Formation of algal bloom in the northern Arabian Sea deep waters during January-March: A study using pooled in situ and satellite data

Abstract

The formation of winter bloom (January-March) was studied in deep waters of the northeastern Arabian Sea (NAS) during 2003 and 2004. Six ship cruises were undertaken at different phases of the bloom to examine its formation. Along with physical and biological observations, optical parameters including profiles of upwelling and downwelling irradiance were collected using a Satlantic underwater radiometer and photosynthetically active radiation (PAR) was calculated using prosoft software. Satellite data sets for chlorophyll concentration, aerosol optical depth (AOD), PAR from the Oceansat-I Ocean Colour Monitor (OCM), and sea winds vectors from the QuikSCAT scatterometer were also used to study the winter algal bloom formation. This study attempts to explain the probable mechanisms that activate the bloom and its dynamics, particularly those due to the influence of wind. The study also presents an intercomparison of relevant parameters during different phases of the event, from the late winter to early spring intermonsoon season. The causes of winter bloom reported by other investigators have been verified with the use of satellite data. An additional observation was attempted that strengthened the explanation of convective mixing and resulting intensification of the bloom. The higher wind speed in February-March 2003 compared to that in 2004 caused a higher aerosol concentration in the overlying atmosphere, thus leading to reduced shortwave solar radiation and PAR over the surface in 2003. This caused a cooling effect in the NAS deep waters. This cooling is thought to be supplementary to the already reported winter cooling, and to cooling due to evaporation. As a result, a substantial increase in surface seawater density was observed, consequently enhancing convective action due to densification and nutrient concentration. The result is that the bloom was homogeneous in space and had a larger concentration during March 2003 than March 2004.