Scientific evidence emphasises the importance of phytoplankton cell size in competition and coexistence dynamics, although these mechanisms are not yet fully clarified. Theoretical frameworks predict that, under low nutrient availability, smaller cells are favoured because of their higher surface-to-volume ratio, whereas larger cells dominate under nutrient-rich conditions. The size and shape of phytoplankton cells influence their movement within the water column, thereby enhancing their efficiency in acquiring nutrients.

This study investigated the combined effects of volume and nutrient supply on phytoplankton community composition and size structure. Water samples from the Acquatina lagoon were used to fill microcosms of 0.5 L, 0.8 L and 15 L, enriched with different nutrient concentrations, control, 1.6 µM and 16.1 µM, with three replicates maintained for seven days. Total and size-fractionated chlorophyll a (pico-, nano-, and micro- phytoplankton), biomass, cell density, taxonomic composition and diversity were analysed at the beginning and end of the experiment to evaluate the effects of the treatments.

Results show that total and size-fractionated chlorophyll a increased with nutrient enrichment and larger experimental volumes, also in the control. Maximum values occurred at 16.1 µM and in the 15 L microcosms. Larger volumes also promoted micro phytoplankton dominance without increasing taxonomic richness. These findings suggest that larger species exploit spatial resources more efficiently, likely due to the relationship between cell size and sedimentation rate. Overall, the study highlights the effect of volume and nutrient on phytoplankton community structure. The experiment therefore confirms that size is a key trait that shapes ecological response patterns.