Suboxic waters of the eastern Arabian Sea shelter secondary chlorophyll maximum dominated by heterotrophic dinoflagellate Pronoctiluca spp. (order Noctilucales)

In the present study, we investigated the dinoflagellate assemblages in the upper water column (< 150-m depth), focusing on the suboxic waters of the eastern Arabian Sea (EAS) along 68°E from 8°N to 21°N during the southwest monsoon 2020 (SWM–2020). Dinoflagellate abundance was higher in the upper water column (0–80-m depth, mean ± SD = 411 ± 903 cells L⁻¹) compared to deeper waters (80–150-m depth, mean ± SD = 128 ± 216 cells L⁻¹). Among 11 identified taxonomic dinoflagellate orders, Peridinales were predominant in the upper waters column (71%, mean ± SD = 285 ± 858 cells L⁻¹). Noctilucales, particularly Pronoctiluca spp., dominated the deeper water column (78%, mean ± SD = 99 ± 223 cells L⁻¹), especially at the southern stations (8–14°N, mean ± SD = 158 ± 270 cells L⁻¹). During SWM–2020, a strong vertical gradient in Pronoctiluca with increased abundance in suboxic, colder waters (< 0.05 mL L⁻¹, < 20 °C) coincided with the secondary chlorophyll maximum layers (12°N, SCMLs ~ 145 m, maximum 832 cells L⁻¹). To compare this observation, samples were taken at 13°N during the winter monsoon (WM-2023), when the SCML peak was prominent (0.3 µg L⁻¹) in suboxic waters. The results revealed an increased abundance of Pronoctiluca close to the SCML depth (~ 117 m) during WM. The canonical correspondence analysis revealed a positive correlation between SCML and Pronoctiluca, suggesting that Pronoctiluca relies on prey, i.e. low-light-adapted smaller phytoplankton. The higher abundance of Pronoctiluca compared to other oceanic regimes highlights the importance of assessing their crucial role in nutrient recycling and remineralisation within the suboxic environments of the EAS.