Progress in Oceanography 128 (2014) 172–184
Contents lists available at ScienceDirect
Progress in Oceanography
journal homepage: www.elsevier.com/locate/pocean
Review
Organic pollutants and ocean fronts across the Atlantic Ocean: A review
Rainer Lohmann ⇑, Igor M. Belkin
Graduate School of Oceanography, University of Rhode Island, 215 South Ferry Road, Narragansett, 02882 RI, USA
a r t i c l e
i n f o
Article history:
Received 5 January 2013
Received in revised form 22 August 2014
Accepted 22 August 2014
Available online 3 September 2014
a b s t r a c t
Little is known about the effect of ocean fronts on pollutants dynamics, particularly organic pollutants.
Since fronts are associated with convergent currents and productive fishing grounds, any possible convergence of pollutants at fronts would raise concerns. The focus here is on relatively persistent organic pollutants, POPs, as non-persistent organic pollutants are rarely found in the open ocean. Results from recent
cruises in the Atlantic Ocean are examined for POP distributions across ocean fronts in (i) the Canary Current; (ii) the Gulf Stream; and (iii) the Amazon and Rio de la Plata Plumes. Few studies achieved a spatial
resolution of 10–20 km, while most had 100–300 km between adjacent stations. The majority of the wellresolved studies measured perfluorinated compounds (PFCs), which seem particularly well suited for
frontal resolution. In the NE Atlantic, concentrations of PFCs sharply decreased between SW Europe
and NW Africa upon crossing the Canary Current Front at 24–27!N. In the Western Atlantic, the PFC concentrations sharply increased upon entering the Amazon River Plume and Rio de la Plata Plume. In the
NW Atlantic, concentrations of several pollutants such as polycyclic aromatic hydrocarbons are very high
in Rhode Island Sound, decreasing to below detection limit in the open ocean. The more persistent and
already phased-out polychlorinated biphenyls (PCBs) displayed elevated concentrations in the Gulf
Stream and Rhode Island Sound, thereby highlighting the importance of ocean fronts, along-front currents, and cross-frontal transport for the dispersal of PCBs.
" 2014 Elsevier Ltd. All rights reserved.
Contents
Introduction: fronts and organic pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Spatial distribution of pollutants in relation to ocean fronts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Canary Current Front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Gulf Stream Front . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
River plumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Introduction: fronts and organic pollutants
Abbreviations: POPs, persistent organic pollutants; PCBs, polychlorinated biphenyls; PFCs, perfluorinated compounds; PAHs, polycyclic aromatic hydrocarbons;
PFOA, perfluorooctanoic acid; PFOS, perfluorooctane sulfonate; HCHs, hexachlorocyclohexanes; HCB, hexachlorobenzene; PBDEs, polybrominated diphenylethers.
⇑ Corresponding author. Tel.: +1 401 874 6612; fax: +1 401 874 6811.
E-mail address: [email protected] (R. Lohmann).
http://dx.doi.org/10.1016/j.pocean.2014.08.013
0079-6611/" 2014 Elsevier Ltd. All rights reserved.
Front is a narrow band of enhanced gradients of physical, chemical and biological properties (temperature, salinity, nutrients, etc.)
that separates distinctly different water bodies (Belkin, 2002).
Cross-frontal ranges (steps) of temperature and salinity of up to
10 !C and 3 psu respectively are routinely observed, though
generally these steps are much smaller, typically 2-to-3 !C and
0.5–1.0 psu. Fronts are typically associated with enhanced
R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
173
Fig. 1. Processes affecting pollutant transport and distribution in the ocean.
productivity at all trophic levels, including fishery grounds; yet a
low-productivity front was also observed (Caldeira et al., 2001).
Fronts are formed by numerous processes, including tides and tidal
mixing, winds, solar heating, current convergence, upwelling/
downwelling, advection, convection, precipitation/evaporation
and sea ice formation (Belkin, 2002).
Fronts and frontal processes play important roles in the spatial
distribution and temporal variability of pollutants (Fig. 1). The five
key oceanic processes associated with fronts (yellow1 arrows in
Fig. 1) are (1) particle sinking, (2) downwelling, (3) turbulent mixing,
(4) convection, and (5) upwelling. Many fronts are convergent (Belkin
et al., 2009), hence associated with downwelling, which enhances
particle sinking. Some fronts feature downwelling on one side and
upwelling on the opposite side, thus exerting opposite effects on particle sinking. Turbulent mixing at fronts can be enhanced by up to
two orders of magnitude vs. ambient ocean (D’Asaro et al., 2011).
Downwelling and upwelling are two components of ocean convection, hence water mass formation and conversion. Cascading along
continental slope (dark blue arrow in Fig. 1) is often associated with
shelf-slope fronts located over the shelf break. The three key atmospheric processes associated with fronts (light blue arrows in Fig. 1)
are (1) dry deposition, (2) wet deposition, and (3) volatilization.
Major fronts associated with the western boundary currents – Gulf
Stream, Kuroshio, etc. – and with the Antarctic Circumpolar Current
can impact the entire lower troposphere up to 1 km above sea surface
(Small et al., 2008), thereby directly affecting dry deposition and wet
1
For interpretation of color in Fig. 1, the reader is referred to the web version of
this article.
deposition rates. Such fronts also modulate near-surface wind stress
(Small et al., 2008), hence volatilization rates. In shallow seas, fronts
extend vertically throughout the entire water column and interact
with bottom currents (dark blue arrow in Fig. 1). Even in the deep
ocean, fronts associated with western boundary currents can generate strong bottom currents (‘‘benthic storms’’) with a speed of
>30 cm/s, leaving ripple marks at depths over 4000 m (Hollister
and McCave, 1984).
