Gupta GVM | Ministry of Earth Sciences (original) (raw)
Papers by Gupta GVM
Carbon uptake rates in a tropical eutrophic estuary (Cochin estuary) and the adjacent coastal Ara... more Carbon uptake rates in a tropical eutrophic estuary (Cochin estuary) and the adjacent coastal Arabian Sea were measured for the first time using 13 C-NaHCO 3 labeling technique. The rates in the estuarine water (pre-monsoon 3.0–20.4; monsoon 2.55–12.4; post-monsoon 3.1–16.3 μmol C l−1 h−1) were significantly higher than the coastal locations
(pre-monsoon 0.02–0.1; monsoon 0.13–3.1; post-monsoon
0.007–0.032 μmol C l−1 h−1). The measured primary productivity
in the estuary was relatively higher at mesohaline locations
influenced with lateral inputs and TN:TP close to the Redfield ratio (16:1). It suggests that in nutrient replete tropical systems, TN:TP plays a major role in controlling the primary productivity rates. The primary productivity data from the coastal Arabian Sea suggests the effect of estuarine discharge on the carbon uptake rate at nearshore regions and
ability of anthropogenic inputs to modulate the coastal biogeochemistry.
Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continent... more Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continental shelf of India in 2012 revealed high concentrations of 45 6 32 nM (off Kochi) and 73 6 63 nM (off Mangalore) during the summer monsoon (SM). N2O concentrations increased nonlinearly during the peak of the SM upwelling, when low O2 (<25 mM) conditions prevailed in the water column. Off Kochi, N2O levels fell gradually from the fall intermonsoon (20 ± 8 nM) to the winter monsoon (8.8 ± 2 nM) and remained low (9.2 ± 5.2 nM) through the spring intermonsoon season. The N2O supersaturation off Kochi (574 ± 720%) was presumably due to its high yield during sediment denitrification, whereas the higher N2O supersaturation observed off Mangalore (1046 ± 885%) was due to its production during denitrification in both the anoxic water column and the underlying sediments. Such distinctive biogeochemical behavior between these two shelf segments is at first augmented by the natural origin of intense upwelling at Mangalore relative to Kochi wherein suboxic to anoxic oxygen minimum zone waters spread from offshore to the shelf of Mangalore, over which the runoff and terrestrial nutrients supply acts in unison. Following new zonal extrapolation approach, our revised estimate of N2O effluxes from the southwestern Indian shelf (7–14°N) was four times lower (0.019–0.039 Tg y-1) than previous estimate. Nevertheless, further studies are needed to refine the N2O effluxes from the entire western Indian shelf to monitor its modification due to expansion/intensification of the coastal low-O2 zones and also to ascertain its actual N2O contribution to the world oceans.
Regional Studies in Marine Science, 2015
The objectives of the present study were to evaluate the ecological status of tropical coastal wa... more The objectives of the present study were to evaluate the ecological status of tropical coastal waters using the temperate benthic indices and examine the effect of seasonal variability on the performance of benthic indices. Macrobenthic samples were collected from northwest to southeast coast of India during 2003–2012 and we tested different univariate indices, ecological strategies, indicator species and multimetric indices. AMBI and multimetric indices performed satisfactorily in evaluating the ecological status. Seasonal variability on the biotic indices was observed during the southwest monsoon and fall intermonsoon period due to recruitment. Therefore , we recommended the non-monsoon period (January–May) as a suitable time of the year to use the indices for effective assessment of the Indian coastal waters. Results show that, the temperate benthic indices are efficient in assessing the tropical environmental status. However, complementary use of different indices is suggested for accurate assessment of the environmental status.
High concentrations of PHC were observed in the inner channels of Visakhapatnam harbour. The esti... more High concentrations of PHC were observed in the inner channels of Visakhapatnam harbour. The estimation of trace metals in surficial sediments indicated higher contamination in Visakhapatnam habour than in Kakinada Bay. Positive correlations between Cu, Zn, Pb and Cd suggests common sources of these metals. Lack of corrleation between Co, Ni with the other metals indicates point sources. High concentrations of chromium reflects intense discharges due to electroplating and battery operation.
