Seasonality in subsurface chlorophyll maxima in the Arabian Sea: detection by IRS-P4/OCM and implication of it to primary productivity

Abstract

Arabian Sea is highly influenced by monsoon systems like SW monsoon (June-September) and NE monsoon (December-February). This affects distribution pattern of phytoplankton, availability of nutrients and changing temperature specially during winter cooling period (February-March). These and other conditions like quality and quantity of light influence phytoplankton (type and concentration) in the Arabian Sea. In our study we have observed monsoon related peaks in phytoplankton and chlorophyll a in the Arabian Sea. These chlorophyll a (Chl a) concentrations detected by OCM sensor onboard IRS-P4 satellite is helping us to work out seasonality of phytoplankton in the Arabian Sea, which is of the great importance in the field of biology and biogeochemistry of this region. However, during these study subsurface chlorophyll a maxima (SCM) was observed as characteristics feature of chlorophyll a distribution in the Arabian Sea. The subsurface chlorophyll a maxima varies from 30m to 55m in the Arabian Sea during various seasons. During November at St. 1 surface chlorophyll a was 1.503 mgm^-3 and subsurface chlorophyll maxima was 12.692 mgm^-3. Similarly, at St. 13 surface chlorophyll a was 0.584 mgm^-3 and surface chlorophyll maxima was 8.517 mgm^-3. During upwelling, nutrients remained unused at sub surface due to shortage of light which may lead to subsurface blooms, detection of which is critical for precise estimation of chlorophyll a from ocean colour sensor. During our 5 year study (covering all the seasons) in northeastern (NE) Arabian Sea, we have observed more than 50 % stations were with subsurface chlorophyll a maxima where chlorophyll a was approximately 10 times higher compared to surface value. The high chlorophyll some time detected by OCM is mainly because of detection of subsurface chlorophyll maxima by the satellite but may not actually sampled during ship studies. The satellite penetration depth (ze), subsurface chlorophyll maxima depth, OCM derived chlorophyll a for different seasons in NE Arabian Sea is presented and discussed in this paper.