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Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Materials and Methods
  6. 4. Results and Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[1] We report the first measurements of new production (15N tracer technique), the component of primary production that sustains on extraneous nutrient inputs to the euphotic zone, in the Bay of Bengal. Experiments done in two different seasons consistently show high new production (averaging around 4 mmol N m−2 d−1 during post monsoon and 5.4 mmol N m−2 d−1 during pre monsoon), validating the earlier conjecture of high new production, based on pCO2 measurements, in the Bay. Averaged over annual time scales, higher new production could cause higher rate of removal of organic carbon. This could also be one of the reasons for comparable organic carbon fluxes observed in the sediment traps of the Bay of Bengal and the eastern Arabian Sea. Thus, oceanic regions like Bay of Bengal may play a more significant role in removing the excess CO2 from the atmosphere than hitherto believed.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Materials and Methods
  6. 4. Results and Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[2] Major international scientific programmes such as JGOFS (Joint Global Ocean Fluxes Study), aimed at assessing the role of oceans as source/sink of atmospheric CO2, concentrated mostly on highly productive regions of the oceans, e.g., Arabian Sea [Smith, 2001]. Intense upwelling during summer and convective mixing due to surface cooling in winter enhance the productivity of the Arabian Sea [Madhupratap et al., 1996]. In contrast, the limited studies carried out in the adjacent Bay of Bengal (henceforth BOB) suggest it to be less productive because of frequent cloud cover and stratification of the surface layers by copious riverine discharge from the subcontinent, inhibiting vertical mixing and the supply of nutrients from below [Prasanna Kumar et al., 2003]. Data of air-sea exchange rates of CO2 for the northern Indian Ocean in general, and BOB in particular, are inadequate in space and time. Limited data [Kumar et al., 1996] during presouthwest monsoon and northeast monsoon of 1991 reveal that a large area of BOB is characterized by pCO2 levels far below the atmospheric value (∼350 μatm), more prominent during northeast monsoon when the air-sea pCO2 gradient sometimes exceeds 100 μatm. Kumar et al. [1996] surmised that biological activity could account for most of the observed pCO2 decrease due to moderately high new production sustained by external nutrients brought in by rivers or atmospheric deposition.

[3] Although the Arabian Sea is more productive than BOB, the time averaged sediment trap data from BOB and the Arabian Sea show comparable organic carbon fluxes [Ittekkot et al., 1991; Lee et al., 1998; Unger et al., 2003], barring the highly productive western Arabian Sea. Ittekkot et al. [1991] have suggested the ballasting of the organic carbon-lithogenic aggregates in the Bay as a possible cause. However, consistent high new production observed by us could be an additional reason as new production and particle sinking are coupled over longer time scales [Eppley et al., 1983]. Here we present the first independent estimates of 15N based new production (defined here as nitrate uptake) for BOB and discuss possible causes and implications.

2. Study Area

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Materials and Methods
  6. 4. Results and Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[4] BOB, a semi-enclosed tropical basin, is a part of northern Indian Ocean and experiences seasonal changes in circulation and climate due to the monsoons. BOB receives a large freshwater influx (1.6*1012 m3 yr−1 compared to 0.3*1012 m3 yr−1 for the Arabian Sea [Subramanian, 1993]) from the rivers draining the subcontinent. This input causes a considerable variation of salinity during and after the monsoons over the whole basin and causes stratification of the sea surface. Also, in contrast to the Arabian Sea, precipitation in BOB exceeds evaporation by 0.80 myr−1. The surface salinity of open ocean stations during Sep–Oct 2002 decreased from south to north (34 psu at 7°N to 32 psu at 16°N) and dropped further by 3 psu at 17°N. The coastal stations also exhibited a similar salinity pattern, but the drop from 16°N to 17°N was higher (34 psu to 21 psu). During Apr–May 2003 the overall variation in salinity was between 32 to 34 psu. Sea surface temperature (SST) during post monsoon along open BOB varied marginally from 28.2 to 29°C from south to north while along the coastal transect it did not show any trend (average ∼30°C). During pre monsoon SST varied from 29 to 31.4°C in the open ocean, and from 29.1 to 30.4°C in coastal locations. The riverine inputs are a major potential source of nutrients such as nitrate, phosphate and silica to the Bay. Additional sources include mixing due to cyclones, frequent in the BOB during post monsoon. The measured nitrate concentrations in the surface Bay during post monsoon, in general, were low (mostly below the detection limit, <0.1 μM; coastal average ∼0.18 μM and open ocean ∼0.10) and for pre monsoon it was mostly around 0.2 μM (coastal average ∼0.28 μM and open ocean ∼0.32; maximum 1.1 μM). However, it increased sharply between 40 and 60 m (maximum ∼15 μM and average ∼7 μM during pre-monsoon).

