Early white paper draft version- October 2013
This preliminary draft is intended as a consultation document.
Comments/edits welcome. Please do not use or cite.
Theme 3: Atmospheric nutrient and particles supply to the surface ocean
Co-authors: Cécile Guieu, Diego Gaiero and Huiwang Gao
1. Brief statement defining the theme:
The ocean receives a broad variety of particles, a range of key macro- and micronutrients, and toxic elements
from the atmosphere. These materials are delivered in chemical forms and amounts that are very different from
the upward supply of internally recycled nutrients from within the ocean. This atmospheric deposition affects vast
regions of the ocean, including sensitive regions far from land. It is a result of both natural processes (e.g. dust
deposition, volcanic eruptions) and, increasingly, by human activities (e.g. increased nitrogen deposition from
pollution). Indirectly, human activity is also likely altering dust emissions/deposition with unknown feedbacks on
climate and biogeochemistry. Despite significant laboratory, field and modelling work over the past decade, the
links between atmospheric deposition, nutrient availability, ocean productivity (up to high trophic levels), carbon
cycling and feedbacks to climate are still poorly understood and modelled.
2. The scientific and societal basis justifying research on this issue. Why is it critical and why does it
need to be done now? What is the end goal? Why is international coordination required?
This theme focuses on the relation between atmospheric input, the carbon cycle and feedbacks to climate. The
consideration of the impacts of atmospheric input on the biogeochemistry of the ocean is quite recent (only few
decades). There is a need to better understand and parameterize the numerous processes involved at a
fundamental level. Work in this area is progressing thanks to recently developed tools (such as measurements of
nutrients and micronutrients at very low levels both in the atmosphere and in seawater and new experimental
enclosures to perform artificial seeding, etc.). Ongoing and future anthropogenic and global changes, both
increasing emissions (e.g., N) and in situ conditions (stratification, pH) may induce changes in atmospheric
deposition fluxes/turn over time in the surface mixed layer and in stoichiometry of the ‘new nutrients’ coming
from the atmosphere. In turn, these changes may result in changes in both biodiversity and microorganism
adaptive strategies for competing for nutrients. Fundamental differences in the response of the microbial
community structure to the input of new nutrients (e.g. stimulation of heterotrophy v. autotrophy) will result in
opposite effect regarding carbon budget depending on the balance between CO2 fixation and respiration. This is
critical because it has direct implications for the way we think about productivity in the ocean and therefore
atmospheric CO2 uptake and fisheries.
This theme has relevance for society as it concerns global and transnational pollution issues, health of the ocean,
and, via potential impacts on fish and higher trophic levels, it touches on issues closely related to deliberate (as
opposed to inadvertent) fertilization of the ocean.
The end goal is to assimilate all information gained from field measurements and laboratory experiments into
more realistic models of deposition and associated mechanisms, taking into account the variable stoichiometry
of atmospheric nutrients and surface ocean biota, with better representation of competitive interactions between
plankton groups. Modeling should also include prediction considering on going and future anthropogenic and
global changes, including both increasing emissions (e.g.., N) and changes in situ conditions (stratification, pH).
International coordination is required considering the vast multi-disciplinary fields involved (atmosphere/ocean
both for biology, chemistry, physic, modeling) and the large spatial variability observed in response to
atmospheric deposition. For example, international coordination is necessary to create a network of existing
time-series data to couple aerosol composition and biogeochemistry of the ocean (e.g. HOT, BATS, CVOO and
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DYFAMED). Such coordination could also allow for the integration/implementation into the network of new
coupled atmosphere/ocean time-series in a number of key areas (Patagonia, Falkland/Malvinas) and also in
HNLC area under the influence of volcano plumes (NE Pacific). The end result will be a large database
integrating all the observations acquired. International coordination could also allow for conducting similar
experiments in different locations, possibly employing Lagrangian studies using tracers or drifting buoys. Trace
element clean mesocosms and tracer-labeled in situ manipulations could also be used to address wholeecosystem effects of atmospheric nutrient input, including particulate organic carbon export.
3. Background – major scientific concepts, key prior work defining the issues:
- Material transported in the atmosphere originates from a variety of natural and anthropogenic sources and
contains both macro- and micronutrients (N, P, C, Si, trace metals including Fe and Cu), and potentially toxic
elements (e.g. Cu, Pb) (GESAMP 1989, Duce et al. 1991, Paytan et al. 2009). Atmospheric transport and
deposition are an important source of new nutrients and particles for large regions of the open ocean (GESAMP,
1989, Duce et al., 1991) and the significance of air-sea exchange to marine nutrient budgets has been
established for nitrogen (Duce et al., 2008), iron (Mahowald et al., 2005; Jickells et al., 2005) and phosphorus
(Mahowald et al., 2008).
- Atmospheric supply of dissolved constituents to the surface ocean depends on particle concentration and size
spectrum, and the solubility of the element-bearing phases in aerosols (Trapp et al., 2010; Baker and Jickells,
2006), which is influenced by atmospheric processing during transport (Krishnamurthy et al., 2009).
