Seagrass meadows are autotrophic ecosystems acting as carbon
sinks, but they have also been shown to be sources of carbon dioxide
(CO2) and methane (CH4). Seagrasses can be negatively affected by
increasing seawater temperatures, but the effects of warming on CO2 and
CH4 fluxes in seagrass meadows have not yet been reported. Here, we
examine the effect of two disturbances on air–seawater fluxes of CO2
and CH4 in Red Sea Halophila stipulacea communities compared to adjacent unvegetated
sediments using cavity ring-down spectroscopy. We first characterized
CO2 and CH4 fluxes in vegetated and adjacent unvegetated
sediments, and then experimentally examined their response, along with that
of the carbon (C) isotopic signature of CO2 and CH4, to gradual warming
from 25 ∘C (winter seawater temperature) to 37 ∘C, 2 ∘C above current maximum temperature. In addition, we assessed
the response to prolonged darkness, thereby providing insights into the
possible role of suppressing plant photosynthesis in supporting CO2 and
CH4 fluxes. We detected 6-fold-higher CO2 fluxes in vegetated
compared to bare sediments, as well as 10- to 100-fold-higher CH4
fluxes. Warming led to an increase in net CO2 and CH4 fluxes,
reaching average fluxes of 10 422.18 ± 2570.12 µmol CO2 m-2 d-1 and 88.11±15.19µmol CH4 m-2 d-1, while CO2 and CH4 fluxes decreased over time in
sediments maintained at 25 ∘C. Prolonged darkness led to an
increase in CO2 fluxes but a decrease in CH4 fluxes in vegetated
sediments. These results add to previous research identifying Red Sea
seagrass meadows as a significant source of CH4, while also indicating
that sublethal warming may lead to increased emissions of greenhouse gases
from seagrass meadows, providing a feedback mechanism that may contribute to
further enhancing global warming.
Introduction
Global warming, as a result of anthropogenic emissions of greenhouse gases,
has led to ocean warming by 0.11 ∘C between 1971 to 2010
(IPCC, 2014), with the global mean sea-surface temperature
predicted to increase further with additional emissions, depending on
emission scenarios (IPCC, 2014). Ocean warming is leading to
a shift in species and ecosystem processes (Hoegh-Guldberg and
Bruno, 2010), including metabolic processes that are under strong thermal
control (Brown et al., 2004;
Garcias-Bonet et al., 2018, 2019b).
Ecosystem metabolism can also be a source of greenhouse gases, depending on
the metabolic balance of the community, where autotrophic communities (net
community production (NCP) >0) act as a sink for carbon dioxide
(CO2), while heterotrophic communities (NCP <0) act as a source of CO2 (Duarte et al.,
2011). Since respiration rates tend to increase faster with warming than
primary production does (Brown et al., 2004;
Harris et al., 2006; Regaudie-De-Gioux and Duarte, 2012), warming may lead to the shifting of typically autotrophic ecosystems, such as seagrass meadows, to being net heterotrophic, thereby switching from acting as sinks to sources of CO2 (Harris et al., 2006). Emissions of metabolic
greenhouse gases with ocean warming may provide a feedback mechanism by
which anthropogenic emissions of greenhouse gases may lead to
warming-dependent emissions by coastal ecosystems, therefore enhancing
climate warming. This feedback effect is particularly likely to occur where
methane (CH4) is released, as CH4 is calculated to have a global
warming potential 28 times larger than CO2 per mole of carbon (C) emitted (Myhre et al., 2013).
Indeed, CO2 and CH4 emissions from some tropical mangrove
forests have been calculated and found to partially offset the capacity of mangroves
to act as C sinks (Rosentreter et al., 2018). Whereas
the emission of CO2 and CH4 from seagrass ecosystems has
received far less attention, seagrass ecosystems have been reported to
support CH4 emissions of the order of 1.4 to 401.3 µmol CH4 m-2 d-1 (cf.
Table 1 in Garcias-Bonet and Duarte, 2017). Provided
estimates of their global extent of seagrass meadows ranging from a
documented 326 000 km2 (Unsworth et
al., 2019) to a predicted 1.6×106 km2
(Jayathilake and Costello, 2018), seagrass meadows may be
important yet hitherto overlooked contributors to CH4 emissions.
Garcias-Bonet and Duarte (2017) reported that seagrasses
could contribute to global CH4 emissions by releasing CH4 at a rate of 0.09 to 2.7 Tg yr-1, which may increase the contribution of
marine global emissions to previously reported global estimates by about 30 % (Garcias-Bonet and Duarte, 2017).
Seagrasses are known to be autotrophic ecosystems, acting as C sinks
(Duarte et al., 2010)
supporting a global burial rate of 27.4 Tg C yr-1
(Duarte et al., 2005b). They store carbon in
their below- and aboveground biomass in the short term, as well as in their
sediment in the long term (Duarte et al., 2005b).
They account for 10 % of the C storage in ocean sediments even though
they only cover 0.2 % of the ocean surface
(Duarte et al., 2005b;
Fourqurean et al., 2012). However, disturbances can lead to the loss of
biomass and the emissions of stored C turning blue carbon ecosystems into C
sources
(Macreadie
et al., 2015; Lovelock et al., 2017; Arias-Ortiz et al., 2018), which will
ultimately contribute to global emissions intensifying the greenhouse
effect. Lyimo et al. (2018) showed that
stressors such as shading and grazing led to an increase of CH4
emissions by seagrass ecosystems by reducing their photosynthetic capacity.
