Characterization of phytoplankton size-structure based productivity, pigment complexes (HPLC/CHEMTAX) and species composition in the Cochin estuary (southwest coast of India): special emphasis on diatoms

•First-time comprehensive study regarding the size-structure based productivity and pigment characterization in the Cochin estuary, located on the southwest coast of India.•Addressed the validation of diatoms, the predominant phytoplankton taxa, in the Cochin estuary using multivariate analytical approaches•The study revealed the predominance of the nanophytoplankton community (in terms of chlorophyll a biomass and primary production) throughout the study region, irrespective of seasons.