Fronts separate water masses with different concentrations of
pollutants. Most major fronts are associated with along-front geostrophic currents that transport pollutants across a wide variety
of scales. Conservative pollutants can be carried by along-front currents across oceans and around the globe. Rate of turbulent energy
dissipation in major frontal zones (Kuroshio, Gulf Stream, Antarctic
Circumpolar Current, etc.) is ‘‘enhanced by one to two orders of
magnitude, suggesting that the front, rather than the atmospheric
forcing, supplied the energy for the turbulence’’ (D’Asaro et al.,
2011), thereby greatly enhancing dissipation of pollutants in these
frontal zones. Surface convergence and downwelling at fronts may
result in reduction of surface concentrations of pollutants; this
effect was dubbed ‘‘self-cleaning’’ (of fronts) by Sherstyankin
et al. (1999). When upwelling develops along one side of a front,
it brings relatively pristine deep waters to the surface. Deep convection associated with some fronts (e.g., formation of the Subantarctic
Mode Water north of the Subantarctic Front) tends to pump surface
contaminants to intermediate and deep layers. Frontal eddies and
intra-thermocline lenses effectuate cross-frontal transfer of pollutants between water masses separated by fronts, acting on the synoptic, meso- and small scales, while frontal interleaving, double
diffusion, and turbulent mixing effectuate cross-frontal flux of
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R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
Fig. 2. Main processes affecting organic pollutant concentrations across fronts.
pollutants on the fine scale. Some physical processes are endemic to
fronts, e.g., interleaving (especially active across density-compensated fronts) and cabbeling (densification and sinking of a mixture
of two water parcels of the same density but different temperature
and salinity). Some chemical processes – even if not truly endemic
to fronts – may intensify in frontal zones owing to either high gradients of properties or vigorous ocean–atmosphere–ice interaction
or elevated biological activity or all of the above – and more. For
example, estuarine fronts act as ‘‘marginal filters’’ (Lisitsyn, 1995)
by trapping fine river sediments that carry contaminants
(Macdonald et al., 2005); the main processes acting at these
marginal filters are ‘‘flocculation and coagulation of dissolved
(colloidal) and suspended matter’’ (Lisitsyn, 1995).
Little is known about the effect of ocean fronts on organic pollutants, but any possible convergence of pollutants in productive
fishing grounds associated with fronts would be a cause for concern, especially given the effect of biomagnification since many
frontal species are top predators, e.g., tuna and billfish. The pioneering work by Tanabe et al. (1991) in the Seto Inland Sea (Japan)
has demonstrated that organic pollutants, particularly those bound
to particles, can be enriched in fronts. Since organic pollutants are
not routinely investigated across ocean fronts, it is appropriate to
first outline the possible effects of fronts on organic pollutant
dynamics. As most fronts are associated with surface convergence
toward the front, we expect a truly dissolved compound to have a
frontal concentration that changes monotonously across the front
(Fig. 2, top) in the same fashion as temperature and salinity. Yet
at the same convergence, floating particles or flotsam, e.g., phytoplankton or floating plastic debris, would have a maximum concentration at the surface, within the front, while the water
masses separated by the front would sink owing to downwelling
circulation along the frontal interface (Fig. 2, bottom). The same
logic applies to pollutants concentrated in the sea surface microlayer (Wurl and Obbard, 2004). The importance of plastic particles
as vectors of organic pollutants is currently under debate (Teuten
et al., 2007; Gouin et al., 2011), while there is agreement that many
organic pollutants strongly sorb to man-made polymers present in
the ocean. Thus, dissolved vs. particle-bound pollutants are
expected to have different cross-frontal distribution patterns at
convergent fronts.
Not all fronts are convergent. Some fronts feature downwelling
on one side and upwelling on another. Typically, the upwelled
deep water is less polluted than surface water, thereby creating a
strong contrast in dissolved organic pollutant concentrations
across surface manifestations of upwelling fronts. Fronts with a
complex multi-layer vertical structure have been reported. For
example, Houghton (2002) performed a dye experiment in the
shelf-break front (SBF) on Georges Bank. At the front, surface convergence caused downwelling while bottom convergence caused
upwelling. The surface-intensified downwelling and bottom-intensified upwelling converged at mid-depth, resulting in mid-depth
divergence or a returning mid-depth flow on both sides of the front
(Houghton, 2002).
Beyond these purely physical mixing processes, there is also
evidence for the enhancement of phytoplankton at fronts (Belkin
and O’Reilly, 2009). For example, Ryan et al. (1999a, 1999b)
observed a strong seasonal chlorophyll enhancement at the shelf
break of the Mid-Atlantic Bight in the spring that lasted into late
June. Such massive chlorophyll blooms along physical fronts must
have profound effects on distribution of organic pollutants. Major
fronts are associated with elevated primary production and
enhanced vertical flux of carbon-rich particles that lead to the
increased vertical export of organic contaminants on sinking particles from the surface layer to intermediate and deep waters (Dachs
et al., 2002; Macdonald et al., 2005).
In terms of organic pollutants, we are focusing on relatively persistent organic pollutants, POPs, as non-persistent organic pollutants are rarely found in the open oceans. We can divide these
persistent pollutants in two categories: (1) those that are predominantly dissolved, with little tendency to bioaccumulate or sorb
significantly; and (2) organic pollutants that bioaccumulate and
sorb to either organic matter such as phytoplankton or floating
particles such as plastic debris (Rios et al., 2007) (Table 1). Whether
a contaminant is predominantly dissolved or bound to organic
matter (particles, but also to colloidal material and dissolved
organic matter, DOM) depends primarily on the compound’s physico-chemical properties (Schwarzenbach et al., 2003). The dissolved vs. sorbed dichotomy is also affected by temperature,
salinity, the abundance of organic matter and particles in the
water. Lastly, the chemical make-up of the particles also affects
the pollutant’s propensity to sorb. This holds true both for natural
particles and floating synthetic polymers. The disposition of compounds to sorb to organic matter is often estimated by relying on
a proxy, such as the compound’s partitioning constant between
octanol and water, Kow (Schwarzenbach et al., 2003). Compounds
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R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
Table 1
Selected organic pollutants as potential tracers of ocean fronts.
