Comparisons with Other Results from The Northern Indian Ocean

Since it is generally assumed that there are problems with the trapping efficiency at water depths of approximately <1,200 m (Scholten et al., 2005; Yu et al., 2001), sediment trap results obtained from shallower water depths were excluded from the following discussion. A comparison of sediment trap results obtained from the northern Indian Ocean shows the highest annual mean POC/PIC ratio in the northern Bay of Bengal (1.75; Fig. 5). Within the Bay of Bengal, the POC/PIC ratios decrease from north to south whereas an a) 16

0 16

0 16

Si 4

300 600 900

Distance [km]

1200

0.00

300 600 900

Distance [km]

1200

Figure 4: (a) Concentrations of silicate averaged for the upper 20 m of the water column along the transect from the Arabian coast towards the central and southern Arabian Sea, 1,300 km offshore during the SW monsoon of 1995 (compare Fig. 1). Nutrient data have been obtained from the US JGOFS datacentre. The blue line shows the contribution of diatoms to the biomass of photoautotrophic plankton during the same time (Garrison et al., 2000). (b) Diatom biomass (blue line) and silicate concentration derived from a box model that is described in the text. (c) Diatom growth rates (red line) and zooplankton grazing rates (blue line) vs. silicate concentration (data are derived from the model) (For colour version, see Colour Plate Section).

Figure 4: (a) Concentrations of silicate averaged for the upper 20 m of the water column along the transect from the Arabian coast towards the central and southern Arabian Sea, 1,300 km offshore during the SW monsoon of 1995 (compare Fig. 1). Nutrient data have been obtained from the US JGOFS datacentre. The blue line shows the contribution of diatoms to the biomass of photoautotrophic plankton during the same time (Garrison et al., 2000). (b) Diatom biomass (blue line) and silicate concentration derived from a box model that is described in the text. (c) Diatom growth rates (red line) and zooplankton grazing rates (blue line) vs. silicate concentration (data are derived from the model) (For colour version, see Colour Plate Section).

Si 4

Table 2: Mean late NE and SW monsoon fluxes in the western Arabian Sea averaged for March (NE monsoon) and between July and September (SW monsoon)

Parameter

Late NE monsoon

SW monsoon

POC

9.15

14.41

PIC

6.38

17.38

Carbonate

53.16

144.80

Biogenic opal

28.48

74.79

Carbonate/biogenic opal

1.87

1.94

POC/PIC

1.43

0.83

Note: Units are 'mgm 2 day 1' except for the ratios that are dimensionless. PIC flux — Carbonate flux x 0.12.

Note: Units are 'mgm 2 day 1' except for the ratios that are dimensionless. PIC flux — Carbonate flux x 0.12.

POC/PIC

0.0 0.4 0.8 1.2 1.6 2.0 -1000 H-'-1—^—^—r"-1-'-1-'-1

POC/PIC

0.0 0.4 0.8 1.2 1.6 2.0 -1000 H-'-1—^—^—r"-1-'-1-'-1

Figure 5: Annual mean POC/PIC ratios vs. water depth. Black circles show results obtained from the US JGOFS sediment trap experiment performed in 1994/1995 (Honjo et al., 1999; Lee et al., 2000), grey circles reveal data derived from the Indo/German sediment trap program in the Arabian Sea (Fig. 1) and those obtained from the Bay of Bengal (Unger et al., 2003) are given in red. Vertical lines representing the mean (solid line) and the range (broken line) of POC/PIC ratios obtained from other sediment trap studies in the carbonate-dominated ocean (Klaas and Archer, 2002) (For colour version, see Colour Plate Section).

Figure 5: Annual mean POC/PIC ratios vs. water depth. Black circles show results obtained from the US JGOFS sediment trap experiment performed in 1994/1995 (Honjo et al., 1999; Lee et al., 2000), grey circles reveal data derived from the Indo/German sediment trap program in the Arabian Sea (Fig. 1) and those obtained from the Bay of Bengal (Unger et al., 2003) are given in red. Vertical lines representing the mean (solid line) and the range (broken line) of POC/PIC ratios obtained from other sediment trap studies in the carbonate-dominated ocean (Klaas and Archer, 2002) (For colour version, see Colour Plate Section).

increasing salinity in the same direction shows diminishing influence of freshwater in the Bay of Bengal (Fig. 1). Accordingly, it was suggested that river discharges, which are the main marine silica sources (Treguer et al., 1995), enhance the efficiency of the biological pump by supplying silica from terrestrial sources (Ittekkot et al., 1991).

During the sediment trap experiment in the northern Bay of Bengal, the sampling site was shifted by approximately 2° to the south between 1990 and 1992 because of logistical reasons (Unger et al., 2003). Results from these trap sites were used to fill gaps within the record from the northern site leading to a time series that covers approximately 7 years. The POC/PIC ratios derived from the combined time series shows an inter-annual variation of 716% exceeding those in the WAST (approximately 710%). In the northern Bay of Bengal, the highest annual mean POC/PIC ratio was determined in year 1988 (2.15) and the lowest in year 1997 (1.52). The southern oscillation index (SOI: http://www.cpc.ncep.noaa.gov) shows strong La Nina and El Nino conditions during the SW monsoon between (JuneSeptember) 1988 and 1997, respectively (Fig. 6). El Nino southern oscillation (ENSO) could, for example, affect the POC/PIC ratios in the northern Bay of Bengal via its influence on precipitation rates in the catchment area

Figure 6: Annual mean POC/PIC ratios obtained from the northern Bay of Bengal at 2,000 m water depth vs. southern oscillation index averaged for the SW monsoon periods of the same year. Sediment trap data are available for years 1988, 1989, 1990, 1991, 1992, 1994 and 1997 (Unger et al., 2003). The open circles indicate the years 1989 and 1994. The solid line represents the regression equation (POC/PIC — 0.15 x SOI+1.91) that excludes the years 1989 and 1994.

Figure 6: Annual mean POC/PIC ratios obtained from the northern Bay of Bengal at 2,000 m water depth vs. southern oscillation index averaged for the SW monsoon periods of the same year. Sediment trap data are available for years 1988, 1989, 1990, 1991, 1992, 1994 and 1997 (Unger et al., 2003). The open circles indicate the years 1989 and 1994. The solid line represents the regression equation (POC/PIC — 0.15 x SOI+1.91) that excludes the years 1989 and 1994.

of the Ganges-Brahmaputra (Ropelewski and Halpert, 1987) and consequently on the river discharges. Nevertheless, there are 2 years (1989 and 1994) that do not fit into the correlation between the POC/PIC ratios and the SOI (Fig. 6). These exceptions point to other factors of relevance masking the link between ENSO and the efficiency of the biological pump during some years in the Bay of Bengal.

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