Seasonal variations of phytoplankton size structure in relation to environmental variables in three Mediterranean shallow coastal lagoons

Seasonal dynamic of Utermöhl fraction of phytoplankton (UFP, cell size > 3 μm) and picophytoplankton (Pico, cell size < 3 μm) were compared in three eutrophic coastal lagoons located in the same geographical area (Western Mediterranean Sea). Considering data on density, biomass, volume and composition of each size class, we focused on three aspects: i) the contribution of UFP and Pico to total phytoplankton; ii) the responses of UFP and Pico to seasonal variations of selected environmental variables; iii) the identification of the main environmental drivers of UFP and Pico seasonal dynamics. Pico contribution to total phytoplankton density was from 20% to 90%, highlighting the potential importance of Pico also in eutrophic ecosystems. Pico contribution to total phytoplankton biomass was lower, from 9% to 16%. Statistically, temperature resulted the strongest environmental factor controlling both the UFP and Pico seasonal variations in all the studied lagoons. In addition, the role of salinity, water transparency and nutrient concentrations was important in relation to the study site and the size class considered. In this work, the effects of temperature, salinity, transparency and nutrient availability on two different phytoplankton size classes have been described in Mediterranean shallow coastal lagoons for the first time, providing new insight for this ecosystem typology worldwide.