What controls the initiation of algal bloom during the winter monsoon in the Arabian Sea??

A winter monsoon cruise was undertaken in the northern Arabian Sea to understand the bio-physical interaction responsible for the occurrence of phytoplankton bloom in the region. The observation shows strong convective mixing with a dense and deeper mixed layer (MLD: 100-140 m) and well-oxygenated upper water column (>95% saturation). The chlorophyll concentration was low (0.1 -0.3 µg/l) despite having ample nitrate (~2.5 µM) in the surface layer. The region, however, was deprived of micro phytoplankton, especially diatomic species and Noctiluca Scintillans, and was dominated by the picophytoplankton (77%-85%). The mean Si/N ratio in the upper 100 m was 0.72 indicating “Silicate stressed” condition for the proliferation of diatoms. Even a deeper mixed layer could not penetrate into the silicicline (~150m) which was deeper than the nitracline (~110m). In addition, the euphotic depth (~49m) was much shallower than the mixed layer depth suggesting the Sverdrup critical depth limitation in the northern Arabian Sea. We further show that the bloom initiated only when the mixed layer shoals towards the euphotic zone. Our observations suggest that two primary factors, the stoichiometric ratio of nutrients, especially Si/N ratio, in the mixed layer and re-stratification of the upper water column, govern the phytoplankton blooming in the northern Arabian Sea during the later winter monsoon.