BGBiogeosciencesBGBiogeosciences1726-4189Copernicus GmbHGöttingen, Germany10.5194/bg-12-1697-2015A halocarbon survey from a seagrass dominated subtropical lagoon, Ria
Formosa (Portugal): flux pattern and isotopic compositionWeinbergI.BahlmannE.enno.bahlmann@zmaw.deEckhardtT.MichaelisW.SeifertR.University of Hamburg, Institute for Biogeochemistry and Marine Chemistry,
Bundesstraße 55, 20146 Hamburg, GermanyE. Bahlmann (enno.bahlmann@zmaw.de)17March2015126169717111May201410July20148January201520January2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://www.biogeosciences.net/12/1697/2015/bg-12-1697-2015.htmlThe full text article is available as a PDF file from https://www.biogeosciences.net/12/1697/2015/bg-12-1697-2015.pdf
In this study we report fluxes of chloromethane (CH3Cl), bromomethane
(CH3Br), iodomethane (CH3I), and bromoform (CHBr3) from two
sampling campaigns (summer and spring) in the seagrass dominated subtropical
lagoon Ria Formosa, Portugal. Dynamic flux chamber measurements were
performed when seagrass patches were either air-exposed or submerged.
Overall, we observed highly variable fluxes from the seagrass meadows and
attributed them to diurnal cycles, tidal effects, and the variety of possible
sources and sinks in the seagrass meadows. The highest emissions with up to
130 nmol m-2 h-1 for CH3Br were observed during tidal
changes, from air exposure to submergence and conversely. Furthermore, during
the spring campaign, the emissions of halocarbons were significantly elevated
during tidal inundation as compared to air exposure.
Accompanying water sampling performed during both campaigns revealed elevated
concentrations of CH3Cl and CH3Br, indicating productive sources
within the lagoon. Stable carbon isotopes of halocarbons from the air and
water phase along with source signatures were used to allocate the
distinctive sources and sinks in the lagoon. Results suggest that CH3Cl
was rather originating from seagrass meadows and water column than from salt
marshes. Aqueous and atmospheric CH3Br was substantially enriched in
13C in comparison to source signatures for seagrass meadows and salt
marshes. This suggests a significant contribution from the water phase on
the atmospheric CH3Br in the lagoon.
A rough global upscaling yields annual productions from seagrass meadows of
2.3–4.5 Gg yr-1, 0.5–1.0 Gg yr-1, 0.6–1.2 Gg yr-1,
and 1.9–3.7 Gg yr-1 for CH3Cl, CH3Br, CH3I, and
CHBr3 respectively. This suggests a minor contribution from seagrass
meadows to the global production of CH3Cl and CH3Br with about 0.1
and 0.7 %, respectively. In comparison to the known marine sources
for CH3I and CHBr3, seagrass meadows are rather small sources.
Introduction
The halocarbons chloromethane (CH3Cl), bromomethane (CH3Br),
iodomethane (CH3I), and bromoform (CHBr3) are prominent precursors
of reactive halogens, which affect the oxidative capacity of the atmosphere
and initiate stratospheric ozone destruction (Saiz-Lopez and von Glasow,
2012,
and references therein). Therefore, during the past decades, the sources and
sinks of these trace gases have been intensively studied.
For CH3Cl, recent atmospheric budget calculations suggest that the known
sinks can be balanced by large emissions from tropical terrestrial sources
(Saito and Yokouchi, 2008; Xiao et al., 2010). Nevertheless, these
calculations still incorporate large uncertainties. The atmospheric budget of
CH3Br still remains unbalanced, with the known sinks exceeding known
sources by about 30 % (Yvon-Lewis et al., 2009). The current emission
estimates for CH3I and CHBr3 are assigned with even larger
uncertainties (Bell et al., 2002; Quack and Wallace, 2003).
Stable carbon isotopes of halocarbons have been applied to further elucidate
their sources and sinks by using individual source signatures (Keppler et
al., 2005). While this was primarily done for CH3Cl, first isotopic
source signatures of naturally-produced CH3Br were recently reported
(Bill et al., 2002; Weinberg et al., 2013). Moreover, the biogeochemical
cycling of halocarbons underlies various transformation processes, which can
be studied by the stable carbon isotope approach in addition to flux and/or
concentration measurements.
Coastal zones are reported as being important source regions of halocarbons.
In these salt water affected systems, halocarbon producers comprise
phytoplankton (Scarratt and Moore, 1998), macroalgae (Gschwend et al.,
1985), salt marshes (Rhew et al., 2000), and mangroves (Manley et al.,
2007).
With a net primary production of 1211 g C m-2 yr-1, seagrass
meadows are one of the most productive ecosystems with a similar global
abundance as mangroves and salt marshes (Duarte et al., 2005). They cover
huge areas of the intertidal and subtidal zone in temperate and
subtropical/tropical regions. Thus, they may represent an additional source
for halocarbons to the atmosphere, which is not yet sufficiently studied.
Seagrass meadows are highly diverse ecosystems with respect to potential
halocarbon producers. Along with the seagrass itself, they comprise epiphytes
such as microalgae and diatoms, and sediment reassembling microphytobenthos
and bacteria communities. All these constituents of the benthic community
have been reported to produce halocarbons (Amachi et al., 2001; Blei et al.,
2010; Manley et al., 2006; Moore et al., 1996; Rhew et al., 2002; Tokarczyk
and Moore, 1994; Urhahn, 2003). While first evidence for the release of
halocarbons from seagrass was obtained by incubation experiments (Urhahn
2003), we could recently confirm this production potential in a field study
of a temperate seagrass meadow in northern Germany (Weinberg et al., 2013).
In order to refine these results, we conducted two field campaigns in the
subtropical lagoon Ria Formosa, Portugal in 2011 and 2012. Here we report
the results of these campaigns comprising dynamic flux chamber measurements
for halocarbons over seagrass meadows during air exposure and tidal
inundation. Using the flux and isotopic data, we present the first insights into
the environmental controls of halocarbon dynamics within this ecosystem.
Materials and methodsSampling site
The Ria Formosa, covering an area of 84 km2, is a mesotidal lagoon at
the south-eastern coast of the Algarve, Portugal (Fig. 1). It is separated
from the Atlantic Ocean by a series of barrier islands and two peninsulas.
About 80 % of the lagoon is intertidal with a semi-diurnal tidal regime
and tidal ranges between 1.3 m during neap tides and 3.5 m during spring
tides (Cabaço et al., 2012). Due to negligible inflow of fresh water and
high exchange of water with the open Atlantic during each tidal cycle, the
salinity within the lagoon is 35 to 36 PSU year round, except for periods of
heavy rainfalls. About a quarter of the intertidal area (13.04 km2) is
covered by dense stands of Zostera noltii Hornem (Guimarães et
al., 2012). Further, but much less abundant, seagrass species in the lagoon
are Zostera Marina L. and Cymodocea nodosa (Ucria)
Ascherson, which are mainly located in shallow parts of the subtidal areas
(Santos et al., 2004). About 30 % of the lagoon's area is covered with
salt marsh communities (R. Santos, personal communication, 2013).
