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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)

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)

Powerful new tools for working with Argo data

Posted by mmaheigan

· Thursday, June 9th, 2022

No single program has been as transformative for ocean science over the past two decades as Argo: the fleet of robotic instruments that collect measurements of temperature and salinity in the upper 2 km of the ocean around the globe. The Argo program has been instrumental in revealing changes to ocean heat content, global sea level, and patterns of ice melt and precipitation. In addition, Biogeochemical Argo—the branch of the Argo program focused on floats with additional biological and chemical sensors—has recently shed light on topics such as regional patterns of carbon production and export, the magnitude of carbon dioxide air-sea flux in the Southern Ocean (thanks to the SOCCOM project), and the dynamics modulating ocean oxygen concentrations and oxygen minimum zones. While Argo data are publicly available in near-real-time via two Global Data Assembly Centers, there tends to be a steep learning curve for new users seeking to access and utilize the data.

To address this issue, a team led by scientists at NOAA’s Pacific Marine Environmental Laboratory developed a software toolbox available in two programming languages for accessing and visualizing Argo data— OneArgo-Mat for MATLAB and OneArgo-R for R. The toolbox includes functions to identify and download float data that adhere to user-defined time and space constraints, and other optional requirements like sensor type and data mode; plot float trajectories and their current positions; filter and manipulate float data based on quality flags and additional metadata; and create figures (profiles, time series, and sections) displaying physical, biological, and chemical properties measured by floats. Examples of figures created using the OneArgo-Mat toolbox are given below (Figure 1).

Figure 1. Example figures created using the OneArgo-Mat toolbox: (A) the trajectory of a float deployed in the North Atlantic from the R/V Johan Hjort in May of 2019, (B) a time series of dissolved oxygen at 80 dbars from that float, and (C) a vertical section plot of nitrate concentrations along the float track from the surface to 300 dbars. The black contour line in panel C denotes the mixed layer depth (MLD) based on a temperature criterion and the red line denotes the depth of the time series shown in panel B. The effects of seasonal phytoplankton blooms are evident in panel C, with mixed layer shoaling in the spring followed by drawdown of nitrate in the surface ocean. Panel B shows that, as the mixed layer deepens through the winter, the oxygen concentration at 80 dbars increases as a result of the oxygenated surface waters reaching that depth. The MATLAB code to download the required data and create all of these plots is shown (D).

The OneArgo-Mat and OneArgo-R toolboxes are intended for newcomers to Argo data, seasoned users, data managers, and everyone in between. For this reason, toolbox functions are equipped with options to streamline float selection, data processing, and figure creation with minimal user coding, if desired. Alternatively, the toolbox also provides rapid and straightforward access to the entire Argo database for experienced users who simply want to download up-to-date profile data for further processing and analysis. The authors hope these new tools will empower current Argo data users and entrain new users, especially as the US GO-BGC Project and US and international Argo partners move toward a global biogeochemical Argo fleet, which will create myriad new opportunities for novel studies of ocean biogeochemistry.

Authors
Jonathan Sharp – Cooperative Institute for Climate, Ocean, and Ecosystem Studies (CICOES) & NOAA Pacific Marine Environmental Laboratory (PMEL)
Hartmut Frenzel – CICOES & NOAA PMEL
Marin Cornec – University of Washington & NOAA PMEL
Yibin Huang – University of California Santa Cruz & NOAA PMEL
Andrea Fassbender – NOAA PMEL

Ocean Acidification drives shifts in global stoichiometry and carbon export efficiency

Posted by mmaheigan

· Friday, November 19th, 2021

Marine food webs and biogeochemical cycles react sensitively to increases in carbon dioxide (CO₂) and associated ocean acidification, but the effects are far more complex than previously thought. A comprehensive study published in Nature Climate Change by a team of researchers from GEOMAR dove deep into the impacts of ocean acidification on marine biota and biogeochemical cycling. The authors combined data from five large-scale field experiments with natural plankton communities to investigate how carbon cycling and export respond to ocean acidification.

The biological pump is a key mechanism in transferring carbon to the deep ocean and contributes significantly to the oceans’ function as a carbon sink. The carbon-to-nitrogen ratio of sinking biogenic particles, here termed (C:N_export), determines the amount of carbon that is transported from the euphotic zone to the ocean interior per unit nutrient, thereby controlling the efficiency of the biological pump. The authors demonstrate for the first time that ocean acidification can change the elemental composition of organic matter export, thereby potentially altering the biological pump and carbon sequestration in a future ocean (Figure 1).

