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A new proxy for ocean iron bioavailability

Posted by mmaheigan

· Monday, July 26th, 2021

In many oceanic regions, iron exerts strong control on phytoplankton growth, ecosystem structure and carbon cycling. Yet, iron bioavailability and uptake rates by phytoplankton in the ocean are poorly constrained.

Recently, Shaked et al. (2020) (see GEOTRACES highlight), established a new approach for quantifying the availability of dissolved Fe (dFe) in natural seawater based on its uptake kinetics by Fe-limited cultured phytoplankton. In a follow up study published in GBC, this approach was extended to in situ phytoplankton, establishing a standardized proxy for dFe bioavailability in low-Fe ocean regions.

As explained in the short video lecture above, Yeala Shaked, Ben Twining, and their colleagues have analyzed large datasets collected during 10 research cruises (including 3 GEOTRACES section and process cruises) in multiple ocean regions. Dissolved Fe bioavailability was estimated through single cell Fe uptake rates, calculated by combining measured Fe contents of individual phytoplankton cells collected with concurrently-measured dFe concentrations, as well as modeled growth rates (Figure). Then the authors applied this proxy for: a) comparing dFe bioavailability among organisms and regions; b) calculating dFe uptake rates and residence times in low-Fe oceanic regions; and c) constraining Fe uptake parameters of earth system models to better predict ocean productivity in response to climate-change.

The data suggest that dFe species are highly available in low-Fe settings, likely due to photochemical reactions in sunlit waters.

Figure 1: The new bioavailability proxy (an uptake rate constant-kin-app) was calculated for ~1000 single cells from multiple ocean regions. For each cell, the iron quota was measured with synchrotron x-ray fluorescence (left panel), a growth rate was estimated from the PISCES model for the corresponding phytoplankton group (right panel), and the dissolved Fe concentration was measured concurrently (middle panel).

Authors:
Y. Shaked (Hebrew University and Interuniversity Institute for Marine Sciences)
B.S. Twining (Bigelow Lab)
A. Tagliabue (University of Liverpool)
M.T. Maldonado (University of British Columbia)
K.N. Buck (University of South Florida)
T. Mellett (University of South Florida)

References:
Shaked, Y., Twining, B. S., Tagliabue, A., & Maldonado, M. T. (2021). Probing the bioavailability of dissolved iron to marine eukaryotic phytoplankton using in situ single cell iron quotas. Global Biogeochemical Cycles, e2021GB006979. https://doi.org/10.1029/2021GB006979

Shaked, Y., Buck, K. N., Mellett, T., & Maldonado, M. T. (2020). Insights into the bioavailability of oceanic dissolved Fe from phytoplankton uptake kinetics. The ISME Journal, 1–12. https://doi.org/10.1038/s41396-020-0597-3

Joint highlight with GEOTRACES – read here.

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)

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)

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)

Timing matters: Correcting float-based measurements of diurnal oxygen variability

Posted by mmaheigan

· Friday, November 6th, 2020

Despite its fundamental importance to the global carbon cycle, climate, and marine ecosystems, oceanic primary production is grossly under-sampled. Autonomous platforms represent an important frontier for expanding measurements of marine primary productivity in time and space, but this requires the establishment of robust, standardized methods to obtain reliable data from these platforms. Using data from profiling floats deployed in the northern Gulf of Mexico, authors of a recent study published in Biogeosciences demonstrated, for the first time, that daily cycles of dissolved oxygen can be observed with Argo-type profiling floats. The floats were instructed to profile continuously, resulting in about one profile every three hours. The floats recorded data both on the ascent (upcast) and the descent (downcast). Adjacent casts showed hysteresis in gradient areas, i.e. a lag in the concentration measurement, due to the slow response time of oxygen sensors.

Figure 1: Example of raw oxygen measurements from a downcast (dark purple line) and an upcast (dark green line) and corrected profiles (lighter purple and green lines) in (a) density and (b) pressure coordinates. (c) Upcasts and downcasts (top 150 m) plotted against each other with raw data (purple) and data corrected according to the new method (red). (d) The root-mean-square difference (RMSD) between the upcast and downcast after correcting casts for a range of time constants (τ), showing an optimal τ value in this case of 76 s (red dot).

To correct for these measurement errors, the authors developed a method to determine sensor response time in situ, using an established process for correcting sensor response time errors. This method requires a timestamp associated with each observation. The response time parameter (τ) was determined by correcting consecutive profiles taken in opposite directions using a range of possible values and finding the minimum root-mean-square-difference between them (Figure 1). In light of these findings, future oxygen measurements from Argo floats should be transmitted with time stamps for a calibration period during which up- and downcasts are recorded to facilitate response time correction. The method developed here will contribute to more accurate measurement of dissolved oxygen, thus improving the quality of derived quantities such as primary productivity.