Map of the lagoon Ria Formosa, Portugal. Asterisk: site of seagrass
meadow studies; triangle: sampling site on the Praia de Faro (upwind
position). Dots with numbers represent sampling points during the transect
cruise.
Sampling
We conducted two sampling campaigns in the western part of the lagoon at the
Ramalhete research station (Centre of Marine Sciences (CCMAR), Universidade
do Algarve) in the vicinity of Faro (37.0∘ N, 7.6∘ W)
(Fig. 1). The sampling was carried out from 23 July–7 August 2011 and
17 Apri–28 April 2012, coinciding with the beginning (2012 campaign) and peak
(2011 campaign) of the seagrass reproductive season. Ambient air temperatures
were distinctively different between both campaigns, ranging from 21 to
27 ∘C (mean 24 ∘C) with almost entirely clear weather in
summer and 13 to 23 ∘C (mean 17 ∘C) in spring with frequent
strong cloud cover. Mean water temperatures were 25.9 ∘C (summer)
and 17.5 ∘C (spring). The prevailing wind direction during both
campaigns was west south-west to with rather low average wind speeds of
4 m s-1 during summer and 5 m s-1 during spring.
During the two campaigns, we used different dynamic flux chamber systems.
During the 2011 campaign, we measured the halocarbon fluxes during air
exposure using a quartz-glass chamber (0.1 m2 surface area, 7 L
enclosure volume) as described in Weinberg et al. (2013) with some
adjustments. For this study, a permanent backup flow
(3 ± 0.2 L min-1) through the flux chamber during sampling and
the change of cryotraps was applied to ensure sufficient mixing. Further, to
overcome analytical problems with the high humidity in the sampled air, the
water content was reduced using a condenser (-15 ∘C). Briefly, the
quartz-glass flux chamber was placed on the seagrass patch and sealed with
surrounding sediment. Two sampling systems were operated simultaneously,
measuring inlet and outlet air of the flux chamber (flow rate
1 ± 0.2 L min-1). Prior to sampling, the flux chamber was
flushed for about 10 min ensuring sufficient equilibration of compounds in
the chamber air.
During the 2012 campaign, we used a dynamic flux chamber system
(0.037 m2 bottom surface area, 8 L enclosure volume) suitable for flux
measurements during both periods of air exposure and tidal immersion. The
properties and setup of this dynamic chamber system is described in detail
elsewhere (Bahlmann et al., 2014). Since this system acts as an ordinary
purge and trap system, the extraction efficiencies were simulated using
halocarbon equilibrated artificial seawater. While the results from these
tests revealed that monohalomethanes were almost completely extracted (≥90 %), the purge efficiencies for CHBr3 were only 33 %. Thus
the reported CHBr3 fluxes determined from seagrass meadows using the
submergible chamber system represent an under-estimate.
Based on the sampling system for the determination of stable carbon isotopes
of halocarbons (Bahlmann et al., 2011), we modified the cryogenic trapping
system for the measurements of halocarbon mixing ratios, in order to
establish a better temporal resolution by reducing the analysis time. This
results in a final air volume of 28 ± 5 L of air at the inlet and the
outlet of the chambers, respectively. The specifications along with the
results from test surveys are given in the Supplement.
The seagrass species sampled was exclusively Z. Noltii. The
seagrass patches sampled had an area coverage of > 95 % and were free
of visible epiphytes such as macroalgae. In this low to medium intertidal
region the epiphytes of Z. Noltii are almost exclusively diatoms,
whose contribution ranges from 0.5 to 4 % of the total seagrass biomass
(Cabaço et al., 2009). We further determined the fluxes from an adjacent
bare sediment spot during the 2011 campaign. On 2 August 2011, these
chamber-based measurements were complemented by atmospheric sampling at a
nearby beach (Praia de Faro, upwind site) about 3 km distant from the lagoon
during the summer campaign 2011 (Fig. 1). At this time the wind direction was
south-westerly, reflecting background air from the coastal ocean.
Discrete water samples for the determination of dissolved halocarbons
concentration and isotopic composition at high tide were taken during both
campaigns. The samples were taken directly above the studied seagrass meadow
using Duran glass bottles (1–2 L volume). Air and sediment intrusions
during water sampling were avoided. The water depth was between 0.3 m and
1 m. On 24 April 2012, a transect cruise through the middle and western part
of the lagoon was conducted during rising waters (Fig. 1). The water samples
were taken from a water depth of 1 m. Dissolved halocarbons were extracted
from seawater using a purge and trap system. Seawater was purged with helium
5.0 (purge flow 1 L min-1) for 30 min. After water vapour
reduction of the purge gas, the compounds were enriched on cryotraps
(submerged in a dry shipper). The shape of the cryotraps used here was the
same as those for flux chamber and atmospheric samples. The water samples
were usually processed within 30 min after sampling. Samples from the
transect cruise were stored in the dark at 4 ∘C and analysed within
8 h. Purge efficiencies of monohalomethanes from lagoon water were
≥ 95 % (1 and 2 L samples). However, the less volatile CHBr3
was only extracted with 50 % (1 L samples) and 30 % (2 L samples).
Therefore, the results of water concentration were corrected for the
respective purge efficiency for this compound.
Measurement and quantification
The measurement procedure is described in detail in the Supplement. Briefly, compounds adsorbed on the cryotraps were thermally
desorbed and transferred to Peltier-cooled adsorption tubes. The analytes
were further desorbed from the adsorption tubes and refocused cryogenically
before injection to the GC-MS system. Air and water samples were measured
on-site at Ramalhete research station using a GC-MS system (6890N/5975B,
Agilent, Germany) equipped with a CP-PorabondQ column (25 m,
0.25 µm i.d., Varian, Germany). The GC-MS was operated in the
electron impact mode. Identification of compounds was executed by retention
times and respective mass spectra. Aliquots of gas standard (Scott EPA TO
15/17, 65 compounds, 1 ppm each in nitrogen, Sigma Aldrich, Germany)
containing CH3Cl, CH3Br, and CHBr3 were applied to quantify
the target compounds. During on-site measurements, CH3I was quantified
using the response factor against CH3Br. The response factor was
determined prior to the campaign. Equivalent amounts of CH3I and
CH3Br from single gas standards were analysed together for the response
factor calculation. The analytical limit of detection was 0.3 ppt
(pmol mol-1) for the halocarbons. The accuracy of the entire sampling
method (sampling, sample treatment, measurement) was derived from test
samples in triplicates. The deviation between the individual samples for
CH3Cl, CH3Br, CH3I, and CHBr3 was 5.4, 6.3, 15.4 and
6.7 %, respectively. A series of procedural blanks (cryotraps and
adsorption tubes) were taken during the sampling campaigns. The occasionally
detected blanks of CH3Cl and CH3Br from these determinations were
≤ 3 % to the “real” samples taken from the seagrass meadows
during sampling campaigns. Therefore, the halocarbon fluxes were not blank
corrected.
