Comprehensive understanding and quantification of different aspects of estuarine carbon (C) cycle are essential to decipher regional and global changes. Here, premonsoon C biogeochemistry of two ...contrasting estuarine systems (Hooghly - anthropogenically influenced and Sundarbans - mangrove dominated) located in the deltaic region of Ganges, India, has been investigated. The Hooghly showed sharper salinity gradient (0.48–32.94) and wider pH range (7.56–8.25) compared to the Sundarbans (salinity: 27.22–29.82; pH: 8.16–8.31). The DIC concentration in the Hooghly (2.27 ± 0.22 mmol L
−1
) was higher than the Sundarbans (1.77 ± 0.06 mmol L
−1
) with relatively depleted δ
13
C
DIC
. The mixing model analysis suggested DIC chemistry in the Hooghly to be principally regulated by respiration of estuarine algae, whereas Sundarbans showed evidences for DIC removal and mangrove-derived DIC contribution. The DOC behaved non-conservatively in both the estuaries. The POC in the Hooghly showed signatures of estuarine algae and marine plankton in the mixing and marine zones, whereas dominance of terrestrial organic matter was found in the freshwater zone of the Hooghly and estuaries of Sundarbans. Both
p
CO
2
(556–5002 μatm) and CH
4
(15.4–445.7 nmol L
−1
) varied over a wider range in the Hooghly compared to the Sundarbans (
p
CO
2
: 268–418 μatm; CH
4
: 41.6–71.5 nmol L
−1
). The Sundarbans acted as a net sink for CO
2
, whereas the Hooghly was a significant source to the regional atmosphere. Both the estuaries acted as source of CH
4
with comparable exchange fluxes. Overall, the Sundarbans was a net C sink and the Hooghly was a net source to regional atmosphere via cumulative fluxes of CO
2
and CH
4
.
Based on a 24h of time-series study, we report the effects of a tidal cycle on carbon biogeochemistry of a mangrove dominated tropical estuary (the Sundarbans) located in the eastern part of India. ...Salinity, dissolved oxygen, and pH showed clear tidal variability with relatively higher values during high tide than low tide. Dissolved inorganic carbon (DIC) concentrations varied over a narrow range (1.92–2.19 mM) with relatively higher values during low tide; reverse trend, however, was noticed for δ13CDIC with significant variability (– 4.28 to – 2.21‰). During low tide, along with estuarine mixing, preliminary evidences for influences of biogeochemical (such as organic carbon mineralization, sulfate reduction, and denitrification) and hydrological processes (porewater exchange) were found on DIC dynamics. The δ13CDIC - DIC relationship suggested respiration of marine plankton to be one of the possible sources for DIC. Dissolved organic carbon showed tidal influence during high tide with a signal of porewater mediated addition during low tide. Both particulate organic carbon and particulate nitrogen concentrations reached the maximum during low tide with stable isotopic compositions showing predominantly marine signature along with the possibility of biogeochemical modifications within the estuary. Marine water contribution together with organic carbon mineralization and possible porewater influx resulted in ~214 μatm higher pCO2 and 1.13 times higher FCO2 during low tide than high tide. On diurnal basis, the estuary released ~1348 mg CO2 per m2 of surface area to the regional atmosphere.
•Quantitative investigation of the effects of tidal cycle on C biogeochemistry of a mangrove-dominated estuary.•DIC increased during low tide whereas DOC showed tidal influence during high tide.•On diurnal basis, estuaries of Indian Sundarbans acted as a source of CO2.
