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.
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 attempts to reveal the impact of salinity on mangrove species in terms of species distribution, and their trend of adapting to salinity changes. The study considers the mangrove ...species of entire Sundarban encompassing India and Bangladesh. The mangrove species distribution depends on a great deal on the salinity regime. Each mangrove species has optimal salinity range for its preferred habitat. This preference, as well as its tolerance level, may alter with the changing environmental regime. Based on this hypothesis, a few indicator species have been identified according to the preferred salinity. Ceriops and Avicennia have been identified as high salinity indicator species which are found in high frequency in the polyhaline zones, whereas Nypa and Heritiera are recognized as low salinity indicator species which show high abundance in oligohaline to mesohaline zones. Exoecaria and Bruguiera, the medium salinity indicator species, are found almost everywhere but are most abundant in the mesohaline zones. This study will help in evaluating the adaptive capacity of mangroves and also could build a functional relationship between their occurrences as indicator species with respect to the salinity.
•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.
Because of the difficulties in setting up arrangements in the intertidal zone for free-air carbon dioxide enrichment experimentation, the responses to increasing atmospheric carbon dioxide in ...mangrove forests are poorly studied. This study applied box model to overcome this limitation, and the relative changes in present level of reservoirs organic carbon contents in response to the future increase of atmospheric carbon dioxide were examined in the Avicennia-dominated mangrove forest at the land-ocean boundary of the northeast coast of the Bay of Bengal. The above- and below-ground biomass (AGB+BGB) and sediment held different carbon stock (53.20±2.87Mg C ha
−1
(mega gram carbon per hectare) versus 18.52±2.77Mg C ha
−1
). Carbon uptake (0.348mg C m
−2
s
−1
) is more than offset by losses from plant emission (0.257mg C m
−2
s
−1
), and litter fall (13.52µg C m
−2
s
−1
) was more than soil CO
2
and CH
4
emission (8.36 and 1.39µg C m
−2
s
−1
, respectively). Across inventory plots, Sundarban mangrove forest carbon storage in above- and below-ground live trees and soil increased by 18.89 and 5.94Mg C ha
−1
between June 2009 and December 2011. Box model well predicted the dynamics of above- and below-ground biomass and soil organic carbon, and increasing atmospheric carbon dioxide concentrations could be the cause of 1.1- and 1.57-fold increases in carbon storage in live biomass and soil, respectively, across Sundarban mangrove forest rather than recovery from past disturbances.
The behavior of dissolved Ca and Mg during mixing of sea and river waters had been studied through a survey in the wastewater fed River Vidyadhari in the Sundarbans estuarine complex, India. The ...percentage variation for Ca was 79.9% during low tide and 95% during high tide with respect to chlorinity (10−3) and the same for Mg was 80% during low tide and 94.6% during high tide. The percentage addition and removal (PA & PR) was calculated with the help of the Theoretical Dilution Line (TDL) concept. A significant negative correlation between PA and PR of both Ca and Mg with chlorinity (10−3) was also observed. On the basis of the insignificant percentage variation (< 8%) of salinity between the surface and the bottom water layers, the Vidyadhari estuary could be characterized as a well-mixed estuary. The Vidyadhari River plays a vital role in draining wastewater with high load of pollutants from Kolkata and surrounding area through a complex riverine system crisscrossing the Sundarban mangrove forest to the Bay of Bengal and hence is a paradigm for studying estuarine mixing patterns along with the behavior of major ions to enrich information to promote sustainable development goal.
MgO is one of the most promising solid base catalysts and has attracted much attention because of its superior performance. The extent of catalytic properties of MgO is highly controlled by its ...morphology, particle size, crystalinity, and surface area. Here, the synthesis of MgO with different morphologies, such as random nanoflakes, arranged nanoflakes toward flower and house of card structure spheres, cubes, and hexagonal plates, through the calcination of magnesium carbonate hydrates (MCH) intermediate is presented. The intermediate MCH has been synthesized under hydrothermal or supercritical hydrothermal as well as solvothermal treatment of clear solution of Mg(NO)3, (NH4)2CO3, or nesquehonite rods in different pH's and amounts of free carbonate ions. A probable reduction mechanism is proposed to explain the formation of the MCH morphologies. The amount of carbonate ion has crucial role in the formation of different morphologies in hydrothermal condition. On calcinations of the synthesized MCH morphologies resulted in MgO with almost identical morphologies as parental MCH. The microstructures of calcined MgO are porous and made of MgO nanoparticle building blocks of the size 4–6 nm. The formed MgO nanoparticles consists of large number of edges and corners, step edges and step corners and numerous base sites of various strength (surface hydroxyl groups, low coordinate O2- sites) which are recognized as active basic sites in heterogeneous catalysis. The calcined MgO microstructures function as a strong solid base catalyst for the solvent-free Claisen-Schmidt condensation of benzaldehyde with acetophenone giving 99% conversion in 4 h. The MgO catalysts are easily recyclable with no significant loss in catalytic activity in the subsequent cycles.
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