During RV Gaveshani cruise No. 235 (August-September, 1993), particulate matter was collected fro... more During RV Gaveshani cruise No. 235 (August-September, 1993), particulate matter was collected from euphotic and deeper waters in the Bay of Bengal and analysed for biogenic silica (BSi), lithogenic silica (LSi) and particulate organic carbon (POC). Mean depth profiles of BSi showed a minimum in the surface waters and a maximum in the middle depths of the euphotic zone. The deepening of BSi maxima from north to south coincides with the deepening of the thermocline and thickening of the surface layer. In coastal waters, the contribution of BSi to total particulate silica was high (> 80 %); in offshore waters it was 60-70 %. BSiIPOC (0.086) and LSiIPOC (0.023) ratios show that particulate matter consists mostly of biogenic material, while the BSiILSi (1.61) ratio revealed the dominance of siliceous phytoplankton in the euphotic zone. High concentrations of BSi (0.49 pmol 1-') and LSi (1.89 pmol 1-') in deeper waters are due to a high sedimentation rate, the resuspension of bottom sediments and the flow of large amounts of riverine siliceous material. Silice biogknique dans le golfe du Bengale pendant la mousson du sud-ouest. Au cours de la campagne 235 du N.O. Guveslzani dans le golfe du Bengale (aoat-septembre 1993), des prClkvements de matikre particulaire ont kt6 effectuCs dans la couche euphotique et dans les eaux profondes : la silice biogknique (BSi), la silice 1ithogCnique (LSi) et le carbone organique particulaire (POC) y ont Ct C analysts. Les profils de silice biogCnique prksentent un minimum dans les eaux superficielles et un maximum aux profondeurs moyennes de la couche euphotique ; les maxima plongent du nord au sud avec la thermocline. Dans les eaux catikres, la silice biogknique constitue plus de 80 % de la silice particulaire totale, mais sa contribution n'est que de 60 a 70 5% dans les eaux du large. Le rapport silice biogkniquelcarbone organique particubire est de 0,086 et le rapport silice 1ithogCniquelcarbone organique particulaire est de 0,023, ce qui indique la nature biogCnique de la matikre particulaire. Le rapport silice biogCniquelsilice lithogknique (1 $1) rkvkle la dominance du phytoplancton siliceux dans la couche euphotique. Dans les eaux profondes, les concentrations ClevCes en silice biogCnique (0,49 pmol I-') et en silice lithogknique (1,89 pmol 1-') sont dues B la skdimentation, a la remise en suspension des sCdiments et B l'apport de matikres siliceuses par les fleuves.
Chemical properties of the water column were examined at the Indian Deep-sea Environment Experime... more Chemical properties of the water column were examined at the Indian Deep-sea Environment Experiment (INDEX) site in the Central Indian Basin (CIB), as a part of baseline studies prior to the benthic disturbance experiment for the environmental impact assessment of mining of polymetallic nodules. The study shows three equatorward moving water masses. (a) The Subsurface Salinity Maximum in the depth range 125–200 m, characterized by high salinity (34.74–34.77 psu) and oxygen minimum associated with weak maxima in nutrients. (b) The Deep Oxygen Maximum (234–245 mM) in the depth range 250–750 m, associated with minima in nutrients and relatively high pH. (c) The Salinity Minimum Water (34.714–34.718 psu) corresponding to the Antarctic Intermediate Water (AAIW) at depths 800–1200 m in the density (s y) range 27.2–27.5. Progressive changes in these characteristics are attributed to mixing with waters above and below, and to oxidation of organic detritus en route. Among the three water masses, the oxygen maximum water shows the lowest changes in its properties, which may suggest that this water mass is moving the fastest. r
A two-gyre circulation system consisting of an anticyclonic gyre (ACG) in the northwestern Bay of... more A two-gyre circulation system consisting of an anticyclonic gyre (ACG) in the northwestern Bay of Bengal and a cyclonic gyre (CG) west of the Andaman Islands is shown by the hydrographic data collected during May}June, prior to the southwest monsoon of 1996. This circulation system leads to an intense southwestward #ow in the central Bay. The circulation pattern characterizes the transition from the pre-monsoon (spring season) ACG to the southwest monsoon CG. A deep chlorophyll maximum (DCM) is found in the depth range 30}50 m in the coastal region and 50}100 m in the open sea. The DCM is located below the surface mixed layer, in the oxycline and nutricline, and above the seasonal thermocline/pycnocline, which coincides with the deep stability maximum (DSM). In the ACG, the DCM is located at 100 m when DSM is at 140 m deep, whereas in the CG, the DCM and DSM rise to 50 and 80 m, respectively. The magnitude of the chlorophyll maximum (CMAX) is nearly twice as high in the CG zone as in the ACG zone. The depths of both DCM and DSM, however, are twice as large in the ACG zone as in the CG zone. This shows that the meso-scale circulation patterns a!ect the spatial distribution of chlorophyll a (both DCM and CMAX) in the Bay of Bengal.
The south eastern Arabian Sea is characterized by moderate coastal upwelling, high biological pro... more The south eastern Arabian Sea is characterized by moderate coastal upwelling, high biological production and subsurface oxygen depletion during the southwest monsoon (June–September). Concurrently, a seasonal closure to trawling activities (15th June–31st July) is implemented here, as a sustainable ecosystem management practise. The effects of monsoon driven environmental changes and consequences of trawling cessation on macrofauna were assessed, based on surveys at 12 sites (30–200 m) preceding and during different phases of the southwest monsoon. Macrofaunal density and biomass increased considerably towards the mid and late monsoon along the inner shelf (30–50 m) where trawling is intense , while no temporal changes were observed along the outer shelf (100–200 m). Density increased four-folds at the 30 m contour and three-folds at 50 m, while biomass nearly doubled at both depths, reflecting a marked increase in density of polychaetes (61–87% of macrofauna). The disproportionate increase in faunal density and biomass along the inner shelf (30–50 m) was due to abundance of juvenile polychaetes and dominance of small-sized opportunists towards late monsoon (August–September). A concurrent hike in nominal species count of polychaetes was also observed in the study area. The increase in polychaete standing stock and high density of planktonic larvae during onset and peak monsoon , coupled with occurrence of juveniles as well as gamete-bearing adults in sediments, indicates that the southwest monsoon is a peak breeding season for the dominant polychaetes in the region. The trawl-ban during this period facilitates the recoupment of benthos by maximising spawning success and larval settlement, thereby enhancing overall ecosystem integrity.
