Phytoplankton communities distribution along a physical gradient in the eastern Indian Ocean based on their pigments and absorption properties

Phytoplankton pigments and absorption properties were measured along 110° E in the southeast Indian Ocean during a research voyage carried out on R/V Investigator in May–June 2019. The data set was collected along a 3300 km transect starting from mesotrophic conditions around 40° S (chlorophyll concentration of about 0.5 mg m⁻³) to oligotrophic conditions (0.04 mg m⁻³) near 10° S. A cluster analysis was applied to phytoplankton absorption data, in which the absorption-based clusters serve as a reference for identifying different phytoplankton pigment assemblages and their depth in the water column. The resulting clusters reflect variations in phytoplankton pigment compositions and degrees of pigment packaging, categorised into distinct groups: mesotrophic waters, surface oligotrophic waters, deeper waters near the deep chlorophyll maxima and low-chlorophyll waters at depths exceeding 100 m. Our results confirm that pigment composition has a significant impact on the absorption spectra for given chlorophyll concentrations. In oligotrophic conditions, the proportions of photosynthetic and photoprotective pigments show large variations along depth. We also found that the phytoplankton absorption coefficient in the blue is lower than predicted by relationships previously established with Chl-a. When pico- and micro-phytoplankton are generally expected to vary in opposite ways from oligo-to meso-trophic waters, with the contribution of nano-phytoplankton remaining quite stable, we here found that pico- and nano-phytoplankton were the varying fractions along the transect, with the contribution of micro-phytoplankton remaining stable and low (about 10%). Our results support the use of optical properties, which are linked to pigment composition, cell size, and intracellular pigment concentration to study phytoplankton communities across varied oceanographic regimes.