Figure 1: Until now, the common assumption is that changes in C:N (and biogeochemistry, in general) are mainly driven by phytoplankton. In a series of in situ mesocosm experiments in different biomes (left), Taucher et al., (2020) found distinct impacts of ocean acidification on the C:N ratio of sinking organic matter (middle). By linking these observations to analysis of plankton community composition, the authors found a key role of heterotrophic processes in controlling the response of C:N to OA, particularly by altering the quality and carbon content of sinking organic matter within the biological pump (right).

Surprisingly, the observed responses were highly variable: C:N_export increased or decreased significantly with increasing CO₂, depending on the composition of species and the structure of the food web. The authors found that heterotrophic processes driven by bacteria and zooplankton play a key role in controlling the response of C:N_export to ocean acidification. This contradicts the widespread paradigm that primary producers are the principal driver of biogeochemical responses to ocean change.

Considering that such diverse pathways, by which planktonic food webs shape the elemental composition and biogeochemical cycling of organic matter, are not represented in state-of-the-art earth system models, these findings also raise the question: Are currently able to predict the large-scale consequences of ocean acidification with any certainty?

Authors:
Jan Taucher (GEOMAR, Kiel, Germany)
Tim Boxhammer (GEOMAR, Kiel, Germany)
Lennart T. Bach (University of Tasmania, Hobart, Australia)
Allanah J. Paul (GEOMAR, Kiel, Germany)
Markus Schartau (GEOMAR, Kiel, Germany)
Paul Stange (GEOMAR, Kiel, Germany)
Ulf Riebesell (GEOMAR, Kiel, Germany)

Contrasting N2O fluxes of source vs. sink in western Arctic Ocean during summer 2017

Posted by mmaheigan

· Wednesday, October 20th, 2021

During the western Arctic summer season both physical and biogeochemical features differ with latitude between the Bering Strait and Chukchi Borderland. The southern region (Bering Strait to the Chukchi Shelf) is relatively warm, saline, and eutrophic, due to the intrusion of Pacific waters that bring heat and nutrients in to the western Arctic Ocean (WAO). Because of the Pacific influence, the WAO is one of the most productive stretches of ocean in the world. In contrast, the northern region (Chukchi Borderland to the Canada Basin) is primarily influenced by freshwater originating from sea ice melt and rivers, and is relatively cold, fresh, and oligotrophic. A frontal zone exists between the southern region and northern region (~73°N) due to the distinct physicochemical contrast between mixing Pacific waters and freshwater. These regions support distinct bacterial communities also, making the environmental variations drivers extremely relevant to nitrous oxide (N₂O) dynamics.

A recent study published in Scientific Reports examined the role of the WAO as a source and a sink of atmospheric N₂O. There are obvious differences in N₂O fluxes between southern Chukchi Sea (SC) and northern Chukchi Sea (NC). In the SC (Pacific water characteristics dominate) N₂O emissions act as a net source to the atmosphere (Figure 1a). In the NC (freshwater dominant) absorption of atmospheric N₂O into the water column suggests that this region acts as a net sink (Figure 1a). The positive fluxes of SC occurred with relatively high sea surface temperature (SST), sea surface salinity (SSS), and biogeochemically-derived N₂O production, whereas the negative fluxes of NC were associated with relatively low SST, SSS, and little N₂O production. These linear relationships between N₂O fluxes and environmental variables suggest that summer WAO N₂O fluxes are remarkably sensitive to environmental changes.

Figure 1. (a) Map of the sampling stations using the Ice Breaking R/V Araon during August 2017. The sampling locations were coloured with N2O fluxes (blue to red gradient, see color bar; sink, air → sea (−), and source, sea → air (+). The southern Chukchi Sea (SC) extends from Bering Strait to Chukchi Shelf and the northern Chukchi Sea (NC) extends from Chukchi Borderland and Canada Basin. The frontal zone arises between SC and NC (black dotted line). (b) Illustration showing future changes in the distribution of the WAO N2O flux constrained by the positive feedback scenario of increasing inflow of Pacific waters and rapidly declining sea-ice extent under accelerating Arctic warming.

This study suggests a potential scenario for future WAO changes in terms of accelerating Arctic change. Increasing inflow of the Pacific waters and rapidly declining sea-ice extent are critical. The increasing inflow of warm nutrient-enriched Pacific waters will likely extend the SC N₂O source region northward, increasing productivity, and thereby intensifying nitrification. All of which would lead to a strengthening of the WAO’s role as an N₂O source. A rapid loss of the sea ice extent could ultimately lead to a sea-ice-free NC, and again, a northward shift, which would result in a diminished role of the NC as an N₂O sink (Figure 1b). While improving our understanding of WAO N₂O dynamics, this study suggests both a direction for future work and a clear need for a longer-term study to answer questions about both seasonal variations in these dynamics and possible interannual to climatological trends.