Cochin backwaters, a micro tidal estuary, undergo a characteristic transformation from a river-do... more Cochin backwaters, a micro tidal estuary, undergo a characteristic transformation from a river-dominated system during summer monsoon to a tide-dominant system during pre-monsoon season. The present study observes that as the river flow weakens after monsoon, the flushing of the estuary diminishes and the nitrogen and phosphorous loadings through anthropogenic activities (industries) and sediment re-suspension alter the nutrient stoichiometry substantially. The increased tidal activity during pre-monsoon changes the estuary into a vertically mixed, eutrophic and flow-restricted system supporting an enhanced organic production. This implies that monsoon-induced hydrology plays an important role in regulating the nutrients, secondary production and even the migrant fauna of the estuary. The system is delicately poised, as continuous release of pollutants including nutrients into this estuary would suppress fish and shell fish production, where only pollution tolerant species can exist.
Detailed measurements of carbon (C) isotopic composition in dissolved inorganic (delta-13CDIC) an... more Detailed measurements of carbon (C) isotopic composition in dissolved inorganic (delta-13CDIC) and particulate organic (delta-13CPOC) fractions were conducted at 18 stations in the Cochin estuary during the post-monsoon season. In general, C biogeochemistry of different regions of the Cochin estuary appears to be regulated by different sources and processes. The northern zone of the estuary appears to be influenced primarily by mixing of sea water enriched in 13CPOC and 13CDIC, and river run-off depleted in the same. In contrast, the southern zone of the estuary was found to be greatly
influenced by local terrestrial sources. Relatively depleted 13CPOC in the freshwater Vembanad lake compared to the main estuary suggests inputs from terrestrial sources along with in situ productivity.
Marine pollution bulletin, Jan 22, 2016
The objectives of the present study were to evaluate the ecological status of tropical coastal wa... more The objectives of the present study were to evaluate the ecological status of tropical coastal waters using the temperate benthic indices and examine the effect of seasonal variability on the performance of benthic indices. Macrobenthic samples were collected from northwest to southeast coast of India during 2003-2012 and we tested different univariate indices, ecological strategies, indicator species and multimetric indices. AMBI and multimetric indices performed satisfactorily in evaluating the ecological status. Seasonal variability on the biotic indices was observed during the southwest monsoon and fall intermonsoon period due to recruitment. Therefore, we recommended the non-monsoon period (January-May) as a suitable time of the year to use the indices for effective assessment of the Indian coastal waters. Results show that, the temperate benthic indices are efficient in assessing the tropical environmental status. However, complementary use of different indices is suggested fo...
Evolution to decay of upwelling and associated biogeochemistry over the southeastern Arabian Sea shelf, Jan 2016
Observations along ten shelf transects in 2012 near 10°N in the southeastern Arabian Sea revealed... more Observations along ten shelf transects in 2012 near 10°N in the southeastern Arabian Sea revealed the usual warm oligotrophic conditions during the winter monsoon (WM) and upwelling of oxygen-deficient, nutrient-rich cool water during the summer monsoon (SM). By changing an oligotrophic to a nutrient-replete condition, the upwelling is the major process that regulates the biogeochemistry of this shelf. Its onset is perceptible at 100 m depth between January and March. The upwelling reaches the surface layer in May and intensifies during June-July but withdraws completely and abruptly by October. Despite the nutrient injection, the primary production during SM, integrated for euphotic zone is comparable to that during the preceding spring intermonsoon (SIM). Again as usual the high oxygen demand coupled with low concentration in the upwelled subsurface waters causes severe oxygen depletion below the shallow pycnocline. The oxygen concentrations/ saturations of 2012 on the mid-shelf are similar from those of mid-1958 to early 1960, except for marginally higher values during the peak upwelling period due to relatively weak upwelling in 2012. This implies little anthropogenic influence on coastal hypoxia unlike many other coastal regions. In 2012, the inner shelf system shifted from net autotrophy in SIM to net heterotrophy in SM but on an annual basis it was net-autotrophic (gross primary production to community respiration ratio, GPP/R: 1.11 ± 0.84) as organic production exceeded consumption.