Air and water samples for determining the isotopic composition of halocarbons
were transferred to adsorption tubes and stored at -80 ∘C until
they were measured. The analysis was conducted using the GC-MS-IRMS system at our
home laboratory (Bahlmann et al., 2011). Additional transport and storage
blanks were processed, which revealed no contamination for all halocarbons
studied.
Calculations
The fluxes were determined with dynamic flux chambers. The chamber is
positioned on the a sampling spot and flushed continuously with ambient air.
The mixing ratios of compounds at the inlet and outlet air are then measured.
The difference of mixing ratios of compounds between inlet and outlet air
along with the flushing rate and the surface area are used for the flux
calculation (FNet, nmol m-2 h-1):
FNet=Q×(Cout-Cin)A×V×1000
Here, Q is the flushing rate of air through the chamber (L h-1),
Cout and Cin are the mixing ratios of target compounds
(ppt) at the outlet and the inlet of the flux chamber. A is the enclosed
surface area of the flux chamber (m2) and V is the molar volume (L) at
1013.25 mbar and 298.15 K.
For calculation of the sea–air fluxes from the lagoon water, the inlet
samples of the flux chamber were used, which reflect the air mixing ratios.
Where no corresponding inlet sample was available, the campaign means were
applied. After conversion of the air mixing ratios to pmol L-1 using
temperature data and the respective molar volume of the ambient air, the
sea–air fluxes (F, nmol m-2 h-1) of halocarbons were
calculated by the equation:
F=kw×(Cw-Ca×H-1)
where kw is the gas exchange velocity (m h-1),
Cw and Ca the water concentration and air
concentration (pmol L-1), respectively, and H the dimensionless and
temperature dependent Henry's law constant taken from Moore (2000) for
CH3Cl, Elliott and Rowland (1993) for CH3Br and CH3I, and
Moore et al. (1995) for CHBr3. Several approximations emerged to
estimate the relationship between the gas exchange velocity k and the wind
speed u for open and coastal oceans (e.g. Nightingale et al., 2000;
Wanninkhof, 1992). These estimations rely on assumptions that trace gas
exchange is based on wind-driven turbulence. This is not applicable in
shallow estuarine and riverine systems where the sea–air gas exchange is
additionally driven by wind-independent currents and the bottom turbulence,
and thus water depth and current velocities further play a major role
(Raymond and Cole, 2001). Studying the sea–air exchange in the Ria Formosa,
these additional factors have to be considered in addition to wind driven
outgassing. Therefore, we used the parameterisation of kw with
the assumption that wind speed and water current driven turbulence are
additive (Borges et al., 2004):
kw=1.0+1.719×w0.5×h-0.5+2.58×u
where w is the water current (cm s-1), h the water depth (m) and
u the wind speed (m s-1). For the calculations of the sea–air flux in
the lagoon, a mean water depth of 1.5 m (Tett et al., 2003) and a mean water
current of 24 cm s-1 (Durham, 2000) was used. The Schmidt number
( Sc) expresses the ratio of transfer coefficients of the kinematic
viscosity of water and gas diffusivity of interest. The gas exchange velocity
kw for each gas was then normalised to a Schmidt number of 660,
assuming a proportionality to Sc-0.5 (Borges et al., 2004).
The individual Schmidt numbers were obtained from Tait (1995) for
CH3Cl, De Bruyn and Saltzman (1997) for CH3Br and CH3I, and
Quack and Wallace (2003) for CHBr3.
Summary of air mixing ratios and water concentrations of halocarbons
in the Ria Formosa and at the background site (Praia de Faro) for the
sampling campaigns in summer 2011 and spring 2012. Values are given as means
(bold) and ranges (in parentheses). Samples from the Ria Formosa are data
from the inlet of the flux chambers with a sampling height of 1 m above
ground (summer: n=36; Praia de Faro: n=5; spring n=47). Given water
concentrations refer to n=8 (summer) and n=10 (spring).
Air mixing ratio Air mixing ratio Water concentration Ria Formosa (ppt)Praia de Faro (ppt)Ria Formosa (pmol L-1)Summer 2011CH3 Cl828 (503–1490)613 (498–685)220 (158–301)CH3 Br22 (8–118)13 (9–19)8 (5–11)CH3 I3 (2–11)1 (0.8–2)12 (4–18)CHBr315(6–31)8 (6–9)102 (67–194)Spring 2012CH3 Cl654 (484–976)–166 (101–267)CH3 Br12 (4–40)–10 (6–28)CH3 I1 (0.4–4.8)–7 (2–16)CHBr32 (0.4–10)–62 (39–133)
Water concentration (pmolL-1) and stable carbon isotope
ratios of halocarbons (‰) obtained from a 2 h transect cruise
on 24 April 2012 (see Fig. 1 for sampling positions).
SampleTimeCH3ClCH3BrCH3ICHBr3(local)pmolL-1‰pmolL-1‰pmolL-1‰pmolL-1‰115:09121-40.95-25.65-20.026-25.8215:50241-42.37-21.25-31.155-18.3315:5896–9–2–21–416:10106–11–5–31–516:21180-44.319-35.914-44.595-18.9616:4672–5–3–18–716:5082–4–5–14–ResultsHalocarbons in the atmosphere and lagoon water
The air mixing ratios in the lagoon were adopted from the inlets of the flux
chambers at 1 m above ground during both campaigns. The results of these
measurements and those of the upwind site outside the lagoon (Praia de Faro)
are presented in Table 1. In summer, elevated air mixing ratios of the
monohalomethanes were observed during periods of easterly winds when air
masses at the sampling site had presumably passed over major parts of the
lagoon. These mixing ratios reached up to 1490 ppt for CH3Cl, 61 ppt
for CH3Br, and 11 ppt for CH3I, reflecting a potent source in this
system. The mixing ratios at the upwind site (Praia de Faro) were
distinctively lower with mean values of 613 ppt (CH3Cl), 13 ppt
(CH3Br), 1 ppt (CH3I), and 8 ppt (CHBr3), further indicating
a source inside the lagoon. In spring 2012, the mean air mixing ratios in the
lagoon were significantly lower than during summer with 654 ppt for
CH3Cl, 12 ppt for CH3Br, 1 ppt for CH3I, and 2 ppt for
CHBr3.
Discrete water samples were taken above the studied seagrass meadow during
tidal inundation (summer n=9; spring n=10). The results are presented in
Table 1. In summer, concentrations ranged from 158 to 301 pmol L-1
(CH3Cl), 5 to 11 pmol L-1 (CH3Br), 4 to 18 pmol L-1
(CH3I), and 67 to 194 pmol L-1 (CHBr3). During the spring
campaign, the water concentrations were 101 to 267 pmol L-1 for
CH3Cl, 6 to 28 pmol L-1 for CH3Br, 2 to 16 pmol L-1
for CH3I, and 39 to 133 pmol L-1 for CHBr3.