Compound
Conservative
Volatile
Reactive
Potential for bioaccumulation
Typical sampling volume
PFOA
PFOS
HCHs
HCB
PCBs (1–3 chlorines)
PCBs (4–10 chlorines)
Low MW PAHsa
High MW PAHs
PBDEs
Yes
Mostly
No
Mostly
Mostly
No
No
No
No
No
No
Yes
Yes
Yes
Little
Yes
Little
Little
No
No
Yes
Little
Little
Little
Yes
Yes
Yes
No
Low
Low
Low
Low
High
Low
Low
High
1L
1L
10s L
100s L
100s L
100s L
100s L
100s L
100s L
PCBs: polychlorinated biphenyls; PAHs: polycyclic aromatic hydrocarbons; PFOA: perfluorooctanoic acid; PFOS: perfluorooctane sulfonate; HCHs: hexachlorocyclohexanes;
HCB: hexachlorobenzene; PBDEs: polybrominated diphenylethers.
a
PAHs up to molecular weight of 202.
with low Kow values prefer to stay dissolved, while those with high
Kow values (>104) tend to bind to organic particles. The sorption of
organic compounds to DOC is more complex, but most studies have
focused on DOC isolated from freshwater and sediments
(Burkhard, 2000). In a study with coastal DOC, Friedman et al.
(2011) suggested that it might sorb PCBs much more strongly than
freshwater DOC.
Examples of predominantly dissolved POPs include the perfluorinated acids and sulfonates, such as perfluorooctanoic acid (PFOA),
perfluorooctane sulfonate (PFOS) and its salts, but also low molecular weight pesticides (hexachlorocyclohexanes, HCHs), chlorinated biphenyls with few chlorines and hexachlorobenzene
(HCB). In contrast, the higher molecular weight polychlorinated
biphenyls (PCBs) with four or more chlorines, but also polybrominated diphenylethers (PBDEs), strongly bind to organic matter in
the water column. A compound that is completely persistent, fully
dissolved, and does neither interact with particles nor volatilizes
constitutes a perfect tracer for water masses. Recently, Yamashita
et al. (2005, 2008) proposed to make use of PFOS and PFOA as
water tracers, as they are persistent, dissolved and display little
tendency to bind to organic matter. The compounds mentioned
above have all been targeted by the Stockholm Convention on Persistent Organic Pollutants, which means they have shown to be
persistent, bioaccumulate (i.e., enrich up the food chain), prone
to long-range transport, and elicit adverse effects. The earliest
group of POPs was also known as the ‘dirty dozen’, consisting of
PCBs, DDT, dioxins and furans, HCB and other organochlorine pesticides [http://www.chem.unep.ch/gpa_trial/01what.htm].
Ocean currents were thought to contribute to the long-range
transport of POPs, as evidenced by the ‘global fractionation’ theory
developed by Wania and Mackay (1993, 1996). Yet the observational programs focused on the global fate of POPs were biased
towards atmospheric vs. oceanic transport (Bidleman et al., 1995;
Jantunen and Bidleman, 1996), mostly due to logistical and technical constraints of measuring POPs in the water. Ship-based measurements are further complicated by the problem of sampling
sufficient volumes of water to overcome detection limits and alleviate contamination concerns on-board ship (Lohmann et al.,
2004).
In the Arctic Ocean, the importance of currents as pollutant
pathways has been recognized, particularly for POP transport. A
good example is the fate of two contrasting isomers of hexachlorocyclohexanes (HCHs). While a-HCH is mostly transported via the
atmosphere, the more water-soluble and less volatile b-HCH (i.e.,
with a smaller Henry’s Law Constant) is thought to be mostly
transported via the ocean (Li et al., 2002; Sahsuvar et al., 2003;
Pucko et al., 2013). Mass-balance model results for a-HCH in the
Arctic Ocean imply that ocean transport has become the dominant
clearance mechanism for a-HCH in the Arctic Ocean (Li et al.,
2004), although microbial degradation dominated a-HCH decrease
in the Western Arctic Ocean (Pucko et al., 2013).
Similarly, the more recent concerns about the presence of perfluorinated compounds PFOS and PFOA in the Arctic Ocean have
pitched their atmospheric transport and oxidation of precursors
against their transport with ocean currents (Armitage et al.,
2006, 2009a, 2009b). Overall, the importance of ocean currents
as a transport vector for certain POPs has been recognized, but
the explicit role of individual fronts in organic pollutant dynamics
and distribution has not been investigated in detail.
Data sources
We surveyed the literature for case studies measuring organic
pollutants in the Oceans. While plenty of studies have reported
organic pollutants in coastal areas and the Baltic, Mediterranean
and other marginal seas, this article focuses on measurements in
the open ocean. Relatively few such studies are available for the
open ocean for legacy compounds, such as PCBs, PCDD/Fs and OCPs
(Table 2). For classical POPs that are present at around 1 pg/L, such
as PCBs (Gioia et al., 2008b), polychlorinated dioxins and furans
(Nizzetto et al., 2010), polybrominated diphenylethers or PBDEs
(Xie et al., 2011a) and many legacy pesticides (e.g., DDTs)
(Lohmann et al., 2009), sampling volumes of around 1000 L need
to be collected to routinely overcome detection limits. This equates
to sampling times of around 24 h, making front resolution in the
ocean difficult. As an approximation of frontal resolution, we
included an estimation of the typical distance between two consecutive sampling points (Table 2). Only few studies achieved a
spatial resolution of 10–20 km, while most had 100–300 km
between adjacent stations. Studies with such coarse resolution
make the detection of fronts difficult. The high cost of trace-level
analysis also prevented continuous water sampling on ocean transects for many years. In the Arctic Ocean, the much higher concentrations of HCHs and HCB enabled their detection in just tens of
liters (Jantunen and Bidleman, 1996). The recent focus on emerging
polar POPs, such as perfluorinated compounds, PFCs (Giesy and
Kannan, 2002), went hand-in-hand with the technological
advances in life sciences and liquid chromatography-based detection systems, enabling the detection of various PFCs in just one
liter of seawater (Yamashita et al., 2005, 2008). For the following
discussion, we chose several recent studies that achieved high spatial resolution of POPs concentrations across the Atlantic Ocean.