Authors:
Jang-Mu Heo (Department of Marine Science, Incheon National University)
Sang-Min Eom (Department of Marine Science, Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)
Hyo-Ryeon Kim (Department of Marine Science, Incheon National University)
Joo-Eun Yoon (Department of Marine Science, Incheon National University)
Il-Nam Kim (Department of Marine Science, Incheon National University)

Extreme events are accelerating coastal carbon cycling

Posted by mmaheigan

· Monday, March 1st, 2021

The world is getting stormier, and recent evidence shows significant impacts on coastal carbon cycling. The upticks in extreme weather events such as tropical cyclones have resulted in enhanced delivery of nutrients and organic matter across the land-ocean continuum. Lagoonal estuaries such as the Albemarle-Pamlico Sound (APS) in North Carolina and Galveston Bay in Texas are key coastal environments in which we can observe the long-term carbon cycling consequences of these events. Residence times of these coastal environments are on the order of months to over a year, providing ample opportunity for biogeochemical processing. Emerging from studies of Atlantic and Gulf of Mexico hurricanes in 2016 and 2017 is a clear example of the role of terrestrial dissolved organic carbon (DOC) as a key reactant driving the observed carbon cycling and ecosystem effects ( Figure 1).

Figure. 1. The impact of hurricanes on CO₂ fluxes (top) and terrestrial DOC decay constants (bottom) demonstrate the sustained effect on the coastal carbon cycle caused by extreme weather events. Top panel shows results from Hurricane Matthew in 2016, where date is month and day and Km downstream represents observations taken along the main axis of the Neuse River Estuary and lower Pamlico Sound, eastern North Carolina. F_CO2 is the daily sea-to-air flux of CO₂ estimated from measurements of temperature, salinity, dissolved inorganic carbon, and wind speed. The results indicate the Sound existed as a weak yet sustained CO2 source to the atmosphere well after the storm. Outgassing of CO₂ is driven by the rapid mineralization of terrestrial DOC. Bottom panel shows the high bioreactivity of flood-derived terrestrial DOC indicated by elevated microbial decay constants for Galveston Bay and the coastal Gulf of Mexico in 2017 as compared to high and low latitude coastal environments.

In coastal North Carolina, 36 tropical cyclones (TCs), including three floods of historical significance in the past two decades, have occurred in the past 20 years. The lingering effects of these storms include extensive periods of carbon dioxide (CO₂) supersaturation. For example, Hurricane Matthew in 2016 caused the lower Pamlico Sound to emit CO₂ for months after the passage of the storm. With similar results documented for the Pamlico Sound for storms in 2011 and 2012, there is solid evidence that shifts in the ecosystem state of this mesotrophic estuary from net autotrophic to net heterotrophic are a major effect of this process.

Reactive DOC from the landscape appears to be driving the shift in ecosystem state. Large plumes of brown-colored DOC are observable from space in numerous satellite images of the Atlantic and Gulf coasts following these storms. The color is part of a phenomenon known as “coastal darkening"—spectroscopic, stable isotopic, and biomarker evidence show this darkening is related to the flushing of wetlands in the flood-plain adjacent to the rivers draining into these estuaries.

Along the Texas coast, Hurricane Harvey produced the largest rainfall event recorded in US history and caused extensive flooding in 2017. Similar to results from coastal North Carolina, flood-derived terrestrial DOC in Galveston Bay exhibited high bioreactivity, with decay constants exceeding those observed for terrestrial DOC across coastal environments from high and low latitudes by almost three-fold. The rapid processing of terrestrial DOC was linked to an active microbial community capable of decomposing aromatic compounds that are abundant in colored DOC as indicated by genomic analyses. These recent studies clearly demonstrate the impacts of large storm events on coastal carbon cycling via the transport of reactive terrestrial DOC into coastal waters. Climate-driven increases in the frequency and intensity of such storm events warrant more sustained capacity to monitor episodic deliveries of carbon and nutrients and their impacts on coastal marine ecosystems.

Authors:
Chris Osburn (North Carolina State University) @closburn
Hans Paerl (University of North Carolina, Institute of Marine Sciences)
Ge Yan (Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences)
Karl Kaiser (Texas A&M University, Galveston Campus)

Citations:

Yan, G., Labonté, J. M., Quigg, A., & Kaiser, K. (2020). Hurricanes accelerate dissolved organic carbon cycling in coastal ecosystems. Frontiers in Marine Science, 7, 248.