The present study focused on understanding differences in the post-monsoon
carbon (C) biogeochemistry of two adjacent estuaries undergoing different
levels of anthropogenic stresses by investigating ...anthropogenically
influenced Hooghly estuary and mangrove-dominated estuaries of the Sundarbans
in the north-eastern India. The salinity of well-oxygenated estuaries of the
Sundarbans (DO: 91 %–104 %) varied over a narrow range
(12.74–16.69) relative to the Hooghly estuary (0.04–10.37). A mixing model
suggested a combination of processes including freshwater intrusion,
carbonate precipitation and carbonate dissolution to be a major factor
controlling dissolved inorganic C (DIC) dynamics in the freshwater regime of
the Hooghly, whereas phytoplankton productivity and CO2 outgassing
dominated in the mixing regime. In the Sundarbans, the removal of DIC (via
CO2 outgassing, phytoplankton uptake and export to the adjoining
continental shelf region) dominated
its addition through mineralization of mangrove-derived organic C. The
concentration of dissolved organic C (DOC) in the Hooghly was ∼40 %
higher than in the Sundarbans, which was largely due to the cumulative effect
of anthropogenic inputs, DOC–POC interconversion and groundwater
contribution rather than freshwater-mediated input. The measured δ13CPOC in the Hooghly suggested particulate organic matter
contributions from different sources (freshwater runoff, terrestrial
C3 plants and anthropogenic discharge), whereas the contribution
from C3 plants was dominant at the Sundarbans. The significant
departure of δ13CPOC from typical mangrove
δ13C in the mangrove-dominated Sundarbans suggested
significant particulate organic C (POC) modification due to degradation by
respiration. The average pCO2 in the Hooghly was higher by
∼1291 µatm compared to the Sundarbans with surface runoff and
organic matter degradation by respiration as dominant factors controlling
pCO2 in the Hooghly and Sundarbans, respectively. The entire
Hooghly–Sundarbans system acted as a source of CO2 to the regional
atmosphere with ∼17 times higher emission from the Hooghly compared to
the Sundarbans. Taken together, the cycling of C in estuaries with different
levels of anthropogenic influences is evidently different, with significantly
higher CO2 emission from the anthropogenically influenced estuary
than the mangrove-dominated ones.
•This study is focused on the understanding distribution of nutrients in the Eastern Equatorial Indian Ocean, their significance on biogeochemical cycles and the impact of water masses on ...nutrients.•The nutrient maxima were recorded at shallower depths.•EEIO surface water was found to be oligotrophic with bio-limiting nitrogen.•An excess of phosphate over nitrate was found between 50-100m depth.•Shallow-depth mineralization was noticed in this region during the study.•Insignificant influence of Indonesian Through Flow (ITF) on nutrient values.•Allochthonous supply of organic matter comes through Indonesian Through Flow (ITF) influencing and augmenting nutrient regeneration.
The seasonal reversal of ocean circulation associated with seasonal change in the direction of prevailing winds and the occurrence of several anomalous events in the Eastern Equatorial Indian Ocean (EEIO) make this region dynamic and complex in terms of its biogeochemical characteristics. Two multidisciplinary cruises were conducted to measure nutrients and associated physicochemical parameters across the water column (up to 1000 m) of the EEIO during boreal summer and winter monsoons to understand the distribution of nutrients and their spatio-temporal variability from a biogeochemical perspective. The seasonality in the thermohaline structure of the region is indistinct except for surface salinity drop during summer monsoon due to more precipitation on-site and in adjoining areas. Low concentrations of chlorophyll at the surface and in the deep chlorophyll maxima represent the oligotrophic nature of this region. Surface water was found nutrient-depleted (0.03–0.4 µM Nitrate, 0.02–0.13 µM Phosphate). The maxima of vertical profiles of nitrate and phosphate were recorded at a shallower depth (150–200 m) when compared to its maxima in usual oceanic conditions, but a silicate maximum was recorded in deeper water. In the surface and upper mixed layer paucity of nutrients resulted in low N:P and N:Si ratios. Therefore, nitrogen limitation is evident. The overall ratio of N:P yielded a mean value of 15.33 and matched with the representative literature value for the Indian Ocean. The minimum oxygen values (<50 µM) in the deep water (150–200 m) indicated a hypoxic condition. No signature of denitrification and a moderate nitrate deficit were observed in deep waters. The negative values of Nitrate anomaly (N-tracer) at 50–100 m depth were attributed to a Watermass influenced by denitrification. The prevailing oligotrophic condition caused limited synthesis of organic matter and subsequently little decomposition in deep water. The maxima in the apparent oxygen utilization (AOU) profile are confined to 150 to 200 m depth and represent the most active zone for regeneration that is limited to shallow depth. Regenerated nutrients reached maxima at shallower depth and primarily control material cycling in this region. Supply of nitrate to the surface water based on the preformed values of prevailing water mass was primarily by Bay of Bengal water. According to the findings of this study, preformed nitrate concentrations between 100 and 200 metres below the surface were found very low, indicating that Indonesian Through Flow (ITF) has little impact on the distribution of nutrients in this area.