Garcias-Bonet and Duarte (2017) reported that CH4
release from seagrass sediments tended to increase with seawater
temperature and suggested that CH4 emissions by seagrass ecosystems
may be under temperature control in the Red Sea. Indeed, some seagrass
ecosystems in the Red Sea have been shown to shift from an autotrophic to a
heterotrophic state during the warmer summer months, indicating that some
seagrass communities might already grow past their thermal optimum
(Burkholz et al., 2019a).
The Red Sea ranks as the warmest sea in the world, with summer seawater
temperatures reaching 35 ∘C, and is warming at higher
rates (0.17±0.07∘C decade-1;
Chaidez et al., 2017) than those of the global
ocean (0.11 ∘C decade-1;
Rhein et al., 2013). Provided
respiration rates and also CH4 fluxes in seagrass ecosystems are likely
to increase with temperature, seagrass meadows in the Red Sea may be close
to shifting from net sinks to net sources of greenhouse gases with further
warming. Emission rates are also dependent on organic carbon supply, as high organic matter in sediments can promote an increase in CH4 production (Sotomayor
et al., 1994; Gonsalves et al., 2011) and organic matter released from
seagrass photosynthesis may also stimulate CO2 and CH4 production
in the sediment community. Indeed, sediments in seagrass ecosystems support
a 1.7-fold increase in organic matter content compared to surrounding bare sediments,
not only due to the slow turnover of biomass but also due to their ability
to trap particles (Duarte et al., 2005a; Kennedy et al.,
2010).
Here, we test the hypothesis that CO2 and CH4 fluxes by seagrass
communities increase with warming. We do so by experimentally examining the
effect of warming and plant activity on air–seawater fluxes of CO2 and
CH4 in a Red Sea seagrass (Halophila stipulacea) community. The tropical seagrass species
Halophila stipulacea (Forsskål) Ascherson is native to the Indian Ocean and is one of the
most common species in the Red Sea (Qurban et al., 2019).
It seems to be highly adaptive to various environments, as it is now found
as an exotic species in the Mediterranean (Lipkin, 1975)
and the Caribbean Sea (Ruiz and Ballantine, 2004), indicating
its high resilience to changing conditions (Por,
1971). We first characterize air–seawater fluxes of CO2 and CH4 in
Red Sea Halophila stipulacea communities compared to adjacent unvegetated sediments and then
experimentally examine their response, along with that of the C isotopic
signature of CO2 and CH4, to gradual warming from 25
to 37 ∘C. In addition, we assess the response to prolonged
darkness, thereby providing insights into the possible role of plant
photosynthesis in supporting CO2 and CH4 fluxes.
Material and methodsStudy site and sample collection
Samples were collected at Al Kharar, a lagoon on the Saudi coast of the
central Red Sea in February 2018. Two H. stipulacea meadows at a depth of 2–3 m, S1
(22∘56′46.5′′ N, 38∘52′40.6′′ E) and S2 (22∘54′44.5′′ N, 38∘53′50.9′′ E), were chosen to represent a range of
organic matter content in the sediment and were selected to evaluate greenhouse gas
fluxes. Moreover, the H. stipulacea meadow in the middle of the lagoon (S2) with higher
biomass density (Table 1) was chosen as the study site to experimentally
assess the role of temperature and darkness in greenhouse gas fluxes. The
seagrass and sediment community was sampled using translucent cylindrical
polyvinyl chloride (PVC) cores (26 cm length and 9.5 cm in diameter). The sharpened edge of the
core was carefully pushed approximately 10 cm into the sediment with a
rubber hammer so that the structure of leaves, roots and sediment stayed
intact. A rubber stopper was then placed on top, before the core was
carefully pulled out of the sediment without disturbing the structure and
another rubber stopper was placed on the bottom of the core. The sediment
cores were immediately transported to the laboratory.
Summary of vegetated sediment, bare sediment and seagrass leaves
characteristics in the study sites (S1 and S2). NA: not available.
S1 S2 VegetatedBareSeagrassVegetatedBareSeagrasssedimentsedimentleafsedimentsedimentleafC concentration (% DW)0.43±0.050.41±0.0317.6±2.720.55±0.030.52±0.0215.32±1.48N concentration (% DW)0.07±0.010.12±0.011.06±0.170.08±0.0020.09±0.010.94±0.07P concentration (% DW)0.03±0.0010.03±00.12±0.010.02±00.02±0.0010.11±0.01Carbonate content (% DW)91.75±0.5691.65±0NA82.61±0.5083.63±0NAOrganic matter (% DW)2.45±0.092.46±0.16NA3.26±0.032.95±0.04NABulk density (g cm-3)1.15±0.021.28±0.03NA1.1±0.071.2±0.04NASeagrass biomass (g DW m-2)60.87±1.24NANA164.66±20.54NANASediment δ13C-Corg (‰)-15.77±0.07-15.94±0.1NA-15.81±0.13-16.36±0.28NASeagrass leaf δ13C-C (‰),-7.96±0.27extracted from Duarte et al. (2018)Sediment and plant characterization
Once the cores were opened, the first 10 cm of the sediment and the plant
biomass from the same cores were collected and dried at 60 ∘C to
a constant dry weight (DW). To characterize the two different H. stipulacea meadows, sediment
and plant biomass samples were then ground to conduct sediment composition and nutrient analyses. A 50 mL tube was filled with
sediment from the first 10 cm, and the contents were dried at 60 ∘C to a constant dry weight and weighed to determine the sediment bulk density (g sediment cm-3). Organic matter content was analyzed by loss on ignition
(LOI; Dean, 1974). Approximately 5 g of dried sediment was placed in
a muffle furnace and burned at 450 ∘C for 5 h. The organic
matter content was calculated as
%OM=pre-ignition weight(g)-post-ignition weight(g)pre-ignition weight(g)×100.
