Lack of seasonality in phytoplankton standing stock (chlorophyll a) and production in the western Bay of Bengal

Abstract

The investigations in the western Bay of Bengal (BoB) during summer, winter and spring intermonsoon periods evidenced lack of pronounced seasonal variation in phytoplankton standing stock (chlorophyll a) and primary production. The supply of riverine materials to the coastal bay is witnessed by the presence of major nutrients (nitrate - NO3, phosphate - PO4 & silicate - SiO4) in the upper layers (up to 20meters) during monsoonal periods. However, it was not found to enhance the phytoplankton production rates to a large extent. The strong stratification due to enormous quantity of fresh water discharge, acts as a barrier to vertical transport of nutrients from the subsurface layers. The average primary production in the inshore and the offshore waters of the BoB were 350±225 and 251±177 mgCm-2 d-1 in summer, 252±210 and 231±150mgCm-2d-1 in winter and 308±120 and 303±95 mgCm-2d-1 in spring intermonsoon periods, respectively. Subsurface chlorophyll maxima (SCM) were observed at 20-50m in the spring intermonsoon, whereas in summer and winter, the chlorophyll maxima occurred in the upper layers of the euphotic zone (<20m). The vertical distribution of phytoplankton (>5µm) was primarily restricted by the pycnocline, and the bulk of phytoplankton assemblage existed within the surface. The seasonal change in cell numbers, however, did not coincide closely with the changes in the chlorophyll a concentration. Diatoms formed the major group (>85%) in the total phytoplankton abundance irrespective of seasons. During spring intermonsoon, large patches of Trichodesmium blooms were observed in the coastal as well as open waters of the western BoB.