Rupa Mukherjee

Bio: Rupa Mukherjee is an academic researcher from Physical Research Laboratory. The author has contributed to research in topics: Brackish water & Biogeochemistry. The author has an hindex of 4, co-authored 6 publications receiving 38 citations.

The post-monsoon carbon biogeochemistry of the Hooghly–Sundarbans estuarine system under different levels of anthropogenic impacts

Abstract: . 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 µ atm compared to the Sundarbans with surface runoff and organic matter degradation by respiration as dominant factors controlling p CO2 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.

Diurnal carbon dynamics in a mangrove-dominated tropical estuary (Sundarbans, India)

Abstract: 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.

Spatial variation of nitrogen uptake rates in the largest brackish water lagoon of Asia (Chilika, India)

Abstract: Inland and coastal water bodies around the world are susceptible to eutrophication due to inputs of anthropogenic nutrients, such as nitrogen (N) and phosphorus (P). Studies related to quantification of uptake rates of these nutrients and the factors governing these rates are limited in tropical aquatic systems, leading to inadequate understanding of N and carbon (C) cycling in these environments. Here, we report the rates of dissolved inorganic N (DIN) uptake and N2 fixation along with N isotopic composition of particulate organic matter (δ15NPOM) in Asia's largest brackish water lagoon (Chilika, India). The experiments were carried out at sixteen different locations in the lagoon where NO3− and NH4+ uptake rates varied from 0.004 to 1.42 μmol N L−1h−1 (average ∼ 0.51 ± 0.51 μmol N L−1h−1) and 0.19–1.76 μmol N L−1h−1 (average ∼ 1.11 ± 0.43 μmol N L−1h−1), respectively. In general, NH4+ was preferred substrate in the lagoon as depicted from relative preference index (RPINH4+ >1). Despite higher DIN concentrations, the lagoon showed diazotrophic activity as evidenced by considerable N2 fixation rates at majority of stations. Preliminary calculations of the lagoon-wide N budgeting indicate N2 fixation and atmospheric deposition to be significant sources of new N to the lagoon. δ15NPOM in the lagoon ranged from 1.34 to 8.31‰ (average ∼ 3.99 ± 1.68‰) indicating contributions from marine and terrestrial sources along with N2 fixation to the organic matter pool.

Sources and transformations of organic matter in sediments of Asia’s largest brackish water lagoon (Chilika, India) and nearby mangrove ecosystem

Abstract: The present study aimed to understand sources and transformations of organic matter along with cycling of nitrogen and carbon in sediments of two geographically close but ecologically distinct wetlands located on the east coast of India viz. Chilika lagoon (Asia’s largest brackish water lagoon) and Bhitarkanika mangrove. The study also investigates potential nitrogen loss pathways in the bottom sediments and explores stables isotopes as a proxy for the source identification of sediment organic matter in shallow aquatic ecosystems. For this purpose, the isotopic compositions of organic carbon and nitrogen (δ13Corg and δ15N) and its contents (% Corg and % N) were measured at different depths in sediment cores collected from the Chilika lagoon (eight cores) and Bhitarkanika mangrove forest (three cores). Overall, the mean δ13Corg and % Corg in the lagoon were − 21.10 ± 0.79‰ and 0.84 ± 0.47%, respectively; whereas the same for mangrove cores were − 24.56 ± 0.80‰ and 1.04 ± 0.26%, respectively. Similarly, average δ15N and % N in the lagoon cores were 4.15 ± 0.63‰ and 0.11 ± 0.05%, respectively; for mangrove cores, the values were 4.28 ± 0.50‰ and 0.07 ± 0.01%, respectively. Isotopic composition and elemental ratios indicated organic matter in the sediments of Bhitarkanika mangrove to be a mixture of terrigenous and marine origin with relative dominance of terrestrial influence. A significant increase in δ13Corg of sediment organic matter compared to suspended particulate organic matter in the Chilika indicated transformation of organic matter in the water or sediment column through mineralization and diagenetic alterations. The δ15N of sediment or particulate organic matter did not show clear evidence of nitrogen loss in the recent past in these two ecosystems through processes such as denitrification. The absence of a relationship between δ13Corg of particulate and sediment organic matter in the Chilika indicated lack of efficient exchange between suspended and sediment organic matter.

Carbon Biogeochemistry of Two Contrasting Tropical Estuarine Ecosystems During Premonsoon

Abstract: 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 δ13CDIC. 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 pCO2 (556–5002 μatm) and CH4 (15.4–445.7 nmol L−1) varied over a wider range in the Hooghly compared to the Sundarbans (pCO2: 268–418 μatm; CH4: 41.6–71.5 nmol L−1). The Sundarbans acted as a net sink for CO2, whereas the Hooghly was a significant source to the regional atmosphere. Both the estuaries acted as source of CH4 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 CO2 and CH4.

