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South Pacific particulate organic carbon fate challenges Martin’s Law

Posted by mmaheigan

· Tuesday, May 14th, 2019

Joint science highlight with GEOTRACES

Carbon storage in the ocean is sensitive to the depths at which particulate organic carbon (POC) is respired back to CO₂ within the twilight zone (100-1000m). For decades, it has been an oceanographic priority to determine the depth scale of this regeneration process. To investigate this, GEOTRACES scientists are deploying new isotopic tools that provide a high-resolution snapshot of POC flux and regeneration across steep biogeochemical gradients in the South Pacific Ocean.

A recent paper in PNAS reported on particulate organic carbon (POC) fluxes throughout the water column (focusing on the upper 1000 m) along the GP16 GEOTRACES section between Peru and Tahiti (Figure 1A). POC fluxes (Figure 1B) were derived by normalizing concentrations of POC to ²³⁰Th following analysis of samples collected by in situ filtration. This work builds on a research theme initiated at the GEOTRACES-OCB synthesis workshop held at Lamont-Doherty Earth Observatory in 2016.

Figure caption: Site map and POC flux characteristics from GEOTRACES GP16 section. Plot A) shows the GP16 station locations as white circles, with nearby sediment trap deployments as black stars, with 2013 MODIS satellite-derived net primary productivity in the background. Plot B) shows POC fluxes from particulate 230Th-normalization from selected stations spanning the zonal extent of the GP16 section. Plot C) shows power law exponent b values for each GP16 station (blue), compared to estimates from bottom-moored sediment traps in the South Pacific (black and red dashed lines), a compilation of sediment traps in the North Pacific (green dashed line), and neutrally buoyant sediment traps in the subtropical North Pacific (yellow shaded band). GP16 regeneration length scales from 230Th-normalization agree most closely with the estimates from neutrally buoyant sediment traps.

The study results show that POC regeneration depth is shallower than anticipated, especially in warm stratified waters of the subtropical gyre. Regeneration depth—expressed in terms of the Martin-curve power-law exponent “b” (Figure 1C)—is shown to be greater than previous estimates (horizontal dashed lines), but similar to values obtained using neutrally buoyant sediment traps at the Hawaii Ocean Time-series Station Aloha. In contrast to the rapid regeneration of POC in warm stratified waters, POC regeneration within the ODZ is below our detection limits. Models have shown that shallower regeneration of POC leads to less efficient carbon storage in the ocean, making the authors speculate that global warming, yielding expanded and more stratified gyres, may induce a reduction of the ocean’s efficacy for carbon storage via the biological pump.

Authors:
Frank J. Pavia, Robert F. Anderson, Sebastian M. Vivancos, Martin Q. Fleisher (Columbia University)
Phoebe J. Lam (University of California Santa Cruz)
B.B. Cael (now at University of Hawai’i Manoa, formerly at MIT)
Yanbin Lu, Pu Zhang, R. Lawrence Edwards (University of Minnesota)
Hai Cheng (University of Minnesota and Xi’an Jiaotong University)

Antarctic Ocean CO2 helped end the ice age

Posted by mmaheigan

· Tuesday, April 2nd, 2019

Many scientists have long hypothesized that the ocean around Antarctica was responsible for changing CO₂ levels during ice ages, but lacked definitive evidence. A new study in Nature provides the most direct evidence of this process to date and provides crucial evidence of the mechanisms—including changing sea ice cover and bipolar seesaw (warming in the Southern Hemisphere during cooling in the Northern Hemisphere) events—that controlled CO₂ and climate during the ice ages.

Using samples of fossil deep-sea corals collected from 1000 m in the Drake Passage (Figure 1a), the authors were able to reconstruct the CO₂ content of the deep ocean. They found that the deep ocean CO₂ record was the “mirror image” of CO₂ in the atmosphere (Figure 1b), with the ocean storing CO₂ during an ice age and releasing it back to the atmosphere during deglaciation. CO₂ rise during the last ice age occurred in a series of steps and jumps associated with intervals of rapid climate change.

