Size-Fractionated Phytoplankton Biomass in Port Blair Bay, South Andaman Island: Spatial Variability and Environmental Control

Island bays show different physico-chemical characteristics due to their connection with open ocean water and small catchment area, and the same is expected with the biological forms. Phytoplankton size structure significantly influences the function of pelagic food webs and is controlled by the ambient nutrient concentrations. To quantify the relationship between these two in a tropical island bay and to understand the controlling factors, we analyzed the chlorophyll-a concentration by partitioning into three size classes (0.2—2.0 µm, 2.0—20.0 µm and > 20.0 µm) for two seasonal observations in the Andaman Islands. During winter, the nano-phytoplankton (2.0 – 20.0 µm) dominated, while in spring, micro-phytoplankton (> 20.0 µm) dominated. It was observed that the bay had abundant nutrients, but low chlorophyll a concentrations in winter, whereas the scenario reversed in spring. The phytoplankton community exhibited significant seasonal differences between the inner and outer bay areas. The inner bay was dominated by the nano-phytoplankton in winter and micro-phytoplankton in spring, while nano-phytoplankton and pico-phytoplankton dominated the outer bay. The micro-phytoplankton showed a significant seasonal variation, while the other two functional groups did not. During winter, the micro-phytoplankton showed a good correlation with the nutrient silicic acid, and the nanoplankton correlated with nitrite, nitrate, silicate and TN (Total Nitrogen) implying nano-phytoplankton’s reliance on most of the nutrients including organic nutrient. During spring, micro-phytoplankton correlated with the nutrient nitrate, and the nano-phytoplankton correlated with the organic nutrient total nitrogen (TN) implying its reliance on the organic nutrient.