Spatio-temporal variation of microphytoplankton in the upwelling system of the south-eastern Arabian Sea during the summer monsoon of 2009

Abstract

The phytoplankton standing crop was assessed in detail along the South Eastern Arabian Sea (SEAS) during the different phases of coastal upwelling in 2009. During phase 1 intense upwelling was observed along the southern transects(8&deg;N and 8.5&deg;N). The maximum chlorophyll <i>a</i> concentration (22.7 mg m^-3) was observedin the coastal waters off Thiruvananthapuram (8.5&deg;N). Further north there was no signature of upwelling, with extensive <i> Trichodesmium erythraeum</i> blooms. Diatoms dominated in these upwelling regions with thecentric diatom <i> Chaetoceros curvisetus</i> being the dominant species along the 8&deg;N transect. Along the 8.5&deg;N transect pennate diatoms like <i> Nitzschia seriata</i> and <i>Pseudo-nitzschia</i> sp. dominated. Duringphase 2, upwelling of varying intensity was observed throughout the study area with maximum chlorophyll <i>a</i> concentrations along the 9&deg;N transect (25 mg m^-3) with <i> Chaetoceros curvisetus</i> as the dominantphytoplankton. Along the 8.5&deg;N transect pennate diatoms during phase 1 were replaced by centric diatoms like <i>Chaetoceros</i> sp. The presence of solitary pennate diatoms <i>Amphora</i> sp. and <i> Navicula</i> sp.were significant in the waters off Kochi. Upwelling was waning during phase 3 and was confined to the coastal waters of the southern transects with the highest chlorophyll <i>a</i> concentration of 11.2 mg m^-3. Along withdiatoms, dinoflagellate cell densities increased in phases 2 and 3. In the northern transects (9&deg;N and 10&deg;N) the proportion of dinoflagellates was comparatively higher and was represented mainly by <i>Protoperidinium</i> spp