Nitrogen in the sea : forms, abundances, and rate processes

Abstract: INTRODUCTION. A HISTORICAL OVERVIEW OF NITROGEN STUDY IN THE SEA. Determination of Nitrogenous Compounds in the Sea. Biological Transformation of Nitrogen. Stoichiometric Model of Organic-Matter Mineralization. Natural 15N Abundance. THE NITROGEN CYCLE. PHYSICO-CHEMICAL PROPERTIES OF NITROGEN. PROPERTIES OF THE NITROGEN ATOM AND BONDING. THERMODYNAMIC FUNCTIONS OF NITROGEN. BIOGEOCHEMICAL SKETCH OF NITROGEN. NITROGEN ISOTOPES. GENESIS OF ISOTOPES OF LIGHT ELEMENTS. ISOTOPE ABUNDANCE AND FRACTIONATION. ISOTOPIC EXCHANGE REACTIONS. KINETIC ISOTOPE EFFECTS. OVERALL ISOTOPIC FRACTIONATION. One-Step Reaction. Unidirectional Two-Step Reaction at Steady State. Two-Step Reaction with a Reversible First Step at a Steady State. Three-Step Reactions. Multistep Reactions. ISOTOPIC EXCHANGE EQUILIBRIA OF NITROGEN. NITROGEN KINETIC ISOTOPE EFFECTS DURING NITRATE REDUCTION. Chemical Systems. Biological Systems. ISOTOPE FRACTIONATION IN BRANCHED REACTIONS AND ITS ECOLOGICAL SIGNIFICANCE. ISOTOPE MASS BALANCE IN MARINE ECOSYSTEMS. Two-Source Model. Three-Source Model. COMPONENTS, SYSTEMS, AND PROCESSES. NITROGENOUS COMPOUNDS IN THE SEA: ABUNDANCE, SOURCES, AND SINK. PHYSICAL FACTORS. Water Movement and Mixing of Water. Solar Radiation. Effects of Temperature and Pressure on Chemical and Biochemical Reactions. Nitrogen Supply by River Runoff and Precipitation. CHEMICAL FACTORS. Chemical Behavior of Nitrogenous Compounds in the Sea. Hydrogen Ion Concentration. Oxidation-Reduction Potential. BIOCHEMICAL PROCESSES. Nitrogen Uptake and Assimilation. Mineralization. Nitrification. Nitrate Respiration and Denitrification. N2 Fixation. ABUNDANCE AND DISTRIBUTION OF NITROGEN IN THE SEA AND THEIR PHYSICAL, CHEMICAL, AND BIOCHEMICAL REGULATION. ROLES OF THE OCEAN IN THE GLOBAL NITROGEN CYCLE. The Buildup of the Biogeochemical Cycle. Nitrogen Inventories. Nitrogen Budgets. DISSOLVED NITROGEN GAS. Atmospheric Pressure. Occlusion of Air Bubbles. Effect of Mixing of Different Water Masses. Biological Effects. AMMONIUM, NITRITE, AND NITRATE. Distribution of Ammonium. Distribution of Nitrite. Distribution of Nitrate. NITROUS OXIDE. UREA. OTHER NITROGENOUS COMPOUNDS IN A WATER COLUMN. NITROGEN UPTAKE BY PHYTOPLANKTON. MECHANISM OF FORMATION OF THE PRIMARY NITRITE MAXIMUM. Nitrification in the Sea. Nitrite Production from Nitrate by Microalgae. Nitrite Assimilation. Formation of Primary Nitrite Maximum. NITROGEN DYNAMICS IN VARIOUS OCEANIC SYSTEMS. HIGH LATITUDINAL AREAS RICH IN NITRATE. OLIGOTROPHIC AREAS. Kuroshio Area. Anticyclonic Gyre Off Shikoku. SUBTROPICAL AND TROPICAL AREAS WITH SUPPLY OF NITRATE. Upwelling Region. Regional Upwelling. SEA AREAS WHERE NITROGEN FIXATION OCCURS. Trichodesmium Ecosystems. Local Ecosystems Characterized by N2 Fixation. SUBSURFACE ECOSYSTEMS. Decomposition of Organic Matter in the Subsurface Water. Nitrogen Diagenesis in Sediments. SEA AREAS WHERE DENITRIFICATION OCCURS. Denitrification in the Subsurface Water. Marine Sediments. NEAR SHORE AND ESTUARINE SYSTEMS. Flooded Soil Ecosystems. Tokyo Bay. VARIATION OF 15N/14N IN NITROGEN CYCLING AND ITS SIGNIFICANCE IN MARINE ENVIRONMENTS. ISOTOPE FRACTIONATION IN THE NITROGEN CYCLE. Isotope Exchange Reactions. Fractionation in Biological Metabolic Processes. DISTRIBUTION OF 15N IN NATURE. General Patterns of 15N Distribution. 15N in Antarctica. 15N Abundance and the Biogeochemical Cycle. INTEGRATED NITROGEN ISOTOPE FRACTIONATION IN ECOSYSTEMS. Enrichment of 15N in a Feeding Process. d15N-d13C Relationship. MARINE ECOSYSTEMS. Phytoplankton and Particulate Organic Matter. The Area Characterized by N2 Fixation. Isotope Biogeochemistry of the Antarctic Ecosystem. Dentrification in the Marine Aquatic System as Investigated Based on 15N Abundance Data. 15N and 13C Abundance in River, Estuarine, and Coastal Areas. Seabird Rookeries. The Deep-Sea World. Animal Behavior. FUTURE STUDIES ON NITROGEN DYNAMICS. HUMAN IMPINGEMENT ON THE NITROGEN CYCLE IN MARINE SYSTEMS. Long-Term Effects. Short-Term Effects. A NEW PARADIGM. Isotope Biogeochemistry. From Components to Systems. REFERENCES. INDEX.

