Introduction
Situated at the interface of land and sea, estuaries are highly susceptible
to anthropogenic inputs and undergo intricate biogeochemical and hydrological
processes. Estuaries play an important role in modulating the global carbon (C)
cycle and the anthropogenic carbon dioxide (CO2) budget (Bauer et al.,
2013; Regnier et al., 2013; Le Quéré et al., 2016). Atmospheric
CO2 is sequestered into terrestrial systems through photosynthesis
and weathering reactions and is transported to the ocean via rivers and
estuaries. Tropical rivers, which constitute ∼66 % of global river
water discharge, deliver ∼0.53 Pg C to the estuaries annually (Huang et
al., 2012). The majority of this exported C is in dissolved form (dissolved
inorganic C (DIC): 0.21 Pg C yr-1 and dissolved organic C (DOC):
0.14 Pg C yr-1) with some contribution as particulate (particulate
organic C (POC): 0.13 Pg C yr-1 and particulate inorganic C (PIC):
0.05 Pg C yr-1) (Huang et al., 2012). Although estuaries are only
∼4 % of the continental shelf regions, CO2 emission flux
from estuarine surface waters is as high as CO2 uptake in
continental shelf regions of the world, albeit with large uncertainty (Borges
et al., 2005; Chen and Borges, 2009; Cai et al., 2006; Cai, 2011). This
suggests estuaries are not only active pathways for transport of C (Ittekkot
and Laane, 1991) but also hotspots for biogeochemical modification of labile
organic matter (OM) (Frankignoulle et al., 1998).
Mangroves covering 137 760 km2 along tropical and subtropical
estuaries and coastlines (Giri et al., 2011) are among the most productive
natural ecosystems in the world with net primary productivity of 218±72 Tg C yr-1 (Bouillon et al., 2008). Fine-root production coupled
with litter fall and wood production are primary sources of mangrove-derived
C to intertidal forest sediment (Bouillon et al., 2008). The fate of this
mangrove-derived C remains poorly understood. Despite taking C burial and
CO2 emission flux across mangrove sediment–atmosphere interface
into account, estimates of global mangrove C budget showed a significant
imbalance between mangrove net primary productivity and its sinks (Bouillon
et al., 2008). Earlier studies reported mangroves to be responsible for
∼10 % of the global terrestrial derived POC and DOC exports to the
coastal zones (Jennerjahn and Ittekkot, 2002; Dittmar et al., 2006). However,
recent studies proposed DIC exchange as a major C export pathway from mangrove
forests, which was ∼70 % of the total mineralized C transport from
mangrove forests to coastal waters (Maher et al., 2013; Alongi, 2014; Alongi
and Mukhopadhyay, 2014). Another study reported groundwater advection from
mangroves to be responsible for 93 %–99 % and 89 %–92 % of
total DIC and DOC exports to the coastal ocean (Maher et al.,
2013). Upon extrapolating these C exports to the global mangrove area, it was
found that the calculated C exports were similar to the missing mangrove C
sink (Sippo et al., 2016). The remaining C that escapes export gets buried in
subsurface sediment layers and participates either in complex anaerobic
processes (linked to production of biogenic trace gases like CH4)
or undergoes long-term sequestration (Jennerjhan and Ittekkot, 2002; Barnes
et al., 2006; Kristensen and Alongi, 2006; Donato et al., 2011; Linto et al.,
2014).
Apart from lateral transport of
dissolved and particulate C, biogeochemical processes such as primary
production, OM mineralization, carbonate precipitation / dissolution and
water–atmosphere CO2 exchange occurring in the estuary also regulate
the inorganic and organic C biogeochemistry of a mangrove-dominated estuary. These processes largely depend upon pH, nutrient availability
and euphotic depth variability, as well as planktonic and bacterial
biodiversity and community compositions. The biogeochemical cycling of
bioavailable elements, such as C and N, in a mangrove-dominated estuary is
largely different from anthropogenically polluted estuary, where much of the
OM is derived from domestic, agricultural and industrial waste. In
anthropogenically affected estuarine systems, heterotrophy generally
dominates over autotrophy (Heip et al., 1995; Gattuso et al., 1998) and a
substantial fraction of biologically reactive OM gets mineralized within the
system (Servais et al., 1987; Ittekkot, 1988; Hopkinson et al., 1997; Moran
et al., 1999). However, this is not always the case, as observed in the
Guanabara Bay, Brazil, which acts as a strong CO2 sink enhanced by
eutrophication (Cotovicz Jr. et al., 2015). The lack of ample rate
measurements of the above-mentioned biogeochemical processes in many regions
of the world restrains biogeochemists from an in-depth understanding of these
processes in different ecological settings. It also leads to an uncertainty
in the estimation of a coastal C budget on a global scale.
Sampling locations at the (a) estuaries of the Sundarbans and (b)
Hooghly estuary.
