The Bay of Bengal is one of a very few areas in the world oceans where active interaction occurs among land, marine and atmospheric systems. Enormous quantities of freshwater and suspended solids are annually drained into the bay where the northeast monsoon is generally accompanied by cyclonic storms in this region. Hence, the Bay of Bengal water column is quite distinct with regard to carbon exchanges with other reservoirs (atmosphere and sediments) and across the equator, and to the receipt of fluvial inputs. The carbon dioxide exchange with atmosphere can be influenced in the Bay of Bengal predominantly by synoptic disturbances. The removal of particulate carbon from the water column is much faster in view of the rapid sedimentation and mineral-organic matter association in the bay. Seasonally changing winds/run-off carry different loads (in forms of aerosols, and dissolved and particulates, respectively) of carbon and nutrients (nitrate) to the bay. This would alter the net carbon fixation by photosynthesis that changes the intensity of the biological pump. Therefore, studies on the carbon dynamics in the bay would enable us better understand the mechanisms of land-air-ocean interactions in regulating the atmospheric carbon dioxide.
Extensive and systematic coverage of various aspects are planned under BOBPS programme in order to comprehensively understand the carbon cycling in the bay, i.e. studies on atmospheric aerosols through internal cycling to mechanisms of sedimentation.
Seven Investigations proposed for biogeochemical studies here are broadly divided in to 3 categories:
Category 1: Influx of materials
One proposal under this category aims to study the inputs of carbon and nutrients from land through the atmosphere and the effect of this external forcing on carbon cycling in the water column of the bay. The investigations and overall objectives are:
(a) Atmospheric aerosols and trace gas studies over the Bay of Bengal
(PRL, Ahmedabad) Rs. 16.50 lakhs.
Objectives:
(i) study the chemical characteristics and deposition fluxes of aerosols over the bay
(ii) Quantify material inputs by terrestrial aerosols and
(iii) understand variability in and regulating factors of air-sea
exchange of trace gases
Category2: Internal cycling of materials
Three proposals are made under this category for studying the internal cycling of carbon in the water column. Temporal and spatial variations in inorganic and organic carbon components will be studied in order to understand the effect of biological pump and sedimentation processes on the internal cycling of carbon. Investigation (a) partly overlaps with this category. The proposed investigations and brief objectives are:
(a) Biogeochemical cycling of carbon and nitrogen in the Bay of Bengal
(NIO, Goa) Rs. 25.25 lakhs.
Objectives:
(i) understand the variability in fluxes of nutrient inputs and its influence on productivity
(ii) factors controlling DOC distribution and
(iii) variability in carbon dioxide air-sea exchange
(b) Biogeochemistry of carbohydrates, amino acids and aminosugars of particulate material in the Bay of Bengal
(NIO, Goa) Rs. 10.63 lakhs.
Objectives:
(i) study variability in particulate (>0.7 m m size) organic components in relation to productivity
(ii) identify sources of organic matter in the bay and
(iii) understanding the diagenetic pathways of particulate organic components in water column and sediments
(c) Role of colloids in carbon turnover in the Bay of Bengal
(Andhra University, Visakhapatnam) Rs. 14.07 lakhs.
Objectives:
(i) study the variability in colloidal organic carbon and assessment of its sources
(ii) understand particle formation mechanism through the relation between colloidal and dissolved constituents of organic carbon and
(iii) quantify the contribution of colloidal organic carbon in carbon cycling in the bay
Category 3: Outflux of materials
This category attempts to quantify the variability in carbon outfluxes from the water column (i.e. carbon emission to atmosphere and its deposition in underlying sediments), as export fluxes across the upper thermocline and burial fluxes near sediment-water interface. Of the three investigations proposed under this category one (a) aims to understand and quantify the export fluxes of particulate carbon within the water column, in the upper 1000 m in particular.The other two (b and c) concentrate on the burial fluxes of carbon. Investigations (a) in categiring 1 and 2 study the trace gas fluxes across the air-sea interface and hence are complimentary to this category.
(a) Radionuclide scavenging and carbon export in the Bay of Bengal water column (PRL, Ahmedabad) Rs. 29.5 lakhs.
Objectives:
(i) understand the relationship between primary productivity, particle export flux and U-Th radioactive disequilibria
(ii) determine carbon export fluxes from the water column and
(iii) study scavenging processes near oceanic margins with particular reference to terrigenous inputs.
(b) Paleoceanography of the Bay of Bengal (³ 1-2 millennia)
(PRL, Ahmedabad) Rs. 3.00 lakhs.Objectives:
(i) study of burial rates of carbon and associated biogenic components
(ii) understand the influence of continental souces on buried carbon and
(iii) reconstruct surface water properties in the past with implications to monsoonal and fluvial fluxes
(c) Burial-fluxes of carbon during the last 22,000 years
(NIO, Goa) Rs. 28.63 lakhs.
Objectives:
(i) Estimate present day depositional fluxes of carbon
(ii) study surface water characteristics and primary productivity and
(iii) study the relation between burial fluxes in fresh sediments and present day sinking fluxes
The original proposals of these investigations, totaling to a sum of Rs.195.40 lakhs, are appended in the following pages.