The carbonate content was estimated using a Pressure
Gauge Calcimeter. Approximately 1 g of sample was placed in the calcimeter, and the
recipient was filled with 10 % hydrochloric acid (HCl). The mass of
CaCO3 in the sample (g) was then calculated as follows:
mCaCO3=p-ba×w,
where p is the pressure recorded (ppm), b is the slope and a the
intercept derived from the calibration curve, and w is the exact weight of
each sediment sample (g). The percentage of CaCO3 in the sample (% DW) was then calculated using Eq. (3):
%CaCO3=mCaCO3w×100.
Dried sediment and plant samples were digested using
US Environmental Protection Agency (EPA) method 3052 and analyzed with nitric acid (HNO3) and HCl using
US EPA method 200.7 following manufacturer instructions. The phosphorus
content (% DW) was analyzed using inductively coupled plasma optical
emission spectroscopy (ICP-OES) on an Agilent 5110 ICP-OES (Agilent
Technologies, Santa Clara, CA, USA). The C and nitrogen (N) concentration of both
plants and sediments was analyzed after acidification with HCl
(Hedges and Stern, 1984), using a FLASH 2000 Organic
Elemental Analyzer (CHNS/O-2, Thermo Fisher Scientific, Waltham, MA, USA). The
isotopic signature of 13C in sediment organic matter was analyzed,
using cavity ring-down spectroscopy (CRDS G2201-I, Picarro Inc., Santa
Clara, CA, USA), from the 13C of CO2 released by a combustion
module (Costech Analytical Technologies Inc., CA, USA) delivering the
CO2 resulting from combusting the sediment organic matter to the CRDS
instrument.
Assessment of carbon dioxide and methane air–seawater fluxes
In February 2018, triplicate cores from vegetated and adjacent bare (about 5 m from the edge of the seagrass patch) sediments were collected from sites
S1 and S2 and transferred to incubation chambers (Percival Scientific Inc.,
Perry, IA, USA) set at 25 ∘C with a 12 h light (up to 70 µmol photons m-2 s-1) : 12 h dark (12 h L : 12 h D) cycle to
measure the greenhouse gas (CO2 and CH4) fluxes supported by these
communities. Before measuring fluxes, the water overlying the sediment
inside the cores was carefully siphoned until only 5 mm of water remained
over the sediment surface; fresh seawater was carefully siphoned in the
core to avoid disturbing the redoxcline, leaving a headspace of approximately 5–6 cm; and the cores were closed again with stoppers containing gas-tight
valves. The cores were left for 1 h to allow for equilibration between
the seawater and the headspace air. We then sampled 10 mL of air from each
core using a syringe and injected the air sample in a cavity ring-down
spectrometer (Picarro Inc., Santa Clara, CA, USA) through a Small
Sample Isotopic Module extension (SSIM A0314, Picarro Inc., Santa Clara, CA,
USA), which provided both the partial pressure and the isotopic carbon
composition of the CO2 and CH4 in the air sample. One sample
from each core was taken at the start (T0), after 12 h of light (T1) and
after 12 h of dark (T2). The daily CO2 and CH4 fluxes were
calculated from the difference between T0 and T2 taking into account the
core surface area (µmol m-2 d-1). Before and after each
sampling, two standards were measured (A: 750 ppm CO2, 9.7 ppm
CH4; B: 250.5 ppm CO2, 3.25 ppm CH4).
Effect of warming on carbon dioxide and methane air–seawater fluxes
In March 2018, we collected 18 vegetated and 18 bare sediment
cores from site S2 to evaluate the response of greenhouse gas fluxes to
warming. The sampling was performed as described above. The cores were
transferred to the Coastal and Marine Resources Core Lab (CMOR, KAUST, Saudi
Arabia). Nine vegetated and nine bare sediment cores each were placed in two
aquaria with flow-through seawater set at in situ temperature (25 ∘C)
and with a 12 h L (up to 200 µmol photons m-2 s-1) : 12 h D cycle.
One aquarium was maintained at 25 ∘C over the entire duration of
the experiment to serve as a control for temperature-independent variability
in fluxes. The temperature in the second aquarium was increased at a rate of
1 ∘C d-1 to allow for acclimatization of the
vegetated and bare cores. CO2 and CH4 fluxes were measured at
every 2 ∘C from 25 to 37 ∘C, with parallel measurements
conducted on the cores maintained at 25 ∘C. After a 1 d
acclimation period at each new temperature, the cores were closed with the
stoppers and transferred to incubation chambers (Percival Scientific Inc.,
Perry, IA, USA) set at the target temperature for CO2 and CH4 flux measurements as described above. After the 24 h measurements, the
cores were returned to the aquaria. An additional core kept at each of the
constant temperature and warming sets was sampled every 4 d (i.e., at 4 ∘C temperature intervals in the warming treatment) to analyze
sediment composition. The cores used for flux estimates were opened after
the final measurement (20 d since collection) to estimate the plant
biomass and analyze the sediment composition at the end of the experiment.
Effect of darkness on carbon dioxide and methane air–seawater fluxes
In May 2018, six vegetated and six bare sediment cores were collected from
site S2 and kept at a constant 25 ∘C with a 24 h dark cycle.
Only during the measurements in the incubators were the cores exposed to a
12 h L : 12 h D cycle, allowing fluxes to be compared with those measured in
cores permanently maintained with the 12 h L : 12 h D photoperiod. CO2
and CH4 fluxes were measured after the first day of acclimation and
then kept in the aquaria until signs of seagrass mortality started to become
apparent, which occurred after 1 week in the dark. CO2 and CH4
fluxes were measured at alternate days as detailed above. At the end of the
experiment (21 d since collection), the cores were opened and sampled to
assess plant biomass and sediment composition.
