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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)

pH: the secrets that you keep

Posted by mmaheigan

· Monday, September 20th, 2021

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

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

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

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

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

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.

Austral summer vertical migration patterns in Antarctic zooplankton

Posted by mmaheigan

· Thursday, October 15th, 2020

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

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

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

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

Will global change “stress out” ocean DOC cycling?

Posted by mmaheigan

· Tuesday, September 29th, 2020

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

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

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

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

Sea ice loss and the changing Arctic carbon cycle

Posted by mmaheigan

· Friday, September 18th, 2020

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

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

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

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

Profiling floats reveal fate of Southern Ocean phytoplankton stocks

Posted by mmaheigan

· Tuesday, September 1st, 2020

More observations are needed to constrain the relative roles of physical (advection), biogeochemical (downward export), and ecological (grazing and biological losses) processes in driving the fate of phytoplankton blooms in Southern Ocean waters. In a recent paper published in Nature Communications, authors used seven Biogeochemical Argo (BGC-Argo) floats that vertically profiled the upper ocean every ten days as they drifted for three years across the remote Sea Ice Zone of the Southern Ocean. Using the floats’ biogeochemical sensors (chlorophyll, nitrate, and backscattering) and regional ratios of nitrate consumption:chlorophyll synthesis, the authors developed a new approach to remotely estimate the fate of the phytoplankton stocks, enabling calculations of herbivory and of downward carbon export. The study revealed that the major fate of phytoplankton biomass in this region is grazing, which consumes ~90% of stocks. The remaining 10% is exported to depth. This pattern was consistent throughout the entire sea ice zone where the floats drifted, from 60°-69° South.

Figure Caption: Southern Ocean Chlorophyll a climatology and floats’ trajectories (top panel). Total losses of Chlorophyll a (including grazing and phytodetritus export, left panel). Phytodetritus export (right panel).

This study region comprises two of the three major krill growth and development areas—the eastern Weddell and King Haakon VII Seas and Prydz Bay and the Kerguelen Plateau—so the observed grazing was probably due to Antarctic krill, underscoring their pivotal importance in this ecosystem. Building upon the greater understanding of ocean ecosystems via satellite ocean colour development in the 1990s, BGC-Argo floats and this new approach will allow remote monitoring of the different fates of phytoplankton stocks and insights into the status of the ecosystem.

Authors:
Sebastien Moreau (Norwegian Polar Institute, Tromsø, Norway)
Philip Boyd (Institute for Marine and Antarctic Studies, Hobart, Australia)
Peter Strutton (Institute for Marine and Antarctic Studies, Hobart, Australia)

Space-based estimates of estuarine dissolved organic carbon flux to the Mid-Atlantic Bight

Posted by mmaheigan

· Wednesday, August 5th, 2020

Dissolved organic carbon (DOC) is a food supplement that supports microorganism growth and plays a major role in the global carbon cycle via the microbial loop, which integrates DOC into the marine food web. DOC from two major estuaries on the US East Coast, Chesapeake (CB) and Delaware Bay (DB), represent major contributors to the adjacent shelf region’s carbon cycle. In a recent study published in Journal of Geophysical Research: Oceans, the authors combined an integrated tracer flux approach, field and satellite data, machine learning, and a physical circulation model to quantify DOC stocks and export fluxes at the mouths of CB and DB.

Figure 1: Model bathymetry for the CB and DB models (a). Twelve‐year (2003–2014) mean MODIS DOC for DB (b) and CB (c) with ROMS grid lines superposed in white and land mask in black. The white dots across the bay mouths are the grid points used in the flux computation. The squares in (a) correspond to the size (50 km × 50 km) and location of the DB and CB MODIS images shown in (b) and (c). The boxes near DB mouth in (b) delimit the cluster of available in situ data stations. The red star, red square, and red diamond near CB mouth in (c) are the locations of in situ data for validation.

Figure 2: Five‐year averaged cross-sections of DOC concentration (top), velocity, and DOC flux at the mouths of Chesapeake Bay (a–c, respectively) and Delaware Bay (d–f, respectively).