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In India, research related to C biogeochemistry of estuarine ecosystems has
been in focus since last two decades with an emphasis on estuaries located in
southern India (e.g. Bouillon et al., 2003; Sarma et al., 2012, 2014;
Bhavya et al., 2017, 2018). The estuaries located in the northern part of
India have received limited attention, including the adjacently located Hooghly
estuary and the estuaries of the Sundarbans, which are part of the
Ganga–Brahmaputra river system (Fig. 1). Characteristically, the Hooghly and
the estuaries of the Sundarbans are different from each other. The Hooghly
estuary experiences significantly higher anthropogenic influence compared to
the mangrove-dominated Sundarbans as evidenced by high nutrient and
freshwater inputs (Table 1). The anthropogenic influences largely include a
supply of the industrial effluents and domestic sewage on a daily basis from
industries and major cities (Kolkata and Howrah) located upstream (Table 1).
The industries along the Hooghly are principally jute
(Corchorus olitorius)-based, which produce fabrics for packaging a
wide range of agricultural and industrial commodities.
General characteristics of the Hooghly estuary and the estuaries of the Sundarbans.
Parameters
Hooghly
Sundarbans
Nutrients (post-monsoon)
DIN: 14.72±1.77 to 27.20±2.05 µM DIP: 1.64±0.23 to 2.11±0.46 µM DSi: 77.75±6.57 to 117.38±11.54 µM Mukhopadhyay et al. (2006)
DIN: 11.70±7.65 µM DIP: 1.01±0.52 µM DSi: 75.9±36.9 µM Biswas et al. (2004)
Chl a (post-monsoon)
2.35–2.79 mg m-3 Mukhopadhyay et al. (2006)
7.88±1.90 mg m-3 Dutta et al. (2015)
Population density
North 24 Parganas and Hooghly: 2500 km-2, Kolkata: 22 000 km-2, Howrah: 3300 km-2, South 24 Parganas: 820 km-2
No major cities and towns
Freshwater discharge(post-monsoon)
3070–7301 million m3 Rudra et al. (2014)
No information available
Catchment area
6 × 104 km2 Sarkar et al. (2017)
No information available
Industrial and municipalwaste-water discharge
1153.8 million L d-1 Ghosh, (1973), Khan (1995)
No information available
Dissolved metal flux
Increased from 230 % to 1770 % annually Samanta and Dalai (2018)
No information available
Earlier, the major focus of biogeochemical studies in the Hooghly and the
estuaries of the Sundarbans had been on the biogeochemistry of trace gases
(Mukhopadhyay et al., 2002; Biswas et al., 2004, 2007; Ganguly et al., 2008,
2009; Dutta et al., 2013, 2015, 2017) with the exception of one comprehensive
study on nutrient budgets at the Hooghly estuary (Mukhopadhyay et al., 2006).
Recently, attempts have been made to understand different aspects of C
cycling in these two estuaries (Samanta et al., 2015; Ray et al., 2015, 2018;
Akhand et al., 2016). Samanta et al. (2015) comprehensively studied DIC
dynamics in the Hooghly estuary, whereas Akhand et al. (2016) focused on DIC
and pCO2 at the Hooghly–Matla estuary. Different aspects
of C cycling in the Hooghly–Sundarbans system have been reported by Ray et
al. (2015, 2018). Barring Samanta et al. (2015), which has wider spatial and
temporal coverages with respect to DIC in the Hooghly, other studies are
severely limited in spatial coverage with focus on the middle to lower parts of the
Hooghly estuary and a few locations in the Sundarbans (one location by Ray et
al., 2015, 2018; three locations by Akhand et al., 2016). Given the vast
expanse of these estuaries, the extrapolation of data from these studies for the
entire ecosystem may lead to overestimation and underestimation.
The primary objective of the present study was to understand differences in
varied aspects of the C cycle (DIC, DOC, POC and CO2) of the Hooghly
and the estuaries of the Sundarbans during the post-monsoon season with relatively better
spatial coverage compared to previous studies. The post-monsoon sampling was
chosen because of relatively stable estuarine conditions for wider spatial
coverage and peak mangrove leaf litter fall during this season (Ray et al.,
2011), which may have an influence on estuarine C dynamics. Considering the
different natures and quantities of supplied OM within these two contrasting
systems, we hypothesized C metabolism in these two estuaries to be very
different with higher CO2 exchange flux from anthropogenically
influenced estuary compared to the mangrove-dominated one. Specifically, the
major aims of the present study were to investigate (a) factors controlling
DIC and DOC dynamics in the region, (b) the sources and fate of POC in these two
contrasting systems, and (c) the partial pressure of CO2
(pCO2) and its controlling mechanisms along with exchange across
the water–atmosphere interface at the Hooghly–Sundarbans during the post-monsoon
period.
