Influence of eddies on phytoplankton composition in the Bay of Bengal

Abstract

Several cyclonic (CE) and anticyclonic (ACE) eddies form in the Bay of Bengal (BoB) and they expect to contain different proportion of nutrients due to divergence and convergence of waters respectively and may have impact on phytoplankton composition. In order to test this hypothesis, one CE and two ACE (north ACE (ACEN) and south ACE (ACES)) were sampled in the BoB during spring intermonsoon. Higher concentration of dissolved inorganic nitrate and lower dissolved inorganic phosphorus was observed in the upper 25 m of water column in the CE and contrasting to that was observed in the ACE regions. Silicate concentrations were above the limiting levels for diatoms growth in the study region. High dissolved organic nitrogen (DON) and phosphorus (DOP) were observed in the upper 10 m in the CE and 80 m in the ACEs. Due to lack of nitrate in the study region DON seems to support phytoplankton growth in the BoB. The phytoplankton groups were estimated using marker pigment composition and dominant groups were evaluated using pigment ratios. Occurrence of autotrophic dinoflagellates were observed only in the ACES, which was formed in the central eastern BoB. Premnesiophytes, chrysophytes were found only in ACEN which was formed in the northeastern shelf region. In addition to this, prochlorophytes and chlorophytes were also found in both ACEN and ACES. The concentration of fucoxanthin, marker pigment for diatoms, was low and confined to the subsurface waters in the CE and ACEN regions due to unavailability of nitrate and their inability to uptake DON. This study suggested that eddies bring different composition of phytoplankton from the region of their origin to the open sea resulting in variations in their diversity. This may have impact on food web dynamics in the BoB. More than 50 eddies form each year in the BoB and their impact on the taxonomic and size composition of phytoplankton and fluxes of carbon to depth needs further studies.