HR: 15:30h
AN: OS43A-07    [Abstracts]
TI: N-uptake and f-ratio Characteristics of the Indian Ocean
AU: * Prakash, S
EM: satyap@prl.res.in
AF: Physical Research Laboratory, Navrangpura, Ahmedabad 380 009 India, Ahmedabad, Guj 380 009, India
AU: Ramesh, R
EM: rramesh@prl.res.in
AF: Physical Research Laboratory, Navrangpura, Ahmedabad 380 009 India, Ahmedabad, Guj 380 009, India
AB: The Indian Ocean, an ideal laboratory for oceanographic studies, consists of three different biogeochemical provinces: the most productive Arabian Sea, Oligotrophic equatorial Indian Ocean and nutrient rich "HNLC" region in south. The present study investigates the N-uptake and f-ratio characteristics of these three provinces of the Indian Ocean and asses its potential role in the Global Carbon Cycle. Here we present ¹⁵N based results from the eastern Arabian Sea, equatorial Indian Ocean and the Southern Indian Ocean. The Arabian Sea, one of the most biologically productive regions of the world ocean, is driven by seasonally reversing Southwest and Northeast monsoons. Development of Noctiluca bloom during winter is well documented in literature but data available on the nitrogen uptake and f-ratios is limited. Our results of ¹⁵N based productivity from the Arabian Sea, in conjunction with earlier reported results, reveal consistent high column N-uptake and f-ratios over consecutive years during bloom conditions; mean N-uptake and f-ratio are $20.1 (± 4) mmol N m^-2 d^-1 and 0.86 (±0.06) respectively. Though N-uptake values are comparable with the values reported (23.2 mmol N m^-2 d^-1), f-ratios are significantly higher suggesting efficient utilization of available nitrate in the water column. Persistent high productivity and f-ratios over years during a bloom suggest more efficient and strong biological pump in the Arabian Sea compared to other regions of the world ocean. The increased production in the surface layer causes formation of the most severe oxygen minimum zone in this part of the world which may cause vigorous denitrification resulting in the formation of radiatively inert N₂ and a potent green house gas N₂O and thus can contribute significantly to the global warming. The Indian Sector of the Southern Ocean is traditionally regarded as a low productive area. Our measurements show that even though the productivity of this region is low, the f-ratio is moderately high (mean=0.49). The plot of total N-uptake (on x-axis) and nitrate uptake (on y-axis) shows very significant correlation between the two: y = (0.63 ± 0.06) x - (0.66 ± 0.42) (coefficient of determination, r² = 0.95). The slope of line of regression (0.63) suggests the maximum possible value of f-ratio for this zone. This signifies that a large part of productivity could get transported to deeper ocean and thus this area has the potential to play a significant role in atmospheric carbon sequestration.
DE: 4806 Carbon cycling (0428)
DE: 4855 Phytoplankton
DE: 4870 Stable isotopes (0454, 1041)
SC: Ocean Sciences [OS]
MN: 2007 Joint Assembly