Gas transfer velocities of CO_2 in three European estuaries (Randers Fjord, Scheldt and Thames)

Abstract: We measured the flux of CO 2 across the air‐water interface using the floating chamber method in three European estuaries with contrasting physical characteristics (Randers Fjord, Scheldt, and Thames). We computed the gas transfer velocity of CO2 (k) from the CO2 flux and concomitant measurements of the air‐water gradient of the partial pressure of CO2 (pCO2). There was a significant linear relationship between k and wind speed for each of the three estuaries. The differences of the y-intercept and the slope between the three sites are related to differences in the contribution of tidal currents to water turbulence at the interface and fetch limitation. The contribution to k from turbulence generated by tidal currents is negligible in microtidal estuaries such as Randers Fjord but is substantial, at low to moderate wind speeds, in macrotidal estuaries such as the Scheldt and the Thames. Our results clearly show that in estuaries a simple parameterization of k as a function of wind speed is site specific and strongly suggest that the y-intercept of the linear relationship is mostly influenced by the contribution of tidal currents, whereas the slope is influenced by fetch limitation. This implies that substantial errors in flux computations are incurred if generic relationships of the gas transfer velocity as a function of wind speed are employed in estuarine environments for the purpose of biogas air‐water flux budgets and ecosystem metabolic studies. Based on organic carbon flux budgets, the overall picture of the net ecosystem metabolism in the coastal ocean is that temperate open continental shelves (bordered by a continental margin) are net autotrophic (net exporters of carbon and thus potential sinks for atmospheric CO2) while near-shore

Carbon Cycling in the World’s Mangrove Ecosystems Revisited: Significance of Non-Steady State Diagenesis and Subsurface Linkages between the Forest Floor and the Coastal Ocean

Abstract: Carbon cycling within the deep mangrove forest floor is unique compared to other marine ecosystems with organic carbon input, mineralization, burial, and advective and groundwater export pathways being in non-steady-state, often oscillating in synchrony with tides, plant uptake, and release/uptake via roots and other edaphic factors in a highly dynamic and harsh environment. Rates of soil organic carbon (CORG) mineralization and belowground CORG stocks are high, with rapid diagenesis throughout the deep (>1 m) soil horizon. Pocketed with cracks, fissures, extensive roots, burrows, tubes, and drainage channels through which tidal waters percolate and drain, the forest floor sustains non-steady-state diagenesis of the soil CORG, in which decomposition processes at the soil surface are distinct from those in deeper soils. Aerobic respiration occurs within the upper 2 mm of the soil surface and within biogenic structures. On average, carbon respiration across the surface soil-air/water interface (104 mmol C m−2 d−1) equates to only 25% of the total carbon mineralized within the entire soil horizon, as nearly all respired carbon (569 mmol C m−2 d−1) is released in a dissolved form via advective porewater exchange and/or lateral transport and subsurface tidal pumping to adjacent tidal waters. A carbon budget for the world’s mangrove ecosystems indicates that subsurface respiration is the second-largest respiratory flux after canopy respiration. Dissolved carbon release is sufficient to oversaturate water-column pCO2, causing tropical coastal waters to be a source of CO2 to the atmosphere. Mangrove dissolved inorganic carbon (DIC) discharge contributes nearly 60% of DIC and 27% of dissolved organic carbon (DOC) discharge from the world’s low latitude rivers to the tropical coastal ocean. Mangroves inhabit only 0.3% of the global coastal ocean area but contribute 55% of air-sea exchange, 14% of CORG burial, 28% of DIC export, and 13% of DOC + particulate organic matter (POC) export from the world’s coastal wetlands and estuaries to the atmosphere and global coastal ocean.

Diurnal carbon dynamics in a mangrove-dominated tropical estuary (Sundarbans, India)

Abstract: 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.