This novel methodology not only improved estimates of combined DB-CB DOC fluxes to the US East Coast, but it also improved quantification of contrasting estuarine properties that affect DOC export such as riverine inputs, timescales of variability, and geomorphology. The combined CB-DB DOC contribution represents 25% of the total organic carbon exported and 27% of the total atmospheric carbon dioxide taken up by the Mid-Atlantic Bight (MAB)—the coastal region extending from Massachusetts to North Carolina. Spatial and, to a lesser extent, temporal covariations of velocity and DOC concentration contributed to the fluxes. The primary drivers of DOC flux differences for these two estuaries are their geomorphologies and volumes of freshwater discharge into the bays (74 billion m³/year for CB and 21 billion m³/year for DB). Terrestrial DOC inputs are similar to the export of DOC at the bay mouths at annual and longer timescales, but diverge significantly at shorter timescales of days to months.

The five-year mean DOC flux for CB and DB are 0.21 (confidence intervals 0.15, 0.27) Tg C/year and 0.05 (0.04, 0.07) Tg C/year, respectively. A flux decomposition analysis showed that temporal and spatial covariations in the DOC flux at the mouth of both bays play a significant role in determining the net export of DOC from the estuaries, which suggests that accurate estimates of estuarine DOC export require information on scales that properly resolve the temporal and spatial variability of water flux and DOC concentration. Neglecting these temporal and spatial covariations in the DOC flux leads to a 40% underestimation of the DOC flux in CB and 28% in DB, which would have a significant impact on the accuracy of carbon budget assessments and the role that these estuaries have on the coastal environment. This combination of satellite and field observations with statistical and numerical models shows great promise for capturing these covariations to better quantify the role of estuaries in the coastal carbon cycle.

Authors:
Sergio R. Signorini (NASA, Goddard Space Flight Center)
Antonio Mannino (NASA, Goddard Space Flight Center)
Marjorie A.M. Friedrichs (VIMS, William and Mary)
Pierre St-Laurent (VIMS, William and Mary)
John Wilkin (Rutgers University)
Aboozar Tabatabai (Rutgers University)
Raymond G. Najjar (The Pennsylvania State University)
Eileen E. Hofmann (Old Dominium University)
Fei Da (VIMS, College of William and Mary)
Hanqin Tian (Auburn University)
Yuanzhi Yao (Auburn University)

Global change impacts soil carbon storage in blue carbon ecosystems

Posted by mmaheigan

· Wednesday, May 20th, 2020

Vegetated coastal “blue carbon” ecosystems, including sea grasses, mangroves, and salt marshes, provide valuable ecosystem services such as carbon sequestration, storm protection, critical habitat, etc.. Many of these services are supported by the ability of blue carbon ecosystems to accumulate soil organic carbon over thousands of years. Rapidly changing climate and environmental conditions will impact decomposition and thus the global reservoir of organic carbon in coastal soils. A recent Perspective article published in Nature Geoscience focused on the biogeochemical factors affecting decomposition in coastal soils, such as mineral protection, redox zonation, water content and movement, and plant-microbe interactions. The authors explored the spatial and temporal scales of these decomposition mechanisms and developed a conceptual framework to characterize how they may respond to environmental disturbances such as land-use change, nutrient loading, warming, and sea-level rise.

Figure caption: Temperate salt marshes (MA, USA). Healthy salt marshes have lush stands of grasses (top). Storms can expose peat deposits that have been buried for thousands of years (bottom). The fate of this soil carbon is unknown, but some fraction will be respired by microbes and returned to the atmosphere as CO₂.

Improved estimates of soil organic carbon in blue carbon systems will require better characterization of these processes from sustained data sets. Furthermore, incorporation of these decomposition mechanisms into ecosystem evolution models will improve our capacity to quantify and predict changes in these soil carbon reservoirs, which could facilitate their inclusion in global budgets and management tools.

Temperate salt marshes (MA, USA). Healthy salt marshes have lush stands of grasses (left/top). Storms can expose peat deposits that have been buried for thousands of years (right/bottom). The fate of this soil carbon is unknown, but some fraction will be respired by microbes and returned to the atmosphere as CO₂.

Authors:
Amanda C Spivak (University of Georgia)
Jon Sanderman (Woods Hole Research Center)
Jennifer Bowen (Northeastern University)
Elizabeth A. Canuel (Virginia Institute of Marine Science)
Charles S Hopkinson (University of Georgia)

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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