The influence of the south-west monsoon upon the nutrient biogeochemistry of the Arabian Sea

Variations in the nutrient concentrations were studied during two cruises t o the Arabian Sea. The situation towards the end of the southwest monsoon s eason (September/October 1994) was compared with the inter-monsoonal season during November and December 1994. Underway surface transects showed the i nfluence of an upwelling system during the first cruise with deep, colder, nutrient-rich water being advected into the surface mixed layer. During the southwesterly monsoon there was an area of coastal Ekman upwelling, bringi ng colder water (24.2 degrees C) into the surface waters of the coastal mar gin. Further offshore at about 350 km there was an area of Ekman upwelling, as a result of wind-stress curl, north of the Findlater Jet axis; this are a also had cooler surface water (24.6 degrees C). Further offshore ( >1000 km) the average surface temperatures increased to >27 degrees C. These wate rs were oligotrophic with no evidence of the upwelling effects observed fur ther inshore. In the upwelling regions nutrient concentrations in the close inshore coastal zone were elevated (NO3 = 18 mu moll(-1), PO4 = 1.48 mu mo ll(-1)); higher concentrations also were measured at the region of offshore upwelling off the shelf, with a maximum nitrate concentration of 12.5 mu m oll(-1) and a maximum phosphate concentration of 1.2 mu moll(-1). Nitrate a nd phosphate concentrations decreased with increasing distance offshore to the oligotrophic waters beyond 1400 km, where typical nitrate concentration s were 35.0 nmoll(-1) (0.035 mu moll(-1))in the surface mixed layer. A CTD section from the coastal shelf, to 1650 km offshore to the oligotrophic wat ers, clearly showed that during the monsoon season, upwelling is one of the major influences upon the nutrient concentrations in the surface waters of the Arabian Sea off the coast of Oman. Productivity of the water column wa s enhanced to a distance of over 800 km offshore. During the intermonsoon p eriod a stable surface mixed layer was established, with a well-defined the rmocline and nitracline. Surface temperature was between 26.8 and 27.4 degr ees C for the entire transect from the coast to 1650 km offshore. Nitrate c oncentrations were typically between 2.0 and 0.4 mu moll(-1) for the transe ct, to about 1200 km where the waters became oligotrophic, and nitrate conc entrations were then typically 8-12 nmoll(-1). Ammonia concentrations for t he oligotrophic waters were typically 130 nmoll(-1), and are reported for t he first time in the Indian Ocean. The nitrogen/phosphorus (N/P) ratios sug gest that phytoplankton production was potentially nitrogen-limited in all the surface waters of the Arabian Sea, with the greatest nitrogen limitatio n during the intermonsoon period. (C) 1999 Elsevier Science Ltd. All rights reserved.