- The main natural source of land-derived particles to the open ocean is wind-blown desert dust, which
constitutes the primary atmospheric source of iron (Jickells et al., 2005) and Figure 1.
- Atmospheric nitrogen is mainly derived from anthropogenic combustion or agricultural sources from densely
populated regions throughout the world (Duce et al., 2008)
- Phosphorus originates from both desert dust and anthropogenic sources (Mahowald et al., 2008)
- The extent to which dust interacts with anthropogenic acids (H 2SO4 and HNO3) during transport increases the
solubility of various elements (Desboeufs et al., 2001) resulting in enrichment of nitrogen (Geng et al., 2009),
and enhanced supply of potentially bioavailable compounds to the surface ocean.
- Post-depositional processes associated with the quantity and quality of dissolved organic matter in seawater
are very important in the bioavailability of atmospheric new (micro)nutrients (Wagener et al., 2010, Bressac and
Guieu, 2013); for example it could result in a strong scavenging of iron on dust instead of dissolution from dust
(Bressac and Guieu, 2013; Wagener et al., 2010; Wuttig et al., 2013).
- The supply of new nutrients to the ocean from external sources such as atmospheric deposition has been
extensively addressed in iron-limited High Nutrient-Low Chlorophyll regions (i.e. Boyd et al., 2007), most of
which receive low atmospheric inputs at the present time (Figure 1). However, much less attention has been
paid to the importance of atmospheric deposition to LNLC regions where it likely represent an important source
of new nutrients for the surface mixed layer (Guieu et al., 2013).
- Impact from volcanoes has been recently emphasized (see for ex. Olgun et al., 2013) with consequences up to
high trophic levels.
- Impact on biota from field and laboratory experiments in LNLC areas indicate positive responses to aerosol
addition, with bacterial production and N2 fixation showing the strongest responses (see review Guieu et al.,
2013). Increases in chlorophyll-a are seen to a lesser extent, however, differential responses among
phytoplankton groups are also apparent (i.e. Paytan et al., 2009; Giovagnetti et al., 2013). Changes in standing
stocks tend to be smaller than changes in metabolic rates (Guieu et al., 2013) (Figure 2).
- The effect of atmospheric deposition on the surface ocean may vary with the degree of oligotrophy of the
receiving waters (Marañon et al., 2010)
- The effect of atmospheric deposition on the surface ocean cannot be seen as a simple fertilization effect
(Guieu et al., 2013) as it results in fundamental differences in the response of the microbial community structure
(e.g. stimulation of heterotrophy v. autotrophy), and hence, in the vertical carbon export and nutrient cycling
(Marañon et al., 2010).
- Aggregation between atmospheric particles and dissolved organic matter can induce a strong and rapid POC
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export independently of a fertilization effect (Ternon et al., 2010; Bressac and Guieu, 2013). This ‘lithogenic
carbon pump’ (Bressac and Guieu, 2013) may significantly augment the export flux resulting from increased
nutrient supply by atmospheric deposition.
- Atmospheric impacts in LNLC regions have been underestimated by models as they typically overlook large
synoptic variations in atmospheric deposition and the associated nutrient and particle inputs (Guieu et al., 2013)
Figure 1. Average estimation of dust deposition (g.m -2. yr-1) over the world oceans (Jickells et al., 2005)
Figure 2. Box-Whisker plots (mean ± standard error and 95% confidence limits) showing the responses of
different biological variables to aerosol additions in LNLC waters (60% of the ocean): synthesized from available
data from a total of 26 field and laboratory aerosol addition bioassay experiments, and mesocosms experiments.
The responses are % changes in the aerosol treatment relative to the control after 2-8 days, with zero indicating
no difference between the aerosol treatment and the control, and a positive response indicating an increase in
the parameter measured for the aerosol treatment relative to the control. Parameters: (BA) Bacteria Abundance,
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(BR) Bacteria Respiration, (BP) Bacteria Production, (Syn.) Synechococcus abundance, (Proc.) Prochlorococcus
abundance, (pico & nano-euks) Nano- and Picoeukaryotes abundance, (nano & microphyto) nano- and microphytoplankton abundance, (Chla) Chlorophyll-a, (PP) primary production, (N2Fix) nitrogen fixation. (Guieu et al.,
2013)
4. Approaches – what will it take to make substantive progress on the issue? What will be achieved in
the 10 years of Future SOLAS?
Systematic measurements are required of atmospheric deposition and nutrients in the surface mixed layer in
regions where atmospheric (natural and/or anthropogenic) supply plays an important role as in Low Nutrient Low
Chlorophyll regions (Guieu et al., 2013) and also in HNLC area such as the NE Pacific. Anthropogenic nitrogen
forcing is primarily a Northern hemisphere phenomenon but, as climate and ocean acidification are global drivers,
there is a requirement for coupled atmosphere marine time-series sampling sites in both hemispheres. Although
reliable measurements of dry deposition remain technically challenging, it will be beneficial to extend wet and dry
deposition measurements and particle characterization to repeat sampling lines across regional deposition
gradients and surface water biogeochemical gradients, using research vessels and voluntary observing ships.
