCO3 along a transect in the North Pacific Ocean. They combined these experiment data with shipboard analyses of the dissolved carbonate system, the 13C-content of dissolved inorganic carbon, and CaCO3 fluxes, to constrain the alkalinity cycle in the upper 1000 meters of the water column. Dissolution rates were too slow to explain alkalinity production or CaCO3 loss from the particulate phase. However, driving dissolution with the metabolic consumption of oxygen brings alkalinity production and CaCO3 loss estimates into quantitative agreement (Figure). The authors argue that a majority of CaCO3 production is likely dissolved through metabolic processes in the upper ocean, including zooplankton grazing, digestion, and egestion, and microbial degradation of marine particle aggregates that contain both organic carbon and CaCO3. This hypothesis stems from the basic fact that almost all marine CaCO3 is biologically produced, placing CaCO3 at the source of the acidifying process (metabolic consumption of organic matter). This process is important because it puts an emphasis on biological processing for the cycling of not only carbon, but also alkalinity, the main buffering component in seawater. These results should help both scientists and stakeholders to understand the fundamental controls on calcium carbonate cycling in the ocean, and therefore the processes that distribute alkalinity throughout the world’s oceans.

Figure Caption: Sinking-dissolution model results compared with tracer-based alkalinity regeneration rates (TA*-CFC, Feely et al., 2002). We also plot alkalinity regeneration rates using updated time transit distribution ages (TA*- and Alk*-TTD). The modeled alkalinity regeneration rate uses our measured dissolution rates for biologically produced calcite and aragonite, and is driven by a combination of background saturation state and metabolic oxygen consumption. The dissolution rate is split up into a calcite component (produced mainly by coccolithophores) and an aragonite component (produced mainly by pteropods). Aragonite does not contribute significantly to the overall dissolution rate. Driving dissolution by metabolic oxygen consumption produces alkalinity regeneration rates that are in quantitative agreement with tracer-based estimates.

Authors:
Adam Subhas (Woods Hole Oceanographic Institution) et al.

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Drivers of recent Chesapeake Bay warming

Posted by mmaheigan

· Friday, August 26th, 2022

Coastal water temperatures have been increasing globally with more frequent marine heat waves threatening marine life and nearshore communities reliant upon these ecosystems. Often, this warming is assumed to be uniform in space and time; however, this is not the case in the Chesapeake Bay, where warming waters play a major role in exacerbating low oxygen levels and indirectly limiting the efficacy of nutrient reduction efforts on land.

New research published in the Journal of the American Water Resources Association combined long-term observations and a hydrodynamic model to quantify the temporal and spatial variability in warming Chesapeake Bay waters, and identify the contributions of different mechanisms driving these historical temperature changes. While winter temperatures have warmed by less than a half a degree over the past 30 years, summer temperatures have warmed by nearly 1.5 °C, with similar increases at the surface and bottom. In cooler months, the atmosphere was the dominant driver of warming throughout the majority of the Bay, but oceanic warming explained more than half of the increased summer temperatures in the southern Bay nearest the Atlantic.

Figure 1: Relative contribution of different factors to warm-month Chesapeake Bay temperature change over the period 1985-2015. Percentages correspond to average main channel contributions for each component.

Warming temperatures have potentially significant implications for the future size of the Chesapeake Bay dead zone, and the marine species directly affected by these low oxygen conditions. Better quantifying warming contributions from the atmosphere, ocean, sea level, and rivers will also help constrain regional temperature projections throughout the estuary. More accurate projections of future Bay temperatures can help coastal managers better understand the potential for invasive species expansion and endemic species loss, impacts to fisheries and aquaculture, and how changes to ecosystem processes may impact coastal communities dependent on a healthy Bay.

Authors:
Kyle E. Hinson (Virginia Institute of Marine Science, William & Mary)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science, William & Mary)
Pierre St-Laurent (Virginia Institute of Marine Science, William & Mary)
Fei Da (Virginia Institute of Marine Science, William & Mary)
Raymond G. Najjar (The Pennsylvania State University)

Nutrient management improves hypoxia in the Chesapeake Bay despite record-breaking precipitation and warming

Posted by mmaheigan

· Friday, August 26th, 2022

Hypoxia is currently one of the greatest threats to coastal and estuarine ecosystems around the world, and this threat is projected to get worse as waters warm and human populations continue to increase. Over the past 35-years, a massive effort has been underway to decrease hypoxia in the Chesapeake Bay by reducing nutrient input from land. Despite this effort, record-breaking precipitation in 2018-2019 fueled particularly large hypoxic volumes in the Bay, calling into question the efficacy of management actions.