As well as helping scientists better understand the ice ages, the new findings also provide context to current CO₂ rise and climate change. Although the CO₂ rise that helped end the last ice age was dramatic in geological terms, CO₂ rise due to human activity over the last 100 years is even larger and about 100 times faster. CO₂ rise at the end of the ice age helped drive major melting of ice sheets resulting in sea level rise of >100 meters. These results bolster the idea that if we want to prevent dangerous levels of global warming and sea level rise in the future, we need to reduce CO₂ emissions as quickly as possible

Authors:
J. W. B. Rae (University of St Andrews, UK)
A. Burke (University of St Andrews, UK)
L. F. Robinson (University of Bristol, UK)
J. F. Adkins (California Institute of Technology)
T. Chen (University of Bristol, UK, Nanjing University, China)
C. Cole (University of St Andrews, UK)
R. Greenop (University of St Andrews, UK)
T. Li (University of Bristol, UK, Nanjing University, China)
E. F. M. Littley (University of St Andrews, UK)
D. C. Nita (University of St Andrews, UK, Babes-Bolyai University, Romania)
J. A. Stewart (University of St Andrews, UK, University of Bristol, UK)
B. J. Taylor (University of St Andrews, UK)

Rapid warming and salinity changes mask acidification in Gulf of Maine waters

Posted by mmaheigan

· Wednesday, February 20th, 2019

Why don’t we see ocean acidification in over a decade of high-frequency observations in the Gulf of Maine? The answer lies in a recent decade of changes that raised sea surface temperature and salinity, and in turn dampened the expected acidification signal and caused the saturation states of calcite minerals to increase. From 2004 to 2014, sea surface temperatures in the Gulf of Maine were higher than any observations recorded in the region over the past 150 years. This greatly impacted both CO₂ solubility and the sea surface carbonate system, as detailed in a recent paper in Biogeochemistry.

Over the 34 years of the time-series, the recent event is extreme, but interannual and decadal salinity and temperature variability also influenced carbonate system parameters, which makes it difficult to isolate and quantify an anthropogenic ocean acidification signal, especially if relying on shorter-term observations (Figure 1).

Figure 1: Modeled Ω_Aragonite (top panel) and pH (bottom panel) anomalies relative to monthly 2004 data. The red lines show trends prior to and after 2004, after which warming accelerated.

For those with a stake in profiting from or managing extractive resources that are susceptible to ocean acidification such as commercially important lobster and bivalves, understanding how ecosystems will be affected is critical. These analyses clearly demonstrate how physical processes can either accelerate or mitigate ocean carbonate system changes, thus confounding the detection of ocean acidification that is expected from increasing atmospheric carbon dioxide. To assess whether an ecosystem or species is at risk or aided by such processes, it is important to observe, understand, and be able to model all sources of carbonate system variability.

Authors:
Joe Salisbury and Bror Jönsson (Both at Ocean Processes Analysis Laboratory, University of New Hampshire)

The past, present, and future of artificial ocean iron fertilization experiments

Posted by mmaheigan

· Wednesday, January 23rd, 2019

Since the beginning of the industrial revolution, human activities have greatly increased atmospheric CO₂ concentrations, leading to global warming and indicating an urgent need to reduce global greenhouse gas emissions. The Martin (or iron) hypothesis suggests that ocean iron fertilization (OIF) could be a low-cost effective method for reducing atmospheric CO₂ levels by stimulating carbon sequestration via the biological pump in iron-limited, high-nutrient, low-chlorophyll (HNLC) ocean regions. Given increasing global political, social, and economic concerns associated with climate change, it is necessary to examine the validity and usefulness of artificial OIF (aOIF) experimentation as a geoengineering solution.

