Phytoplankton carbon biomass: Insights from the eastern Indian Ocean

Abstract

The study of phytoplankton carbon biomass is limited but has the potential to provide important insights into several oceanographic processes, such as the vertical carbon export and yield from the oceanic food web. In this study, we estimated the carbon biomass of the whole phytoplankton community in the eastern Indian Ocean (EIO) during spring of 2021 using flow cytometry and microscopy. We also examined the effect of physicochemical parameters on the spatial distribution of phytoplankton carbon biomass (phytoplankton-C). In the EIO, the range of phytoplankton-C was between 0.03 and 33.96 μg C L-1, with picophytoplankton (≤2 μm) accounting for 92.41 ± 8.95% and 89.06 ± 13.21% of the total phytoplankton-C in the surface and deep chlorophyll maximum (DCM) layer, respectively. Among various phytoplankton groups, Prochlorococcus and picoeukaryotes accounted for a major part of the phytoplankton-C in the EIO, contributing (57 ± 12%) and (25 ± 7%) to the depth-integrated carbon biomass, respectively. This was followed by Synechococcus (6 ± 4%), diatoms (6 ± 4%) and dinoflagellates (3 ± 1%). Phytoplankton-C exhibited significant variations in both horizontal and vertical distribution within the study area. Horizontally, the distribution of phytoplankton-C was markedly influenced by physical events in the EIO, such as freshwater inputs in the Bay of Bengal, Wyrtki Jet at the Equator and Southern Equatorial Current in the southern EIO. Vertically, different phytoplankton groups exhibited varying distribution patterns in carbon biomass with increasing depths, primarily due to their diverse responses to the vertical variation of irradiance and nutrients in the water column. The total phytoplankton-C in the surface layer (9.56 ± 3.01 μg C L-1) and the DCM layer (9.07 ± 3.75 μg C L-1) did not show a significant difference between each other (t-test, p > 0.1, n = 42), indicating that DCM didn't correspond to a phytoplankton-C maximum but rather a physiological adaptation of C: Chl a ratios of phytoplankton cells in the EIO. Therefore, vertical profiles of Chl a should be interpreted with caution when establishing their ecological significance in the study area. The carbon biomass of the entire phytoplankton community in the EIO was reported in this study, which could enhance our understanding of the contribution of different phytoplankton groups to the carbon pool and their role in the biogeochemical cycle in the EIO.