3. Materials and Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Materials and Methods
  6. 4. Results and Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[5] Sampling was performed along the cruise track shown in Figure 1 at nine different stations onboard ORV Sagar Kanya for two seasons: post monsoon (17th Sep–11th Oct 2002; SK-182) and pre monsoon (16th Apr–6th May 2003; SK-191). Water samples were collected by a CTD rosette fitted with 30-L Go-Flo bottles. Four sampling depths during post monsoon and six sampling depths during pre monsoon were chosen to cover the photic zone (deeper in the pre monsoon). Ambient nitrate concentration was measured by the column reduction technique [Strickland and Parson, 1972]. Ambient ammonium and urea were estimated indirectly using zooplankton biomass regeneration method [Wiebe et al., 1975] and were found to be less than 0.04 μM (ammonium) and 0.01 μM (urea). While nitrate tracer was added at 10% of the ambient concentrations, constant amounts of ammonium and urea were added for all depths (0.01 μM and 0.03 μM, respectively). For the calculation of uptake rates for ammonium and urea it was assumed that tracer added was the only source for the planktons and hence conservative estimates were obtained. The analysis was performed using a CarloErba elemental Analyzer interfaced via conflo III to a Finnigan Delta Plus mass spectrometer using a technique for sub-microgram level 15N determination [Owens and Rees, 1989]. All water samples were processed in duplicate, and the maximum difference in the measured particulate organic nitrogen of duplicate samples was found to be <10%. The coefficient of variation in 15N atom% measurement was less than 1% for samples that used nitrate or urea substrates and 3.4% for those using ammonium (caption to Table 1). Uptake rates were calculated using the equation of Dugdale and Wilkerson [1986].

image

Figure 1. The cruise track followed during the study. PP are the productivity stations.

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Table 1. Column Integrated Uptake Rates in Coastal and Open Bay of Bengal During September–October 2002 (Cruise SK 182, Odd Rows) and April–May 2003 (Cruise SK 191, Even Rows)a
Stn.Lat (°N)Long (°E)Uptake Rate (mmol N m−2 d−1)Surface Nitrate (μM)
NitrateAmmoniaUreaTotal
  • a

    Uptake rates are in mmol N m−2 d−1. Four to six depths were sampled at each station. Measured photic depths (Secchi disc) were uniformly ∼60 m (SK182) and 60–100 m (SK 191). Blanks (atom% 15N) were 0.368 ± 0.001, 0.397 ± 0.011 and 0.411 ± 0.036, respectively, for nitrate, ammonia and urea experiments; all data are blank-corrected. Two standards IAEA-NO-3 (KNO3, no. 213) and IAEA-N-2 ([NH4]2SO4, no. 342) gave values 0.3674 ± 0.0009 (n = 37) and 0.3727 ± 0.0008 (n = 6) respectively (expected values are 0.3681 and 0.3738 respectively).

  • b

    Coastal stations.