Tidal transport from coastal wetlands (“outwelling”), together with riverine fluxes, provide the most important sources of terrigenous organic matter (OM) to the ocean. The flux of dissolved organic ...carbon (DOC) from the mangrove swamps accounts for 10% of the terrestrial DOC flux to the coastal water. This study examines the sources, distribution, and export of dissolved OM at interannual, seasonal, and diurnal bases along the estuaries located at the Sundarbans, the world’s largest deltaic mangrove and heritage site. Sampling was carried out from the riverine (Hooghly) and mangrove-dominated tidal estuaries (Saptamukhi, Thakuran, Matla), covering all three seasons (pre-monsoon, monsoon, and post-monsoon) during 2012–2017. DOC varied at a broad range, from 109 to 462 μM (n = 146), with higher concentration observed in the Hooghly (383 ± 120 μM, n = 35) than the mangrove estuaries (246 ± 82 to 266 ± 120 μM, n = 111). Non-conservative mixing of DOC along the salinity gradient attested to mangrove input, particularly in the polyhaline waters. Upper and mid estuarine zones of the mangrove estuaries showed slightly higher DOC concentration (270 ± 92 μM, n = 84, salinity range 18–25) than in the mouth (250 ± 85 μM, n = 27, salinity range 26–27), because of the dilution with marine waters having low DOC concentration and shorter water residence time downstream. Seasonally, higher DOC concentration during the post-monsoon might be linked to higher litterfall, promoting leaching of organic compounds to the water. In that connection, colored dissolved organic matter (CDOM) could be a by-product of mangrove litter leaching, and its absorption coefficient (at 350 nm) exhibited non-conservative mixing pattern at wide ranges from 2.5 to 7.6 m−1 (n = 40). CDOM enrichment was observed in the surface water during the low tide when outwelling maximized. Overall, the central and eastern parts of the Indian Sundarbans showed enrichment of more terrigenous type CDOM (evident from optical proxies, e.g., S275–295 and SUVA254) than the western part, probably due to greater mangrove productivity in the eastern side. Flux estimates of DOM resulted in higher yield and export of mangrove-derived DOC but lower export of CDOM to the Bay of Bengal as compared to their riverine transport.
The world’s largest mangroves ecosystem, the Sundarbans, being highly productive and a place for extensive organic matter cycling, is considered to be the hotspot for biogeochemical studies in the ...tropical estuarine environment. Hence, the spatial and temporal dynamics of the biogenic gases (CO
2
, CH
4
, and N
2
O), also known as radiatively active gases, were measured in mangrove-dominated estuaries of the system. In addition to spatial and seasonal observation, three full tidal cycles were observed at one site. Results showed that the air/water gas saturations were widely distributed and highly variable along the stretch. The gas saturations showed varying responses to salinity and tidal fluctuations. This indicated that localized biogeochemical processes may be more influential than simple mixing and dilution processes in controlling the variability of these gases. The surface waters were always supersaturated with CH
4
(Up to 13,133%) relative to the atmosphere. However, N
2
O ranged from 8 to 1,286% and CO
2
from 30 to 2075%. N
2
O fluxes were ∼4.8 times higher in the pre-monsoon than the post-monsoon. CH
4
fluxes were ∼3.6 times higher in the pre-monsoon than both the monsoon and the post-monsoon. CO
2
fluxes were ∼10 times higher in the monsoon than both the pre-monsoon and the post-monsoon. The seasonality in the gas saturation could be linked more to the availability of substrates than physicochemical parameters. Overall, air/water CH
4
fluxes varied maximally (0.4–18.4 μmol m
−2
d
−1
), followed by CO
2
fluxes (−0.6–10.9 mmol m
−2
d
−1
), and N
2
O fluxes varied the least of all (−0.6–5.4 μmol m
−2
d
−1
). Interestingly, CH
4
and N
2
O fluxes were positively correlated to each other (
p
< 0.05), suggesting organic matter decomposition as the key factor in the production of these two gases. Finally, these water–air CO
2
, CH
4
, and N
2
O flux estimates show that the estuaries are a modest source of CH
4
but fluctuate between sources and sinks for CO
2
and N
2
O gases.
