Roles of upwelling and riverine influx on the summer phytoplankton bloom in the Western Bay of Bengal

Abstract

The Bay of Bengal (BoB) is generally considered a low-productive region. However, several phytoplankton blooms (high Chlorophyll a) are frequently reported, including the one observed between Kakinada (KKD) and Visakhapatnam (VSKP) during the Southwest Monsoon (SWM) of 2019. Hydrographic observations along the two adjacent cross-shore sections of Visakhapatnam (VSKP) and Kakinada (KKD) revealed the presence of different water masses in these regions. VSKP transect was influenced by intense coastal upwelling that advected towards the slope and offshore area. In contrast, the coastal and slope locations of the KKD transect was dominantly influenced by the Godavari River input, while there was a warm-core eddy in the offshore region. There was an increase in the Chlorophyll a (Chl a) biomass in the coastal locations of KKD 4.37 ± 2.04 mg/m3) caused by the freshwater input, but the magnitude was substantially high in the coastal and slope regions of VSKP (8.52 ± 2.86 mg/m3) extending further towards offshore associated with upwelling and their advection processes. Column integrated Chl a concentration was 2–3​ folds higher in the offshore regions of VSKP due to the presence of subsurface Chl a Maxima (SSCM), while it was less prominant in KKD. Based on the vertical patterns of the Chl a, the present study could observe two less known patterns in the distribution of Chl a, such as (1) double Chl a maxima (DoCM) and (2) extended Chl a maxima (ECM). This study elaborates on the coastal and oceanic processes causing the change in the hydrography leading to the development of massive surface blooms and their distinct vertical patterns in the WBoB