Discussion
Based on the results obtained during the present study, below we discuss
different aspects of the C cycle within the Hooghly–Sundarbans system.
Major drivers of DIC dynamics
DIC concentrations observed in this study for the Hooghly were higher than
that reported by Samanta et al. (2015) for the same season (DIC: 1700 to
2250 µM), whereas observed δ13CDIC were
within their reported range (δ13CDIC: -11.4 ‰ to -4.0 ‰). Statistically significant
correlations between DIC–salinity (r2=0.43, p = 0.015) and
δ13CDIC–salinity (r2=0.58,
p = 0.003) in the Hooghly suggested the potential influence of marine
and freshwater mixing on DIC and δ13CDIC in the
estuary (Fig. 3a, b), rationalizing the application of a two-endmember mixing
models. A two-endmember mixing model
to decipher processes influencing DIC chemistry has been applied earlier in
the Hooghly estuary (Samanta et al., 2015).
(a) DIC–salinity in the Hooghly, (b)
δ13CDIC–salinity in the Hooghly, (c)
ΔDIC–Δδ13CDIC in the Hooghly,
(d) DIC–salinity in the Sundarbans and (e)
δ13CDIC–salinity in the Sundarbans.
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Based on the methodology discussed earlier, calculated ΔC for
DIC (ΔDIC ∼-0.27 to 0.17) predicted a dominance of DIC addition
(n=4) over removal (n = 2) in the freshwater regime of the Hooghly,
whereas only removal was evident in the mixing regime. In the case of Δδ13C for DIC (Δδ13CDIC), values were
mostly positive (n = 9), i.e. measured δ13CDIC was higher compared to estimated
δ13CDIC due to conservative mixing. A deviation plot
(ΔDIC vs. Δδ13CDIC; Fig. 3c) for samples
of the Hooghly showed the following patterns: (a) a decrease in ΔDIC with
increasing Δδ13CDIC (n = 5), indicating
phytoplankton productivity and/or outgassing of CO2 across the
water–atmosphere interface, (b) a decrease in ΔDIC with decreasing
Δδ13CDIC (n = 4), indicating carbonate
precipitation and (c) increase of ΔDIC with increasing Δδ13CDIC (n = 4) representing carbonate dissolution
within the system.
Based on these calculations, both organic and inorganic processes
(productivity, carbonate precipitation and dissolution) along with physical
processes (CO2 outgassing across the water–atmosphere interface)
appeared to regulate DIC chemistry in the Hooghly estuary. Spatially,
phytoplankton productivity and/or outgassing of CO2 appeared to
regulate DIC in the mixing regime (n = 5 out of 7) of the Hooghly.
Earlier studies have advocated for high phytoplankton productivity in
non-limiting nutrient conditions during the
post-monsoon season in the Hooghly (Mukhopadhyay et al., 2002., 2006). However,
based on the present data, particularly due to a lack of direct primary
productivity measurements, it was difficult to spatially decouple individual
contributions of primary productivity and CO2 outgassing in the
mixing regime. In contrast to the mixing regime, carbonate precipitation and
dissolution appeared to be dominant processes affecting DIC chemistry in the
freshwater regime of the Hooghly.
In mangrove-dominated estuaries of the Sundarbans, observed δ13CDIC during this study were within the range (δ13CDIC: -4.7±0.7 ‰) reported by Ray et al.
(2018), whereas observed DIC concentrations were lower than their estimates
(DIC: 2130±100 µmol kg-1). Our data also showed
similarity with Khura and Trang rivers, two mangrove-dominated rivers of
peninsular Thailand flowing towards the Andaman Sea, although from hydrological
prospective these two systems are contrasting in nature (Sundarbans: narrow
salinity gradient (12.74 to 16.69) vs. Khura and Trang rivers: sharp salinity
gradient (∼0 to 35); Miyajima et al., 2009). Like Hooghly, the
δ13CDIC–salinity relationship was statistically
significant (r2=0.55, p = 0.009) for the Sundarbans, but the DIC–salinity relationship remained insignificant (p = 0.18) (Fig. 3d, e).
Given the dominance of mangroves in the Sundarbans, the role of mangrove-derived organic carbon (OC) mineralization may be important in regulating DIC
chemistry in this ecosystem. Theoretically, ΔCMangrove for DIC (ΔDICMangrove) estimated
based on DIC (ΔDICM1) and
δ13CDIC (ΔDICM2) should be
equal. The negative and unequal values of ΔDICM2 (-41
to 62 µM) and ΔDICM1 (-186 to
11 µM) indicate a large DIC outflux over influx through
mineralization of mangrove-derived OC in this tropical mangrove system. The
removal mechanisms of DIC include CO2 outgassing across estuarine
water–atmosphere boundary, phytoplankton uptake and export to the adjacent
continental shelf region (northern BOB, Ray et al., 2018). The evidence for
CO2 outgassing was found at almost all locations covered during the
present study (10 out of 11 locations covered; see Sect. 4.4). Also, a recent
study by Ray et al. (2018) estimated DIC export
(∼3.69 Tg C yr-1) from the estuaries of the Sundarbans as the
dominant form of C export. Although data for primary productivity is not
available for the study period, earlier studies have reported post-monsoon as
the peak season for phytoplankton productivity (Biswas et al., 2007; Dutta et
al., 2015). Given the evidence for the presence of DIC removal processes in the
Sundarbans, a comprehensive study that measures rates of these processes with
higher spatial and temporal coverages is desirable to understand the balance
between influx and outflux of DIC in the Sundarbans.