Measurements of carbon dioxide and methane air–seawater fluxes
The concentration of CO2 in the seawater after equilibrium was
calculated based on the concentration of CO2 in the headspace (ppm)
measured by CRDS according to Sea
et al. (2018) and Wilson et al. (2012):
[CO2]w=10-6βmapdry,
where β is the Bunsen solubility coefficient of CO2 (mol mL-1 atm-1), ma is the CO2 concentration measured in
the headspace (ppm) and pdry is the atmospheric pressure of dry air
(atm). The Bunsen solubility coefficient of CO2 was calculated using
Eq. (5):
β=Hcp×RT,
where Hcp is the Henry constant (mol mL-1 atm-1) calculated
using the marelac R package (Soetaert et al., 2016). R is the ideal gas constant
(0.082057459 atm L mol mL-1 K-1), and T is the standard temperature
(273.15 K).
The atmospheric pressure of dry air (pdry) was calculated as follows:
pdry=pwet1-%H2O,
where pwet is the atmospheric pressure of wet air. Boyle's law
was applied as gas was collected several times from the same core.
The concentration of dissolved CO2 in seawater before equilibrium was
then calculated using Eq. (7):
[CO2]aq=[CO2]wVw+10-6maVaVw,
where Vw is
the volume of seawater (mL) and Va is the volume of the headspace
(mL). The units were then converted to nanomolar (nM):
[CO2]aq=109×pdry[CO2]aqRT.
The daily
CO2 fluxes were calculated from the difference between T0 and T2 taking
into account the core surface area (µmol m-2 d-1).
Daily CH4 fluxes were estimated using the same calculations as for the
CO2 fluxes with the exception of the Bunsen solubility coefficient. The
Bunsen solubility coefficient was calculated as a function of the seawater
temperature and salinity following Wiesenburg and
Guinasso (1979). The total CO2 greenhouse-equivalent fluxes were
calculated assuming CH4 to have a greenhouse potential 28-fold greater
than that of CO2 per mol of C (Myhre
et al., 2013).
Isotopic composition of carbon dioxide (δ13C-CO2) and methane (δ13C-CH4)
The isotopic signature of CO2 and CH4 produced during the
incubations was estimated using Keeling plots following
Garcias-Bonet and Duarte (2017). δ13C of
CO2 and CH4 produced in our incubations was extracted from the
intercept of the linear regression between the inverse of the gas
concentration (ppm-1) and the isotopic signature measured from the
discrete samples in the CDRS instrument.
Data analysis
The data were analyzed for normality using the Shapiro–Wilk test.
The Mann–Whitney test and Student's t test were used to test for differences in seagrass and
sediment composition between sites and between vegetated and bare sediments,
and analysis of variation (ANOVA) and the Kruskal–Wallis test were used to test for differences between
vegetated and bare sediments and both sites. To assess differences in
greenhouse gas fluxes between different H. stipulacea communities, differences in
CO2 and CH4 fluxes were analyzed between sites and between
vegetated and bare sediment by using the Kruskal–Wallis test. Trends in the flux
between the communities experiencing warming and the ones maintained at 25 ∘C, as well as in the isotopic signature of δ13C-CO2 and δ13C-CH4 over time, were analyzed by
linear regression. When assessing the effect of darkness on greenhouse gas
fluxes, the trend of CO2 and CH4 fluxes and their isotopic
signatures were analyzed by linear regression. The statistical analyses were
conducted in PRISM 5 (GraphPad Software, La Jolla, CA, USA) and JMP Pro
13.1.0 (SAS Institute Inc., Cary, NC, USA). The data are openly available
from Burkholz et al. (2019b).
ResultsSeagrass and sediment composition
Carbon, nitrogen and phosphorus (P) concentrations in seagrass leaves
were low, but they were 4- to 40-fold higher than vegetated and bare
sediment concentrations (Table 1). Seagrass sampled in site S1 had the
highest C, N and P concentrations in the leaves, while sediment C and P
concentrations differed significantly between sites (ANOVA, p<0.05,
and Kruskal–Wallis, p<0.001, respectively), with the highest C and
the lowest P concentrations found in the sediment of S2 (Table 1). There
were no consistent differences in C, N and P concentration in bare and
vegetated sediments (Table 1).
The sediments had high, but variable, carbonate concentrations, which
differed between sites (Kruskal–Wallis, p<0.0001; Table 1). The
organic matter content was slightly increased in S2 compared to S1, in both
vegetated (t test, p<0.0001) and bare (t test, p<0.001)
sediments (Table 1). Sediment bulk density was similar in both S1 and S2
sites, but vegetated sediments in S1 showed significantly lower bulk density
compared to bare sediments (t test, p<0.05; Table 1). Seagrass
biomass density was significantly higher in S2 compared to S1 (t test, p<0.05). The isotopic signature of sediment organic carbon ranged across sites
from -15.77±0.07 ‰, in vegetated sediments, to
-16.36±0.28 ‰, in bare sediments (Table 1). The
carbon isotopic signature of seagrass leaves from the same location has been
recently reported as -7.96±0.27 ‰ by
Duarte et al. (2018).