Remarks on possible linkages:
1) Drifting sediment traps: NIO and PRL to collaborate
2) POC measurements: NIO and PRL to collaborate
3) Organic constituent analysis in particles: NIO and Andhra University to collaborate
4) Large volume sampling through pumps: PRL and Andhra University to collaborate
5) Paleostudies: NIO
and PRL to collaborate.
Biogeochemical cycling of carbon and nitrogen in the Bay of Bengal
Participants
S. N. de Sousa Prinicpal Investigator
S. S. Sardessai
National Institute of Oceanography , Dona Paula, Goa.
Objectives
In order to understand the physical and biological controls operating in the Bay of Bengal, on the biogeochemical cycling of carbon and nitrogen we propose to undertake this investigation, as a part of the Bay of Bengal Process Study Programme, with the following objectives:
run-off and upwelling, and its influence on the productivity patterns
seasonal variability
The Bay of Bengal, one of the two northern arms of the Indian Ocean, has distinctive characteristics. A massive freshwater discharge of 1.6 x 1012 m3/year from the seven major rivers (Subramanian, 1993) result, together with precipitation, in an excess freshwater input over evaporation (Venkateshwaran, 1956) of 1500 x 109 m3 leading to a positive water balance in the bay (Ramanathan and Pisharoty, 1972) . A freshwater discharge of such a magnitude in a small area causes strong stratification in the upper layers. This slows down the rate of exchange of nutrients between the nutrient-enriched deep waters and the euphotic layers. However, there are conflicting reports on the productivity of the bay. One of the questions needs to be addressed is the sources and extent of nutrient supply into the bay.
Along with the enormous influx of freshwater, the Bay of Bengal receives 1387 x 106 tons of suspended solids annually (Subramanian, 1993). Organic matter associated with this massive discharge of freshwater and suspended solids, is expected to contribute significantly to the carbon and nitrogen pools of the Bay of Bengal and may have a control over their biogeochemical cycling. The rate of organic matter decomposition within the water column is reported to be low due to the faster sedimentation of organic matter (2.04 - 3.59 gC m-2 yr-1; Ittekkot et al., 1991) consequent to its incorporation into the rapidly sinking terrigeneous material in the Bay of Bengal (Kumar et al., 1998).
The near equatorial oceans are generally found to be sources of atmospheric carbon dioxide. In agreement with this we found that the Arabian Sea acts as a perennial source of atmospheric carbon dioxide (Sarma et al. 1998); a result of our efforts under JGOFS (India)-Arabian Sea Programme . However, the freshwater discharge into the bay seems to make this region a seasonal sink (Kumar et al., 1996). Nevertheless, the variability in this exchange process is clearly unknown. On the other hand, the water column chemistry of carbon is only little known (George et al., 1994). The north-south gradients in carbon dioxide, as occurred in the Arabian Sea, are not evident. Moreover, there is no information on the dissolved organic carbon in the Bay of Bengal despite the fact that rivers pour in large quantities of dissolved organic matter. On the other hand, even in the sub-oxic waters of the Bay of Bengal no active denitrification is found to occur (Naqvi et al., 1994; Rao et al., 1994) In view of these, it is essential to study and understand the carbon cycle in detail in the bay where land-ocean-atmosphere interact vigorously, the extent of which vary with seasons. Therefore, we propose to extend our investigations on carbon and nitrogen cycling from the Arabian Sea, carried out under JGOFS (India) Programme, to the Bay of Bengal, under BOBPS Programme.
Method
Measurements of dissolved oxygen, pH, alkalinity, dissolved organic carbon (DOC), transparent exopolymer particles (TEP), total carbon dioxide (TCO2), nitrate, nitrite, phosphate and silicate will be carried out.
The determination of dissolved
oxygen will be done based on the spectrometric/winkler method; pH and alkalinity
by multiwavelength spetrophotometric methods using cresol red (Byrne and
Breland, 1989) and bromo cresol green indicators, respectively. Total carbon
dioxide will be analysed by a coulometric method (George et al.
1994; Sarma et al., 1996). Dissolved organic carbon (DOC) will be
measured by the high temperature catalytic oxidation method. The nutrients
(nitrate, nitrite, phosphate and silicate) will be analysed using auto-analyser
(Grasshoff, 1983) while the transparent exopolymer particles (TEP) will
be estimated by a spectrometric technique involving Alcian blue staining
(Passow and Alldredge, 1995)
References
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George, M. D., Kumar, M. D., Naqvi, S. W. A., Nenerjee, S., de Sousa, S. N., Narvekar, P. V. and Jayakumar, D. A. (1994). A study of the carbondioxide system in the northern Indian ocean during premonsoon. Mar. Chem., 47, 243-254.
Grasshoff, K., (1983). Manual of methods of seawater analysis, second edition, Verlag Chemie, Weihein, D-6940, GMBH Ittekkot, V., Nair, R. R., Honjo, S., Ramaswamy, V., Bartsch, M., Manganini, S. and Dessai, B.N., (1991). Enhanced particle fluxes in the Bay of Bengal induced by injection of freshwater. Nature, 351, 385-387.
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