Figure 1. The number of days of additional hypoxia (O2 < 3 mg L^-1) that would have occurred in the Chesapeake Bay if the 35 years of nutrient reductions never occurred, as calculated by differences between a realistic numerical model simulation and one with 1985 nitrogen levels. This management effort has had the greatest impact at the northern and southern edges of the hypoxia in the Bay, where there would have been an additional 60-90 days of O2 < 3 mg L^-1 if nutrient reductions never occurred.

In a recent paper published in Science of the Total Environment, researchers used empirical and numerical modeling to quantify the impact of nutrient management efforts on hypoxia in the Chesapeake Bay. Results suggest that if the nutrient reduction efforts beginning in 1985 had not taken place, hypoxia would have been ~50–120% greater during the average discharge years of 2016–2017 and ~20–50% greater during the wet years of 2018–2019. The management impact was most pronounced in regions of the Bay where the hypoxia season would have been 60-90 days longer if nutrient reductions did not occur (Figure 1).

Although these results suggest that management has reduced hypoxic conditions in the Bay, additional analysis revealed that warming temperatures have already offset 6-34% of this improvement. This highlights the importance of factoring in climate change when setting future management goals.

Figure 2. The number of days of additional hypoxia (O2 < 3 mg L^-1) that would have occurred in the Chesapeake Bay if the 35 years of nutrient reductions never occurred, as calculated by differences between a realistic numerical model simulation and one with 1985 nitrogen levels. This management effort has had the greatest impact at the northern and southern edges of the hypoxia in the Bay, where there would have been an additional 60-90 days of O2 < 3 mg L^-1 if nutrient reductions never occurred.

Authors:
Luke T. Frankel (Virginia Institute of Marine Science, William & Mary)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science, William & Mary)
Pierre St-Laurent (Virginia Institute of Marine Science, William & Mary)
Aaron J. Bever (Anchor QEA)
Romuald N. Lipcius (Virginia Institute of Marine Science, William & Mary)
Gopal Bhatt (Pennsylvania State University; Chesapeake Bay Program)
Gary W. Shenk (USGS; Chesapeake Bay Program)

Integrated analysis of carbon dioxide and oxygen concentrations as a quality control of ocean float data

Posted by mmaheigan

· Friday, August 26th, 2022

A recent study in Communications Earth & Environment, examined spatiotemporal patterns of the two dissolved gases CO₂ and O₂in the surface ocean, using the high-quality global dataset GLODAPv2.2020. We used surface ocean data from GLODAP to make plots of carbon dioxide and oxygen relative to saturation (CORS plots). These plots of CO₂ deviations from saturation (ΔCO₂) against oxygen deviations from saturation (ΔO₂) (Figure 1) provide detailed insight into the identity and intensity of biogeochemical processes operating in different basins.

Figure 1: Relationships between ΔCO₂ and ΔO₂ in the global ocean basins based on surface data in the GLODAPv2.2020 database. The black dashed lines are the least-squares best-fit lines to the data; unc denotes the uncertainty in the y-intercept value with 95% confidence; r is the associated Pearson correlation coefficient; n is the number of data points.

In addition, data in all basins and all seasons shares some common behaviors: (1) negative slopes of best fit lines to the data, and (2) near-zero y-intercepts of those lines. We utilized these findings to compare patterns in CORS plots from GLODAP with those from BGC-Argo float data from the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program. Given that the float O₂ data is likely to be more accurate than the float pH data (from which the float CO₂ is calculated), CORS plots are useful for detecting questionable float CO₂ data, by comparing trends in float CORS plots (e.g. Figure 2) to trends in GLODAP CORS plots (Figure 1). As well as the immediately detected erroneous data, we discovered significant discrepancies in ΔCO₂-ΔO₂ y-intercepts compared to the global reference (i.e., GLODAPv2.2020 y-intercepts, Figure 1). The y-intercepts of 48 floats with QCed O₂ and CO₂ data (at regions south of 55°S) were on average greater by 0.36 μmol kg⁻¹ than the GLODAP-derived ones, implying the overestimations of float-based CO₂ release in the Southern Ocean.

Figure 2. CORS plots from data collected by SOCCOM floats F9096 and F9099 in the high-latitude Southern Ocean. Circles with solid edges denote data flagged as ‘good’, whereas crosses denote data flagged as ‘questionable’.