Data quality
Data quality is of paramount importance when interpreting the
spatial trends of organic tracers across the Oceans. There are
numerous challenges to trace-level analysis of organic pollutants
in the ocean, which fall into the categories of accuracy and precision of results. Accuracy (‘trueness’) describes how correct the stated concentration is relative to the ‘true’ value (if it was known).
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R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
Table 2
Studies of organic pollutants’ spatial distribution in the oceans.
a
Pollutant
Ocean
Resolutiona (km)
Reference
PCBs
PCBs, PAHs
PCBs
PCBs
PCBs
PCBs, HCB
PCBs, OCPs
PAHs
PAHs, OCPs
PAHs
PCDD/Fs
Phthalates
PBDEs
PBDEs
OCPs
OCPs
OCPs
OCPs
OCPs
OCPs
OCPs
OCPs
OCPs
OCPs
OCPs
OCPs
PFCs
PFCs
PFCs
PFCs
PFCs
PFCs
PFCs
PFCs
PFCs
Global oceans
North Atlantic
North Atlantic, Arctic
North Atlantic, Arctic
Atlantic Ocean
Pacific ocean
Atlantic Ocean
Atlantic Ocean
North Atlantic, Arctic
Atlantic Ocean
Atlantic Ocean
North Atlantic, Arctic
Atlantic Ocean
Atlantic Ocean
North Pacific, Arctic
Atlantic, Southern ocean
Arctic Ocean
North Pacific, Arctic
North Pacific, Arctic
Arctic
South Atlantic, Southern ocean
Atlantic Ocean
Arctic Ocean
Arctic Ocean
Atlantic Ocean
Atlantic Ocean
Global oceans
Global oceans
Southern oceans
Atlantic Ocean
Atlantic Ocean, Southern ocean
North Atlantic Ocean
Arctic Ocean
Atlantic Ocean, Arctic
North Pacific, Arctic
100–300
>300
50–300
>100
50–100
300
30–50
30–50
>200
30–50
>300
>100
100–150
30–50
50–100
100–200
>50
30–50
>200
>100
>50
>200
50–100
30–50
>300
300
100–200
50–100
>200
100
50–100
10–20
30–50
30–50
30–50
Iwata et al. (1993)
Schulz-Bull et al. (1998)
Sobek and Gustafsson (2004)
Gioia et al. (2008a)
Gioia et al. (2008b)
Zhang and Lohmann (2010)
Lohmann et al. (2012)
Nizzetto et al. (2008)
Lohmann et al. (2009)
Lohmann et al. (2013a)
Nizzetto et al. (2010)
Xie et al. (2007)
Xie et al. (2011a)
Lohmann et al. (2013b)
Cai et al. (2012a)
Xie et al. (2011b)
Wong et al. (2011)
Cai et al. (2010)
Ding et al. (2007)
Bidleman et al. (2007)
Jantunen et al. (2004)
Lakaschus et al. (2002)
Yao et al. (2002)
Jantunen and Bidleman (1995, 1998)
Schreitmüller and Ballschmiter (1995)
Fischer et al. (1991)
Yamashita et al. (2005)
Yamashita et al. (2008)
Wei et al. (2007)
Ahrens et al. (2009)
Ahrens et al. (2010b)
Ahrens et al. (2010a)
Busch et al. (2010)
Benskin et al. (2012a, 2012b)
Cai et al. (2012b)
Best resolution achieved in the given study. We compiled studies covering oceanic transects, looking for frontal resolution in the sampling design.
Laboratories demonstrate the validity of their methods by including standard reference materials (which come with certified
concentration ranges) in their analytical runs. Participation in
round-robin studies is another way of demonstrating accuracy of
results. A major concern regarding shipboard measurements is
the contamination of samples during sampling or processing of
samples on-board, which would lead to deviations from accurate
results. The inclusion of laboratory and field blanks is a necessary,
but not sufficient, quality control step in achieving accurate results.
Precision deals with the ability of a laboratory to demonstrate
reproducible results. For the scope of this paper, samples need
not be accurate, but need to be precise in their results to detect
the influence of fronts on the distribution of tracers. Precision
can be demonstrated through repeat analysis of the same sample,
having multiple samples taken at the same time or, at the very
least, through repeat injections of the same extract. Typical results
of repeat analysis are in the tens of percent, which means that a
change in POP concentration of a factor of 2 safely identifies variability outside of analytical uncertainty. As an example, coefficients of variation for triplicate analyses by the same laboratory
(‘precision’) were 14–20% for PFOA and PFOS in seawater
(Benskin et al., 2012a,b). Interlaboratory agreement (closer to
‘accuracy’) for PFCs in seawater were generally within a factor of
2 (Benskin et al., 2012a,b). Sampling of the more hydrophobic POPs
is more challenging (Muir and Lohmann, 2013), but similar precision can be achieved. For the purpose of this paper, we did not perform a statistical analysis of data, as would be appropriate, e.g., for
algorithmic detection of fronts from satellite data, etc. Yet as evident from the Discussion and Figures, in most instances cross-fron-
tal concentration changes were steep enough that no statistical
test was needed to identify the location of the front.
