Plankton as indicators of coastal water bodies during south-west to north-east monsoon transition at Kalpakkam

In the coastal waters of Kalpakkam, a water body characterized by high salinity, rich nutrients and low temperature (southern-sub surface water SSW) was seen during the early half of September when the pole-ward current prevailed. This was replaced by low saline, low nutrient and relatively high temperature water (northern-surface water NSW) during early October when the current was equator-ward. This reversal in current direction is induced by the north-east monsoon winds during the SW to NE monsoon transition. It is also observed that SSW in the Kalpakkam coast during the later half of September was characterized by the presence of pennate diatoms Nitzschia closterium, Asterionella glacialis and Thalassionema nitzschiodes. However, NSW in this coast in early October is characterized by the presence of centric diatom Skeletonema costatum. The two water bodies also have a characteristic Zooplankton population, the SSW is rich in meroplankton and the NSW is poor in meroplankton. Thus, the pennate diatom N. closterium, A. glacialis and T. nitzschiodes would be considered as indicators of SSW and centric diatom S. costatum as the indicator of NSW.