Our study demonstrates CORS plots’ ability to identify questionable data (data shown to be questionable by other QC methods) and to reveal issues with supposed ‘good’ data (i.e., quality issues not picked up by other QC methods). CORS plots use only surface data, hence this QC method complements existing methods based on analysis of deep data. As the oceanographic community becomes increasingly reliant on data collected from autonomous platforms, techniques like CORS will help diagnose data quality, and immediately detect questionable data.

Authors:
Yingxu Wu (Polar and Marine Research Institute, Jimei University, Xiamen, China; University of Southampton)
Dorothee C.E. Bakker (University of East Anglia)
Eric P. Achterberg (GEOMAR Helmholtz Centre for Ocean Research Kiel)
Amavi N. Silva (University of Southampton)
Daisy D. Pickup (University of Southampton)
Xiang Li (George Washington University)
Sue Hartman (National Oceanography Centre, Southampton)
David Stappard (University of Southampton)
Di Qi (Polar and Marine Research Institute, Jimei University, Xiamen, China)
Toby Tyrrell (University of Southampton)

Why are sand lance embryos so sensitive to future high CO2-oceans?

Posted by mmaheigan

· Friday, August 26th, 2022

Two decades of ocean acidification experiments have shown that elevated CO₂ can affect many traits in fish early life stages. Only few species, however, show direct CO₂-induced survival reductions. This may partly reflect a bias in our current empirical record, which is dominated by species from nearshore tropical-to-temperate environments. There, these organisms already experience highly variable CO₂ conditions. In contrast, fishes from more offshore habitats, especially at higher latitudes are adapted to more CO₂-stable conditions, which could make them more CO₂-sensitive. This group of fishes is still underrepresented in the literature, despite its enormous commercial and ecological importance.

To help address this gap, we conducted new experimental work on northern sand lance Ammodytes dubius, a key forage fish on offshore Northwest Atlantic sand banks with trophic links to more than 70 different predator species of fish, squid, seabirds, and marine mammals. On Stellwagen Bank in the southern Gulf of Maine, sand lance are the ‘backbone’ of the eponymous National Marine Sanctuary.

We followed up on the intriguing findings of a pilot study a few years ago. Over two years and two trials, we again produced embryos from wild, Stellwagen Bank spawners and reared them at several pCO₂ levels (~400−2000 μatm) in combination with static and dynamic temperatures. Again, we observed consistently large CO₂-induced reductions in hatching success (-23% at 1,000 µatm, -61% at ~2,000 µatm), but this time the effects were temperature-independent.

Intriguingly, we again saw that many sand lance embryos at high CO₂ treatments did not merely arrest in their development (indicative of acidosis), but appeared to develop fully to hatch but were somehow incapable of doing so. We show several lines of evidence supporting the hypothesis that CO₂ directly impairs hatching in this species. Most fish rely on hatching enzymes that help embryos break the chorion (egg shell), but these ubiquitous enzymes may work less efficiently under high CO₂, low pH conditions.

For additional context, we also derived long-term, seasonal pCO₂ projections specifically for Stellwagen Bank, which together with the experimental data suggested that increasing CO₂ levels alone could reduce sand lance hatching success to 71% of contemporary levels by the year 2100.

We believe that the importance of sand lances as forage fishes across most northern hemisphere shelf ecosystem warrants a strategic effort of OA researchers to begin testing other sand lance species or populations to understand the magnitude of the problem and its underlying mechanisms.

Authors:
Hannes Baumann (University of Connecticut)
Lucas Jones (University of Connecticut)
Christopher Murray (University of Washington)
Samantha Siedlecki (University of Connecticut)
Michael Alexander (NOAA Physical Sciences Laboratory)
Emma Cross (Southern Connecticut State University)

Seaweeds for carbon dioxide removal: Forensic accounting is essential

Posted by mmaheigan

· Friday, August 26th, 2022

Terrestrial forests are well known as major carbon stores and are already used in carbon credit and offset schemes. In the coastal zone, seaweeds have a similar functional role to trees (carbon dioxide fixation, primary production) and on this basis are being promoted for use as carbon offsets. However, compared to terrestrial forests in which CO₂ is taken up directly from the atmosphere and can be readily accounted for in long-lived biomass (wood, roots), accrediting carbon sequestration to seaweeds is extremely complex. This is because:

1) Seaweeds do not store carbon as living biomass on time scales relevant to sequestration (> 100 years) because individuals and seaweed beds have a rapid turnover time (< 10 years).