Figure 1. (a) Global annual distribution of surface chlorophyll concentrations (mg m^-3) with locations of 13 aOIF experiments. Maximum and initial values in (b) maximum quantum yield of photosynthesis (Fv/Fm ratios) and (c) chlorophyll-a concentrations (mg m^-3) during aOIF experiments. (d) Changes in primary productivity (ΔPP = [PP]_{post-fertilization (postf)} ‒ [PP]_{pre-fertilization (pref)}; mg C m^-2 d^-1). (e) Distributions of chlorophyll-a concentrations (mg m^-3) on day 24 after iron addition in the Southern Ocean iron experiment-north (SOFeX-N) from MODIS Terra Level-2 daily image and on day 20 in the SOFeX-south (SOFeX-S) from SeaWiFS Level-2 daily image (white dotted box indicates phytoplankton bloom during aOIF experiments). (f) Changes in nitrate concentrations (ΔNO₃^– = [NO₃^–]_postf ‒ [NO₃^–]_pref; μM). (g) Changes in partial pressure of CO₂ (ΔpCO₂ = [pCO₂]_postf ‒ [pCO₂]_pref; μatm). The color bar indicates changes in dissolved inorganic carbon (ΔDIC = [DIC]_postf ‒ [DIC]_pref; μM). The numbers on the X axis indicate the order of aOIF experiments as given in Figure 1a and are grouped according to ocean basins; Equatorial Pacific (EP) (yellow bar), Southern Ocean (SO) (blue bar), subarctic North Pacific (NP) (red bar), and subtropical North Atlantic (NA) (green bar).

A review paper published in Biogeosciences on aOIF experiments provides a thorough overview of 13 scientific artificial OIF experiments conducted in HNLC regions over the last 25 years. These aOIF experiments have demonstrated that iron addition stimulates substantial increases in phytoplankton biomass and primary production, resulting in drawdown of macro-nutrients and dissolved inorganic carbon (Figure 1). Many of the aOIF experiments have also precipitated community shifts from smaller (pico- and nano-) to larger (micro) phytoplankton. However, the impact on the net transfer of CO₂ from the atmosphere to below the winter mixed layer via the biological pump is not yet fully understood or quantified and appears to vary with environmental conditions, export flux measurement techniques, and other unknown factors. These results, including possible side effects, have been debated among those who support and oppose aOIF experimentation, and many questions remain about the effectiveness of scientific aOIF, possible side effects, and international aOIF law frameworks. Therefore, it is important to continue undertaking small-scale, scientifically controlled studies to better understand natural processes in the HNLC regions, assess the associated risks, and lay the groundwork for evaluating the potential effectiveness and impacts of large-scale aOIF as a geoengineering solution to anthropogenic climate change. Additionally, this paper suggests considerations for the design of future aOIF experiments to maximize the effectiveness of the technique and begin to answer open questions under international aOIF regulations.

Authors:
Joo-Eun Yoon (Incheon National University)
Il-Nam Kim (Incheon National University)
Alison M. Macdonald (Woods Hole Oceanographic Institution)

Efficient carbon drawdown allows for a high future carbon uptake in the North Atlantic

Posted by mmaheigan

· Wednesday, November 7th, 2018

As one of the major carbon sinks in the global ocean, the North Atlantic is a key player in mediating and ameliorating the ongoing global warming. Current projections of the North Atlantic carbon sink in a high-CO₂ future vary greatly among models, with some showing that a slowdown in carbon uptake has already begun and others predicting that this slowdown will not occur until nearly 2100. To ensure the credibility of future projections as needed for finding adequate mitigation strategies, it is important to address and reduce this uncertainty.

Percentage of anthropogenically altered carbon stored in the deep North Atlantic (left panel) and North Atlantic uptake of anthropogenically altered carbon (right panel) as simulated by 11 different Earth System Models for a high CO2-future. Black x and line in left panel mark the observational estimate and its uncertainties, while blue and red shading reflect model spread, including models that simulate deep ocean storage within (blue) and outside (red) these observational uncertainties.

A new study in the Journal of Climate identified some of the mechanisms behind this ambiguity by analyzing the output of 11 Earth System Models for a high-CO₂ future. The authors show that discrepancies among models largely originate around high-latitude gateways from the surface to the deep ocean. Through biological production, deep convection and subsequent transport via the deep western boundary current, these gateways remove carbon from the upper ocean. When enough carbon is removed to maintain a lower oceanic pCO₂ relative to atmospheric pCO₂, the ocean continues to take up carbon. The study reveals that the fraction of anthropogenic carbon that is stored below 1000 m depth is not only an indicator of current carbon removal from the upper ocean but also a predictor of future ocean carbon uptake. When models that lie outside the range of observational uncertainties in deep carbon storage (red shading, left panel of figure) were excluded, revised projections showed higher North Atlantic carbon uptake in the future with lower associated uncertainties (blue shading, right panel of figure). This result highlights the need to depart from the concept of more or less randomly chosen models when reporting on future projections and their uncertainties. Results that are more reliable and hence of better use for mitigation strategies can be gained by focusing solely on models that simulate key mechanisms within observational uncertainties.

