Effect of monsoonal perturbations on the occurrence of phytoplankton blooms in a tropical bay

Abstract

In this study, the influence of intraseasonal variations in rainfall and the resultant freshwater flux (monsoon perturbations) on phytoplankton bloom dynamics were evaluated by quantifying live phytoplankton at a fixed station (Dona Paula Bay, west coast of India) every day during the 2008 southwest monsoon season (June-September). Pre-processing of the sample for live phytoplankton analysis using FlowCAM through fluorescence-based quantification of phytoplankton size fractions is described for the first time. Six diatom blooms of autochthonous origin were encountered during the observations, coinciding with nutrient enrichment and a lull in river runoff. The blooms observed at the beginning (1st bloom) and the end of the season (6th bloom) were dominated by nano- and picophytoplankton, and the intervening blooms by microphytoplankton. All blooms coincided with flood tide or high tide under optimal salinity (>15) and light (depth of light penetration: >50 cm; solar radiation: 30-70 mW cm^-2) conditions following heavy rainfall and nutrient flux. Termination of blooms coincided with nitrate exhaustion. Dinoflagellate (2nd dominant group) abundance was positively associated with depth of light penetration >100 cm and low nutrient concentrations. Bloom duration of 1-6 d was recorded, indicating that such events are widespread and can significantly influence the system’s metabolic balance. The average net photic zone production of the season was positive (0.11 ± 0.67 g O₂ m^-2 d^-1), and a quarter of the monsoon season was net autotrophic. Although bloom production was underutilized (up to 63%), much of the system’s carbon requirement (up to 70%) was met by allochthonous supply.