PP18°59′58.9″87°58′43.1″0.430.330.491.250.14
   0.990.670.281.940.4
PP211°58′57.9″87°57′58.2″8.850.811.3310.990.04
   3.270.910.774.950.2
PP315°0′9.3″87°58′24″1.440.801.143.380.08
   1.581.070.613.260.4
PP417°56′33.1″87°54′38.6″1.530.280.872.680.08
   8.461.800.9111.170.2
PP5b20°0′0.4″88°0′1.8″2.130.490.973.590.11
   2.470.950.153.570.2
PP6b18°59′44.8″85°30′4.8″4.480.490.995.960.2
   10.681.080.5112.270.8
PP7b16°59′29.7″83°30′3″3.870.481.135.480.17
   4.350.960.255.560.2
PP8b15°0′20″81°29′47″0.170.470.841.480.11
   7.801.550.8410.190.2
PP9b11°59′37.3″81°0′9.8″0.630.280.261.170.21
   9.441.671.0612.170.50

4. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Materials and Methods
  6. 4. Results and Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[6] Results of nitrate, ammonium and urea uptakes for both the seasons at nine stations are shown in Table 1. Using the average Redfield ratio (C/N) of 6.6, we convert the total nitrogen uptake to carbon uptake; the total productivity in the BOB varied from 90 to 870 mgC m−2 d−1 during Sep–Oct. 2002, with an average value of 316 mg C m−2 d−1. The total productivity during Apr–May 2003 varied from 154 to 975 mg C m−2 d−1 with an average of 575 mg C m−2 d−1. Also, the new production during Apr–May 2003 (overall average ∼433 with coastal average of 552 and open ocean average of 284 mg C m−2 d−1) was higher than that during Sep–Oct 2002 (overall average ∼318 with coastal stations averaging ∼281 and open ocean ∼364 mg C m−2 d−1). These values are comparable to the new production off India, 400 ± 160 mg C m−2 d−1 reported for the Arabian Sea [Sambrotto, 2001]. There is a very significant correlation between new (y) and total production (x): y = 0.88 x − 0.89 (coefficient of determination, r2 = 0.99) in BOB. The relationship remains significant (r2 = 0.96) with a similar slope, when ammonium concentration from zooplankton regeneration is considered for uptake calculation during Sep–Oct 2002 (Figure 2). Data for the Arabian Sea [Watts and Owens, 1999] also show such a correlation, but with some scatter: y = 0.33 x − 0.36 (r2 = 0.86) (Figure 2). However, this relationship becomes somewhat better (y = 0.46 x − 0.43; r2 = 0.93) when conservative estimates (i.e., no ambient ammonium) were considered. During this calculation the average ammonium concentration was taken as 0.13 μM [Woodward et al., 1999]. The significance of this linear relationship lies in the quantification of new production from satellite data on total productivity. In both the seas, the x-intercept, i.e., minimum amount of regenerated production in the total absence of extraneous nitrate supply is ∼1 mmol N m−2 d−1 (equivalently ∼80 mg C m−2 d−1).

image

Figure 2. Relationship between total and new production. Left panel represents the Arabian Sea data from Watts and Owens [1999] where filled circles are the original data and open circles are the re-calculated conservative estimates. In the right panel (present study), rectangles are for Sep–Oct 2002, triangles for Apr–May 2003 and circles are for Sep–Oct 2002 after taking into account the ambient ammonium estimated from zooplankton biomass.