Authors:
N. Goris (Bjerknes Centre for Climate Research, Norway)
J.F. Tjiputra (Bjerknes Centre for Climate Research, Norway)
A. Olsen (University of Bergen and Bjerknes Centre for Climate Research, Norway)
J. Schwinger (Bjerknes Centre for Climate Research, Norway)
S.K. Lauvset (Bjerknes Centre for Climate Research, Norway)
E. Jeansson (Bjerknes Centre for Climate Research, Norway)

When marine-terminating glaciers ‘pump’ the ocean

Posted by mmaheigan

· Wednesday, October 10th, 2018

How will increasing meltwater from Greenland affect the biogeochemistry of the ocean? Release of meltwater into the ocean has physical and biogeochemical effects on the local water column. With respect to nutrient availability, meltwater supplies the bioessential nutrients iron and silicic acid but is deficient in nitrate and phosphate. However, despite very low meltwater nitrate and phosphate concentrations, pronounced summertime phytoplankton blooms are observed in many, though not all, of Greenland’s large fjord systems. These unusual summertime blooms are associated with meltwater from marine-terminating glaciers. So if the meltwater itself is not supplying nitrate and phosphate that these blooms require, what is the source of the nutrients that support these blooms?

An illustration of how changing the depth of a glacier affects downstream productivity

A recent study published in Nature Communications shows that when meltwater is released below sea level under large marine-terminating glaciers, it rises rapidly towards the surface in buoyant discharge plumes. As these plumes rise, they entrain large quantities of deep, nutrient-rich seawater. This vertical transport, or ‘pumping’, of these nutrients to the surface sustains unusually high summertime productivity in Greenland’s fjords. Conversely, when meltwater is released at the ocean surface, primary production is reduced because the meltwater itself lacks the nitrate and phosphate required to fuel phytoplankton blooms. Consequently, the inland retreat of Greenland’s large marine-terminating glaciers is ultimately bad news for summertime marine phytoplankton communities. As the depth of the marine-terminating glaciers shoals, their associated nutrient ‘pumps’ collapse, which will likely have negative effects on primary production and associated inshore fisheries.

Authors:
M.J. Hopwood (GEOMAR)
D. Carroll (Jet Propulsion Laboratory)
T.J. Browning (GEOMAR)
L. Meire (Royal Netherlands Institute for Sea Research and Greenland Climate Research Centre)
J. Mortensen (Greenland Climate Research Centre)
S. Krisch (GEOMAR)
E.P. Achterberg (GEOMAR)

Marine Snowfall at the Equator

Posted by mmaheigan

· Thursday, July 19th, 2018

The continual flow of organic particles such as dead organisms and fecal material towards the deep sea is called “marine snow,” and it plays an important role in the ocean carbon cycle and climate-related processes. This snowfall is most intense where high primary production can be observed near the surface. This is the case along the equator in the Pacific and Atlantic Oceans. However, it is not well known how particles are distributed at depth and which processes influence this distribution. A recent study published in Nature Geoscience involved the use of high-resolution particle density data using the Underwater Vision Profiler (UVP) from the equatorial Atlantic and Pacific Oceans down to a depth of 5,000 meters, revealing that several previously accepted ideas on the downward flux of particles into the deep sea should be revisited.

Figure 1. The Underwater Vision Profiler (UVP) during a trial in the Kiel Fjord. The UVP provided crucial data for the new study. Photo: Rainer Kiko, GEOMAR

It is typically assumed that the largest particle density can be found close to the surface and that density attenuates continuously with depth. However, high-resolution particle data show that density increases again in the 300-600-meter depth range. The authors attribute this observation to the daily migratory behavior of organisms such as zooplankton that retreat to these depths during the day, contributing to the particle load via defecation and mortality.

Another surprising result is the observation of many small particles below 1,000 meters depth that contribute a large fraction of the bathypelagic particle flux. This observation counters the general assumption, especially in many biogeochemical models, that particle flux at depth comprises fast sinking particles such as fecal pellets. Diminished remineralization rates of small particles or increased disaggregation of larger particles may contribute to the elevated small particle fluxes at this depth.