2) Most seaweed production is consumed within the system (herbivory) or transported laterally out of the system as dissolved organic carbon, particulate organic carbon, or CO₂. Tracking the fate and storage of seaweed carbon has not been quantified to date.

3) Seaweeds take up CO₂ from surrounding seawater on time scales of seconds, but the re-equilibration of atmospheric CO₂ into surface waters occurs on time scales of weeks (coastal ocean) to years (open ocean). Hence, timescales of CO₂equilibration are the ultimate driver of CDR, but tracking the fate of the seawater from which CO₂ has been taken up by seaweeds is very difficult as it is a ‘moving target’ that can be advected offshore, moved into heterotrophic systems (e.g. an oyster bed), or subducted.

4) A proportion of seaweed carbon is likely stored in sediments on geological timescales but this storage potential has not been accurately quantified.

Figure 1: Key physical and biological processes that require quantification in Forensic Carbon Accounting for seaweeds. Although the rate of CO2 uptake by seaweeds is well known, the lateral advection of seaweed carbon to other systems, including the deep ocean and sediments, as dissolved and particulate organic carbon (DOC, POC) and respired CO2 are at present poorly quantified. It is also important to account for time scales of air-ocean CO2 equilibrium, and the fate of the seawater parcel which bears the signature of CO2 uptake.

Forensic Carbon Accounting (FCA) is a new tool for assessing the role of coastal habitats in carbon dioxide removal (CDR). FCA is essential to rigorously assess the potential of these systems in carbon credit and offset frameworks that takes into account the complexities of tracking the dual fates of seaweed carbon and the seawater that carries the associated CO₂ deficit that results from seaweed photosynthesis (Figure 1). FCA can be applied to all coastal systems, and should be undertaken in any system (natural or aquaculture, onshore or offshore) prior to claiming carbon credits or offsets.

Author:
Catriona L. Hurd (Institute for Marine and Antarctic Studies, University of Tasmania)

Is it a shift of the surface ecosystem toward small cells in the eastern North Pacific subtropical gyre?

Posted by mmaheigan

· Friday, August 26th, 2022

The eastern North Pacific subtropical gyre (NPSG) ecosystem contains a large proportion of the ocean surface, resulting in a significant impact on the global ocean primary production and export production. The NPSG is influenced by the interannual climate variabilities of the Pacific decadal oscillation and North Pacific gyre oscillation (NPGO). In particular, a recent report associated the NPGO mode with favorable physical conditions for phytoplankton growth in the eastern NPSG environment. However, the NPGO mode-driven changes in the relationship between primary production and export production and the processes driving these changes remain unclear in the eastern NPSG environment.

A study published in Frontiers in Marine Science investigated the temporal changes in the physical-biogeochemical-ecological responses in the surface eastern NPSG to the NPGO mode, using a suite of oceanographic measurements from Station ALOHA (22.75°N and 158°W) from 1992–2018. When the NPGO mode was in positive phases (N₂⁺: 1998‒2004, N₄⁺: 2007‒2013), the eastern region of NPSG was related to deepened mixed layer depth, conducive to high primary production. Accordingly, the N₂⁺ phase showed a high export production, associated with increase in nano-sized phytoplankton group and inorganic and organic nitrogen-to-phosphorus (N:P) ratios. However, even under the conditions of deep mixed layer depth and increased primary production, the N₄⁺phase has a low export production. This difference is attributed to multiple physical and biogeochemical factors, such as the depression of thermocline, shift toward the pico-sized phytoplankton group, increase in smallest-sized mesozooplankton and heterotrophic bacteria, and decrease in N:P ratios. Under high stratification induced by prolonged warming, these results suggest that the future surface eastern NPSG may experience a permanent shift toward small cells (Figure 1), leading to a reduction in the biological pump.

Authors:
Joo-Eun Yoon (University of Cambridge)
Ju-Hyoung Kim (Kunsan National University)
Il-Nam Kim (Incheon National University)

How does the competition between phytoplankton and bacteria for iron alter ocean biogeochemical cycles?

Posted by mmaheigan

· Friday, August 26th, 2022

Free-living bacteria play a key role in cycling essential biogeochemical resources in the ocean, including iron, via their uptake, transformation, and release of organic matter throughout the water column. Bacteria process half of the ocean’s primary production, remineralize dissolved organic matter, and re-direct otherwise lost organic matter to higher trophic levels. For these reasons, it is crucial to understand what factors limit the growth of bacteria and how bacteria activities impact global ocean biogeochemical cycles.