Environmental Monitoring and Assessment, 2015
ABSTRACT Characteristics of a cold-core eddy and its influence on the mesozooplankton community w... more ABSTRACT Characteristics of a cold-core eddy and its influence on the mesozooplankton community were studied along the central (87° E) Bay of Bengal during winter monsoon (November 2008) based on in situ data. The thermo-haline structure and the satellite-derived sea level anomaly maps showed the presence of a cyclonic eddy between 16° N and 20° N. The nutrient enhancement due to the eddy pumping in the euphotic column (∼50 m) had resulted in high chlorophyll a concentration, a factor of 8 times higher than that outside the eddy, which led to higher mesozooplankton biovolume (0.35 ± 0.36 ml m(-3)) and abundance (276 ± 184 ind m(-3)). The northern cyclonic eddy (NCE) seems to exist for approximately 6 months between July and January. During summer, the NCE is forced by local wind stress curl and the resultant Ekman pumping, whereas during fall and early phase of the winter, it is sustained by westward propagating semi-annual Rossby waves. The longer existence of NCE in the study region, which originated 6 months prior to the present observation, provides a favourable environment for the mesozooplankton community to grow and reproduce, resulting in noticeable increase in the biovolume. Hence, the persistent and longer existence of NCE significantly influences the biological production of the generally oligotrophic BoB, making it locally biologically 'active'.
Estuaries and Coasts, 2015
Quantification of nitrogen (N) transformation rates in tropical estuarine-coastal waters coupled ... more Quantification of nitrogen (N) transformation rates in tropical estuarine-coastal waters coupled systems undergoing anthropogenic disturbances is scant. A thorough understanding of these metabolic rates is required to evolve a mitigation strategy to save such systems from further degradation. Here, we report the first measurements of ammonium (NH4+) and nitrate (NO3-) uptake along with N2 fixation rates in the Cochin estuary, a tropical eutrophic ecosystem along the west coast of India, and two transects (off Cochin and off Mangalore) in the coastal Arabian Sea. In general, the Cochin estuary sustained higher uptake rates of NH4+ (0.32-0.91 mol N l-1 h-1) and NO3- (0.01-0.38 mol N l-1 h-1) compared to coastal waters. The N uptake in the nearshore waters of Cochin transect (NH4+: 0.34 mol N l-1 h-1 and NO3-: 0.18 mol N l-1 h-1) was influenced more by estuarine discharge than was in the Mangalore transect (NH4+: 0.02 mol N l-1 h-1 and NO3- : 0.03 mol N l-1 h-1). Despite high dissolved inorganic nitrogen (DIN) concentrations, the Cochin estuary also showed higher N2 fixation rates (0.59 - 1.31 nmol N l-1 h-1) than the coastal waters (0.33-0.55 nmol N l-1 h-1). NH4+ was the preferred substrate for phytoplankton growth, both in the Cochin estuary and coastal waters, indicating the significance of regenerative processes in primary production. A significant negative correlation between total nitrogen (TN): total phosphorus (TP) and NH4+ uptake (as well as N2 fixation) rates in the estuary suggests that nutrient stoichiometry plays a major role in modulating N transformation rates in the Cochin estuary.
International Journal of Oceanography, 2013
ABSTRACT Symbiotic diatom-diazotrophic cyanobacteria association (DDA) of Rhizosolenia hebetata a... more ABSTRACT Symbiotic diatom-diazotrophic cyanobacteria association (DDA) of Rhizosolenia hebetata and Rhizosolenia formosa with endosymbiotic cyanobacteria Richelia intracellularis was noticed and documented for the first time during a bloom of the cyanobacterium Trichodesmium erythraeum in the oligotrophic shelf waters along Kochi and Mangalore transects, southeastern Arabian Sea (SEAS), during spring intermonsoon (April 2012). Although the host is frequently seen, the symbiont is rarely reported in the Indian EEZ. The presence of nitrogen-fixing symbiotic association of Rhizosolenia-Richelia along with Trichodesmium erythraeum highlights the significance of DDAs on the nutrient and energy budgets of phytoplankton in the oligotrophic environments of the Arabian Sea during spring intermonsoon.
Estuarine, Coastal and Shelf Science, 2008
“Biolog” plates were used to study the changes in the metabolic capabilities of bacterioplankton ... more “Biolog” plates were used to study the changes in the metabolic capabilities of bacterioplankton over a complete tidal cycle in a tropical ecosystem (Cochin Estuary) along southwest coast of India. The pattern of utilization of carbon sources showed a definite shift in the community metabolism along a salinity gradient. Multivariate statistical analysis revealed two communities, namely allochthonous bacterioplankton sensitive to salinity and autochthonous bacterioplankton, which are tolerant to wide salinity fluctuations. ...
Estuarine, Coastal and Shelf Science, 2007
... phytoplankton and zooplankton have been established in estuarine systems ([Sautour et al., 19... more ... phytoplankton and zooplankton have been established in estuarine systems ([Sautour et al., 1996], [Perissinotto et al., 2000] and [Tan et al ... Maximum number of microzooplankton species (seven) observed was at stations 1 and 6 only, but species like Tintinnidium incertum and ...
Estuarine, Coastal and Shelf Science, 2008
... lacking. The Cochin Estuary (CE) is one of the vulnerable ecosystems in India, which is prese... more ... lacking. The Cochin Estuary (CE) is one of the vulnerable ecosystems in India, which is presently undergoing environmental deterioration due to increased anthropogenic activities ([Menon et al., 2000] and [Qasim, 2003]). The ...
