Environmental influence on the picophytoplankton community structure in the central and northern Bay of Bengal

Abstract

The Bay of Bengal (BoB) is a low productive, oligotrophic ecosystem. The picophytoplankton (Pico; <3μm) play a significant role in the biogeochemistry of such ecosystems. The information on the Pico is exiguous from this Bay. Here, the seasonal and spatial variations in abundance and carbon biomass of the Pico groups were investigated (July’08 to April’09) in the central and northern BoB in relation to the environmental conditions. Surface samples were collected from twenty-two stations across the Chennai–Port Blair–Kolkata sector. The Synechococcus group was dominant during most of the year, both in abundance and biomass, with few exceptions when Prochlorococcus dominated the Pico community. The Synechococcus abundance and carbon biomass were higher in the nutrient-rich coastal regions and correlated negatively with salinity. The picoeukaryotes exhibited a distribution pattern similar to Synechococcus, although relatively lower in numbers and biomass. The Prochlorococcus abundance and biomass were higher in the open ocean regions of the central BoB (CBoB). The relatively lower abundance of Prochlorococcus in the northern BoB implies an influence of the freshwater influx, as depicted from the positive correlation with salinity. During the winter monsoon, a cyclonic storm coincided with a deep mixed layer characterized by high concentrations of nutrients and chlorophyll-a in the CBoB. The Pico abundance and biomass were relatively higher during this period, with the Prochlorococcus dominating the former and Synechococcus, the latter. The real-time observations of the surface distribution of Pico abundance and carbon biomass in the BoB revealed seasonal variations that were modulated by the episodic mesoscale features.