The results obtained from samples of the transect cruise covered in 2012
(Fig. 1) are given in Table 2. We observed an approximate two-fold increase of
concentration for CH3Cl (from 121 to 241 pmol L-1) and CHBr3
(from 26 to 55 pmol L-1) between position 1 (Faro-Olhão inlet) and
position 2 (near to the seagrass meadows studied). The increase was less
pronounced for CH3Br (5 to 7 pmol L-1) and not notable for
CH3I. The seawater at positions 6 and 7, the nearest to the Ancão
inlet, revealed rather low concentrations for all compounds. We further
observed rising concentrations for all halocarbons along positions 3, 4, and
5 with increasing distance to the Ancão inlet. They increased from 96 to
180 pmol L-1 for CH3Cl, from 9 to 19 pmol L-1 for
CH3Br, 2 to 14 pmol L-1 for CH3I, and 21 to
95 pmol L-1 for CHBr3.
Fluxes from seagrass meadows, sediment, and sea–air exchange
The mean fluxes and ranges of CH3Cl, CH3Br, CH3I, and
CHBr3 from seagrass meadows, sediment, and from sea–air exchange
calculations obtained from the two sampling campaigns are given in Table 3.
During the summer campaign (air exposure), we observed highly variable
emission and deposition fluxes ranging from -49 to
74 nmol m-2 h-1 and -5.7 to 130 nmol m-2 h-1 for
CH3Cl and CH3Br, respectively. The variability was less pronounced
for CH3I (0.5 to 2.8 nmol m-2 h-1) and CHBr3 (-0.6
to 5.7 nmol m-2 h-1) where predominantly emissions were
measured. Strongly elevated fluxes up to 130 nmol m-2 h-1 for
CH3Br were recorded in conjunction with tidal change, from air exposure
to inundation and conversely. These high fluxes were substantiated by a
concurrent enhanced atmospheric mixing ratio, ranging from 23 to 118 ppt
(campaign median 14 ppt). Omitting these compound-specific tidal phenomena,
the fluxes of CH3Cl and CH3Br were positively correlated (R2
0.55 ,p< 0.05). There were no significant correlations between CH3I
and CHBr3 and the other investigated halocarbons. Due to the inherent
high variability of the fluxes, halocarbon fluxes were poorly correlated with
solar radiation (R2≤0.20).
Mean net fluxes (bold) and ranges of halocarbons from flux chamber
experiments seagrass meadows and sediments, as well as those from sea–air
exchange calculations. Data were obtained during the summer 2011 and spring
2012 campaigns in the Ria Formosa.
The flux chamber measurements over the sediment during air exposure
predominantly revealed emissions of all four halocarbons (n=5). These fluxes were:
3.6 ± 4.3 nmol m-2 h-1 (CH3Cl);
0.6 ± 0.5 nmol m-2 h-1 (CH3Br);
0.3 ± 0.2 nmol m-2 h-1 (CH3I); and
0.8 ± 1.0 nmol m-2 h-1 (CHBr3). Except for CH3I,
the halocarbon fluxes were statistically significant different from zero
(Mann-Whitney-U test; p< 0.05). Hence, the bare sediment may contribute
to the overall emissions above the seagrass by about 10 to 20 % for
CH3Cl and CH3Br, and 45 % for CHBr3.
During the 2012 spring campaign, the halocarbon fluxes from seagrass meadows
were determined during both periods of air exposure and periods of tidal
immersion. Furthermore, the measurements were complemented by other trace
gases including hydrocarbons and sulphur containing compounds (Bahlmann et
al., 2014). As in the summer campaign, the seagrass meadows were a net source
for all halocarbons studied, but on a lower level. The individual ranges of
air exposure measurements were: -30 to 69 nmol m-2 h-1
(CH3Cl); -0.8 to 3.9 nmol m-2 h-1 (CH3Br); -0.6 to
2.6 nmol m-2 h-1 (CH3I); and -0.5 to
1.3 nmol m-2 h-1 (CHBr3). On average, the seagrass meadows
were a net source also under submerged conditions ranging from: -58 to
100 nmol m-2 h-1 for CH3Cl; -1.6 to
8.3 nmol m-2 h-1 for CH3Br; 0.1 to
8.0 nmol m-2 h-1 for CH3I; and -0.4 to
10.6 nmol m-2 h-1 for CHBr3. Due to the low purge
efficiency of CHBr3 during high tide measurements, the fluxes determined
with the submergible chamber are underestimated for this compound. Despite
this high variability in production/decomposition during air exposure and
inundation, the monohalomethanes were significantly correlated to each other
(R2≥0.50). These correlations were enhanced compared to those found
when the seagrass meadows were air-exposed (R2≥0.56). In this case, only
CH3I and CH3Br were significantly correlated (R20.51).
CHBr3 was only slightly correlated to the monohalomethanes.
While deposition fluxes of CH3Cl and CH3Br of air-exposed seagrass
meadows occurred predominantly during periods of low irradiance in summer,
no obvious relation to the time of day and/or solar radiation was observed
during spring, when deposition fluxes were frequently detected. For CH3I
and CHBr3, uptake was only occasionally observed and situations of
emission clearly dominated.
As in the summer campaign, we observed some remarkable tidal effects on
halocarbon fluxes during the spring campaign. Firstly, the highest fluxes of
all halocarbons were measured when the lagoon water was just reaching the
sampling site. Occasionally this was also observed from air exposure to tidal
inundation, although less pronounced. However, these short-timed effects were
not as strong as during the summer campaign. Secondly, at tidal maximum we
observed deposition fluxes for CH3Cl and CH3Br and deposition
fluxes or very weak emissions for CH3I and CHBr3. Before and after
this period, emission fluxes during incoming tide and ebb flow dominated.
The lagoon water was a net source for all investigated halocarbons to the
atmosphere during both campaigns. In summer, the flux ranges were:
13–45 nmol m-2 h-1 (CH3Cl);
0.6–1.7 nmol m-2 h-1 (CH3Br);
0.5–3.2 nmol m-2 h-1 (CH3I); and
1.0–8.0 nmol m-2 h-1 (CHBr3). The respective fluxes in
spring were 3.5–32 (CH3Cl), 0.5–4.1 (CH3Br), 0.3–3.7
(CH3I), 3.8–24a,nmol m-2 h-1 (CHBr3).
Compilation of stable carbon isotope values of halocarbons (%)
from the two sampling campaigns. Source signatures of seagrass meadows were
calculated using a coupled mass and isotope balance (Weinberg et al., 2013).
Stable carbon isotope ratios of halocarbons were determined for selected
samples of both campaigns (Table 4). Isotopic source signatures from
seagrass meadows for CH3Cl and CH3Br were calculated using a
coupled isotope and mass balance without integration of a possible sink
function (Weinberg et al., 2013).