Carbon uptake rates in a tropical eutrophic estuary (Cochin estuary) and the adjacent coastal Ara... more Carbon uptake rates in a tropical eutrophic estuary (Cochin estuary) and the adjacent coastal Arabian Sea were measured for the first time using 13 C-NaHCO 3 labeling technique. The rates in the estuarine water (pre-monsoon 3.0–20.4; monsoon 2.55–12.4; post-monsoon 3.1–16.3 μmol C l−1 h−1) were significantly higher than the coastal locations
(pre-monsoon 0.02–0.1; monsoon 0.13–3.1; post-monsoon
0.007–0.032 μmol C l−1 h−1). The measured primary productivity
in the estuary was relatively higher at mesohaline locations
influenced with lateral inputs and TN:TP close to the Redfield ratio (16:1). It suggests that in nutrient replete tropical systems, TN:TP plays a major role in controlling the primary productivity rates. The primary productivity data from the coastal Arabian Sea suggests the effect of estuarine discharge on the carbon uptake rate at nearshore regions and
ability of anthropogenic inputs to modulate the coastal biogeochemistry.
Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continent... more Repeat measurements of dissolved nitrous oxide (N2O) along two transects of the western continental shelf of India in 2012 revealed high concentrations of 45 6 32 nM (off Kochi) and 73 6 63 nM (off Mangalore) during the summer monsoon (SM). N2O concentrations increased nonlinearly during the peak of the SM upwelling, when low O2 (<25 mM) conditions prevailed in the water column. Off Kochi, N2O levels fell gradually from the fall intermonsoon (20 ± 8 nM) to the winter monsoon (8.8 ± 2 nM) and remained low (9.2 ± 5.2 nM) through the spring intermonsoon season. The N2O supersaturation off Kochi (574 ± 720%) was presumably due to its high yield during sediment denitrification, whereas the higher N2O supersaturation observed off Mangalore (1046 ± 885%) was due to its production during denitrification in both the anoxic water column and the underlying sediments. Such distinctive biogeochemical behavior between these two shelf segments is at first augmented by the natural origin of intense upwelling at Mangalore relative to Kochi wherein suboxic to anoxic oxygen minimum zone waters spread from offshore to the shelf of Mangalore, over which the runoff and terrestrial nutrients supply acts in unison. Following new zonal extrapolation approach, our revised estimate of N2O effluxes from the southwestern Indian shelf (7–14°N) was four times lower (0.019–0.039 Tg y-1) than previous estimate. Nevertheless, further studies are needed to refine the N2O effluxes from the entire western Indian shelf to monitor its modification due to expansion/intensification of the coastal low-O2 zones and also to ascertain its actual N2O contribution to the world oceans.
Regional Studies in Marine Science, 2015
The objectives of the present study were to evaluate the ecological status of tropical coastal wa... more The objectives of the present study were to evaluate the ecological status of tropical coastal waters using the temperate benthic indices and examine the effect of seasonal variability on the performance of benthic indices. Macrobenthic samples were collected from northwest to southeast coast of India during 2003–2012 and we tested different univariate indices, ecological strategies, indicator species and multimetric indices. AMBI and multimetric indices performed satisfactorily in evaluating the ecological status. Seasonal variability on the biotic indices was observed during the southwest monsoon and fall intermonsoon period due to recruitment. Therefore , we recommended the non-monsoon period (January–May) as a suitable time of the year to use the indices for effective assessment of the Indian coastal waters. Results show that, the temperate benthic indices are efficient in assessing the tropical environmental status. However, complementary use of different indices is suggested for accurate assessment of the environmental status.
High concentrations of PHC were observed in the inner channels of Visakhapatnam harbour. The esti... more High concentrations of PHC were observed in the inner channels of Visakhapatnam harbour. The estimation of trace metals in surficial sediments indicated higher contamination in Visakhapatnam habour than in Kakinada Bay. Positive correlations between Cu, Zn, Pb and Cd suggests common sources of these metals. Lack of corrleation between Co, Ni with the other metals indicates point sources. High concentrations of chromium reflects intense discharges due to electroplating and battery operation.