Spatial distribution of pollutants in relation to ocean fronts
Global distribution of pollutants is believed to be determined
largely by two factors: (1) atmospheric transport and (2) oceanic
transport. Most studies to date focused on atmospheric transport,
whereas few focused on oceanic transport. The problem of pollutant transport partitioning between atmosphere and ocean can
be looked at from different angles, e.g., from theoretical considerations or numerical simulations with global coupled ocean–atmosphere circulation models. Yet one of the most promising
approaches is an exploratory analysis of long oceanic transects that
might hold clues. In this section we review published data
obtained along such transects, looking for front signals and linking
them to physical fronts. Biological fronts (e.g., fronts in chlorophyll
field) must play an important role, which sometimes might rival
the role played by physical fronts. Yet the science of biological
fronts is still in its infancy (e.g., Belkin and O’Reilly, 2009), therefore here we focus on physical fronts in temperature, salinity and
density fields.
Our analysis is based on an intuitively obvious notion that
atmospherically-dominated surface distributions of pollutants
are, at least initially, spatially smooth. The inherent smoothness
of atmospheric fields (compared with oceanic fields) stems mostly
from the former’s relatively large temporal variability. While bearing certain similarities with oceanic fronts, the atmospheric fronts
R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
(Berry et al., 2011) lack stability: Their spatio-temporal scales of
variability and corresponding magnitudes are drastically different
from those of oceanic fronts. Therefore, the atmospheric fronts
are much less likely to leave a lasting imprint on the ocean. Most
atmospheric fronts are relatively short-lived (a few days or weeks),
although some of them, e.g. the famous Mei-yu Front in East Asia,
may persist for up to a few months, and only a couple of fronts in
the Northern Hemisphere are semi-permanent, namely the Polar
Front and Arctic Front, while the Polar Front in the Southern Hemisphere is probably the only truly permanent atmospheric front.
Nonetheless, pollutants can be transported by various mechanisms, including those linked to atmospheric fronts, e.g. cyclones
traveling along these fronts. Sometimes pollutants are transported
across oceans as long filaments reminiscent of fronts or as isolated
blobs of air (Wilkening et al., 2000) similar to oceanic rings
spawned by fronts. The atmospheric fronts also play an important
role in wet deposition of pollutants. Indeed, up to 90% of rainfall in
major storm-track bands is associated with atmospheric fronts
(Catto et al., 2012). The bulk of long-distance moisture transport
is carried by front-like atmospheric rivers (Newell et al., 1992;
Rutz et al., 2014) linked to heavy rainfalls (Lavers et al., 2011).
Therefore, atmospheric rivers must be crucial to wet deposition
of pollutants. Each of the above features can leave a distinct
event-like signature in the surface layer of the ocean. Yet the inherently high variability of these atmospheric features precludes their
long-term impact on the ocean. It also means that time-averaged
atmospheric deposition of any substance onto the sea surface is
bound to be spatially smooth. Hence stepwise discontinuities in
spatial distributions of pollutants along oceanic transects are likely
linked to oceanic discontinuities, i.e. fronts. To prevent contamination of our analysis by step-like features at the sea surface caused
by transient atmospheric phenomena, we emphasize the importance of repeat oceanographic transects that allow the researcher
to distinguish quasi-stationary features from transients. To date,
by far the most complete archive of pollutant measurements along
repeat transects has been assembled – and is appended annually –
thanks to regular Antarctic voyages by RV Polarstern. Even though
there is no central repository of pollutant data, Polarstern data are
promptly reported in peer-reviewed journal papers accompanied
by supplementary materials that include data tables. The below
analysis is mostly based on these data, particularly those for perfluorinated alkyl acids and sulfonates. Owing to our reliance on
data reported from cruises by RV Polarstern, the emphasis is on
fronts in the Eastern Atlantic Ocean.
Canary Current Front
The Antarctic voyages of RV Polarstern follow the same pattern,
departing from Bremerhaven and having first stations occupied in
the North Sea, English Channel or Bay of Biscay. Here we focus on
the northern segments of these tracks as the ship proceeds from
European coastal waters southward into much less polluted waters
off NW Africa and farther south into even less polluted waters of
the Equatorial Atlantic (albeit elevated concentrations of some
banned POPs have been observed off West Africa (Gioia et al.,
2008b, 2011), possibly due to a combination of illegal waste dumping coupled to atmospheric emissions). As we are about to see
below, transitions between these waters are not gradual. Instead,
concentrations of individual pollutants decrease southward in a
stepwise fashion as the ship crosses over sharp fronts associated
with major oceanic currents. These fronts act as water mass
boundaries. While thermohaline signatures of these fronts have
been studied for decades, this is the first time that these fronts
are identified in distributions of pollutants.
During the 2007 voyage, RV Polarstern has crossed a sharp front
between 38!N and 36!N, which manifests in north–south
177
distributions of individual PFC concentrations (Fig. 3). For example,
PFHxA and PFNA exhibit little variability from Sta. 1 up to Sta. 10,
where PFHxA drops 6-fold to St. 11, while PFNA drops twofold.
These drastic changes are collocated with the Azores Current
(Front), which is known to extend zonally along 34–35!N (Gould,
1985). Coincident with these sharp drops is the PFHpA emergence
at Sta. 11, after which PFHpA remains fairly constant up to Sta. 20.
This location marks the point where the Canary Current veers offshore from the African coast westward.
The sharp drops in PFC concentrations seem to contrast with
the rather gradual southward increase of SST along the ship track
until 20!N (Ahrens et al., 2009), which can be explained by the
divergent nature of the NW African upwelling area, one of the largest and most persistent eastern boundary upwelling regions in the
World Ocean. This phenomenon illustrates the profound difference
between the largely divergent nature of eastern boundaries and
largely convergent nature of western boundaries. Indeed, the western boundary regions feature convergences of cold and warm currents that create the largest SST gradients in the World Ocean (e.g.,
Labrador Current and Gulf Stream; Falkland/Malvinas Current and
Brazil Current; Oyashio and Kuroshio). A study in contrast, the
mostly divergence-dominated environments along eastern boundaries of the Atlantic, Indian, and Pacific oceans are not conducive to
forming exceptionally strong SST fronts. Thus, the rather gradual
north–south increase in SST along the Polarstern track is not at
all surprising.