Carbon dioxide and methane air–seawater fluxes
The daily CO2 flux was up to 6-fold higher in vegetated compared to
bare sediments and tended to be generally higher in S2 compared to S1, where
bare sediments showed net CO2 uptake, although differences were not
significant (Kruskal–Wallis, p>0.05; Fig. 1a). At both sites, S1
and S2, the daily net CH4 flux was 10- to 100-fold higher in vegetated
compared to adjacent bare sediments with generally higher fluxes at S2
(Kruskal–Wallis, p<0.01; Fig. 1b). The total CO2 equivalent (CO2-eq) fluxes varied between sites and were higher in the
vegetated compared to the bare sediments (Kruskal–Wallis, p<0.01;
Fig. 1c).
Mean + SE (a)CO2, (b)CH4 and (c)CO2-eq fluxes in
vegetated (green) and adjacent bare (yellow) sediments at two sites (S1 and
S2) in the central Red Sea.
Effect of warming on carbon dioxide and methane air–seawater fluxes
The CO2 fluxes in vegetated sediments increased greatly with warming
(R2=0.38, p<0.001) but decreased over time when the
community was maintained at 25 ∘C (R2=0.30, p<0.01; Fig. 2a, Table S1 in the Supplement), shifting from sediments showing net CO2
emission to net CO2 uptake. Similar responses were observed in the bare
sediments, where CO2 fluxes increased with warming (R2=0.54,
p<0.0001), while the community tended to shift over time from
supporting net CO2 emission to net CO2 uptake when maintained
at 25 ∘C (Fig. 2b). The net CO2 flux – i.e., the difference
between the CO2 fluxes in warming sediments and those at sediments
maintained at 25 ∘C – increased significantly with warming in both
the vegetated and the bare sediment (R2=0.74 and p<0.05, and
R2=0.91 and p<0.001, respectively; Fig. 2c).
Mean ± SE CO2 (left) and CH4 (right) fluxes in (a, d) vegetated and (b, e) bare sediments. Symbols indicate each
replicate of the community experiencing warming from 25 to 37 ∘C
(red) and the community maintained at 25 ∘C (blue) over the
experimental period (number of days since the onset of the experiment). An
outlier at 33 ∘C in vegetated sediments in shown in green. (c, f) Mean ± SE (c)CO2 and (f)CH4 net fluxes in vegetated
(green) and bare (yellow) sediments over the experimental period (number of
days since the onset of the experiment). The second x axis indicates the
experimental temperature for the community exposed to warming from 25 to 37 ∘C. Lines represent a fitted linear equation.
CH4 fluxes declined over time when the sediments were maintained at 25 ∘C, both in vegetated (R2=0.43, p<0.001; Fig. 2d) and, less strongly, bare sediments (R2=0.24, p<0.01;
Fig. 2e, Table S2). In contrast, CH4 fluxes tended to increase with
temperature in vegetated (Fig. 2d) and bare (Fig. 2e) sediments gradually
warmed from 25 to 37 ∘C, although it was not
significant (p>0.05 and p>0.05, respectively; Table S2). The net CH4 fluxes – i.e., the difference between the CH4
fluxes in sediments exposed to warming and sediments maintained at
25 ∘C – increased significantly over time (i.e., with warming) in
vegetated (R2=0.69, p<0.05) but not in bare sediments (p>0.05; Fig. 2f). An outlier in the vegetated sediment at 33 ∘C supporting extreme emissions (CO2 flux of 55 170 µmol CO2 m-2 d-1 and CH4 flux of 699.8 CH4µmol m-2 d-1) was observed on day 14 in one of the replicates of
the warming treatment where seagrass had died (Fig. 2a and d); it was
excluded from the regression analyses reported above.
Despite CO2 and CH4 fluxes showing the same response to warming in
both types of sediment, vegetated sediments held higher fluxes than bare
sediments. The relationship between net CO2 and CH4 fluxes in
bare vs. vegetated sediments showed that both bare and vegetated communities
tended to act as net CO2 sinks at 25 ∘C, but tended to act
as CO2 sources at warmer temperatures (Fig. 3a), whereas net CH4 fluxes were 3- to 8-fold higher in vegetated compared to bare sediments (Fig. 3b). The CH4 / CO2 ratio declined in the vegetated sediments
exposed to warming from 7 to 0.8 %. For CO2 and CH4 fluxes in
vegetated sediments, the Q10 value for the temperature range 25–37 ∘C was 9 and 1.5, respectively, while the Q10 value for bare
sediments was 13.8 and 4.2, respectively.
Relationship between vegetated and bare sediments for (a)CO2
and (b)CH4 fluxes. Symbols indicate different temperatures ranging
from 25 to 37 ∘C; the dashed line indicates 1 : 1; and dotted
lines show 2 : 1, 4 : 1 and 8 : 1.
Effect of darkness on carbon dioxide and methane air–seawater fluxes
The vegetated sediment shifted over time from showing net CO2 uptake to
net CO2 emission when maintained in the dark (R2=0.70, p<0.05), while the increase in the bare sediment was not significant
(p>0.05; Fig. 4a, Table S3). In contrast, when vegetated
sediment was maintained at 25 ∘C with a 12 h L : 12 h D
photoperiod, the community shifted from net CO2 emission to net
CO2 uptake (Mann–Whitney, p<0.05; Fig. 5a). In bare sediments,
CO2 fluxes showed the same trend in cores maintained at 25 ∘C with a 12 h L : 12 h D photoperiod as under dark conditions (Fig. 5b).
Mean ± SE (a)CO2 and (b)CH4 fluxes in vegetated
(green) and bare (yellow) sediments of communities exposed to prolonged
darkness over the experimental period (number of days since the onset of the
experiment).