During RV Gaveshani cruise No. 235 (August-September, 1993), particulate matter was collected fro... more During RV Gaveshani cruise No. 235 (August-September, 1993), particulate matter was collected from euphotic and deeper waters in the Bay of Bengal and analysed for biogenic silica (BSi), lithogenic silica (LSi) and particulate organic carbon (POC). Mean depth profiles of BSi showed a minimum in the surface waters and a maximum in the middle depths of the euphotic zone. The deepening of BSi maxima from north to south coincides with the deepening of the thermocline and thickening of the surface layer. In coastal waters, the contribution of BSi to total particulate silica was high (> 80 %); in offshore waters it was 60-70 %. BSiIPOC (0.086) and LSiIPOC (0.023) ratios show that particulate matter consists mostly of biogenic material, while the BSiILSi (1.61) ratio revealed the dominance of siliceous phytoplankton in the euphotic zone. High concentrations of BSi (0.49 pmol 1-') and LSi (1.89 pmol 1-') in deeper waters are due to a high sedimentation rate, the resuspension of bottom sediments and the flow of large amounts of riverine siliceous material. Silice biogknique dans le golfe du Bengale pendant la mousson du sud-ouest. Au cours de la campagne 235 du N.O. Guveslzani dans le golfe du Bengale (aoat-septembre 1993), des prClkvements de matikre particulaire ont kt6 effectuCs dans la couche euphotique et dans les eaux profondes : la silice biogknique (BSi), la silice 1ithogCnique (LSi) et le carbone organique particulaire (POC) y ont Ct C analysts. Les profils de silice biogCnique prksentent un minimum dans les eaux superficielles et un maximum aux profondeurs moyennes de la couche euphotique ; les maxima plongent du nord au sud avec la thermocline. Dans les eaux catikres, la silice biogknique constitue plus de 80 % de la silice particulaire totale, mais sa contribution n'est que de 60 a 70 5% dans les eaux du large. Le rapport silice biogkniquelcarbone organique particubire est de 0,086 et le rapport silice 1ithogCniquelcarbone organique particulaire est de 0,023, ce qui indique la nature biogCnique de la matikre particulaire. Le rapport silice biogCniquelsilice lithogknique (1 $1) rkvkle la dominance du phytoplancton siliceux dans la couche euphotique. Dans les eaux profondes, les concentrations ClevCes en silice biogCnique (0,49 pmol I-') et en silice lithogknique (1,89 pmol 1-') sont dues B la skdimentation, a la remise en suspension des sCdiments et B l'apport de matikres siliceuses par les fleuves.
Chemical properties of the water column were examined at the Indian Deep-sea Environment Experime... more Chemical properties of the water column were examined at the Indian Deep-sea Environment Experiment (INDEX) site in the Central Indian Basin (CIB), as a part of baseline studies prior to the benthic disturbance experiment for the environmental impact assessment of mining of polymetallic nodules. The study shows three equatorward moving water masses. (a) The Subsurface Salinity Maximum in the depth range 125–200 m, characterized by high salinity (34.74–34.77 psu) and oxygen minimum associated with weak maxima in nutrients. (b) The Deep Oxygen Maximum (234–245 mM) in the depth range 250–750 m, associated with minima in nutrients and relatively high pH. (c) The Salinity Minimum Water (34.714–34.718 psu) corresponding to the Antarctic Intermediate Water (AAIW) at depths 800–1200 m in the density (s y) range 27.2–27.5. Progressive changes in these characteristics are attributed to mixing with waters above and below, and to oxidation of organic detritus en route. Among the three water masses, the oxygen maximum water shows the lowest changes in its properties, which may suggest that this water mass is moving the fastest. r
A two-gyre circulation system consisting of an anticyclonic gyre (ACG) in the northwestern Bay of... more A two-gyre circulation system consisting of an anticyclonic gyre (ACG) in the northwestern Bay of Bengal and a cyclonic gyre (CG) west of the Andaman Islands is shown by the hydrographic data collected during May}June, prior to the southwest monsoon of 1996. This circulation system leads to an intense southwestward #ow in the central Bay. The circulation pattern characterizes the transition from the pre-monsoon (spring season) ACG to the southwest monsoon CG. A deep chlorophyll maximum (DCM) is found in the depth range 30}50 m in the coastal region and 50}100 m in the open sea. The DCM is located below the surface mixed layer, in the oxycline and nutricline, and above the seasonal thermocline/pycnocline, which coincides with the deep stability maximum (DSM). In the ACG, the DCM is located at 100 m when DSM is at 140 m deep, whereas in the CG, the DCM and DSM rise to 50 and 80 m, respectively. The magnitude of the chlorophyll maximum (CMAX) is nearly twice as high in the CG zone as in the ACG zone. The depths of both DCM and DSM, however, are twice as large in the ACG zone as in the CG zone. This shows that the meso-scale circulation patterns a!ect the spatial distribution of chlorophyll a (both DCM and CMAX) in the Bay of Bengal.
The south eastern Arabian Sea is characterized by moderate coastal upwelling, high biological pro... more The south eastern Arabian Sea is characterized by moderate coastal upwelling, high biological production and subsurface oxygen depletion during the southwest monsoon (June–September). Concurrently, a seasonal closure to trawling activities (15th June–31st July) is implemented here, as a sustainable ecosystem management practise. The effects of monsoon driven environmental changes and consequences of trawling cessation on macrofauna were assessed, based on surveys at 12 sites (30–200 m) preceding and during different phases of the southwest monsoon. Macrofaunal density and biomass increased considerably towards the mid and late monsoon along the inner shelf (30–50 m) where trawling is intense , while no temporal changes were observed along the outer shelf (100–200 m). Density increased four-folds at the 30 m contour and three-folds at 50 m, while biomass nearly doubled at both depths, reflecting a marked increase in density of polychaetes (61–87% of macrofauna). The disproportionate increase in faunal density and biomass along the inner shelf (30–50 m) was due to abundance of juvenile polychaetes and dominance of small-sized opportunists towards late monsoon (August–September). A concurrent hike in nominal species count of polychaetes was also observed in the study area. The increase in polychaete standing stock and high density of planktonic larvae during onset and peak monsoon , coupled with occurrence of juveniles as well as gamete-bearing adults in sediments, indicates that the southwest monsoon is a peak breeding season for the dominant polychaetes in the region. The trawl-ban during this period facilitates the recoupment of benthos by maximising spawning success and larval settlement, thereby enhancing overall ecosystem integrity.