These transects should ideally accommodate rate measurements and nutrient manipulation experiments to gain
insight into the proximal controls of plankton composition and process rates. Linking time-series studies of
aerosol composition with oceanic time-series data (Schultz et al., 2012) is valuable for constraining the response
of the marine ecosystem to deposition events. Existing time series stations that monitor both atmosphere and
ocean properties (e.g. HOT, BATS, CVOO, DYFAMED) could become focal points for more detailed
experiments and process studies, possibly employing Lagrangian studies using tracers and/or drifting buoys.
Trace-element clean mesocosms and tracer-labeled in situ manipulations could also be used to address wholeecosystem impacts of atmospheric nutrient input, including particulate organic carbon export. If such
international effort is deployed (allowing comparative studies and data collection from time-series and data
sharing, the SOLAS community should be able, within 10 years, to provide the necessary information for their
assimilation into realistic models of deposition and associated mechanisms, taking into account the variable
stoichiometry of atmospheric nutrients and surface ocean biota, with better representation of competitive
interactions between plankton groups and aerosols/organic matter aggregation processes. Transport, deposition
and biogeochemical models require thorough testing and validation against in-situ time series datasets and
remote-sensing observations. In addition, methodological intercalibration, sample sharing, common reference
materials and standardization of techniques are all required to ensure global coherence and quality control.
5. Community readiness – is there an existing community engaged on this issue? Are there institutional
or other barriers to progress? Is infrastructure or human capacity building required in order to achieve
the goals?
An existing research community is addressing these issues but it is not well coordinated due to the multidisciplinary nature of the research. In addition, in general, each group works in a dedicated area with no
geographical connection with other groups (e.g.,. Sargasso Sea, East tropical Atlantic, off Patagonia, Western
Pacific, Mediterranean Sea). International coordination is a difficult task because it has to be maintained over
time. Recently the existing marine time-series started to coordinate their effort (Karl et al., 2003) for the marine
biogeochemistry. Recently Schultz et al. 2012 proposed to set up a ‘Marine Atmospheric network’ for the longterm observation of the link between dust/iron and marine biogeochemistry. They recommended focusing on an
HNLC area because of the impacts of dust in such regions. The Schultz et al. (2012) work results from GESAMP
Working Group 38 ‘The Atmospheric Input of Chemicals to the ocean’. Coordinated aerosol – marine
biogeochemistry measurements could also be implemented for other atmospheric nutrients that impact LNLC
areas. In addition to encouraging international coordination in a “Marine Atmosphere time-series network”,
SOLAS could help reach the presented goals for this theme by encouraging a large group of experts to setup a
common processes studies project in a dedicated area that would be chosen by all the experts to fulfill a large
number of remaining questions. Finally, research related to this theme is evolving with time; for example the
potential fertilizing effect of volcanic ash is the subject of many new projects that produce important literature.
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Multi-disciplinary meetings where people with different interests (ex. dust, volcanic ashes, pollution) can share
their experience and point of view should be encouraged.
Considering all those different tasks, a dedicated person to organize a successful international effort will be
necessary.
6. External connections – what partnerships are required in order to achieve the goals? What
mechanisms will be used to accomplish the interactions?
To setup a network and/or initiate a joint process studies project, SOLAS could organize a joint workshop with
coordinators of GESAMP working group 38, BioGEOTRACES, ISAR (International Society for Aeolian
Research*), and IMBER. Another important community is the one dedicated to understanding atmospheric
deposition fluxes to the ocean. Still models yield a wide range of estimates of the ratios of wet-to-dry deposition.
The knowledge of deposition over the ocean is based upon a few limited experimental data sets that are today
mostly discontinued (e.g., SEAREX, AEROCE). Fostered by the World Meteorological Organization (WMO), a
few monitoring stations exist that are equipped with sophisticated instrumentation (e.g., Malta and Izaña), and
some programmes in Africa and Asia (e.g. AMMA, SAMUM, ACE-ASIA) and ship measurements have provided
shorter-term information.
* (ISAR) scientists undertaking research in aeolian processes, landforms, and modeling, to stimulate scientific
research in aeolian topics and related fields and contribute largely to understand the emission process of
dust/aerosols. This community is very active in trying to improve our knowledge in subjects like the amount of
dust/aerosol emitted and its embedded nutrients and largely contribute on having reasonable information on the
geographic distribution of dust sources.
7. Sustainability – articulate relationship (if any) between this project and the FE goals of Global
Development and Transformation Towards Sustainability.
As detailed in section 3, the project will first tackle fundamental scientific questions on the impact of atmospheric
deposition in biogeochemistry and (end to end) ecosystem functioning. Because we consider (via experimental
approach and modelling) how ongoing anthropogenic and natural changes will modify the present functioning
and how this will impact on carbon storage- one of the most important ecosystem service that ocean is providing
to Man- this project can indeed be articulated with the objectives of FE.
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