Ecological Indicators (Oct 2023)
Variability of the phytoplankton biomass in the southwestern Bay of Bengal from 2003 to 2018
Abstract
The Bay of Bengal (BoB) has distinctive hydrological characteristics, and various physical mechanisms strongly influence the growth of phytoplankton in this region. In this work, we investigated the monthly and interannual variations of phytoplankton biomass (PB) in the southwestern (SW) BoB (78–88°E, 0–15°N) utilizing satellite data from 2003 to 2018. Bivariate wavelet coherence (BWC) and multiple wavelet coherence (MWC) analyses were conducted to assess the temporal fluctuations in various physical oceanographic parameters. The findings revealed that wind speed (WSP) and sea surface temperature (SST) were the main drivers for the monthly variations of chlorophyll-a (Chl-a). The southwest monsoon promoted the formation of upwelling by offshore Ekman transport via the southwest monsoon current (SMC) and the upward Ekman pumping induced by the strong positive wind stress curl, which brought nutrient-rich, cold waters from the subsurface to the surface, stimulating phytoplankton growth. The BWC analysis results identified WSP as the most effective individual factor accounting for the interannual fluctuations in Chl-a concentrations. The MWC analysis results unraveled that the combinations of WSP–SST, WSP–Niño 3.4, WSP–EPV, and WSP–DMI exhibited the strongest relationships with the changes in Chl-a in the SW BoB. The PB in 2015 demonstrated abnormally low values. The El Niño event in 2015 substantially weakened the southwest monsoon and current, possibly weakening the Ekman transport on the southern coast of Sri Lanka and India. Moreover, an abnormally intensified anticyclonic eddy occurred in this region. These processes disrupted the vertical exchange of nutrients, obstructing the vertical transport of nutrient-enriched subsurface waters and leading to abnormally decreased levels of Chl-a in 2015.