Cochin backwaters, a micro tidal estuary, undergo a characteristic transformation from a river-do... more Cochin backwaters, a micro tidal estuary, undergo a characteristic transformation from a river-dominated system during summer monsoon to a tide-dominant system during pre-monsoon season. The present study observes that as the river flow weakens after monsoon, the flushing of the estuary diminishes and the nitrogen and phosphorous loadings through anthropogenic activities (industries) and sediment re-suspension alter the nutrient stoichiometry substantially. The increased tidal activity during pre-monsoon changes the estuary into a vertically mixed, eutrophic and flow-restricted system supporting an enhanced organic production. This implies that monsoon-induced hydrology plays an important role in regulating the nutrients, secondary production and even the migrant fauna of the estuary. The system is delicately poised, as continuous release of pollutants including nutrients into this estuary would suppress fish and shell fish production, where only pollution tolerant species can exist.
Detailed measurements of carbon (C) isotopic composition in dissolved inorganic (delta-13CDIC) an... more Detailed measurements of carbon (C) isotopic composition in dissolved inorganic (delta-13CDIC) and particulate organic (delta-13CPOC) fractions were conducted at 18 stations in the Cochin estuary during the post-monsoon season. In general, C biogeochemistry of different regions of the Cochin estuary appears to be regulated by different sources and processes. The northern zone of the estuary appears to be influenced primarily by mixing of sea water enriched in 13CPOC and 13CDIC, and river run-off depleted in the same. In contrast, the southern zone of the estuary was found to be greatly
influenced by local terrestrial sources. Relatively depleted 13CPOC in the freshwater Vembanad lake compared to the main estuary suggests inputs from terrestrial sources along with in situ productivity.
Marine pollution bulletin, Jan 22, 2016
The objectives of the present study were to evaluate the ecological status of tropical coastal wa... more The objectives of the present study were to evaluate the ecological status of tropical coastal waters using the temperate benthic indices and examine the effect of seasonal variability on the performance of benthic indices. Macrobenthic samples were collected from northwest to southeast coast of India during 2003-2012 and we tested different univariate indices, ecological strategies, indicator species and multimetric indices. AMBI and multimetric indices performed satisfactorily in evaluating the ecological status. Seasonal variability on the biotic indices was observed during the southwest monsoon and fall intermonsoon period due to recruitment. Therefore, we recommended the non-monsoon period (January-May) as a suitable time of the year to use the indices for effective assessment of the Indian coastal waters. Results show that, the temperate benthic indices are efficient in assessing the tropical environmental status. However, complementary use of different indices is suggested fo...
Evolution to decay of upwelling and associated biogeochemistry over the southeastern Arabian Sea shelf, Jan 2016
Observations along ten shelf transects in 2012 near 10°N in the southeastern Arabian Sea revealed... more Observations along ten shelf transects in 2012 near 10°N in the southeastern Arabian Sea revealed the usual warm oligotrophic conditions during the winter monsoon (WM) and upwelling of oxygen-deficient, nutrient-rich cool water during the summer monsoon (SM). By changing an oligotrophic to a nutrient-replete condition, the upwelling is the major process that regulates the biogeochemistry of this shelf. Its onset is perceptible at 100 m depth between January and March. The upwelling reaches the surface layer in May and intensifies during June-July but withdraws completely and abruptly by October. Despite the nutrient injection, the primary production during SM, integrated for euphotic zone is comparable to that during the preceding spring intermonsoon (SIM). Again as usual the high oxygen demand coupled with low concentration in the upwelled subsurface waters causes severe oxygen depletion below the shallow pycnocline. The oxygen concentrations/ saturations of 2012 on the mid-shelf are similar from those of mid-1958 to early 1960, except for marginally higher values during the peak upwelling period due to relatively weak upwelling in 2012. This implies little anthropogenic influence on coastal hypoxia unlike many other coastal regions. In 2012, the inner shelf system shifted from net autotrophy in SIM to net heterotrophy in SM but on an annual basis it was net-autotrophic (gross primary production to community respiration ratio, GPP/R: 1.11 ± 0.84) as organic production exceeded consumption.
