Natural isotopic composition of nitrogen in suspended particulate matter in the Bay of Bengal

Abstract. We present the first measurement of nitrogen isotopic composition (δ¹⁵N) in suspended particulate matter (SPM) of the surface Bay of Bengal (BOB) at 24 different locations during pre- (April–May 2003) and post- (September–October 2002) monsoon seasons. The δ¹⁵N of particulate organic nitrogen (PON) in surface suspended matter of coastal as well as northern open BOB shows signatures of a two end-member mixing between continental inputs and marine sources. Dilution by the organic and detrital continental material brought in by rivers leads to consistently lower δ¹⁵N, evident from the relationship between surface salinity and δ¹⁵N. δ¹⁵N of surface PON of open ocean locations during both seasons, and also at coastal locations during pre-monsoon suggest the nitrate from deeper waters as a predominant source of nutrient for planktons. The depth profiles of δ¹⁵N of SPM during pre-monsoon season at nine different locations are also presented. These indicate an increase in δ¹⁵N by a maximum of 2.8‰ between euphotic depth and 300 m, which is lower than that observed in the eastern Indian Ocean, indicating the role of higher sinking rates of particles ballasted by aggregates of organic and mineral matter in BOB.