Data from the 2008 voyage of RV Polarstern (Fig. 4) reveal a different picture, devoid of sharp fronts. It is hard to rationalize the
drastic change of pattern. The most obvious interpretation of this
striking metamorphosis is temporal variability of either the front
itself, varying concentrations of PFCs upstream of the cruise track
and/or trends masked by varying sampling stations. Either way,
this is a topic of a separate investigation, which is well beyond
the scope of this study.
There was no sampling of PFCs during the 2009 voyage. The PFC
sampling resumed during the 2010 voyage, when, again, sharp
fronts, albeit at different locations (compared with 2009), were
observed (Fig. 5). Two fronts stand out, the Canary Current Front
between Stas. 12–13 (20–25!N) and South Equatorial Current Front
between Stas. 19–20 (3–7!S). The best indicator of the Canary Current Front is PFOA: Its concentration drops precipitously below
MDL across this front. Concentrations of PFHxA and PFHpA also
drop across this front, albeit less abruptly. The best indicator of
the South Equatorial Front is PFHxA, whose concentration drops
below MDL across this front.
Data collected by RV Polarstern in 2010 across the Canary Current Front are consistent with observations in the 2007 cruse of
RV Oden (Fig. 6). All four PFCs dropped precipitously across this
front between Stas. 6–7 (23.6–27.3!N), immediately SW of the
Canary Islands. In both cruises (Oden-2007 and Polarstern-2010)
the front was detected at approximately the same location south
or southwest of the Canary Islands. However, the PFOS signatures
of this front observed in 2007 and 2010 were quite different:
Whereas in 2007 the PFOS (alongside with PFOA) was an excellent
tracer of the front, in 2010 the PFOS concentrations have not changed across this front – unlike the PFOA concentrations that fell
below MDL across the front. Another prominent feature along this
section sampled by RV Oden in 2007 is the Rio de La Plata Plume in
the SW Atlantic revealed by maximum concentrations of all PFCs
(especially PFOS and PFOA) except for PFHpA.
Measurements of HCH from RV Polarstern in November 2008
(Xie et al., 2011b, Fig. 1b therein) revealed the same sharp transition from polluted European waters to relatively pristine waters
off West Africa. In 2008, this sharp transition occurred in two
steps (38–31!N and 25–16!N) (Xie et al., 2011b, Fig. 1b and
Table 3 therein), which is consistent with the location of the
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R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
Fig. 3. North–south distribution of surface PFCs along the 2007 track of RV Polarstern (inset). Data from Ahrens et al. (2009, supporting information, Table S4).
Fig. 4. North–south distributions of surface PFCs along the 2008 track of RV Polarstern (inset). Data from Ahrens et al. (2010b, supplementary material, Table S3).
same transition based on our measurements from RV Oden in
2007 (27–24!N) (Fig. 6). Measurements of polycyclic aromatic
hydrocarbons (PAHs) in October 2005 along a similar north–
south transect in the NE Atlantic (Nizzetto et al., 2008, Fig. 1b
therein) revealed a stepwise decline in dissolved PAHs concentrations south of the Canary Islands, between 22!N and 17!N,
slightly south of the front location in 2007 and 2008. Lakaschus
et al. (2002) compiled measurements of HCHs made from RV
Polarstern between the North Sea and Antarctica in 1987, 1989,
1991, 1993, 1995, 1997, and 1999. These data consistently show
a sharp decline in a-HCH from the Portugal Current to the Canary
Current (Lakaschus et al., 2002, supporting info). The best spatial
resolution along this track was achieved in 1999, revealing a
steep a-HCH drop (front) between 25!N and 20!N (Lakaschus
et al., 2002, Fig. 3a). Taking together, these data show that notwithstanding the manifold decrease in a-HCH over the last decades, the contaminant’s spatial pattern remains fairly robust,
featuring a sharp transition from European waters to the subtrop-
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R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
Fig. 5. North–south distributions of surface PFCs along the 2010 track of RV Polarstern (inset). Data from Zhao et al. (2012, supporting information, Table S4).
Fig. 6. North–south distributions of surface PFCs along the 2007 track of RV Oden (inset). Data from Benskin et al. (2012b, supporting information, Table S12).
Table 3
Sea surface concentrations of selected organic contaminants, temperature (SST) and salinity (SSS) along the RV Endeavor 2009 transect from the Sargasso Sea towards
Narragansett Bay. Data from Lohmann et al. (2012, 2013a, 2013b). DL, detection limit.
Region
Sta.
Lat. (!N)
Lon. (!W)
SST (!C)
SSS (psu)
a-HCH (pg/L)
c-HCH (pg/L)
Sum PAHs (pg/L)
Sum BDEs (pg/L)
Sargasso Sea
Gulf Stream
Slope Sea – South
Slope Sea – North
New England Shelf
53
54
55
56
57
36.0
37.3
38.6
39.7
40.5
70.0
70.3
70.6
71.0
71.2
26.7
28.3
24.7
24.1
20.3
36.3
36.1
35.3
35.0
33.2
<DL
<DL
12
22
47
<DL
<DL
4.9
4.6
9.6
5.58
<DL
3.46
84.76
972
<DL
<DL
<DL
<DL
9819
180
R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
Fig. 7. North–south distributions of surface PFCs along the 2009 RV Endeavor cruise 464 track (insets). Data from Benskin et al. (2012b, supporting information, Table S11).
Fig. 8. North-south surface distributions of PFCs, PCBs, and HCB along the 2009 RV Endeavor cruise 464 track. Shown are samples representing the Sargasso Sea–Gulf Stream–
Rhode Island Sound transect. The cruise track is shown in Fig. 7 (left inset). Data from Lohmann et al. (2012) (supporting information, Table S18 for PCBs and OCPs).
ical gyre. The temperature step of a few degrees across the Canary Front cannot cause a noticeable change in HCH concentrations. Strong atmospheric deposition of POPs can leave an
oceanic imprint such as elevated aqueous PCB concentrations
off West Africa, likely from atmospheric emissions (including
those from illegal waste dumping of banned POPs) rather than
ocean currents (Gioia et al., 2008b, 2011).