Environmental Monitoring and Assessment, 2015
ABSTRACT Characteristics of a cold-core eddy and its influence on the mesozooplankton community w... more ABSTRACT Characteristics of a cold-core eddy and its influence on the mesozooplankton community were studied along the central (87° E) Bay of Bengal during winter monsoon (November 2008) based on in situ data. The thermo-haline structure and the satellite-derived sea level anomaly maps showed the presence of a cyclonic eddy between 16° N and 20° N. The nutrient enhancement due to the eddy pumping in the euphotic column (∼50 m) had resulted in high chlorophyll a concentration, a factor of 8 times higher than that outside the eddy, which led to higher mesozooplankton biovolume (0.35 ± 0.36 ml m(-3)) and abundance (276 ± 184 ind m(-3)). The northern cyclonic eddy (NCE) seems to exist for approximately 6 months between July and January. During summer, the NCE is forced by local wind stress curl and the resultant Ekman pumping, whereas during fall and early phase of the winter, it is sustained by westward propagating semi-annual Rossby waves. The longer existence of NCE in the study region, which originated 6 months prior to the present observation, provides a favourable environment for the mesozooplankton community to grow and reproduce, resulting in noticeable increase in the biovolume. Hence, the persistent and longer existence of NCE significantly influences the biological production of the generally oligotrophic BoB, making it locally biologically 'active'.
Estuaries and Coasts, 2015
Quantification of nitrogen (N) transformation rates in tropical estuarine-coastal waters coupled ... more Quantification of nitrogen (N) transformation rates in tropical estuarine-coastal waters coupled systems undergoing anthropogenic disturbances is scant. A thorough understanding of these metabolic rates is required to evolve a mitigation strategy to save such systems from further degradation. Here, we report the first measurements of ammonium (NH4+) and nitrate (NO3-) uptake along with N2 fixation rates in the Cochin estuary, a tropical eutrophic ecosystem along the west coast of India, and two transects (off Cochin and off Mangalore) in the coastal Arabian Sea. In general, the Cochin estuary sustained higher uptake rates of NH4+ (0.32-0.91 mol N l-1 h-1) and NO3- (0.01-0.38 mol N l-1 h-1) compared to coastal waters. The N uptake in the nearshore waters of Cochin transect (NH4+: 0.34 mol N l-1 h-1 and NO3-: 0.18 mol N l-1 h-1) was influenced more by estuarine discharge than was in the Mangalore transect (NH4+: 0.02 mol N l-1 h-1 and NO3- : 0.03 mol N l-1 h-1). Despite high dissolved inorganic nitrogen (DIN) concentrations, the Cochin estuary also showed higher N2 fixation rates (0.59 - 1.31 nmol N l-1 h-1) than the coastal waters (0.33-0.55 nmol N l-1 h-1). NH4+ was the preferred substrate for phytoplankton growth, both in the Cochin estuary and coastal waters, indicating the significance of regenerative processes in primary production. A significant negative correlation between total nitrogen (TN): total phosphorus (TP) and NH4+ uptake (as well as N2 fixation) rates in the estuary suggests that nutrient stoichiometry plays a major role in modulating N transformation rates in the Cochin estuary.
International Journal of Oceanography, 2013
ABSTRACT Symbiotic diatom-diazotrophic cyanobacteria association (DDA) of Rhizosolenia hebetata a... more ABSTRACT Symbiotic diatom-diazotrophic cyanobacteria association (DDA) of Rhizosolenia hebetata and Rhizosolenia formosa with endosymbiotic cyanobacteria Richelia intracellularis was noticed and documented for the first time during a bloom of the cyanobacterium Trichodesmium erythraeum in the oligotrophic shelf waters along Kochi and Mangalore transects, southeastern Arabian Sea (SEAS), during spring intermonsoon (April 2012). Although the host is frequently seen, the symbiont is rarely reported in the Indian EEZ. The presence of nitrogen-fixing symbiotic association of Rhizosolenia-Richelia along with Trichodesmium erythraeum highlights the significance of DDAs on the nutrient and energy budgets of phytoplankton in the oligotrophic environments of the Arabian Sea during spring intermonsoon.
Estuarine, Coastal and Shelf Science, 2008
“Biolog” plates were used to study the changes in the metabolic capabilities of bacterioplankton ... more “Biolog” plates were used to study the changes in the metabolic capabilities of bacterioplankton over a complete tidal cycle in a tropical ecosystem (Cochin Estuary) along southwest coast of India. The pattern of utilization of carbon sources showed a definite shift in the community metabolism along a salinity gradient. Multivariate statistical analysis revealed two communities, namely allochthonous bacterioplankton sensitive to salinity and autochthonous bacterioplankton, which are tolerant to wide salinity fluctuations. ...
Estuarine, Coastal and Shelf Science, 2007
... phytoplankton and zooplankton have been established in estuarine systems ([Sautour et al., 19... more ... phytoplankton and zooplankton have been established in estuarine systems ([Sautour et al., 1996], [Perissinotto et al., 2000] and [Tan et al ... Maximum number of microzooplankton species (seven) observed was at stations 1 and 6 only, but species like Tintinnidium incertum and ...
Estuarine, Coastal and Shelf Science, 2008
... lacking. The Cochin Estuary (CE) is one of the vulnerable ecosystems in India, which is prese... more ... lacking. The Cochin Estuary (CE) is one of the vulnerable ecosystems in India, which is presently undergoing environmental deterioration due to increased anthropogenic activities ([Menon et al., 2000] and [Qasim, 2003]). The ...