Nitrification is a vital biogeochemical process during which ammonium (NH
4
+
) is oxidized to nitrite (NO
2
−
) and nitrate (NO
3
−
) using oxygen. Nitrification is particularly important in ...estuaries, where nitrification rates have significant implications for ecosystem functioning. Our study investigated seasonal and spatial variations of nitrification rates in both water and sediment phases of mangrove ecosystems in the Indian Sundarbans, integrating the three major estuaries (Saptamukhi, Thakuran, and Matla). We used on board incubation to measure nitrifying activity i.e., ammonium oxidation rate (Ra) and nitrite oxidation rate (Rn) in the water column. We found that average Ra and Rn rates were similar, ranging from 31.1 to 31.8 nmol N L
−1
h
−1
, respectively. Water column nitrification rates showed an inverse relationship with salinity (
r
= − 0.51,
p
< 0.001), while substrate concentrations of ammonium had a direct impact on nitrification rates. Annual dissolved inorganic nitrogen (DIN) concentrations were 11.7 ± 7.65 µM, with an average nitrate (NO
3
−
) concentration of 4.5–16.3 µM and average ammonium (NH
4
+
) concentrations from 2.1 to 5 µM. Suspended particulate matter and chlorophyll showed positive (
r
= 0.38,
p
= 0.009) and negative (
r
= − 0.31,
p
= 0.034) correlations, respectively, with water column nitrification rates. For water, both Ra and Rn occurred at similar rates in our study area. In sediment, nitrifying activity (SEDn) rates ranged from 0.34 to 8.69 nmol N g wet wt
−1
h
−1
. SEDn showed positive and negative correlations with sediment temperature (temp) and pH, respectively. Based on our observations, the average water column nitrification potential ranged from 43 to 420 nmol N L
−1
h
−1
considering three estuaries, while the sediment nitrification potential estimated at Saptamukhi estuary showed a value of 69 nmol N g wet wt
−1
h
−1
.
Bacterioplankton abundances in three major estuaries of the Indian Sundarbans (Saptamukhi, Thakuran and Matla) were determined by flow cytometry and fluorescence microscopy in a post-monsoonal study ...followed by microbial community composition (16S rRNA clone library), during the period 2012–2016. All three estuaries were oxygenated and meso - to polyhaline. They had similar levels of dissolved inorganic carbon and nutrients and were net heterotrophic, as evidenced by net negative community productivity. Mean post-monsoon bacterioplankton abundances (x 109 cells L−1) in these estuaries were: Saptamukhi (1.30 ± 0.78 × 109 cells L−1); Matla (1.38 ± 1.34 × 109 cells L−1); Thakuran, (0.92 ± 0.46 × 109 cells L1). Bacterioplankton abundance was associated with water temperature coupled with dissolved inorganic nutrients and carbonate parameters. Highest abundances were at the most upstream sites in the Saptamukhi and Matla estuaries, while a mid-estuarine peak was evident in Thakuran. In the Saptamukhi and Thakuran the bacterioplankton was dominated by Proteobacteria (mostly Deltaproteobacteria), with a contribution from Bacteroidetes in the Saptamukhi and from Cyanobacteria and Actinobacteria in the Thakuran. Despite the oxygenated nature of these estuaries, clones resembling the sulfate reducer Desulfovibrio sp. were detected, and this could be a consequence of anoxic pore water exchange across the sediment-water interface. Sulfate reduction may thus be an important pathway for carbon metabolism in these ecosystems.