Other than biogeochemical processes, factors such as groundwater and
porewater exchange to the estuary might also play a significant role in
estuarine DIC chemistry (Tait et al., 2016). High pCO2 and DIC,
along with low pH and TAlk/DIC are general characteristics of groundwater,
especially within carbonate aquifer region (Cai et al., 2003). Although all
the parameters of groundwater inorganic C system (like pH, TAlk and
pCO2) were not measured during the present study, groundwater DIC
was ∼5.57 and ∼3.61 times higher compared to mean surface water DIC
in the Sundarbans and Hooghly. The markedly higher DIC in groundwater, as
well as the similarity in its isotopic composition with estuarine DIC, may
stand as a signal for the influence of groundwater on estuarine DIC, with a
possibly greater influence in the Sundarbans than Hooghly as evident from the
slope of the TAlk–DIC relationships (Hooghly: 0.98, Sundarbans: 0.03). In
the Sundarbans, to the best of our knowledge, no report exists regarding
groundwater discharge. Contradictory reports exist for the Hooghly, where
Samanta et al. (2015) indicated groundwater contribution at a low-salinity
regime (salinity < 10, same as our salinity range) based on Ca
measurement, which was not observed based on the Ra isotope measurement in an
earlier study (Somayajulu et al., 2002). Porewater DIC in the Sundarbans was
∼7.63 times higher than the estuarine water, indicating the possibility
of DIC input from the adjoining mangrove system to the estuary through
porewater exchange depending upon changes in hypsometric gradient during
tidal fluctuation (i.e. tidal pumping). By using porewater-specific discharge
and porosity at 0.008 cm min-1 and 0.58 (Dutta et al., 2013, 2015),
respectively, during the post-monsoon season and extrapolating the flux value on a
daily basis (i.e. for 12 h as tides are semidiurnal in nature), mean
FDIC during the post-monsoon season was calculated as
∼770.4 mmol m-2 d-1. However, the significant impact of
porewater on DIC may be limited only in mangrove creek water (samples not
collected) as evident from the narrow variability of estuarine TAlk and DIC
as well as no significant correlation between them (p = 0.93). A
comprehensive investigation that measures rates of ground- and
porewater-mediated DIC additions is needed to thoroughly understand their
importance in controlling the DIC chemistry of the Hooghly–Sundarbans
system.
From the above discussion, it appears that higher DIC in the Hooghly
compared to the Sundarbans may be due to cumulative interactions between
freshwater content to the individual estuaries as well as the degree of
biogeochemical and hydrological processes. A relatively higher freshwater
contribution in the Hooghly compared to the Sundarbans (as evident from
salinity), as well as significant negative relationship between DIC and salinity, proved the significant impact of freshwater on DIC pool in the Hooghly.
However, quantifications of other biogeochemical and hydrological processes
are needed to decipher dominant processes affecting DIC dynamics in the
Hooghly–Sundarbans system.
DOC in the Hooghly–Sundarbans
In the Hooghly, DOC concentrations observed during this study was higher
than the range (226.9±26.2 to 324±27 µM) reported by
Ray et al. (2018), whereas observed DOC in the Sundarbans was comparable
with their estimates (262.5±48.2 µM). The marine water and freshwater mixing did not appear to exert major control over DOC in the
Hooghly–Sundarbans system as evident from the lack of significant correlations
between DOC and salinity (Hooghly freshwater regime: r2=0.33,
p = 0.23; Hooghly mixing regime: r2=0.10, p = 0.50;
Sundarbans: r2=0.27, p=0.10, Fig. 4a). Our observation showed
similarity with other Indian estuaries (Bouillon et al., 2003), with opposite
reports from elsewhere (Raymond and Bauer, 2001; Abril et al., 2002). This
indicates that DOC in this subtropical estuarine system is principally
controlled by processes other than the mixing of two water masses.
(a) DOC–salinity in the Hooghly–Sundarbans system,
(b) DOC–pCO2 in the Hooghly, (c)
DOC–pCO2 in the Sundarbans and (d) DOC–POC in the
Hooghly–Sundarbans system.