In 2011, the difference in atmospheric mixing ratios of CH3Cl and
CH3Br between within the lagoon and the upwind position (Praia de Faro)
was accompanied by a shift of δ13C values. More 13C depleted
values were found for CH3Cl in the lagoon (-42 ± 2 ‰)
compared to the upwind position (-39 ± 0.4 ‰). In contrast,
the δ13C values of CH3Br were significantly enriched in
13C by about 10 ‰ inside the lagoon
(-29 ± 5 ‰) as compared to the upwind site
(-38 ± 3). These δ13C values found in air samples in the
lagoon roughly correspond to the δ13C values of CH3Cl
(-43 ± 3 ‰) and CH3Br (-23 ± 3 ‰) found
in samples of lagoon waters.
Atmospheric CH3Cl and CH3Br were on average more enriched in
13C in spring than in summer by 4 and 6 ‰, respectively. While
the δ13C values of CH3Cl in the lagoon water were quite
similar between both periods of the year, those of CH3Br were on average
more depleted in 13C during spring, suggesting certain changes in
production/decomposition processes. The isotopic composition of CH3I in
lagoon water was quite similar between summer (-39 ± 9 ‰)
and spring (mean -37 ± 7 ‰). As for CH3Br, the
δ13C values of CHBr3 were more enriched in 13C in summer
when compared with those of the spring campaign.
The difference in concentration along the transect cruise was accompanied by
variations in the carbon isotopic composition of all compounds (Table 1,
Fig. 2). The most 13C depleted values of CH3Cl, CH3Br, and
CH3I were detected at the position furthest from the inlet.
Interestingly, CHBr3 showed the opposite trend with more 13C
enriched values in the lagoon (-25.8 ‰ vs.
∼-18 ‰).
Using the fluxes and δ13C values from the inlet and outlet of the
flux chamber, we were able to calculate the source signatures of seagrass
covered areas. The resulting source signatures of CH3Cl from seagrass
meadows were similar during both campaigns (-51 ± 6 and
-56 ± 2 ‰, respectively) and independent from the strength
of emission. For CH3Br, we observed most depleted δ13C values
of -53 and -58 ‰ at increased emission fluxes in summer, but
values of -26 and -29 ‰ during periods of low emission. This
corroborates the findings of isotopically heavy CH3Br produced within
the seagrass meadows (-29 ‰) in spring 2012, when all samples
analysed for the isotopic composition were taken at situations of low
emission.
Mean concentrations (bold) and ranges of dissolved halocarbons
(pmolL-1) from the subtropical lagoon Ria Formosa in summer 2011
(n=9) and spring 2012 (n=10) in comparison to published data from coastal
Atlantic waters.
LocationCH3ClCH3BrCH3ICHBr3Faro, Portugal (summer)1220 (158–301)8 (5–11)12 (4–18)102 (66–194)Faro, Portugal (spring)1166 (101–267)10 (6–28)7 (2–16)62 (39–133)East Atlantic 2,#–––68.3 (36.6–102.0)Roscoff, France3,#––12.9 (9.0–31.8)217.4 (124.8–519.4)Greenland, NW Atlantic 4104–260–0.2–16.1–Norfolk, UK 5–3.2 (1.7–8.7)––Menai Strait, UK 6,#––6.7 (0.0–80.0)214.2 (3.0–3588.4)Mace Head, Ireland 7,#–3.7 (1.7–5.7)15.3 (10.9–19.2)388.0 (221.8–554.3)West Atlantic 888.4 (61.5–179.0)1.9 (0.8–5)––North West Atlantic 971.0 (55.0–106.0)–––Nova Scotia, Canada 10––4–6–Gulf of Maine, UK 11,#––8–1840–1240
1 This study; 2 Carpenter et al. (2009); 3 Jones et al. (2009);
4 Tait et al. (1994); 5 Baker et al. (1999); 6 Bravo-Linares and
Mudge (2009); 7 Carpenter et al. (2000); 8 Hu et al. (2010);
9 MacDonald and Moore (2007); 10 Moore and Groszko (1999);
11 Zhou et al. (2005); # macroalgae dominated
DiscussionDissolved halocarbons
A comparison of halocarbon concentrations in the lagoon water to other
measurements of the coastal Atlantic found in the literature is displayed in
Table 5. The lagoon waters appeared to be highly enhanced in CH3Cl.
Except for one early study of Tait et al. (1994), our measurements gave the most
elevated concentrations for this compound. Enhanced concentrations in the
lagoon waters were also found for CH3Br. Given the mean concentrations
from other coastal Atlantic studies (Baker et al., 1999; Carpenter et al.,
2000; Hu et al., 2010), we recorded higher concentration by a factor of 2 to
3 at our sampling site. The average water concentrations in the lagoon of
CH3I were in the same range as reported from other parts of the Atlantic
(Moore and Groszko 1999; Zhou et al., 2005). However, especially those
regions where macroalgae are the dominating source organisms possess higher
maximum values (Bravo-Lineares and Mudge, 2009; Jones et al., 2009). This is
even more pronounced for CHBr3, for which the seawater concentration
within or in the vicinity of macroalgae beds are strongly elevated
(Bravo-Lineares and Mudge, 2009; Carpenter et al., 2000; Jones et al., 2009).
The area occupied by the prevalent macroalgae species Enteromorpha
spp. and Ulva spp. in the Ria Formosa is estimated to be
2.5 km2 (Duarte et al., 2008), considerably below that of other
abundant sources such as seagrass meadows. We cannot exclude that
phytoplankton contributes significantly to the water concentration of
halocarbons, but the predominantly low chlorophyll concentrations
(3.06 µg L-1 from long-term measurements, Brito et al., 2012)
and low water volumes seem to limit the impact from this source.
Despite the short residence time of the lagoon water masses, of which
50–75 % is exchanged during one tidal cycle (Brito et al., 2010), the
transect cruise along the main channels revealed a successive enrichment of
halocarbon concentration in the water with increasing distance from the main
inlets (Fig. 1 and Table 2). Therefore, the net halocarbon production in the
lagoon appears to clearly exceed that outside the lagoon. This is supported
by the distinctively increased air mixing ratios of halocarbons in the lagoon
as compared to the upwind site (Table 1).
Overall, the lagoon seems to comprise highly potent halocarbon sources in
the water column for CH3Cl and CH3Br but not for CH3I and
CHBr3.
Diurnal variation of mean halocarbon fluxes (triangles) from
seagrass meadows during periods of air exposure in summer 2011 (a:
CH3Cl, b: CH3Br, c: CH3I, d: CHBr3). Error bars refer to
standard deviations. Circles are solar radiation values. Note that the scales
on the y axis are different for each compound.
