The phosphate levels of the major surface currents of the Indian Ocean

Abstract

The principal surface currents of the north Indian Ocean are much richer in phosphate (greater than 0.25 µg-atom/l) than those of the south Indian Ocean (less than 0.15 µg-atom/I). In summer large areas of the surface waters of the south-east Indian Ocean have a very low phosphate content (less than 0.10 µg-atom/l). These waters are by far the lowest in phosphate of the whole Indian Ocean. Their salinity-temperature- phosphate relations show that waters from two regions, the South Equatorial Current in the north and the high salinity belt around 30-35º S., contribute to their formation. Waters of this high salinity belt are carried northward into the low phosphate region by the West Australian Current in summer. These high-salinity waters most probably form by evaporation of an upper 50-m mixed layer of waters of the south-east Atlantic drifting eastward in the south Indian Ocean at an average rate of 15 cm per sec. In the eastern Indian Ocean north of 10ºS., surface phosphate levels in summer are governed by the circulation of the richer phosphate waters of the counter current. In winter the circulation of richer phosphate waters of the South-west Monsoon Current governs the phosphate level.