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Although it is difficult to accurately decipher processes influencing DOC
without δ13CDOC data, some insights may be
obtained from estimated ΔC of DOC (ΔDOC). The estimated
ΔDOC in the Hooghly indicated both net addition (n = 3) and
removal (n = 3) of DOC in the freshwater regime (ΔDOC = -0.16 to 0.11), whereas only net addition was evident throughout the mixing
regime (ΔDOC = 0.08 to 1.74). In the Sundarbans, except the lower
Thakuran (St. T3, Δ DOCM1=-20 µM), net addition of mangrove-derived DOC was
estimated throughout (ΔDOCM1=2 to 134 µM).
In an estuary, DOC can be added through in situ production (by benthic and
pelagic primary producers), lysis of halophobic freshwater phytoplankton
cells and POC dissolution. DOC can be removed through bacterial
mineralization, flocculation as POC and photo-oxidation (Bouillon et al.,
2006). At the Hooghly–Sundarbans system, no evidence for freshwater
phytoplankton (δ13C: -33 ‰ to -40 ‰;
Freitas et al., 2001) was found from δ13CPOC,
ruling out its potential effect on DOC. Although an indirect signal for
phytoplankton productivity was observed in the freshwater regime from the
δ13CDIC and POC relationship (r2=0.68, p=0.05), further evaluation of its impact on DOC was not possible due to the
lack of direct measurement. Contradictory results exist regarding the
influence of phytoplankton productivity on DOC. Some studies did not find a
direct link between DOC and primary productivity (Boto and Wellington, 1988),
whereas a significant contribution of
phytoplankton production to building a DOC pool
(∼8 % to 40 %) has been reported by others (Dittmar and Lara,
2001; Kristensen and Suraswadi, 2002).
In a nutrient-rich estuary like Hooghly, the lack of a significant relationship
between DOC and pCO2 (freshwater regime: p = 0.69, mixing
regime: p = 0.67, Fig. 4b) suggested either inefficient bacterial DOC
mineralization or significant DOC mineralization compensated by phytoplankton
CO2 uptake. However, a significant positive relationship between
these two in the Sundarbans (r2=0.45, p = 0.02, Fig. 4c)
indicated an increase in aerobic bacterial activity with increasing DOC. In
mangrove ecosystems, leaching of mangrove leaf litter as DOC is as fast as
∼30 % of mangrove leaf litter leaching as DOC is reported within the
initial 9 days of degradation (Camilleri and Ribi, 1986). In the Sundarbans,
mangrove leaf litter fall peaks during the post-monsoon season (Ray et al., 2011) and its
subsequent significant leaching as DOC was evident during the present study
from relatively higher DOC compared to POC (DOC : POC = 0.50 to 3.39,
mean: 1.79±0.94 %). Our interpretation for Sundarbans corroborated
with that reported by Ray et al. (2018) for the same system as well as with
Bouillon et al. (2003) for the Godavari estuary, southern India.
Despite high-water residence time in the Hooghly (∼40 days during the
post-monsoon season; Samanta et al., 2015) and in mangrove ecosystems like the
Sundarbans (Alongi et al., 2005; Singh et al., 2016), DOC photo-oxidation may
not be so potent due to unstable estuarine conditions in the
Hooghly–Sundarbans system (Richardson number < 0.14) having
intensive vertical mixing with longitudinal dispersion coefficients of
784 m2 s-1 (Goutam et al., 2015; Sadhuram et al., 2005). The
unstable condition may not favour DOC–POC interconversion as well but
mediated by charged complexes and repulsion–attraction interactions, the
interconversion partly depends upon variation in salinity. More specifically,
the interconversion is efficient during initial mixing of freshwater (river) and
seawater and the coagulation mostly completes within the salinity range 2–3.
This appeared to be the case in the Hooghly, where DOC and POC were
negatively correlated in the freshwater regime (r2=0.86,
p = 0.007, Fig. 4d) but not in the mixing regime (p = 0.43) or in
the Sundarbans (p = 0.84).
Although estimated ΔDOC indicated largely net DOC addition to the
Hooghly–Sundarbans system, except leaf litter leaching in the Sundarbans, no
significant evidence for other internal sources was found. This suggested a
potential contribution from external sources that may include industrial
effluents and municipal waste-water discharge (i.e. surface runoff) in the
freshwater regime of the Hooghly (Table 1). However, there are no direct DOC
influx data available to corroborate the same. Relatively higher amounts of DOC compared
to POC (DOC : POC > 1) at some locations (H2, H5, H6) of the
freshwater regime may stand as a signal for higher DOC contribution at those
locations but it is not prudent to pinpoint its sources due to a lack of
isotopic data. Considering significantly high DOC levels in waste water
effluent (Katsoyiannis and Samara, 2006, 2007), along with fast degradation of
biodegradable DOC (∼80 % within 24 h; Seidl et al., 1998) and
residence time of Hooghly water (mentioned earlier), Samanta et al. (2015)
suggested the possibility of anthropogenic DOC biodegradation during its
transport in the estuary. Although anthropogenic inputs were mostly confined
to the freshwater regime, relatively higher DOC in the mixing regime of the
Hooghly compared to the freshwater regime suggested DOC input via some
additional pathway, possibly groundwater discharge. The contribution of
groundwater to the Hooghly estuary within the salinity range observed during
the present study has been reported (Samanta et al., 2015). However, there is
no report of groundwater-mediated DOC influx to the estuary. For
mangrove-dominated ecosystems like the Sundarbans, a recent study by Maher et
al. (2013) estimated ∼89 %–92 % of the total DOC export to be
driven by groundwater advection. To understand spatial variability of DOC
chemistry in the Hooghly–Sundarbans system, a thorough investigation that
measures rates of groundwater and surface-runoff-mediated DOC addition is
warranted.