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[7] The possible sources of nutrient for higher new production in the Bay could be (i) the supply of nutrients from the adjacent land due to the river discharge [Ittekkot et al., 1991]. However, to quantify the riverine contribution to the nutrient pool of the BOB river borne nutrient data is needed. The nitrogen flux into the BOB was indirectly calculated by Kumar et al. [1996] based on the global flux of fresh water (37.4*1012 m3 yr−1 [Martin and Whitfield, 1983]) and nitrogen (50*1012 gN yr−1 [Duce et al., 1991]) by world rivers to ocean, to be 2.17*1012gN yr−1. If the fresh water from the Indian rivers is assumed to spread over total area of BOB (2.2*1012 m2), the above influx works out to be 2.68 mgN m−2 d−1. (ii) The contribution to the nitrogen pool of BOB through wet deposition of nitrogenous aerosols is reported to be around 1.53 mg N m−2d−1 [Kumar et al., 1996] using an annual precipitation rate of 2 m over BOB [Tomczak and Godfrey, 1994]. Thus, the total (river + atmosphere) external input of nitrogen to the Bay is ∼4.21 mg N m−2 d−1. However, a higher estimate for the external nitrogenous nutrients to BOB was given by Schafer et al. [1993], where the fluvial input (particulate and dissolved) was estimated to be 5.2*1012 gN yr−1, amounting to 6.36 mgN m−2 d−1. Total atmospheric nitrogen input was 6.83 mgN m−2 d−1, the same order of magnitude as the fluvial input; thus the total extraneous nitrogen (fluvial + atmospheric) to the Bay is ∼13.19 mgN m−2 d−1 (∼0.94 mmol N m−2 d−1). Our results for the two seasons show an average new production of 4.7 mmol N m−2 d−1; therefore, the estimated nitrogen inputs from rivers and atmosphere can at best supply only ∼20% of the total required nitrogenous inputs. The rest (80%) of the required nitrogen has to come from the supply of nitrate from deeper waters. These nutrients could be brought to the surface by the cyclones or high speed winds, very frequent in the BOB during post monsoon (Sep–Dec), in varying amounts, depending on location, intensity and residence time. Data from India Meteorological Department show that 25 and 15% of the total cyclones in BOB occur during Sep–Oct and Apr–May respectively [Das, 1995]. The shallow nitracline in BOB [Madhupratap et al., 2003] might have allowed the nitrate to come up, especially when aided by churning due to cyclonic winds in the post monsoon season. Also, the stratification due to freshwater discharge reduces towards the south and is too weak to prevent nutrients from coming up [Vinaychandran et al., 2002]. During this study, wind speed of ∼20 m/s was encountered over the Bay (Qscat wind speed data; http://www.ssmi.com/qscat/qscat_browse.html). The higher average new production at coastal stations during Apr–May may be due to East India Coastal Current, a poleward current along the western boundary of BOB, active at north of 10°N, bringing cooler, more saline water with nutrients to the surface [Shetye et al., 1993]. This western boundary current of anticyclonic subtropical gyre, best developed during March–April, decays only by June, which includes the study period. Also, the typical vertical profiles of nitrate uptake (Figure 3) during Apr–May indicate significant uptake rates below 40 m, where nutrients are available in plenty (average ∼7 μM) along with light (euphotic zone ∼80 m). Such a condition could lead to photosynthetic rates several fold higher than that at sea surface [Pollehne et al., 1993]. A unique possibility of nutrient supply is sub-marine ground water discharge. Dowling et al. [2003] found an average nitrate concentration of 43 μM in the Bengal basin ground water, the main source for sub-marine ground water discharge into BOB. Further studies are required to quantify this.

image

Figure 3. Typical profiles of uptake rates and nitrate (PP9) during pre and post monsoon in the Bay of Bengal.

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5. Conclusion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Materials and Methods
  6. 4. Results and Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[8] Our results show that even moderately productive oceans such as BOB, may be capable of high new production, and thus can be efficient in removing the atmospheric CO2 on longer time scales. High new production could cause low surface pCO2, and higher organic carbon flux as observed by sediment traps. This study underscores the need for more 15N based new production measurements in different oceans hitherto considered low or moderately productive. The observed linear relationship between new and total production could be useful for making new production maps for the region from satellite data.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Materials and Methods
  6. 4. Results and Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

[9] We thank M. Madhupratap, S. Prasanna Kumar, M. Sudhakar and other NIO colleagues; the Department of Ocean Development for providing ship time for this work, funded by ISRO-GBP Department of Space, Government of India.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Materials and Methods
  6. 4. Results and Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information
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Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Study Area
  5. 3. Materials and Methods
  6. 4. Results and Discussion
  7. 5. Conclusion
  8. Acknowledgments
  9. References
  10. Supporting Information

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