Monsoon driven changes in phytoplankton populations in the eastern Arabian Sea as revealed by microscopy and HPLC pigment analysis

Massive outbreaks of Noctiluca scintillans blooms in the Arabian Sea due to spread of hypoxia

“…Two ml aliquots of the concentrate were carefully added to five purged and five control bottles with minimal exposure to atmospheric O 2 . In addition, 2 ml aliquots of the concentrate were filtered in triplicate onto GF/F filters, frozen in liquid nitrogen and transported to the shore for estimation of biomass as Chl a by high-performance liquid chromatography (HPLC) 7,24 .…”

“…In 2011, we conducted grazing experiments on board that estimated clearance rates of 200 to 800 ml per hour for the salp Pegea confoederata, suggesting that high concentrations of salps (up to hundreds of zooids per m 2 ) could rapidly consume dense aggregations of N. scintillans. Using pigments 7,24 as a measure of food consumed in the first 3 h, we observed that single salps can remove on average 71% of Chl a and 78% of Chl b from seawater dominated by N. scintillans cells with the deposition of large pellets ( Supplementary Fig. 3a).…”

“…Two ml of this concentrate was added to each of the 10 bottles containing FSW, followed by the NaH 14 CO 3 solution. Samples were also collected for estimation of the biomass (as Chl a) of the o200 mm fraction by HPLC 7,24 . All bottles were incubated on deck (at B30% of ambient direct sunlight using neutral density screening, measured with a LI-190 Quantum sensor) in running seawater to approximate in situ conditions.…”

“…Live salps were collected separately from which single adult salps were carefully picked, transferred into FSW for about 2 h to empty their guts and then into the buckets containing the N. scintillans bloom. After a 4-h incubation period when the buckets were incubated in running seawater in the dark, 50 ml samples were collected in triplicate for post incubation pigment analysis by HPLC 7,24 . In addition, the faecal pellets that were formed were carefully picked and placed in petri dishes.…”

“…Most contemporary estimates of phytoplankton biomass (estimated as Chlorophyll a, Chl a), productivity and carbon cycling have come from satellite-based observations 4,5 or coupled biophysical models 6 . Beginning in 2003, our field studies have provided large datasets of the taxonomic structure 4,5,7 of winter phytoplankton communities which show a drastic shift from diatom-dominated blooms to thick and widespread blooms composed primarily of the green dinoflagellate, N. scintillans. Unlike the red form reported mostly in temperate coastal waters worldwide 8 , this Arabian Sea N. scintillans is green because of its large endosymbiotic population of free-swimming cells of Pedinomonas noctilucae (Class Prasinophyceae, Division Chlorophyta) 9,10 (Fig.…”