Gulf Stream Front
One of the strongest fronts of the World Ocean – the Gulf
Stream Front – was sampled during the 2009 cruise 464 of RV
Endeavor, EN 464 (Figs. 7–9). The cross-Gulf Stream pattern
revealed by Endeavor was quite peculiar. The warm (southern)
front of the Gulf Stream was not detected in SST, apparently
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R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
Fig. 9. North–south distributions of surface PCBs, SST and SSS along the 2009 RV Endeavor cruise 464 track. The cruise track is shown in Fig. 7 (inset). Data from Lohmann
et al. (2012) (supporting information, Table S18 for PCBs and Table S7 for SST and SSS.
because the SST signature of the Gulf Stream is extremely weak in
late July when summer heating all but obliterates surface fronts.
The salinity signature of the Gulf Stream was still noticeable
(Benskin et al., 2012b, supporting information, Table S9) although
salinity data were rudimentary. All four PFCs peaked as Endeavor
left the Gulf Stream by crossing over the cold (northern) front
bounding the Gulf Stream between Sta. 29–30 (37.3–38.2!N). Concentrations of all four PFCs remained very high at Stas. 30–31
(38.2–39.0!N), decreased as Endeavor proceeded northward, and
peaked over the New England Shelf, in the proximity of Rhode
Island Sound and Narragansett Bay.
Along the same RV Endeavor transect in 2009, samples were
also taken for hydrophobic organic compounds, including various
organochlorine pesticides, PCBs, PAHs and PBDEs. Results show
that several pollutants are still predominantly land-derived, i.e.
those where on-going emissions from urban/industrial areas (manifested in enhanced concentrations in coastal regions) exceed concentrations in the open ocean. Examples of these pollutants are
PAHs, PBDEs, a-HCH and lindane with higher concentrations in
Rhode Island Sound, decreasing to concentrations below detection
limit in the open ocean (Table 3). As noted above, there is only a
small (with respect to physico-chemical properties) temperature
gradient (around 7 K) between the Sargasso Sea and Rhode Island
Sound, so the sharp decrease in concentration of most pollutants
is due to removal (by particle settling, photolysis and biodegradation) and dilution, rather than redistribution. For example, the dissolved concentration of a-HCH decreased roughly 4-fold between
the Gulf Stream and Rhode Island Sound, while the temperature
difference affects air–water partitioning by less than a factor of 2
(Table 3).
In contrast, the more persistent and already phased-out PCBs
and HCB displayed concentration distributions that suggest ocean
fronts’ importance in maintaining concentration gradients
(Fig. 8): PCB 28 and HCB had higher concentrations in the Gulf
Stream (probably originating from the Gulf of Mexico) than in
Rhode Island Sound. The higher molecular weight PCBs 52, 101
and 118 displayed a distribution that shows a decrease in concentrations between the Gulf Stream and Rhode Island Sound, possibly
due to sorption to particles in-between these two regions. For PCBs
Table 4
P
Change (D) in concentrations of the sum of perfluorinated compounds ( PFCs) in
Atlantic Ocean surface water samples affected by freshwater outflows of the Amazon
River and Rio de la Plata. Data from Benskin et al. (2012b). SSS, sea surface salinity.
P
P
Station
SSS (psu)
PFCsa (pg/L)
Freshwater (%)
D PFCsa (pg/L)
Amazon River plume (RV Endeavor transect in 2009)
15
37
110
14
30
140
19
13
29
77
21
12
25
170
32
11
34
170
6
10
36
110
Mean:
160
!160
190
970
Rio de la Plata plume (RV Oden transect in 2007)
24
36
110
25
34
540
7
26
34
350
7
27
35
85
Mean:
6500
3700
290
5100
P
PFCs = sum of perfluorohexanoic acid (PFHxA), perfluoroheptanoic acid
(PFHpA), perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA), perfluorodecanoic acid (PFDA), perfluoroundecanoic acid (PFUnA), perfluorododecanoic acid
(PFDoA), perfluorotridecanoate (PFTrA), perfluorotetradecanoic acid (PFTA), perfluorobutanesulfonate (PFBS), perfluorohexane sulfonate (PFHxS), perfluorooctane
sulfonate (PFOS), and perfluorooctane sulfonamide (PFOSA).
a
28 and 52, the Gulf Stream carried higher concentrations than the
Sargasso Sea, while the opposite was true for PCBs 101 and 118.
Elevated concentrations of HCB and PCBs 28 and 52 could originate
from the Gulf of Mexico/Mississippi River. Taken together, the
transect off the U.S. Northeast showed the importance of the Gulf
Stream with its associated fronts coupled with the pollutant characteristics (dominated by primary or secondary emissions). The
sampling was not detailed enough to find evidence of other fronts
closer to shore (mid-shelf front, shelf-slope front).
River plumes
River plumes are notable features crossed by research vessels
traversing the Atlantic Ocean (Benskin et al., 2012b). Two outstanding river plumes in the western Atlantic Ocean were crossed
182
R. Lohmann, I.M. Belkin / Progress in Oceanography 128 (2014) 172–184
by RV Endeavor in 2009, namely the Amazon River and Rio de la
Plata plumes. We estimated the influence of the Amazon River
and Rio de la Plata plumes based on salinity differences for PFCs
(Table 4). We applied a two-endmember approach to estimate
the source strength of the river plumes, assuming that any increase
of PFCs in the plume was solely due to the river outflow. The background (fully marine) PFC concentration was subtracted, and the
remaining difference normalized to the freshwater fraction of the
sample (Table 4) to obtain the concentration delivered by the river’s plume.
In the case of the Amazon River plume, dissolved concentrations
increased by up to 10 pg L!1 relative to the concentrations in fully
marine waters, while salinity was at 27 psu (a dilution of seawater
(36.4 psu in this region) by 26%). As evident from Table 4, the four
consecutive samples taken in the Amazon River plume revealed
substantial differences in concentrations (Benskin et al., 2012b).