Comparison of mean ± SE CO2 (left) and CH4 (right)
fluxes in (a, c) vegetated and (b, d) bare sediments maintained at 25 ∘C with a 12 h L : 12 h D photoperiod (white) and communities kept
at 25 ∘C with a 24 h D period (black) over the experimental period
(number of days since the onset of the experiment). Dots indicate mean
values, and error bars indicate standard error of the mean.
When vegetated sediments were kept in the dark, net CH4 fluxes
decreased 5-fold over time (R2=0.99, p<0.0001; Fig. 4b,
Table S4). However, the CH4 fluxes did not differ significantly between
vegetated cores maintained at 25 ∘C in the 12 h L : 12 h D
photoperiod or in the dark (Mann–Whitney, p>0.05), showing the
same trend of decreasing CH4 fluxes (Fig. 5c). In the bare sediment,
CH4 fluxes in sediments kept in the dark were higher than those at 25 ∘C with a 12 h L : 12 h D photoperiod, with significant
differences only observed on days 14 and 20 (Mann–Whitney, p<0.05 and p<0.05, respectively; Fig. 5d).
Isotopic composition of carbon dioxide (δ13C-CO2) and
methane (δ13C-CH4)
The isotopic signature of the δ13C-CO2 became heavier with
warming in the bare sediment, increasing from -22.36±-4.97 ‰ δ13C at 25 ∘C to -9.01±0.98 ‰ δ13C at 37 ∘C (R2=0.91, p<0.001), while the other treatments showed similar values
over time, ranging from a minimum average of -17.89±1.81 ‰ to a maximum average of -11.55±5.32 ‰ δ13C (Fig. 6a–d).
Mean ± SE isotopic signature of CO2 (δ13C-CO2) and CH4 (δ13C-CH4) in the
communities experiencing warming from 25 to 37 ∘C (left) and the
communities maintained at 25 ∘C (right). (a–d)δ13C-CO2 is shown for the (a, b) vegetated and (c, d) bare sediments over the experimental period (number of days since the onset of
the experiment). (e–h)δ13C-CH4 is shown for the (e, f) vegetated and (g, h) bare sediment over the experimental period. The
second x axis indicates the temperature increase for the community
experiencing warming.
The isotopic signature of δ13C-CH4 decreased over time in
both vegetated and bare sediments, whether they were maintained at constant
temperature or experienced warming (Fig. 6e–h). The isotopic signature in
the vegetated sediment exposed to warming decreased significantly from -50.8 ‰
to -54.06 ‰ (R2=0.67, p<0.001).
The δ13C isotopic composition of both CO2 and CH4 became heavier over time when the community was kept in the dark
(Fig. 7), with a significant increase of δ13C-CH4 in bare
sediments (R2=0.94, p<0.01; Fig. 7d).
Mean ± SE isotopic signature of CO2 (δ13C-CO2, left) and CH4 (δ13C-CH4, right)
in (a, c) vegetated and (b, d) bare sediments exposed to prolonged
darkness over the experimental period (number of days since the onset of the
experiment).
DiscussionCarbon dioxide and methane air–seawater fluxes
The values reported for CO2 and CH4 fluxes varied greatly between
the two sites studied here, with higher fluxes in the more organic sediments
with higher biomass density (S2). CO2 and CH4 fluxes were also highly
variable over time at the studied site, as the first evaluation of fluxes at
the same location delivered rates up to 100-fold above the rates of
the second measurement 1 week later. Hence, organic matter availability
along with temperature may account for the large variation in CO2 and
CH4 fluxes. Additionally, the variability of CO2 and CH4 fluxes could also be supported by infaunal species present in the cores
that were not recorded in this study. These trends were similar to results
reported in previous studies, as a high variability between species and
locations was found (cf. Table 1 in Garcias-Bonet and Duarte,
2017).
Even though there were some differences, carbon, nitrogen and phosphorus
concentrations were generally similar, and they did not seem to have an
effect on CO2 and CH4 fluxes. Carbon, nitrogen and phosphorus
concentrations were low compared to mean values (carbon: 33.6±0.31 % DW; nitrogen: 1.92±0.05 % DW; phosphorus: 0.23±0.011 % DW) reported for seagrass leaves by
Duarte (1990).
Serrano et al. (2018) explained the
discrepancy between Red Sea data and global data with the extreme conditions
in the Red Sea, such as low nutrient input and high temperatures, as well as
a limited data set favoring high carbon stocks in the Mediterranean.
The results presented here add to the findings of Garcias-Bonet and
Duarte (2017) identifying Red Sea seagrass communities as a significant
source of CH4. The presence of seagrass resulted in a larger organic
matter supply to the sediments, favoring the presence of methanogens, which
led to higher CH4 fluxes compared to those fluxes supported in bare
sediments (Barber and Carlson,
1993; Bahlmann et al., 2015), consistent with the up-to-100-fold-higher
CH4 fluxes supported by vegetated compared to bare sediments in this
study. Additionally, higher fluxes in vegetated cores could be an indicator
of direct effects resulting from the presence of seagrass, as vascular
plants on land have been shown to have varying effects on methane emissions
caused by differences in biomass and gross photosynthesis
(Öquist and Svensson, 2002).
Similar trends were also seen by Garcias-Bonet and Duarte (2017), who reported an increase in CH4 fluxes with increasing organic
matter content in Red Sea seagrass sediments. They reported organic matter
contents in Red Sea seagrass sediments ranging from 2.33±0.07 %
(Halodule uninervis) to 12.42±0.23 % (Enhalus acoroides), including a mixed meadow with H. stipulacea and H. uninervis
showing slightly increased organic matter content of 3.51±0.17 %
compared to vegetated sediments at S2. Moreover, they found the highest
CH4 fluxes in meadows with the highest biomass, confirming our
findings of higher fluxes at study site S2.