Flux pattern from seagrass meadows
The halocarbon fluxes from seagrass meadows were characterised by a high
variability with deposition and emission fluxes occurring at all sampling
spots. The same was observed within other studies investigating halocarbon
fluxes in coastal environments (e.g. Blei et al., 2010; Manley et al., 2006;
Rhew et al., 2000). Halocarbon dynamics in coastal systems where multiple
sources and sinks interact are complex, and it should be noted that the
fluxes discussed here refer to the entire benthic community constituting the
seagrass meadows. Thus, some variability may relate to the activity of
distinct source organisms, which may be stimulated by different environmental
factors. To gain insights into the common environmental controls for this
ecosystem, we discuss the following factors: (i) diurnal variations; (ii) tidal
effects; and (iii) seasonal dependence.
Diurnal variations. The correlation analysis with solar radiation
resulted in only a weak association with the magnitude of fluxes. However,
after grouping by daytime, our data provide some indication for a diurnal
pattern (Fig. 2). For CH3Cl, there was the most obvious relationship
between time of day and actual emissions. Highest emissions were observed
during day periods with increased sunlight (midday and afternoon). In
contrast, deposition fluxes were exclusively recorded during periods of low
radiation and at nighttime. The same was also observed for CH3Br. However,
highest mean emissions of this compound seemed to be shifted towards the
afternoon. CH3I was constantly emitted from the seagrass covered spot,
revealing a weak diurnal dependence. The emissions did not cease during
periods of low irradiance and darkness. Nevertheless, elevated mean emissions
were observed in the afternoon. Except for one occasion, CHBr3 was emitted
throughout the sampling periods. Mean emissions were higher around midday and
afternoon than during the night.
Several studies from salt marshes reported a diurnal trend of halocarbon
emissions initiated by irradiance (Dimmer et al., 2001; Rhew et al., 2000,
2002; Drewer et al., 2006). The flux data of halocarbons from the summer
campaign with elevated fluxes during midday and afternoon suggest a similar
pattern also in seagrass meadows. However, this was more obvious for
CH3Cl and CH3Br than for CH3I and CHBr3. The lower
production of CH3I during the time of highest light intensity cannot
fully be explained. Blei et al. (2010) reported that the main environmental
association in salt marsh emissions of CH3Cl and CH3Br was with
ambient temperature rather than light. However, during the summer campaign,
temperature variations (day/night) were too low to explain the observed
emission/deposition pattern of CH3Cl and CH3Br.
It is known that coastal sediments can act as a sink for CH3Cl and
CH3Br, mainly due to microbial degradation (Miller et al., 2004; Oremland
et al., 1994). This would support our findings of the deposition fluxes
during nighttime, where production above the sediment is presumably lower
than during daytime (summer campaign). While, in general, the deposition
fluxes of CH3Cl and CH3Br occurred more frequently during the
spring campaign, they did not exhibit a day–night relationship. Moreover, the
dependence of light intensity on the magnitude of emission fluxes of
halocarbons seemed to have a minor effect during this period of the year.
Tidal effects. During the spring campaign, mean fluxes derived from
submerged seagrass meadows were elevated by factors of 17 (CH3Cl), 5
(CH3Br), 3 (CH3I), and 8 (CHBr3) when compared to the average
fluxes during air exposure. This clearly higher production of halocarbons
under submerged conditions was quite unexpected, as in general it is believed
that the production of trace gases during low tide exceeds that during
inundation. For halocarbons, this was suggested for example by Carpenter et
al. (1999) and Jones et al. (2009) from atmospheric measurements over
intertidal macroalgae beds in Mace Head, Ireland. Nevertheless, in accordance
with our results from halocarbon measurements we also observed higher primary
productivity by increased CO2 uptake during submerged conditions
(Bahlmann et al., 2014). Therefore, the higher productivity may reflect
higher enzymatic activity (e.g. methyltransferases) within the organisms of
the seagrass community, by which monohalomethanes are presumably formed.
Furthermore, the correlation analysis revealed a different behaviour of
halocarbons between the two tidal states, with stronger correlations between
monohalomethanes during tidal inundation than air exposure. Obviously the
change in environmental conditions was accompanied with a shift in the
halocarbon production-decomposition pattern of the benthic community and/or
different source organisms were stimulated.
An interesting outcome of both campaigns is the observation of strongly
elevated halocarbon fluxes during tidal change, from air exposure to
submergence and reversely (Table 3). Continuous high-time resolution CO2
and methane flux measurements performed in spring 2012 (Bahlmann et al.,
2014) principally support this observation. At the particular moment when the
water reached the sampling site, we observed a distinct peak flux of methane
and CO2. This may be evidence for processes in the sediments
attributable to changes in hydrodynamic pressures, resulting in the release of
trace gases trapped in sedimentary pore spaces (Bahlmann et al., 2014). The
remarkable deposition flux of CH3Cl and CH3Br during the maximum
water level (Table 3) was accompanied by highest emissions of other trace
gases, such as methanethiol and hydrogen sulfide, as discussed by Bahlmann et
al. (2014). These compounds are effective nucleophiles, which could have
contributed to the degradation of halocarbons as described in Barbash and
Reinhard (1989). This suggests a significantly different biogeochemistry
during this period compared with tide and ebb flow.
Overall, while there is evidence for a tidal control on halocarbon
production and decomposition, additional research is needed to further
elucidate these phenomena.
Seasonal dependence. There are considerable differences between the
results from the spring and summer campaigns. We observed elevated mixing ratios
for all halocarbons in ambient air, as well as higher water concentrations for
CH3Cl, CH3I, and CHBr3 compounds in summer (Table 1). This
observed signal of general increased halocarbon production in the lagoon
during summer might be attenuated by enhanced degradation in the water phase
and sediments at higher temperatures. Nevertheless, given the calculated
sea–air flux there is only little evidence for a pronounced seasonal
relationship in halocarbon volatilisation to the atmosphere from the lagoon
water. While the fluxes of CH3Cl appeared to be enhanced in summer,
those of CH3Br and CH3I seemed to be quite similar between spring
and summer. CHBr3 emissions were actually higher in spring than in
summer due to higher water concentrations. Comparing the data obtained from
air-exposed sites during the two campaigns, the fluxes in summer were
strongly enhanced by factors of 16 (CH3Cl and CH3Br), 2
(CH3I), and 5 (CHBr3). Moreover, the halocarbon fluxes showed a
distinct diurnal cycle during summer but not during spring. The differences
of ambient conditions between the campaigns with lower air temperatures and
lower solar radiation in spring may have contributed to the differences in
the emission patterns of halocarbons. That these environmental conditions can
substantially influence the magnitude of fluxes was reported from other
ecosystems such as salt marshes (Blei et al., 2010; Manley et al., 2006).
However, further studies covering the entire seasoning are necessary to fully
unravel the annual halocarbon emissions from seagrass meadows.