Overall, on average, the concentration of DOC in the Hooghly was
∼40 % higher than in the Sundarbans, which appeared to be due to the
cumulative effect of contributions from freshwater and groundwater, higher
anthropogenic inputs and DOC–POC interconversion. However, DOC inputs via
other pathways may be dominant over freshwater-mediated input as evident from
the insignificant DOC–salinity relationship during the present study. To
quantitatively understand the relative control of the above-mentioned
contributors to the DOC pool in the Hooghly–Sundarbans system, the individual
components need to be studied in detail.
Major drivers of particulate organic matter
The average POC during this study was relatively higher than the range
(Hooghly: 40.3±1.1 to 129.7±6.7 µM, Sundarbans: 45.4±7.5 µM) reported by Ray et al. (2018) for the
Hooghly–Sundarbans system. However, it was within the range (51 to
750 µM; Sarma et al., 2014) reported for a large set of Indian
estuaries. No significant SPM–salinity or POC–salinity relationships were
observed during the present study (Fig. 5a, b), except for a moderate
negative correlation between POC and salinity (r2=0.62,
p = 0.06) in the freshwater regime of the Hooghly. This inverse
relationship may be linked to freshwater-mediated POC addition. Also, as
described earlier, the contribution of POC via surface runoff is also a
possibility in this regime due to the presence of several industries and large
urban population (St. H2: Megacity Kolkata) that discharge industrial
effluents and municipal waste water to the estuary on a regular basis (Table 1).
A signal for surface-runoff-mediated POC addition was evident in the
freshwater regime where ∼61 % and ∼43 % higher POC were
observed at H3 and H4, respectively, compared to an upstream location
(St. H2). However, based on the present data, it was not possible to decouple
freshwater and surface-runoff-mediated POC inputs to the Hooghly estuary.
A relatively lower contribution of POC to the SPM pool of the Sundarbans
(0.66 % to 1.23 %) compared to the Hooghly (0.96 % to 4.22 %;
Fig. 5c) may be due to low primary production owing to a high SPM load
(Ittekkot and Laane, 1991) as observed in the mangrove-dominated Godavari
estuary in southern India (Bouillon et al., 2003).
(a) SPM–salinity in the Hooghly–Sundarbans system,
(b) POC–salinity in the Hooghly–Sundarbans system, (c)
% POC / SPM–salinity in the Hooghly–Sundarbans system, (d)
ΔPOC–Δδ13CPOC in the Hooghly,
(e) POC–pCO2 in the Hooghly and (f)
POC–pCO2 in the Sundarbans.
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In general, wide ranges for δ13C (rivers ∼-28 ‰
to -25 ‰; marine plankton ∼-22 ‰ to -18 ‰; C3 plant ∼-32 ‰ to -24 ‰; C4 plants ∼-13 ‰ to -10 ‰;
freshwater algae and their detritus ∼-30 ‰ to -40 ‰) have been reported in the ecosystem (Smith and Epstein, 1971;
Cerling et al., 1997; Bouillon et al., 2003; Bontes et al., 2006; Kohn, 2010;
Marwick et al., 2015). In the Hooghly, our measured
δ13CPOC suggested an influx of POC via freshwater
runoff as well as terrestrial C3 plants. Additionally, the estuary
was also anthropogenically stressed during the post-monsoon season with measured δ13CPOC within
the range reported for sewage (δ13CPOC ∼-28 ‰ to -14 ‰, Andrews et al., 1998;
δ13CDOC ∼-26 ‰, Jin et al., 2018).
In the mixing regime of the Hooghly, significantly lower
δ13CPOC at H11 and H12 compared to other sampling
locations may be linked to localized 13C-depleted organic C influx
to the estuary from adjacent mangroves and anthropogenic discharge,
respectively.
