Seasonality in the hydrography and biogeochemistry across the Pakistan margin of the NE Arabian Sea

A series of cruises were conducted across the Pakistan continental shelf-edge and upper slope in the NE Arabian Sea between March and October 2003 coinciding with spring inter-monsoon and late-to-post SW monsoon conditions. Water column hydrographic profiling together with nutrient chemistry show the upper boundary of the oxygen minimum zone (OMZ) rose during the SW monsoon by approximately 80 m leading to an inundation of a substantial area of the continental shelf with low-oxygen water while the lower boundary showed only slight movement. The core of the OMZ, identified by the presence of a secondary nitrite maximum and high nitrate deficits, rose by approximately 200 m. Nitrite concentrations within the secondary nitrite maximum approximately doubled over the period of study but this was not matched with an increase in the nitrate deficit. Dissolved oxygen concentrations in the core of the OMZ showed little or no substantial change during the course of the study. Hydrographic profiling showed greatest change in water masses around the 200–600 m depth range probably resulting from the northward movement of the West India Undercurrent.