Delineating morphological traits of oceanic micro-phytoplankton as potential ecological indicators

The micro-phytoplankton (>20 μm) adaptations and resilience were assessed using morphological traits (shape, surface-to-volume ratio; S:V, and greatest-axial-linear-dimension; GALD) from sea-surface and different SCML-depths (shallow:20-50 m, intermediate:50-100 m, and deep:100-140 m) across different bioregions of Indian Ocean. The dominant simple elongated phytoplankton-geometric-shapes (PGSs) and morphological traits showed distinct north-south distribution and varied with light and nutrient availability. Further, SCML and corresponding sea-surface PGS will be similar or dissimilar if the former is located within or deeper than mixed-layer depth. Also, simple and complex PGS contribution gradually decreases and increases with increasing depth. Additionally, shallow SCML-PGS showed low-S:V and high-GALD while vice-versa for intermediate/deep SCML-PGS due to phenotypic plasticity behavior. Overall, only simple-PGS (cylinder, elliptic-prism, and prism-on-parallelogram) showed strong adaptive behavior through phenotypic plasticity and were highlighted as potential ecological tracers to address ecological impact of oceanographic processes (including coastal eutrophication, and aerosol deposition) linked to nutrient and light availability in predicted ocean change scenarios.