Longitudinal patterns of spring-intermonsoon phytoplankton biomass, species compositions and size structure in the Bay of Bengal

Vertical distributions of phytoplankton biomass, compositions and size structure were investigated during the spring-intermonsoon (April 22 to 30) of 2010 along transact 10°N of the Bay of Bengal, northern Indian Ocean. Surface phytoplankton biomass (Chl a) was (0.065±0.009) μg/L, being greater than 80% of which was contributed by pico-phytoplankton (<3 μm). The Chl a concentration vertically increased to the maximal values at deep chlorophyll maximum (DCM) layer that shoaled eastwards from 75 to 40 m. The Chl a biomass at DCM layer generally varied between 0.2 and 0.4 μg/L, reaching the maximum of 0.56 μg/L with micro-phytoplankton cells (>20 μm) accounting for 58% and nano-(3-20 μm) or pico-cells for 15% and 27%, respectively. In particular, the cells concentration coupling well with phosphate level was observed at middle layer (75-100 m) of 87° to 89°E, dominated by micro-cells diatoms (e.g., Chaetoceros atlanticus v. neapolitana, Chaetoceros femur and Pseudonitzschia sp.) and cyanobacteria (i.e., Trichodesmium hildebrandtii), with the cells concentration reached as high as 4.0×10⁴ and 4.3×10⁴ cells/L. At the rest of the transact however, dinoflagellates (e.g., Amphisdinium carterae and Prorocentrum sp.) were the dominant species, with the cells concentration varying from 0.3×10³ to 6.8×10³ cells/L. Our results also indicate that the regulation of large cells (micro-, nano-) on phytoplankton biomass merely occurred at DCM layer of the Bay.