In the estuaries of the Sundarbans, isotopic signatures of POC showed similarity
with terrestrial C3 plants. Interestingly, despite being a
mangrove-dominated estuary (salinity: 12.74 to 16.55), no clear signature of
either freshwater or mangrove-borne (δ13C: mangrove leaf
∼-28.4 ‰, soil ∼-24.3 ‰, Ray et al., 2015, 2018)
POC was evident from δ13CPOC values,
suggesting the possibility of significant POC modification within the
system. Modification of POC within the estuaries of Indian subcontinent has
been reported earlier (Sarma et al., 2014). Inter-estuary comparison revealed
relatively lower average δ13CPOC at the Hooghly
(mean δ13CPOC: -24.87±0.89 ‰)
compared to the Sundarbans (mean δ13CPOC: -23.36±0.32 ‰), which appeared to be due to differences in degree of
freshwater contribution, anthropogenic inputs (high in Hooghly vs. little/no
in Sundarbans), the nature of terrestrial C3 plant material (mangrove
in the Sundarbans vs. others in Hooghly), as well as responsible processes for
POC modification within the system.
To decipher processes involved in POC modification, estimated ΔC
for POC (ΔPOC) in the Hooghly indicated both net addition
(n = 3) and removal (n = 3) of POC in the freshwater regime
(ΔPOC = -0.45 to 0.48), whereas removal (n = 6)
dominated over addition (n = 1) in the mixing regime (ΔPOC = -0.39 to 0.07). In an estuary, POC may be added through
freshwater- and surface-runoff-mediated inputs, phytoplankton productivity
and DOC flocculation. The removal of POC is likely due to settling at
subtidal sediment, export to the adjacent continental shelf region,
modification via conversion to DOC and degradation by respiration in the case of
an oxygenated estuary.
(a) ECO2–AOU in the Sundarbans, (b)
pCO2–salinity in the Sundarbans, (c)
ECO2–AOU in the Hooghly, (d) pCO2–salinity
in the Hooghly, (e) FCO2–salinity in the
Hooghly and (f) FCO2–salinity in the
Sundarbans.
![]()
The plot between Δδ13C for POC (Δδ13CPOC) and ΔPOC (Fig. 5d) indicated that different
processes are active in different regimes of the Hooghly estuary. The decrease
in ΔPOC with an increase in Δδ13CPOC
(n = 4 for the mixing regime and n = 1 for the freshwater regime)
suggested degradation of POC by respiration. This process did not appear to
significantly impact the estuarine CO2 pool as evident from the
POC–pCO2 relationship (freshwater regime: p = 0.29, mixing
regime: p = 0.50; Fig. 5e). A decrease in both ΔPOC and
Δδ13CPOC (n = 2 for mixing regime and
n = 2 for freshwater regime) supported the settling of POC to subtidal
sediment. Despite high-water residence time, this process may not be
effective in the Hooghly due to unstable estuarine conditions (described
earlier). An increase in ΔPOC with a decrease in Δδ13CPOC (n = 2 for the freshwater regime)
indicated POC inputs via surface and freshwater runoff as well as
phytoplankton productivity. An increase in both ΔPOC and Δδ13CPOC (n = 1 for the mixing regime and
n = 1 for the freshwater regime) may be linked to DOC-to-POC conversion
by flocculation.
In the Sundarbans, negative and lower ΔPOCM2 (-209 to
-28 µM) compared to ΔPOCM1 (-35 to
327 µM) suggested DIC-like behaviour, i.e. simultaneous removal or
modification along with the addition of mangrove-derived POC. No evidence for in
situ POC–DOC exchange was found based on the POC–DOC relationship; however,
the signal for degradation of POC by respiration was evident in the Sundarbans
from the POC–pCO2 relationship (r2=0.37, p = 0.05,
Fig. 5f). Similarly to the Hooghly, despite high-water residence time in
mangroves (Alongi et al., 2005; Singh et al., 2016), unstable estuarine
conditions may not favour efficient settlement of POC at subtidal sediment.
The export of POC from the Hooghly–Sundarbans system to the northern BOB,
without significant in situ modification, is also a possibility. This
export has been estimated to be ∼0.02 to 0.07 and ∼0.58 Tg
annually for the Hooghly and Sundarbans, respectively (Ray et al., 2018).
