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

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 O 2 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.