Halocarbons sources in the lagoon: an isotopic perspective
The results from the atmospheric sampling of Praia de Faro air (upwind) and
lagoon air revealed differences regarding the mixing ratios and δ13C values of CH3Cl and CH3Br (Tables 1 and 4). We observed
elevated concentrations in the lagoon for both compounds, whereby the higher
concentrations were accompanied with shifts towards isotopically light
CH3Cl but heavy CH3Br. Sources other than the studied seagrass
meadows, for example the abundant salt marshes, may have contributed
substantially to the elevated mixing ratios. Assuming atmospheric stable
conditions with negligible sinks in the atmosphere, the difference of air
mixing ratios and δ13C values between upwind air and lagoon air
should reflect the isotopic source signature within the lagoon. Therefore, as
a first approach, an isotope mass balance was used by integrating mean data
from both sampling sites (Tables 1 and 4). The resulting source signatures
within the lagoon are -49 ‰ for CH3Cl and -16 ‰ for
CH3Br.
Isotopic source signatures of CH3Cl from seagrass meadows during chamber
incubations (air exposure) in the Ria Formosa were -51 ± 6 ‰
(summer) and -56 ± 2 ‰ (spring). During the summer campaign,
CH3Cl emissions from the salt marsh plant Spartina maritima
were determined with δ13C values of -66 and -72 ‰. These
values are in good agreement with those of Bill et al. (2002) from a
Californian salt marsh (-69 to -71 ‰, daytime values).
Unfortunately, we do not have isotopic data for the inundated periods from
seagrass meadows, but the δ13C values of CH3Cl in the water
phase (-42 ± 2 ‰) come close to those measured in the
atmosphere. An abiotic production mechanism has been reported for CH3Cl
from senescent plant material (Hamilton et al., 2003). While we cannot
generally exclude additional CH3Cl generation via this pathway, the
isotopic data obtained in the Ria Formosa do not mirror strongly 13C
depleted values (δ13C of -135 ± 12 ‰, Keppler et
al., 2004) as expected for compounds from this production mechanism. Overall,
this rather indicates a stronger imprint of the seagrass meadows and/or water
column on the atmospheric CH3Cl than from salt marshes or abiotic
processes.
With δ13C values of -42 ± 17 ‰, the source
signature of CH3Br from seagrass meadows tend to be more depleted in
13C compared with the calculated source signature from the atmospheric
samples. It should be noted that the δ13C values for this compound
were more depleted in 13C during periods of increased emission
(-55 ‰) than during low emission (-28 ‰). This shift
can most likely be explained by degradation processes in the sediments, which
occurred simultaneously. This corroborates our observations from northern
Germany with subsequent recalculation of a sedimentary sink function from
accompanying sediment measurements (Weinberg et al., 2013). Reported source
signatures of CH3Br from salt marshes range from -59 to
-65 ‰ (day time values, Bill et al., 2002). Our own measurements
in the Ria Formosa indicate similar δ13C values (-65 ‰)
or even more depleted ones (unpublished data). In any case, neither source
signatures from seagrass meadows nor salt marshes seem to match the overall
source signature estimated from the atmospheric samples. Therefore, it is
most likely that the atmospheric CH3Br is strongly influenced by
CH3Br emissions from the surface waters (δ13C values in water
phase (summer): -23 ± 3 ‰). Even during periods of low tide
the water remains in the deep channels, which may be sufficient to have an
impact on the local atmosphere. Thus, despite the sources in the lagoon
presumably producing isotopically light CH3Br, δ13C values in
the atmosphere strongly reflect decomposed CH3Br, whose residual fraction
is actually enriched in 13C. Accordingly, aqueous CH3Br appears to
become rapidly degraded by biotic/abiotic processes such as hydrolysis,
transhalogenation, and microbial degradation with strong isotopic
fractionation (King and Saltzman, 1997; Miller et al., 2004). These
decomposition mechanisms are temperature dependent, with increasing
destruction as seawater temperature rises (King and Saltzman, 1997).
This is most likely the reason why the δ13C values in the lagoon
waters in summer are more enriched in 13C, compared with those from
the spring campaign.
Compilation of mean emissions (bold black vertical lines) and ranges
from different sources in coastal environments for CH3Cl (upper panel),
CH3Br (middle panel) and CH3I (lower panel). Note the different
scales. Published data adopted from: 1 this study; 2 Weinberg et
al. (2013); 3 Blei et al. (2010); 4 Cox et al. (2004); 5
Dimmer et al. (2001); 6 Drewer et al. (2006); 7 Valtanen et al. (2009); 8 Rhew and Mazéas (2010); 9 Manley et al., (2006);
10 Rhew et al. (2000); 11 Manley et al. (2007); 12 Carpenter
et al. (2000); 13 Leedham et al. (2013). Note that the data of
CH3Cl from subtropical salt marshes are downscaled by a factor of 10 for
visualisation reasons. Where multiple references were used, the individual
study means were averaged and presented along with the resulting ranges.
Thus, ranges of halocarbon fluxes in each single study are not covered.
Studies reporting a strong species dependency in magnitude of fluxes were
averaged over all species groups for simplicity. Macroalgae emissions given
in g fresh weight per hour were converted by using the species' fresh
weights and spatial coverage in the coastal belt in Mace Head, Ireland for
CH3Br (Carpenter et al., 2000) and the Malaysian coastline for CH3I
(Leedham et al., 2013), respectively.
To the best of our knowledge, this is the first report of δ13C
values of CH3I in the water phase. As shown by the water samples from
the transect cruise, the sources in the lagoon may produce isotopic light
CH3I. Given this, CH3I seems, to some extent, to follow the δ13C values of CH3Cl. These sources may be biotic, by e.g.
phytoplankton, seagrass meadows, or bacteria. On the other hand, Moore and
Zafirou (1994) reported a photochemical source for CH3I by radical
recombination of iodine with seawater dissolved organic matter. Due to the
lack of isotopic source signatures and fractionation factors for production
(and consumption), it is difficult to draw conclusions from the data.
The δ13C values of CHBr3 were more depleted in 13C from
the lagoon inlet towards the parts deeper inside. This suggests a different
combination of sources in water masses coming from the Atlantic. Moreover,
this potential variation of source contribution can be further assumed by the
certain change between summer and spring where e.g. macroalgae are more
abundant in the latter period (Anibal et al., 2007). Already reported source
signatures of phytoplankton, macroalgae, and seagrass meadows cover the range
of -10 to -23 ‰ (Auer et al., 2006; Weinberg et al., 2013), thus
demonstrating differences in their isotopic fingerprint. We cannot exclude
that degradation might also have an effect on the δ13C values
determined in lagoon waters. As for CH3I, there is still need for
further research on the CHBr3 cycling utilising stable carbon isotopes.
Magnitude of fluxes and comparison to other coastal measurements and first
estimate of global source strength
The area-based fluxes of CH3Cl, CH3Br, and CH3I from seagrass
meadows in comparison to emission data of other coastal sources are presented
in Fig. 3. In comparison to the emissions from a temperate seagrass meadow in
late summer in northern Germany (Weinberg et al., 2013), fluxes were elevated
in the subtropical lagoon in summer during air exposure. This was more
pronounced for CH3Br (factor 33) than for CH3Cl (factor 2),
CH3I (factor 2), and CHBr3 (factor 5). In contrast, fluxes from
air-exposed seagrass meadows recorded during spring are comparable to those
determined in northern Germany. Thus, the difference between fluxes from
temperate and subtropical regions is less pronounced than reported for salt
marshes with emissions from subtropical regions, exceeding those from
temperate regions by up to two orders of magnitude for CH3Cl and
CH3Br (Blei et al. 2010; Cox et al., 2004; Dimmer et al., 2001; Drewer
et al., 2006; Manley et al., 2006; Rhew and Mazéas, 2010; Rhew et al.,
2000, 2014; Valtanen et al., 2009). Beside this regional (climatic)
difference, several authors attributed this to a highly species dependent
emission potential.