Osburn, C. L., Rudolph, J. C., Paerl, H. W., Hounshell, A. G., & Van Dam, B. R. (2019). Lingering carbon cycle effects of Hurricane Matthew in North Carolina's coastal waters. Geophysical Research Letters, 46(5), 2654-2661.

Species loss alters ecosystem function in plankton communities

Posted by mmaheigan

· Monday, February 8th, 2021

Climate change impacts on the ocean such as warming, altered nutrient supply, and acidification will lead to significant rearrangement of phytoplankton communities, with the potential for some phytoplankton species to become extinct, especially at the regional level. This leads to the question: What are phytoplankton species’ redundancy levels from ecological and biogeochemical standpoints—i.e. will other species be able to fill the functional ecological and/or biogeochemical roles of the extinct species? Authors of a paper published recently in Global Change Biology explored these ideas using a global three-dimensional computer model with diverse planktonic communities, in which single phytoplankton types were partially or fully eliminated. Complex trophic interactions such as decreased abundance of a predator’s predator led to unexpected “ripples” through the community structure and in particular, reductions in carbon transfer to higher trophic levels. The impacts of changes in resource utilization extended to regions beyond where the phytoplankton type went extinct. Redundancy appeared lowest for types on the edges of trait space (e.g., smallest) or those with unique competitive strategies. These are responses that laboratory or field studies may not adequately capture. These results suggest that species losses could compound many of the already anticipated outcomes of changing climate in terms of productivity, trophic transfer, and restructuring of planktonic communities. The authors also suggest that a combination of modeling, field, and laboratory studies will be the best path forward for studying functional redundancy in phytoplankton.

Figure caption: Examples of the modelled ecological and biogeochemical responses to the extinction of different phytoplankton species.Figure caption: Examples of the modelled ecological and biogeochemical responses to the extinction of different phytoplankton species.

Authors:
Stephanie Dutkiewicz (Massachusetts Institute of Technology)
Philip W. Boyd (Institute for Marine and Antarctic Studies, University of Tasmania)
Ulf Riebesell (GEOMAR Helmholtz Centre for Ocean Research Kiel)

Ice sheets mobilize trace elements for export downstream

Posted by mmaheigan

· Thursday, January 7th, 2021

Trace elements are essential micronutrients for life in the ocean and also serve as valuable fingerprints of chemical weathering. The behaviour of trace elements in the ocean has gained interest because some of these elements are found at vanishingly low concentrations that limit ecosystem productivity. Despite delivering >2000 km³ yr^-1 of freshwater to the polar oceans, ice sheets have largely been overlooked as major trace element sources. This is partly due to a lack of data on meltwater endmember chemistry beneath and emerging from the Greenland and Antarctic ice sheets, which cover 10% of Earth’s land surface area, and partly because meltwaters were previously assumed to be dilute compared to most river waters.

In a study published in PNAS, authors analysed the trace element composition of meltwaters from the Mercer Subglacial Lake, a hydrologically active subglacial lake >1000 m below the surface of the Antarctic Ice Sheet, and a meltwater river emerging from beneath a large outlet glacier of the Greenland Ice Sheet (Leverett Glacier). These subglacial meltwaters (i.e., water travelling along the ice-rock interface beneath an ice mass) contained much higher concentrations of trace elements than anticipated. For example, typically immobile elements like iron and aluminium were observed in the dissolved phase (<0.45 µm) at much higher concentrations than in mean river or open ocean waters (up to 20,900 nM for Fe and 69,100 nM for Al), but exhibited large size fractionation between colloidal/nanoparticulate (0.02 – 0.45 µm) and soluble (<0.02 µm) size fractions (Figure 1). Subglacial physical and biogeochemical weathering processes are thought to mobilize many of these trace elements from the bedrock and sediments beneath ice sheets and export them downstream. Antarctic subglacial meltwaters were more enriched in dissolved trace elements than Greenland Ice Sheet outflow, which is likely due to longer subglacial residence times, lack of dilution from surface meltwater inputs, and differences in underlying sediment geology.

These results indicate that ice sheet systems can mobilize large quantities of trace elements from the land to the ocean and serve as major contributors to regional elemental cycles (e.g., coastal Southern Ocean). In a warming climate with increasing ice sheet runoff, subglacial meltwaters will become an increasingly dynamic source of micronutrients to coastal oceanic ecosystems in the polar regions.

