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Revisiting the Noctiluca scintillans paradox in northern Arabian Sea

In 2015, a Noctiluca scintillans bloom and associated water column properties were studied in the northern Arabian Sea. Our observations showed photic depth limited to 30 m with uniform oxygen concentration of ~223 M. In general, the dissolved oxygen ranged between 180 and 223 M within the top 80 m indicating saturated mixed layer. Chlorophyll a varied between 0.24 and 2.4 mg m within the core of the bloom and <0.1 mg m outside. We further examined Argo oxygen data from 2006 to 2013 to delineate possible surface water hypoxia associated with the initiation of N. scintillans bloom. Oxygen profiles from Argo data suggest oxic upper water column (~ 50 m) with strong seasonal shoaling. Our results do not indicate any mixed layer oxygen depletion associated with the N. scintillans bloom or any evidence of surface water hypoxia in the past. However, examination of silicate/nitrate (Si/N) climatology suggests strong longitudinal variation. The silicate in the surface waters in the northwestern Arabian Sea is depleted much earlier (Si/N < 1) compared with the eastern part, resulting in a strong spatial trend. This presumably facilitates easy community transition to a N. scintillans bloom. This is supported by the heterotrophic nature of the species which, under detectable and belowdetectable nitrate conditions, gives it a competitive advantage over other phytoplankton communities.