In a recent study, Pham and colleagues used a global ocean ecosystem model to dive into how iron limits the growth of free-living marine bacteria, how bacteria modulate ocean iron cycling, and the consequences to marine ecosystems of the competition between bacteria and phytoplankton for iron.

Figure 1: (a) Iron limitation status of bacteria in December, January, and February (DJF) in the surface ocean. Low values (in blue color = close to zero) mean that iron is the limiting factor for the growth of bacteria; (b) Bacterial iron consumption in the upper 120m of the ocean and (c) Changes (anomalies) in export carbon production when bacteria have a high requirement for iron.

Through a series of computer simulations performed in the global ocean ecosystem model, the authors found that iron is a limiting factor for bacterial growth in iron-limited regions in the Southern Ocean, the tropical, and the subarctic Pacific due to the high iron requirement and iron uptake capability of bacteria. Bacteria act as an iron sink in the upper ocean due to their significant iron consumption, a rate comparable to phytoplankton. The competition between bacteria and phytoplankton for iron alters phytoplankton bloom dynamics, ocean carbon export, and the availability of dissolved organic carbon needed for bacterial growth. These results suggest that earth system models that omit bacteria ignore an important organism modulating biogeochemical responses of the ocean to future changes.

Authors:
Anh Le-Duy Pham (Laboratoire d’Océanographie et de Climatologie: Expérimentation et Approches Numériques (LOCEAN), IPSL, CNRS/UPMC/IRD/MNHN, Paris, France)
Olivier Aumont (Laboratoire d’Océanographie et de Climatologie: Expérimentation et Approches Numériques (LOCEAN), IPSL, CNRS/UPMC/IRD/MNHN, Paris, France)
Lavenia Ratnarajah (University of Liverpool, United Kingdom)
Alessandro Tagliabue (University of Liverpool, United Kingdom)

Carbon fluxes in the coastal ocean: Synthesis, boundary processes and future trends

Posted by mmaheigan

· Friday, August 26th, 2022

A vital part of mitigating climate change is the coastal and open ocean carbon sink, without this, it is not possible to meet the target set by the Paris Agreement. More research is needed to better understand the ocean carbon cycle and its future role in the uptake of anthropogenic carbon. A review provides an analysis of the current qualitative and quantitative understanding of the coastal ocean carbon cycle at regional to global scales, with a focus on the air-sea CO₂ exchange. It includes novel findings obtained using the full breadth of methodological approaches, from observation-based studies and advanced statistical methods to conceptual and theoretical frameworks, and numerical modeling.

Figure 1: Updated sea-air CO₂ flux density (mol C m⁻² year⁻¹) in the global coastal oceans that reveals that the global coastal ocean is an integrated CO₂ sink with the strongest CO₂ uptake at high latitudes. The challenges associated with identifying current and projected responses of the coastal ocean and it source/sink role in the global carbon budget require observational networks that are coordinated and integrated with modeling programs; development of this capability is a priority for the ocean carbon research and management communities.

Based on a new quantitative synthesis of air-sea CO₂ exchange, this study yields an estimate for the globally integrated coastal ocean CO₂ flux of −0.25 ± 0.05 Pg C year⁻¹, with polar and subpolar regions accounting for most of the CO₂ removal (>90%). A framework that classifies river-dominated ocean margin (RiOMar) and ocean-dominated margin (OceMar) systems is used in to conceptualize coastal carbon cycle processes. Ocean carbon models are reviewed in terms of the ability to simulate key processes and project future changes in different continental shelf regions. Concurrent trends and changes in the land-ocean-atmosphere coupled system introduce large uncertainties into projections of ocean carbon fluxes, in particular into defining the role of the coastal carbon sink and its evolution, both of which are of fundamental importance to climate science and climate policies developed before and after achievement of net-zero CO₂ emissions. The major gaps and challenges identified for current coastal ocean carbon research have important implications for climate and sustainability policies. This study is a contribution to the Regional Carbon Cycle Assessment and Processes Phase 2 supported by the Global Carbon Project.

Authors:
M. H. Dai, J. Z. Su, Y. Y. Z., E. E. Hofmann, Z. M. Cao, W.-J. Cai, J. P. Gan, F. Lacroix, G. G. Laruelle, F. F. Meng, J. D. Müller, P. A.G. Regnier, G. Z. Wang, and Z. X. Wang

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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