Dissolved aluminium in the surface microlayer of the eastern Arabian Sea

Abstract

Measurements of dissolved aluminium (Al) in surface microlayer (SML) samples from the eastern Arabian Sea during the southwest (summer) and northwast (winter) monsoon periods have revealed much higher concentrations (23-657 nmol kg sup(-1)) than in the sub-surface layer (SSL; collected at 1 m depth). Higher concentrations observed during the summer monsoon and directional trends in the distribution, with conentration maxima situated away from the influence of fluvial discharge, are best explainEd. by the atmospheric deposition of mineral dust aerosal and its subsequent dissolution in the surface seawater. The low Al concentrations in the SSL (11-296 nmol kg sup(-1)) varied in line with those in the SML. However, Al was significantly higher there than in the rest of the water column (up to 3000 m), indicating that much of the dissolved aerosal Al was removed in the upper few metres of the water column. The non-nutrient-type behaviour of dissolved Al did not support a mechanism of Al removal by biological uptake. High Al levels in the SML observed during the summer monsoon were associated with low primary productivity. The considerably lower values observed the winter monsoon, a priod of higher primary productivity in the Arabian Sea, could be due to removeal from solution by adsorption onto biogenic particles