Average emissions of CH3Cl from the air-exposed seagrass meadows in
summer are in the same range than those determined in temperate salt marshes
(Blei et al. 2010; Cox et al., 2004; Dimmer et al., 2001; Drewer et al.,
2006; Valtanen et al., 2009). In contrast, subtropical counterparts of these
macrophytes are distinctively stronger emitters of this compound by at least
one order of magnitude (Manley et al., 2006; Rhew and Mazéas, 2010; Rhew
et al., 2000, 2014). Greenhouse grown mangroves produce significantly more
CH3Cl than seagrass meadows, revealing a higher emission potential for
these plant species on a per area basis (Manley et al., 2007).
Fluxes of CH3Br from subtropical seagrass meadows during air exposure
exceed those of temperate macroalgae from Mace Head, Ireland (Carpenter et
al., 2000) and temperate salt marshes (Blei et al. 2010; Cox et al., 2004;
Dimmer et al., 2001; Drewer et al., 2006; Valtanen et al., 2009). However,
the CH3Br fluxes from seagrass meadows are distinctively lower than
those of subtropical salt marsh plants (Manley et al., 2006; Rhew and
Mazéas, 2010; Rhew et al., 2000). Mangroves seem to have a similar
emission potential as seagrass meadows (Manley et al., 2007).
For CH3I, seagrass meadows are a minor source in comparison to the high
release of macroalgae in subtropical areas (Leedham et al., 2013). Except for
salt marshes from Tasmania (Cox et al., 2004), plant-related communities such
as mangroves (Manley et al., 2007) and salt marshes (Dimmer et al., 2001) are
more pronounced emission sources of this compound. The same holds true for
CHBr3, where macroalgae communities from temperate and
subtropical/tropical regions dominate the emissions of polyhalomethanes on a
per area basis (e.g. Carpenter et al., 2000; Gschwend et al., 1985; Leedham
et al., 2013).
Many uncertainties arise from a limited number of emission data to estimate
the global relevance of seagrass meadows. There may be high variation in
space and time, high heterogeneity of seagrass meadows, species dependent
emission potential, and errors regarding the global seagrass abundance.
Therefore, the scale-up of our data gives only a first rough approximation;
it was undertaken as follows. Since we did not measure a full annual cycle,
we assumed that seagrass measurements during the summer campaign represent
emissions from the reproductive season (May–September). The remaining
period of the year (October–April) was calculated with emission data from
the spring campaign. The emission data were weighted to tidal states using 8
and 16 h per day as durations when seagrass meadows are air-exposed
or submerged, respectively. Due to the lack of flood tide emission data in
summer, we used those derived from the sea–air exchange. The resulting
average annual emissions from seagrass meadows of
150 µmol m-2 yr-1 (CH3Cl),
18 µmol m-2 yr-1 (CH3Br),
14 µmol m-2 yr-1 (CH3I), and
25 µmol m-2 yr-1 (CHBr3) were scaled-up with the
current estimates of a global seagrass area, ranging from
0.3 × 1012 m2 (Duarte et al., 2005) to
0.6 × 1012 m2 (Charpy-Roubaud and Sournia, 1990).
The tentative estimate yields annual emissions of 2.3–4.5 Gg yr-1 for
CH3Cl, 0.5–1.0 Gg yr-1 for CH3Br, 0.6–1.2 Gg yr-1
for CH3I, and 1.9–3.7 Gg yr-1for CHBr3. Based on the recent
global budget calculations (Xiao et al., 2010; Montzka and Reimann, 2011),
these ranges are equivalent to 0.06–0.11 and 0.45–0.89 %, for
CH3Cl and CH3Br, respectively. Seagrass meadows would therefore
cover a portion of 1.4–2.8 % of the missing sources for CH3Br
reported in the most recent World Meteorological Organization (WMO) report (36.1 Gg yr-1; Montzka and
Reimann, 2011). Given the emissions from oceanic sources (e.g. Butler et al.,
2007; Quack and Wallace, 2003, and references therein), CH3I and
CHBr3 emissions from seagrass meadows are rather insignificant on a
global scale.
Conclusions
Our data are the first to report detailed halocarbon fluxes from seagrass
meadows. The fluxes of halocarbons were highly variable with increased
fluxes when the seagrass meadows were submerged, and distinct emission peaks
when lagoon waters were just arriving or leaving the sampling site. For
CH3Cl and CH3Br, we observed a diurnal dependence on the fluxes
with increased emissions during midday/afternoon and deposition fluxes
during periods of low radiation. Generally, diurnal variations (during air
exposure), atmospheric mixing ratios, and emission rates of halocarbons were
smaller in spring than in summer, suggesting a seasonal dependence. Our
results indicate that on a global scale, seagrass meadows are a minor source
of halocarbons, but that they will have an imprint on the local and regional
budgets, particularly on subtropical coastlines, where seagrass meadows
belong to the most abundant ecosystems.
Our stable carbon isotope results suggest that CH3Cl originates
predominantly from the water column and/or seagrass meadows, rather than
from adjacent salt marshes or abiotic formation processes. Atmospheric and
aqueous CH3Br in the lagoon was substantially enriched in 13C
pointing towards degradation processes and re-emission into the atmosphere.
Future studies should focus on halocarbon emissions from seagrass-based
systems from different regions in order to refine the global relevance.
Since the magnitudes of fluxes are often species dependent, budget
calculations would benefit from a more detailed investigation of fluxes from
different seagrass species. More work is also required to identify other
elements of these ecosystems, such as the sediments, which are capable of
acting as both a sink and a source of halocarbons.
The Supplement related to this article is available online at doi:10.5194/bg-12-1697-2015-supplement.
Acknowledgements
The authors thank the German Federal Ministry of Education and Research
(BMBF) for funding (grants 03F0611E and 03F0662E). The stay at the Ramalhete
research station in Faro, Portugal was co-funded by ASSEMBLE EU FP7 research
infrastructure project. Rui Santos, João Reis, and Bruno Fragoso (CCMAR,
Universidade do Algarve) are greatly acknowledged for their extensive
support during sampling site selection and sampling. Our technical staff
members Sabine Beckmann and Ralf Lendt are thanked for their valuable help.
We express gratitude to three anonymous reviewers and especially the
Associate Editor Jens-Arne Subke for their comments and suggestions, which
considerably improved the quality of the manuscript.
Edited by: J.-A. Subke
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