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

We present the first measurements 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 values of particulate organic nitrogen (PON) in surface suspended matter of coastal and northern open BOB show mixing between continental inputs and marine sources. Dilution by continental material brought in by rivers leads to consistently lower δ15N, evident from the relationship between surface salinity and δ15N. δ15N values of surface PON of open ocean locations during both seasons, and also at coastal locations during premonsoon suggest nitrate from deeper waters to be a predominant source of nutrients for phytoplankton. The depth profiles of δ15N of PON during the premonsoon season at nine different locations indicate an increase in δ15N by a maximum of 2.8‰ between euphotic depth and 300 m. These changes are less than those observed in the far eastern Indian Ocean, indicating higher sinking rates of particles ballasted by aggregates of organic and mineral matter in BOB.