Figure caption: Leverett Glacier (Greenland Ice Sheet) and Mercer Subglacial Lake (Antarctic Ice Sheet) dissolved elemental concentrations (<0.45 µm) normalized to mean non-glacial riverine trace element concentrations (Gaillardet et al., 2014) and major element concentrations (Martin and Meybeck, 1979). Grey regions indicate ±50 % of the riverine mean. Although major elements can be significantly depleted compared to non-glacial rivers, trace elements are commonly similar to or enriched.

Authors:
Jon R. Hawkings (Florida State Univ and German Research Centre for Geosciences)
Mark L. Skidmore (Montana State Univ)
Jemma L. Wadham (Univ of Bristol, UK)
John C. Priscu (Montana State Univ)
Peter L. Morton (Florida State Univ)
Jade E. Hatton (Univ of Bristol, UK)
Christopher B. Gardner (Ohio State Univ)
Tyler J. Kohler (École Polytechnique Fédérale de Lausanne, Switzerland)
Marek Stibal (Charles University, Prague, Czech Republic)
Elizabeth A. Bagshaw (Cardiff Univ, UK)
August Steigmeyer (Montana State Univ)
Joel Barker (Univ of Minnesota)
John E. Dore (Montana State Univ)
W. Berry Lyons (Ohio State Univ)
Martyn Tranter (Univ of Bristol, UK)
Robert G. M. Spencer (Florida State Univ)
SALSA Science Team

A new Regional Earth System Model of the Mediterranean Sea biogeochemical dynamics

Posted by mmaheigan

· Thursday, November 19th, 2020

The Mediterranean Sea is a semi-enclosed mid-latitude oligotrophic basin with a lower net primary production than the global ocean. A west-east productivity trophic gradient results from the superposition of biogeochemical and physical processes, including the biological pump and associated carbon and nutrient (nitrogen, phosphorus) fluxes, the spatial asymmetric distribution of nutrient sources (rivers, atmospheric deposition, coastal upwelling, etc.), the estuarine inverse circulation associated with the inflow of Atlantic water through the Gibraltar Strait. The complex and highly variable interface between land and sea throughout this basin add a further layer of complexity in the Mediterranean oceanic and atmospheric circulation and on the associated biogeochemistry dynamics, emphasizing the need for high-resolution truly integrated Regional Earth System Models to track and understand fine-scale processes and ecosystem dynamics.

In a recent paper published in the Journal of Advances in Modeling Earth System, the authors introduced a new version of the Regional Earth System model RegCM-ES and evaluated its performance in the Mediterranean region. RegCM-ES fully integrates the regional climate model RegCM4, the land surface scheme CLM4.5 (Community Land Model), the river routing model HD (Hydrological Discharge Model), the ocean model MITgcm (MIT General Circulation model) and the Biogeochemical Flux Model BFM.

A comparison with available observations has shown that RegCM-ES was able to capture the mean climate of the region and to reproduce horizontal and vertical patterns of chlorophyll-a and PO₄(the limiting nutrient in the basin) (Figure 1). The same comparison revealed a systematic underestimation of simulated dissolved oxygen (which will be fixed by the use of a new parametrization of oxygen solubility), and an overestimation of NO₃, possibly due to uncertainties in initial and boundary conditions (mostly traced to river and Dardanelles nutrient discharges) and an overly vigorous vertical mixing simulated by the ocean model in some parts of the Basin.

Figure.1 Distributions of chlorophyll-a mg/m³ (top) and PO₄ mmol/m³ (bottom) in the Mediterranean Sea as simulated by RegCM-ES.

Overall, this analysis has demonstrated that RegCM-ES has the capabilities required to become a powerful tool for studying regional dynamics and impacts of climate change on the Mediterranean Sea and other ocean basins around the world.

Authors:
Marco Reale (Abdus Salam International Centre for theoretical physics-ICTP, National Institute of Oceanography and Experimental Geophysics-OGS)
Filippo Giorgi (Abdus Salam International Centre for theoretical physics-ICTP)
Cosimo Solidoro (National Institute of Oceanography and Experimental Geophysics-OGS)
Valeria Di Biagio (National Institute of Oceanography and Experimental Geophysics-OGS)
Fabio Di Sante (Abdus Salam International Centre for theoretical physics-ICTP)
Laura Mariotti (National Institute of Oceanography and Experimental Geophysics-OGS)
Riccardo Farneti (Abdus Salam International Centre for theoretical physics-ICTP)
Gianmaria Sannino (Italian National Agency for New Technologies, Energy and Sustainable Economic Development-ENEA)

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)

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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