Monsoon driven interplay between phytoplankton size classes and trophic status in a tropical coastal lagoon.

Tropical coastal lagoons influenced by monsoonal hydrology experience seasonal shifts in freshwater influx that significantly alter their trophic state. However, the response of phytoplankton size classes (PSC), Pico-, Nano-, and Micro-phytoplankton to such variability remains poorly understood. This study investigates the relationship between PSC distribution and the Trophic Level Index (TLI) in the Chilika Lagoon, India, using comprehensive monthly data from 2018 to 2019 across a salinity continuum. Phytoplankton size classes were characterized using HPLC aided pigment chemotaxonomy, which enabled the reliable detection of Nano- and Pico-phytoplankton often overlooked by conventional microscopy. The lagoon shifted from eutrophic (TLI = 4.61) in 2018 to mesotrophic (TLI = 3.98) in 2019, driven by reduced nutrient concentrations and increased turbidity. Across all seasons and sectors, Pico-phytoplankton dominated (66-76 %), even under eutrophic conditions, indicating their adaptive advantage under fluctuating nutrient and light regimes. Micro-phytoplankton showed seasonal increases, particularly during the monsoon in 2019, likely driven by silicate availability and turbidity. Redundancy Analysis (RDA) revealed strong spatiotemporal associations of PSC with environmental variables, Pico-phytoplankton with salinity, transparency, and nutrients; Micro-phytoplankton with turbidity and total suspended matter. The trophodynamic implications suggest that Pico-phytoplankton dominance during pre- and post-monsoon seasons may support a microbial food web, while the increased presence of Micro-phytoplankton during the monsoon potentially indicates a shift toward a mixed trophic structure. Such seasonal variations in size structured phytoplankton communities are ecologically significant, as they influence energy transfer efficiency and, in turn, affect the overall productivity of Chilika Lagoon.