This most likely reflected the heterogeneity of the plume. This subject is poorly studied, largely because it requires high-resolution
observations across a river plume and such data sets are extremely
rare. Another reason is the plume scale. Small plumes naturally
tend to be more uniform than large plumes created by such rivers
as the Amazon River. The Amazon (freshwater) plume that we
sampled several hundred km offshore contributed an additional
tens-to-hundreds pg L!1 of the various PFCs towards the Atlantic
Ocean (Fig. 7). The average discharge of the Amazon River is ca.
1.6 " 105 m3 s!1 or 5 " 1012 m3 yr!1 (Salisbury et al., 2011). Consequently, ca. 1.4 t of PFCs are flushed annually into the open Atlantic
Ocean (Table 4).
Waters affected by the Rio de la Plata plume were sampled during the RV Oden cruise in 2007 at two stations (Benskin et al.,
2012b), as evidenced by decreases in salinity in samples 24 and
25 (by 2 psu), coupled to a marked increase in PFCs (Fig. 6). Concentrations of sum PFCs increased from #100 pg/L outside the
plume to 350–540 pg/L in the plume. The dominant PFCs were
PFOS, which increased 4-fold to 140–170 pg/L in the plume, and
PFOA, which increased at least 10-fold to 100 pg/L in the plume.
Other notable PFCs were PFUnA, which reached 90 pg/L in the
plume, and PFHxA, which increased three- to tenfold in the plume
(Table 4).
The average discharge of the Rio de la Plata is ca.
2.2 " 104 m3 s!1 or 7 " 1011 m3 yr!1 (Framiñan and Brown,
1996). Combining the increase attributed to the freshwater with
the river’s annual discharge results in an annual delivery of 3.5 t
of PFCs from the Rio de la Plata into the South Atlantic Ocean
(Table 4). The striking differences between the Amazon River vs.
Rio de la Plata discharges of PFCs likely reflect profound differences
in demographics and economic geography of their respective
watersheds. Indeed, the Amazon River Basin is populated by just
about 5 million people spread over 7 million km2. A study in contrast, the Rio de la Plata drains the Buenos-Aires megalopolis, with
14 million people and most of Argentina’s industrial capacity
(Colombo et al., 2011), and the Montevideo agglomeration (Uruguay) with 2 million people.
To put the total mass of PFCs delivered to the Atlantic into perspective, we calculated the outflow of PFCs from Narragansett Bay,
where concentrations of PFCs is very high (5.8 ng/L, after Benskin
et al., 2012b). Yet long-term average annual freshwater discharge
of the Bay is only 107.5 m3/s (or 3.4 " 109 m3/yr) (Ries, 1990),
resulting in an average annual delivery of total PFCs of ca. 77 kg/
yr. This amount pales in comparison with the amounts of PFCs discharged by the Amazon River and Rio de la Plata.
The same RV Endeavor transect in 2009, which reported PFC
concentrations across the Amazon River plume, has also reported
PCBs, pesticides, PAHs and PBDEs (Lohmann et al., 2012, 2013a,
2013b). In most cases, no significant change in concentrations
was found. Concentrations of PCBs increased slightly across the
Amazon plume (Fig. 9), but the scatter in PCB concentrations is
clearly elevated relative to that in PFCs, making it difficult to draw
firm conclusions.
On a more general note, river plumes have the potential to be
important pathways into the oceans for water-soluble and persistent compounds (Li and Daler, 2004) beyond PFCs, such as herbicides (Alegria and Shaw, 1999), pharmaceuticals (Zhang et al.,
2012) and personal care products (Qi et al., 2014). For example,
the more recalcitrant artificial sweetener sucralose has been
detected in part of the Gulf Stream already (Mead et al., 2009).
Conclusions
Examining recent cruise results of organic pollutant concentrations across the Atlantic Ocean revealed the importance of major
fronts for their dispersal, coupled with pollutant-specific characteristics linked to their sources, partitioning and persistence.
Strong increases in concentrations of the sum of perfluorinated
P
compounds ( PFCs) were observed in two river plumes: from
100 pg/L outside the plumes to 540 pg/L in the Rio de la Plata
plume and to 170 pg/L in the Amazon River plume. A sharp transition (front) from polluted European waters to the relatively pristine trade winds zone could be observed on a transect from the
Bay of Biscay to southern Argentina. North of the front (which is
linked to the Canary Current), concentrations of PFOA, PFOS and
PFHxA were >100 pg/L, decreasing to <50 pg/L (and below detection limit for PFHxA) south of the front. The same transition across
the Canary Current was observed for hydrophobic organic
pollutants, such as hexachlorocyclohexanes and even polycyclic
aromatic hydrocarbons. More complex pollutant patterns were
observed between the Gulf Stream and Rhode Island Sound. The
more persistent polychlorinated biphenyls and hexachlorobenzene
actually displayed higher concentrations in the Gulf Stream than in
the Sargasso Sea or Rhode Island Sound. In contrast, the emerging
perfluorinated compounds displayed a strong increase from the
Gulf Stream towards Rhode Island Sound. The sampling campaigns
we re-evaluated were not detailed enough to assess the effect of
mid-shelf and shelf-break fronts on pollutant dynamics, though
the possibility of detecting perfluorinated compounds in one liter
samples opens the door to a much more detailed understanding
of the interplay between fronts and organic pollutant dynamics
in the oceans.
Acknowledgements
R.L. acknowledges support from NSF’s Office of Polar Programs
(ARC 1203486) to study the transport pathways of organic pollutants into the Arctic Ocean. I.B.’s interest in POPs and fronts was
stimulated by participation in the SETAC Asia Pacific 2010 Conference in Guangzhou; the travel support of the South China Sea Institute of Oceanology, Chinese Academy of Sciences, is greatly
appreciated.
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