Authors
Christopher Gordon (Dalhousie University)
Katja Fennel (Dalhousie University)
Clark Richards (Fisheries and Oceans Canada)
Nick Shay (University of Miami)
Jodi Brewster (University of Miami)

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)

Estuarine sediment resuspension drives non-local impacts on biogeochemistry

Posted by mmaheigan

· Friday, September 18th, 2020

Sediment processes, including resuspension and transport, affect water quality in estuaries by altering light attenuation, primary productivity, and organic matter remineralization, which then influence oxygen and nitrogen dynamics. In a recent paper published in Estuaries and Coasts, the authors quantified the degree to which sediment resuspension and transport affected estuarine biogeochemistry by implementing a coupled hydrodynamic-sediment transport-biogeochemical model of the Chesapeake Bay. By comparing summertime model runs that either included or neglected seabed resuspension, the study revealed that resuspension increased light attenuation, especially in the northernmost portion of the Bay, which subsequently shifted primary production downstream (Figure 1). Resuspension also increased remineralization in the central Bay, which experienced higher organic matter concentrations due to the downstream shift in primary productivity. When combined with estuarine circulation, these resuspension-induced shifts caused oxygen to increase and ammonium to increase throughout the Bay in the bottom portion of the water column. Averaged over the channel, resuspension decreased oxygen by ~25% and increased ammonium by ~50% for the bottom water column. Changes due to resuspension were of the same order of magnitude as, and generally exceeded, short-term variations within individual summers, as well as interannual variability between wet and dry years. This work highlights the importance of a localized process like sediment resuspension and its capacity to drive biogeochemical variations on larger spatial scales. Documenting the spatiotemporal footprint of these processes is critical for understanding and predicting the response of estuarine and coastal systems to environmental changes, and for informing management efforts.

Figure 1: Schematic of how resuspension affects biogeochemical processes based on HydroBioSed model estimates for Chesapeake Bay.

Authors:
Julia M. Moriarty (University of Colorado Boulder)
Marjorie A. M. Friedrichs (Virginia Institute of Marine Science)
Courtney K. Harris (Virginia Institute of Marine Science)

Also see the Geobites piece “Muddy waters lead to decreased oxygen in Chesapeake Bay” on this publication, by Hadley McIntosh Marcek

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)

Surface bacterial communities respond to rapid changes in the western Arctic

Posted by mmaheigan

· Tuesday, January 7th, 2020

During the western Arctic summer open water season, latitudinal differences in the physical and biogeochemical features of the surface water are apparent from the Bering Strait to the Chukchi Borderland. Lower latitude regions (i.e. Bering Strait to Chukchi Shelf) are primarily driven by the inflow of Pacific waters that supply nutrients and heat, leading to high primary production. Conversely, the higher latitude regions (i.e. Chukchi Borderland and Canada Basin) are relatively cold, fresh, and oligotrophic because the surface layer is influenced by freshwater inputs from melting ice and rivers via the Beaufort Gyre. Mixing of the two surface water masses in the western Arctic produces a physicochemical frontal zone (FZ) in the Chukchi Sea.

In a recent study published in Scientific Reports, authors used observations from summer 2017 to investigate latitudinal variations in bacterial community composition in surface waters between the Bering Strait and Chukchi Borderland and the underlying processes driving the changes. Results indicate three distinctive communities: 1) Southern Chukchi (SC) bacterial communities are associated with nutrient-rich conditions, including genera such as Sulfitobacter; 2) a northern Chukchi (NC) bacterial community that dominated by SAR clades, Flavobacterium, Paraglaciecola, and Polaribacter, genera associated with low nutrients and sea ice conditions. If climate-driven changes in the western Arctic continue along the same trajectory, it’s likely we will see altered bacterial communities. If the impact of warm, nutrient-rich Pacific water inflows dominates, it is likely that the productive SC region will expand and the FZ will move northward, leading to nutrient enrichment in the western Arctic (Figure 1). In response, bacterial communities would be dominated by organic matter decomposers, such as Sulfitobacter, due to high primary productivity. However, if the impact of sea-ice meltwater dominates, then the oligotrophic NC region will expand and the FZ will move southward, leading to nutrient depletion in western Arctic surface waters (Figure 1). Continued monitoring in this region will enhance our understanding of how bacterial communities respond (Figure 1b) to a rapidly changing western Arctic Ocean.

Figure 1. (a) Map of the August 2017 Ice Breaker RV Araon western Arctic Ocean sampling stations used in this study. The basemap shows the Chl-a concentration contour (blue to red background colors). Pink, green, and blue circles represent stations in the South Chukchi (SC), Frontal Zone (FZ), and Northern Chukchi (NC) regions. (b) Schematic diagram of surface bacterial community distribution in response to future western Arctic Ocean changes.

Authors:
Il-Nam Kim (Department of Marine Science, Incheon National University)
Sung-Ho Kang (Korea Polar Research Institute)
Eun Jin Yang (Korea Polar Research Institute)

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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