Variability in phytoplankton shape dominance in marine pelagic systems: prevalence of different adaptive strategies in the eastern Arabian Sea during the winter monsoon

Phytoplankton communities from pelagic systems were assessed to explore the potential of using commonly used traits (such as cell geometry and taxa) as ecological function indicators from the data generated during the winter monsoon in the eastern Arabian Sea (AS). Altogether, data from two oceanic, i.e., convective mixing influenced non-oligotrophic northeastern-AS (NEAS-O) and Rossby wave-influenced oligotrophic southeastern-AS (SEAS-O) and one coastal (NEAS-C) cruises were utilized to decipher the ecological inferences. Overall phytoplankton shapes showed a high level of redundancy by selecting only a few dominant shapes (5 of 22 shapes), though taxonomic diversity was rich (164 species). The taxonomic and morphological approach adopted revealed high species and shape diversity in NEAS-O than in high-abundance NEAS-C and low-abundance SEAS-O. Also, the shape diversity and dominant shapes (cylinder, elliptic-prism, and prism-on-parallelogram) remained the same in oceans than NEAS-C where combined (cylinder + 2 half-sphere) and simple (elliptic-prism) shapes dominated. Additionally, the Rossby-wave front and its reminiscence in SEAS-O and sea-surface-temperature fronts in NEAS-C favored simple and combine shaped phytoplankton, respectively. The morphological properties assessment revealed that the dominant shapes adapted the strategy to conserve the optimal surface-to-volume ratio (S:V) irrespective of changes in greatest-axial-linear-dimension (GALD) in NEAS-O and SEAS-O but not in NEAS-C. However, the dominant shapes in the NEAS-O and SEAS-O opted for high S:V with low GALD and low S:V with high GALD, respectively, while high S:V with no relation with GALD in NEAS-C suggests the prevalence of different adaptive strategies to cope with the respective hydrographic conditions, particularly nutrient availability.