<inline-formula><mml:math id="M738" display="inline"><mml:mi>p</mml:mi></mml:math></inline-formula><inline-formula><mml:math id="M739" display="inline"><mml:mrow class="chem"><mml:msub><mml:mi mathvariant="normal">CO</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> and <inline-formula><mml:math id="M740" display="inline"><mml:mrow class="chem"><mml:mi>F</mml:mi><mml:msub><mml:mi mathvariant="normal">CO</mml:mi><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:mrow></mml:math></inline-formula> in the Hooghly–Sundarbans
The estimated pCO2 values for the Hooghly–Sundarbans system
during this study were in the range (Cochin estuary: 150 to
3800 µatm, Gupta et al., 2009; Mandovi–Zuari estuary: 500 to
3500 µatm, Sarma et al., 2001) reported for other tidal estuaries
of India. In the Sundarbans, barring three locations (S3, T3 and M2), a
significant negative correlation between pCO2 and percent
saturation of DO (r2= 0.76, p = 0.005; figure not given)
suggested the presence of processes, such as degradation of OM by
respiration, responsible for controlling both CO2 production and
O2 consumption in the surface estuarine water. Furthermore,
significant positive correlation between ECO2 and AOU (ECO2= 0.057 AOU + 1.22, r2= 0.76, p = 0.005, n = 8;
Fig. 6a) confirmed the effect of OM degradation by respiration on
CO2 distribution, particularly in the upper region of the
Sundarbans. Our observations were in agreement with a previous study in the
Sundarbans (Akhand et al., 2016) as well as another subtropical estuary,
Pearl River estuary, China (Zhai et al., 2005). However, a relatively lower
slope for ECO2–AOU relationship (0.057) compared to the slope for
Redfield respiration in a HCO3--rich environment
((CH2O)106(NH3)16H3PO4 + 138O2+18HCO32-→124CO2+140H2O + 16NO3-+HPO42-; ΔCO2: (-ΔO2) = 124/138 = 0.90, Zhai et al., 2005) suggested lower
production of CO2 than expected from Redfield respiration. This may
be linked to the formation of low molecular weight OM instead of the final
product (CO2) during aerobic OM respiration (Zhai et al., 2005).
Moreover, the pCO2–salinity relationship (p = 0.18, Fig. 6b)
confirmed no significant effect of freshwater and marine water contribution on
variability of pCO2 in the Sundarbans. Other potential sources of
CO2 to the mangrove-dominated Sundarbans could be groundwater (or
pore water) exchange across the intertidal mangrove sediment–water interface.
Although based on our own data set, it is not possible to confirm the same.
However, relatively higher pCO2 levels during low tide compared
to high tide at the Matla estuary in the Sundarbans (Akhand et al., 2016) as well
as in other estuarine mangrove systems worldwide (Bouillon et al., 2007; Call
et al., 2015; Rosentreter et al., 2018) suggested groundwater (or pore water)
exchange to be a potential CO2 source in such systems.
Unlike the Sundarbans, the ECO2–AOU relationship did not confirm
the significant impact of OM degradation by respiration on CO2 in
either freshwater (p = 0.50) or mixing regimes (p = 0.75) of the
Hooghly (Fig. 6c). Overall, pCO2 in the freshwater regime of the
Hooghly was significantly higher compared to the mixing regime (Table 3),
which may be linked to additional CO2 supply in the freshwater
regime via freshwater or surface runoff from adjoining areas (Table 1).
Inter-estuary comparison of pCO2 also revealed higher average
pCO2 in the Hooghly by ∼1291 µatm compared to the
Sundarbans, which was largely due to significantly higher pCO2 in
the freshwater regime of the Hooghly (Tables 2 and 3). A lack of negative
correlation between pCO2 and salinity in freshwater regime
(Fig. 6d) of the Hooghly suggested a limited contribution of CO2 due
to freshwater input. Therefore, CO2 supply via surface runoff may
be the primary reason for higher pCO2 in the Hooghly estuary.
Positive mean FCO2 clearly suggested the
Hooghly–Sundarbans system to be a net source of CO2 to the regional
atmosphere post-monsoon (Fig. 6e, f). Specifically, from regional
climate and environmental change perspectives, the anthropogenically influenced
Hooghly estuary was a relatively greater source of CO2 to the
regional atmosphere compared to the mangrove-dominated Sundarbans
([FCO2]Hooghly:
[FCO2]Sundarbans= 17). However, despite
being a CO2 source, FCO2 measured for the
estuaries of the Sundarbans were considerably lower compared to global mean
FCO2 reported for the mangrove-dominated estuaries
(∼43 to 59 mmol C m-2 d-1; Call et al., 2015). Similarly,
FCO2 measured for the Hooghly estuary were relatively
lower compared to some Chinese estuarine systems (Pearl River inner estuary:
46 mmol m-2 d-1, Guo et al., 2009; Yangtze River estuary:
41 mmol m-2 d-1, Zhai et al., 2007).
The difference in FCO2 between the Hooghly and Sundarbans
may be due to variability in pCO2 level as well as
micrometeorological and physicochemical parameters controlling gas transfer
velocity across the water–atmosphere interface. Quantitatively, the difference in
k values for the Hoogly and Sundarbans were not large (kSundarbans-kHooghly ∼0.031 cm h-1).
Therefore, large difference in FCO2 between these two
estuarine systems may be due to a difference in pCO2. Taken
together, supporting our hypothesis, it appears that differences in land use
and degrees of anthropogenic influence have the potential to alter the C
biogeochemistry of aquatic ecosystems with anthropogenically stressed aquatic
systems acting as a relatively greater source of CO2 to the
regional atmosphere than mangrove-dominated ones.