•During post-monsoon (2012-2016) three major estuaries of Indian Sundarbans were well oxygenated and well mixed.•Estuaries exhibited analogous levels of dissolved nutrients and carbonate parameters.•Bacterioplankton abundance of three estuaries varied from 0.92 ± 0.46 × 109 cells L-1 to 1.38 ± 1.34 × 109 cells L-1.•Bacterioplankton abundance was controlled by multiple physico-chemical factors (such as water temperature, pCO2) etc.•Bacterioplankton community contained strains associated with sulfate reduction, such as Desulfovibrio sp.•Anaerobic microbes in well oxygenated estuaries indicate role of exchange of pore-water in shaping the microbiota.
We investigated diurnal-scale variations in the abundances of bacterioplankton, high and low nucleic acid (HNA, LNA) cells, and associated physico-chemical variables in a mangrove dominated estuary ...of the Indian Sundarbans, during pre-monsoon and monsoon conditions. Bacterioplankton abundance (BA) varying from 1.83 ± 0.47 to 0.34 ± 0.11 × 109 cells L−1 with lowest abundance during the monsoon, was mainly dominated by LNA cells (74.46 ± 12.79% to 74.36 ± 20.51%). In two time series studies bacterial productivity (BP), total bacterioplankton abundance and HNA cell numbers increased towards high tide (BA; 0.21–0.04 × 109 cells L−1, HNA; 0.11–0.06 × 109 cells L−1) while LNA cell abundance varied. Total bacterioplankton, HNA and LNA cell numbers were also highest during the night (total bacterioplankton; 0.32–0.04 × 109 cells L−1, HNA; 0.31–0.01 × 109 cells L−1, LNA; 0.01–0.03 × 109 cells L−1). Highest dissolved oxygen and pH was at high tide (high salinity), while other carbonate system parameters and dissolved inorganic nutrients (except DIP) were higher at low tide. The overall abundances of bacterioplankton, HNA and LNA cells were controlled by a combination of factors, water temperature and dissolved oxygen being the most important controls on day-night and tidal variation of bacterioplankton.
•Bacterioplankton is mainly dominated by LNA cells in mangrove water of Indian Sundarbans.•Abundance of bacterioplankton and HNA cells increase at high tide and night.•Bacterioplankton and sub-groups are regulated by multiple physico-chemical factors.
A detailed study on potential sources, variation, and environmental effects of the rainwater ions was carried out at Lothian Island, Sundarban mangrove forest, India, during the southwest monsoon ...(June–September) in 2019. On an event basis, the maximum rainwater precipitation was observed 17.65 mm Day
−1
and a minimum of 1.02 mm Day
−1
. The maximum amount of total precipitation was recorded in the month of July (237 mm). The volume weighted mean (VWM) concentration shows that the total ionic composition was 93.7 μeq L
−1
, whereas the percentage contribution of the total ionic concentration is found to be 45.97% to anions and 54.02% to the cations. Temporal variation was observed between early (June- July) and late monsoon (August—September), which shows a high concentration of major ions in early monsoon and low concentration in late monsoon due to the washout of atmospheric particles with the frequent and increasing precipitation. The pH values of the 78% samples show neutral pH and neutralization factors (NF) followed a sequence of NF
Ca
˃NF
Mg
˃ NF
NH4
with factors of 0.77, 0.34, and 0.14 indicating Ca
2+
was the most potential species to balance the acidic ions (NO
3
−
, SO
4
2−
) over the study area. Source apportionment study indicates the significant influence of marine actions (long-range transport by monsoonal wind from marine origin, Sea spray, salty soil profile of mangrove) as the major source of ions over Sundarban. The rate of nutrient wet deposition in the form of rainwater was estimated and average monsoonal nitrogen flux was observed 0.87 kg ha
−1
where NO
3
contributes the most (0.60 kg ha
−1
). N and P deposition flux also showed a simultaneous pattern with the seasonal nutrient concentration of surrounding river water, which may be an indication of a possible contribution of atmospheric wet deposition in the spike of monsoonal nutrient concentration in river water.
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