Figure 2. Zonal current velocity and Particulate Organic Carbon (POC) content across the equatorial Atlantic at 23˚W as observed in November 2012. From left to right: Zonal current velocity, POC content in small particle fraction and POC content in large particle fraction (adapted from Kiko et al. 2017).

This study highlights the importance of coupled biological and physical processes in understanding and quantifying the biological carbon pump. Further work on this important topic can now also be submitted to the new Frontiers in Marine Science research topic “Zooplankton and Nekton: Gatekeepers of the Biological Pump” (https://www.frontiersin.org/research-topics/8114/zooplankton-and-nekton-gatekeepers-of-the-biological-pump; Co-editors R. Kiko, M. Iversen, A. Maas, H. Hauss and D. Bianchi). The research topic welcomes a broad range of contributions, from individual-based process studies, to local and global field observations, to modeling approaches to better characterize the role of zooplankton and nekton for the biological pump.

Authors:
R. Kiko (GEOMAR)
A. Biastoch (GEOMAR)
P. Brandt (GEOMAR, University of Kiel)
S. Cravatte (LEGOS, University of Toulouse)
H. Hauss (GEOMAR)
R. Hummels (GEOMAR)
I. Kriest (GEOMAR)
F. Marin (LEGOS, University of Toulouse)
A. M. P. McDonnell (University of Alaska Fairbanks)
A. Oschlies (GEOMAR)
M. Picheral (Laboratoire d’Océanographie de Villefranche-sur-Mer, Observatoire Océanologique)
F. U. Schwarzkopf (GEOMAR)
A. M. Thurnherr (Lamont-Doherty Earth Observatory,)
L. Stemmann (Sorbonne Universités, Observatoire Océanologique)

Building ocean biogeochemistry observing capacity, one float at a time: An update on the Biogeochemical-Argo Program

Posted by mmaheigan

· Thursday, July 5th, 2018

By Ken Johnson (MBARI)

The Biogeochemical-Argo (BGC-Argo) Program is an international effort to develop a global network of biogeochemical sensors on Argo profiling floats that has emerged from over a decade of community discussion and planning. While there is no formal funding for this global program, it is being implemented via a series of international research projects that harness the unique capabilities provided by BGC profiling floats. The U.S. Ocean Carbon and Biogeochemistry (OCB) Program maintains and supports a U.S. BGC-Argo subcommittee as a focal point for U.S. community input on the implementation of the global biogeochemical float array and associated science program development.

Figure 1. Steve Riser deploying a SOCCOM float from the R/V Palmer

About BGC-Argo Floats
BGC-Argo floats can carry a suite of chemical and bio-optical sensors (Figure 1 – Float Schematic). They have enough energy to make about 250 to 300 vertical profiles from 2000 m to the surface. At a cycle time of 10 days, that corresponds to a lifetime near 7 years. The long endurance allows the floats to resolve seasonal to interannual variations in carbon and nutrient cycling throughout the water column. These time scales are difficult to study from ships and ocean interior processes are hard to resolve from satellites. BGC profiling floats extend the capabilities of these traditional observing systems in significant ways.

Figure 2. Images of Navis and APEX floats used in the SOCCOM program. These floats carry oxygen, nitrate, pH, and bio-optical (chlorophyll fluorescence and backscatter) sensors.

All of the data from profiling floats operating as part of the Argo program must be available in real-time with no restrictions on access. The Argo Global Data Assembly centers in France and the USA both provide complete listings of all BGC profiles (argo_bio-profile_index.txt) and access to the data. Extensive documentation of the data processing protocols is available from the Argo Data Management Team. Individual research programs, such as SOCCOM (see below), may also provide direct data access to the observed data along with value added products such as best estimates of pCO₂ derived from pH sensor data.

