Seasonal variations in bio-optical properties of phytoplankton in the Arabian Sea

Data collected during three expeditions to the Arabian Sea are examined to characterise the changes in phytoplankton properties between the intermonso on and monsoon seasons. The results are used to elucidate the key changes t hat lead to the occurrence of phytoplankton blooms following the onset of t he monsoon winds. Contrary to expectations, changes in the assimilation num ber and the initial slope of the photosynthesis-light curve decreased in th e SW monsoon season, apparently in the wrong sense for them to account for the incidence of blooms in the Arabian Sea during this season. On the other hand, there were strong differences between the intermonsoon and monsoon v alues of the chlorophyll-specific absorption coefficient for the phytoplank ton, probably arising from seasonal changes in cell size and community comp osition. The sense of these changes was such as to increase substantially t he maximum quantum yield for photosynthesis during the monsoon season compa red with the intermonsoon. When these results are applied in a simple expre ssion for the daily, primary production of the water column, they can expla in the rapid accumulation of phytoplankton observed in the Arabian Sea foll owing the start of the monsoon season. Thus, the occurrence of seasonal blo oms in the Arabian Sea depends on combined variations in both the optical a nd photosynthetic characteristics of phytoplankton in the region. (C) 1999 Elsevier Science Ltd. All rights reserved.