In terms of CO2 equivalents, only the bare sediment
maintained at 25 ∘C seemed to act as a C sink over the
experimental period, while the vegetated sediments, both maintained at 25 ∘C and exposed to warming, acted as sources of greenhouse gases.
A sublethal disturbance, such as warming below the lethal threshold, can
therefore lead to a shift of seagrass ecosystems from acting as net sinks to
net sources of greenhouse gases, as demonstrated experimentally here.
Effect of warming
Both CO2 and CH4 fluxes were higher in vegetated compared to
adjacent bare sediments, indicating elevated remineralization rates in
vegetated sediments as well as a higher susceptibility of seagrass sediment
to increasing temperatures. Vegetated sediments exposed to warming shifted
from acting as a CO2 sink to an increasingly intense source, while the
CO2 fluxes in vegetated sediments maintained at 25 ∘C
decreased over time. Warming leads to an increase in both community
photosynthesis and respiration, with respiration's faster rate of increase
(Harris et al., 2006) explaining the shift to a CO2
source in sediments exposed to a thermal stressor. However, the fluxes
maintained at 25 ∘C showed a net CO2 uptake with a mean of
464.78±156.6µmol CO2 m-2 d-1 (Table S1),
while those reported in a mixed Halodule sp. and Halophila sp. meadow in India showed a net
CO2 release (dry season: 1190±1600µmol CO2 m-2 d-1; wet season: 18 400 ± 8800 µmol CO2 m-2 d-1; Banerjee et al., 2018). Both
values reported were measured at higher temperatures (dry season: 30±0.68∘C; wet season: 27.94±0.72∘C;
Banerjee et al., 2018) compared to our fluxes measured at
25 ∘C, also indicating that temperature might lead to higher
fluxes. An initial high CO2 flux measured on day two after sampling
could be an indicator for the experienced disturbance due to sample
collection and transportation even though we allowed the cores some time to
adapt.
Mean CH4 fluxes at in situ temperature (25 ∘C) in vegetated
sediments were lower than the mean value of 85.1±27.8µmol CH4 m-2 d-1 reported for other seagrass meadows in the Red
Sea (Garcias-Bonet and Duarte, 2017). In contrast, the
community exposed to warming reached a maximum average CH4 flux almost
4-fold higher than the community held at 25 ∘C and showed a
clear increase with warming relative to sediments held at 25 ∘C.
The increase in CH4 fluxes with warming was consistent with reports
from Barber and Carlson (1993) for a Thalassia testudinum community in Florida Bay
and from Garcias-Bonet and Duarte (2017) for Red Sea seagrass
communities, who reported higher CH4 fluxes at higher temperatures.
Additionally, previous research has shown that methanogenesis has a higher
thermal dependence than respiration and photosynthesis (Yvon-Durocher et al., 2014),
confirming the trends seen here with increasing fluxes at higher
temperatures. We also reported a 10-fold decline in CH4 fluxes over
time for sediment communities maintained at 25 ∘C, which could be
attributable to increased sulfate reduction, reduced CH4 production or
a combination of both. Methane is produced under anoxic conditions in marine
sediments, yet only a small portion is released, as CH4 production by
methanogens is compensated for by CH4 oxidation by sulfate-reducing
bacteria (Barnes and Goldberg, 1976). Similar to the
trends seen in CO2 fluxes, the decrease in CH4 fluxes could be
attributable to an initial stress response to the disturbance caused by
sample collection and transportation. While reduced photosynthetic activity
and a degradation in biomass could result in higher CH4 fluxes
(Lyimo et al., 2018), the cores maintained
at 25 ∘C might show the effect of healthy conditions.
Increasing water temperature led to a decrease in the CH4 / CO2
ratio. While there was ∼7 % of sequestered carbon released
as CH4 to the atmosphere in vegetated sediments at 25 ∘C (on
day two ), it decreased to ∼0.8 % in vegetated sediments at
37 ∘C. In contrast, Banerjee et al. (2018)
reported ∼1 % of carbon being released as CH4.
The isotopic composition of CO2 in all treatments showed generally
heavier isotopic signatures compared to previous reports of seagrass carbon (average δ13C value of -7.73±0.11 ‰ for Red Sea seagrass and -7.57±0.15 ‰ for H. stipulacea in the Red Sea; Duarte et
al., 2018), indicating various organic matter sources such as macroalgae
blades (13.38±0.3 ‰), mangrove leaves (26.58±0.13 ‰) and seston (25.43±0.42 ‰; Duarte et al., 2018).
However, the mean δ13C value of Red Sea seagrass sediments was
reported to be -13.36±0.4 ‰ (Garcias-Bonet et al., 2019a),
similar to the results found in this study. Our chosen study sites were
located in an enclosed lagoon with a high abundance of mangrove forests,
leading to the conclusion that mangroves might be a major source of organic
matter for our study sites. However, a recent study applying stable isotope
mixing models found the major contributors to the organic matter in seagrass
sediments in the Red Sea to be seagrass leaves and macroalgae blades, with
contributions of 43 % and 37 %, respectively
(Garcias-Bonet et al., 2019a).
The isotopic signature of CO2 released from bare sediments shifted
with warming, suggesting a shift from seston, mangroves and macroalgae to seagrass carbon as the source
of CO2. In the vegetated cores, the isotopic composition of CO2 stayed rather constant, indicating several sources of organic carbon with
no clear shift, regardless of warming.