Regional Deployments
In 2018, it is projected that 127 profiling floats with biogeochemical sensors are will be deployed, including ~40 floats by U.S. projects. Most of the U.S. deployments (30+) will be carried out by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project (Figure 2 – Float Deployment). These floats will carry oxygen, nitrate, pH, chlorophyll fluorescence, and backscatter sensors. As part of the NOAA Tropical Pacific Observing System (TPOS) program, Steve Riser’s group (Univ. Washington) will deploy 3 BGC-Argo floats per year in the equatorial Pacific over the next 4 years. These floats will be equipped with oxygen, pH, bio-optical sensors and Passive Acoustic Listener (PAL) sensors, which provide wind speed estimates at 15-minute intervals while the floats are parked at 1000 m. Wind speed is derived from the noise spectrum of breaking waves. Steve Emerson (Univ. Washington), with NSF support, is also deploying floats equipped with oxygen, nitrate and pH sensors in the equatorial Pacific. With funding from NSF, Andrea Fassbender (MBARI) will deploy two floats at Ocean Station Papa in the northeast Pacific in collaboration with the EXPORTS program. These floats will also carry oxygen, nitrate, pH, and bio-optical sensors.

Nearly 90 BGC floats will be deployed in 2018 by other nations in multiple ocean basins. Much of this effort will focus on the North Pacific and North Atlantic. The sensor load on these floats is somewhat variable. Some will be deployed with only oxygen sensors or bio-optical sensors for chlorophyll fluorescence and particle abundance. Others will carry the full suite of six sensors (oxygen, nitrate, pH, chlorophyll fluorescence, backscatter, and irradiance) that are outlined in the BGC-Argo Implementation Plan (BGC-Argo, 2016). These floats will contribute to the existing array of 305 biogeochemical floats (Figure 3 BGC Argo Map).

Community Activities
In response to the tremendous interest in the scientific community in the capabilities of profiling floats, OCB is sponsoring a Biogeochemical Float Workshop at the University of Washington in Seattle from July 9-13, 2018 to begin the process of transferring this expertise to the broader oceanographic community, bringing together potential users of this technology to discuss biogeochemical profiling float technology, sensors, and data management and begin the process of the intelligent design of future scientific experiments. The workshop will provide participating scientists direct access to the facilities of the Float Laboratory operated by Riser. This workshop builds on a previous OCB workshop Observing Biogeochemical Cycles at Global Scales with Profiling Floats and Gliders (Johnson et al., 2009). BGC-Argo will also have a prominent presence at the 6th Argo Science Workshop (October 22-24, 2018, Tokyo, Japan) and OceanObs19 (September 16-20, 2019, Honolulu, HI).

Figure 3. May 2018 map of the location of BGC-Argo floats that have reported in the previous month and sensor types on these floats. From jcommops.org.

BGC-Argo Publications
Several resources now highlight the capabilities of profiling floats to accomplish scientific observing goals. A web-based bibliography of biogeochemical float papers hosted on the Biogeochemical-Argo website currently includes >100 publications and continues to grow. A special issue of Journal of Geophysical Research: Oceans focused on the SOCCOM program is in progress with 11 papers now available and a dozen more forthcoming. These papers include summaries of the technical capabilities of floats and the biogeochemical sensors, comparisons of float bio-optical data with satellite remote sensing observations, seasonal and interannual assessments of air-sea oxygen flux, under-ice biogeochemistry, carbon export, comparisons of pCO₂ estimated from floats with pH vs. time-series data, and net community production. The connection of float observations with numerical models is a special focus of the program and this is highlighted in several papers, including a description of the Biogeochemical Southern Ocean State Estimate (SOSE), which is a data assimilating BGC model. Results from Observing System Simulation Experiments (OSSEs) used to assess the number of floats needed for large-scale observations are also reported. The BGC-Argo steering committee is developing a community white paper for the OceanObs19 conference in September 2019. BGC-Argo also develops and distributes a community newsletter.

For more information, visit the BGC-Argo website or reach out to the U.S. BGC-Argo Subcommittee.

Long-term coastal data sets reveal unifying relationship between oxygen and pH fluctuations

Posted by mmaheigan

· Thursday, June 7th, 2018

Coastal habitats are critically important to humans, but without consistent and reliable observations we cannot understand the direction and magnitude of unfolding changes in these habitats. Environmental monitoring is therefore a prescient—yet still undervalued—societal service, and no effort better exemplifies this than the work conducted within the National Estuarine Research Reserve System (NERRS). NERRS is a network of 29 U.S. estuarine sites operated as a partnership between NOAA and the coastal states. NERRS has established a system-wide monitoring program with standardized instrumentation, protocols, and data reporting to guide consistent and comparable data collection across all NERRS sites. This has resulted in high-quality, comparable data on short- to long-term changes in water quality and biological systems to inform effective coastal zone management.

