A temporal increase of ~5‰ in the average nitrogen isotopic composition (δ<sup>15</sup>N<sub>PON</sub>) of surface particulate organic nitrogen was observed in the open north-eastern Arabian Sea during January to late February-early March 2003, despite the presence of <i>T. erithraeum</i> (up to ~11%), a diazotroph that fixes atmospheric N<sub>2</sub>, in the latter period. Hydrographic conditions and residence time of nitrate in the water column suggested that this increase could be a combined effect of denitrification in the subsurface layer and inefficient utilization of nitrate entrained in the water column during January.