The isotopic signature of CH4 in vegetated sediments confirmed its
biogenic source as previous reports have shown that the isotopic signature
of CH4 from biogenic sources can range from -40 ‰ to -80 ‰, while the isotopic signature of CH4 from
geological and thermogenic sources ranges from -30 ‰ to -50 ‰ (Reeburgh, 2014). The isotopic
composition of CH4 in bare sediments was generally at the lower end
of this range, with no clear shift with increasing temperature.
The isotopic composition of CH4 can be determined by the production of
CH4 (methanogenesis) leading to lower δ13C values and the
oxidation of CH4 (methanotrophy) leading to higher δ13C
values (Whiticar, 1990). Garcias-Bonet and Duarte (2017) reported fluctuations in the
isotopic signature of CH4 in Red Sea seagrass meadows, suggesting an
indication of both processes. With exposure to increasing temperatures, we
observed a shift to a lighter isotopic signature of CH4 in vegetated
sediments, thereby indicating an increasing CH4 production by
methanogens with warming.
Effect of prolonged darkness
Communities maintained at 25 ∘C with a 12 h L : 12 h D photoperiod
showed continuous net CO2 uptake, while the communities kept in the
dark shifted, as expected, to a heterotrophic state, acting as a CO2
source. The net CO2 production corresponded to community respiration
rates, while that with a 12 h L : 12 h D photoperiod corresponded to the net
community production.
We found, however, no effect of prolonged darkness on CH4 fluxes,
suggesting that the elevated CH4 fluxes in vegetated sediments were not
directly supported by fresh photosynthetic products, but rather by the
elevated organic matter content in vegetated sediments compared to bare
ones. These findings were in contrast to those reported by
Lyimo et al. (2018), who found increased
CH4 fluxes under shading, indicating that degradation of belowground
biomass might have been the key factor related to increased CH4 fluxes.
However, they also reported varying results for different shading
intensities, with low intensity having similar fluxes compared to their
control group (Lyimo et al., 2018). In
contrast, Öquist and Svensson (2002) found
that photosynthesis might be regulating methane fluxes in a subarctic
peatland ecosystem, with lower photosynthesis resulting in lower methane
fluxes.
Implications
Reports on greenhouse gas fluxes by seagrass ecosystems are limited
(Oremland,
1975; Barber and Carlson, 1993; Alongi et al., 2008; Deborde et al., 2010;
Bahlmann et al., 2015; Garcias-Bonet and Duarte, 2017; Banerjee et al.,
2018; Lyimo et al., 2018), and no reports had been previously published on
how increasing seawater temperatures might affect greenhouse gas fluxes by
seagrass ecosystems. Blue carbon ecosystems have been shown to turn into C
sources when disturbances lead to mortality (Macreadie
et al., 2015; Lovelock et al., 2017; Arias-Ortiz et al., 2018), consistent
with the very large CO2 and CH4 fluxes observed in one vegetated
sediment where the seagrass died when warmed to 33 ∘C. However,
even where seagrass remained alive, warming led to elevated greenhouse
fluxes. Additionally, the elevated nutrient and high organic matter stock in
seagrass meadows, which supports 1.7 times more organic matter content
than surrounding bare sediments, can promote an increase in CO2and
CH4 fluxes following disturbance (Gonsalves
et al., 2011; Sotomayor et al., 1994). Our results suggest that this stock
in seagrass sediments may be remineralized to support net greenhouse gas
fluxes at the warmer temperatures reached and with further warming of the
Red Sea. Hence, warming may, as other disturbances
(Lovelock et al., 2017), shift seagrass ecosystems
from net sinks to net sources of greenhouse gases, thereby providing a
feedback mechanism that may contribute to further enhancing global warming.
Conclusion
In summary, this study reports, for the first time, experimental evidence
that warming leads to increased greenhouse gas (CO2 and CH4)
fluxes in a H. stipulacea meadow in the Red Sea, and it may lead to seagrass meadows
shifting from acting as sinks to sources of greenhouse gases. Increased
fluxes at higher temperatures can be an indication of higher
re-mineralization rates and a higher susceptibility of vegetated sediments to
temperature. The elevated organic matter content, higher biomass density and greater
plant activity in vegetated sediments led to increased CO2 and CH4 fluxes in vegetated compared to bare sediments and a much steeper increase
in CO2 and CH4 fluxes with warming. In addition, prolonged
darkness led to an increase in CO2 fluxes, while CH4 fluxes
decreased over time, also indicating organic matter to be the driver.
However, we also found a high variability in fluxes over time, indicating
that other factors, such as infaunal species, could play a role as well.
While the current focus is on conserving blue carbon ecosystems from losses due to deteriorated water quality or mechanical damage, our results show that
sublethal warming may also lead to emissions of greenhouse gases from
seagrass meadows, contributing to a feedback between ocean warming and
further climate change.
Data availability
All data are accessible in the repository Pangea (10.1594/PANGAEA.905687, Burkholz et al., 2019b).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-17-1717-2020-supplement.
Author contributions
NGB, CMD and CB designed the project. CB collected the samples and
conducted the experiments. NGB, CMD and CB analyzed the results; CMD and CB
wrote the first draft of the manuscript; and all authors contributed
substantially to the final draft. All authors approved the final
submission.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank Paloma Carrillo de Albornoz, Mongi Ennasri and Vijayalaxmi Dasari for their help
with analyses. We also thank Katherine Rowe and the Coastal and Marine
Resources Core Lab (CMOR) for their assistance during field work and CMOR
for their support with the experimental setup.
Financial support
This research has been supported by the King Abdullah University of Science and Technology through baseline and CARF funding (grant nos. FCC/1/1973-32-01 and BAS/1/1071-01-01).
Review statement
This paper was edited by Aninda Mazumdar and reviewed by two anonymous referees.
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