Figure 1: Using dissolved oxygen and salinity, monthly mean pH can be predicted within and across coastal systems due to the unifying metabolic coupling of oxygen and pH.

In a recent study published in Estuaries and Coasts, Baumann and Smith (2017) used a subset of this unique data set to analyze short- and long-term variability in pH and dissolved oxygen (DO) at 16 NERRS sites across the U.S. Atlantic, Caribbean, Gulf of Mexico, and Pacific coasts (> 5 million data points). They observed that large, metabolically driven fluctuations of pH and DO are indeed a unifying feature of nearshore habitats. Furthermore, mean pH or mean diel pH fluctuations can be predicted across habitats simply from salinity and oxygen levels/fluctuations (Fig.1). These results provide strong empirical evidence that common metabolic principles drive diel to seasonal pH and DO variations within and across a diversity of estuarine environments. As expected, the study did not yield interannual, monotonic trends in nearshore pH conditions; rather, interannual fluctuations were of similar magnitude to the pH decrease predicted for the average surface ocean over the next three centuries (Fig.2). Correlations of weekly anomalies of pH, oxygen, and temperature yielded strong empirical support for the hypothesis that coastal acidification—in addition to being driven by eutrophication and atmospheric CO₂ increases—is exacerbated by warming, likely via increased community respiration.

Figure 2: Interannual variations in temperature, pH, and dissolved oxygen (DO) anomalies in 16 NERRS sites across the US Atlantic, Gulf of Mexico, Caribbean, and Pacific coasts.

Analyses of these long-term data sets have provided important insights on biogeochemical variability and underlying drivers in nearshore environments, highlighting the value and utility of long-term monitoring efforts like NERRS. Sustained, high-quality data sets in these nearshore environments are essential for the study of environmental change and should be prioritized by funding agencies. The observed metabolically driven pH and DO fluctuations suggest that local measures to reduce nutrient pollution can be an effective management tool in support of healthy coastal environments, a boon for both the habitats and humans.

Authors:
Hannes Baumann (University of Connecticut)
Erik M. Smith (North Inlet-Winyah Bay National Estuarine Research Reserve, University of South Carolina)

Unexpected acidification of deep waters in the Sea of Japan due to global warming

Posted by mmaheigan

· Tuesday, May 22nd, 2018

Oceans worldwide are warming up, and thermohaline circulation is expected to slow down. At the same time, ocean acidity is increasing due to the influx of anthropogenic carbon dioxide (CO₂) from the atmosphere, a phenomenon called ocean acidification that has primarily been documented in shallow waters. In general, deeper waters contain less anthropogenic CO₂, but predicted reductions in ventilation of deep waters may impact deep ocean chemistry, as described in a recent study in Nature Climate Change.

Figure caption: Secular trend of total scale pH at in-situ temperature and pressure at various depths between 1965 and 2015 in the Sea of Japan.

The Sea of Japan is a marginal sea with its own deep- and bottom-water formation that maintains relatively elevated oxygen levels. However, time-series data from 1965-2015 (the longest time-series available) reveal that oxygen concentrations in these deep waters are declining, indicating a reduction in ventilation that increases their residence time. As organic matter decomposition in these waters continues to accumulate more CO₂, the pH decreases. As a result, the acidification rate near the bottom of the Sea of Japan is 27% higher than at the surface. As a miniature ocean with its own deep- and bottom-water formation, the Sea of Japan provides insight into how future warming might alter deep-ocean ventilation and chemistry.

Authors:
Chen-Tung Arthur Chen (National SunYat-sen University, Taiwan and Second Institute of Oceanography, China)
Hon-Kit Lui (National SunYat-sen University and Taiwan Research Institute)
Chia-Han Hsieh (National SunYat-sen University, Taiwan)
Tetsuo Yanagi (International Environmental Management of Enclosed Coastal Seas Center, Japan)
Naohiro Kosugi (Japan Meterological Agency)
Masao Ishii (Japan Meterological Agency)
Gwo-Ching Gong (National Taiwan Ocean University)

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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