Phytoplankton geometric shapes along Mediterranean seasonal environmental variability in natural and artificial lakes

The amazing variability in morphological traits and the relative trade-offs represent the best adaptive strategy of phytoplankton to cope with the strong variability of the pelagic environment. In this work, phytoplankton species were grouped in geometric shapes. Field multiannual data on cell density, biomass, volume, surface and surface-to-volume ratio were analyzed to study the seasonal dynamic of phytoplankton geometric shapes in relation to environmental variability in two different Mediterranean lakes, a natural and an artificial lake. The affirmation of specific geometric shapes in precise seasonal environmental conditions was detected in both ecosystems, with the following common highlights: 1) Cone + Half Sphere was the only complex shape, and with a low surface-to-volume ratio contributed most to the total phytoplankton abundance in winter, when the mixing depth-to-euphotic depth ratio and the nutrient concentrations were the highest; 2) Cylinder represented the most elongated and the largest shape. Its contribution was higher in summer, when the mixing depth-to-euphotic depth ratio was the lowest and the water column was thermally stratified; 3) the geometric shape with the highest surface-to-volume ratio (Sphere in the natural lake, Cube in the artificial lake) contributed most to the total phytoplankton abundance in spring, when the lowest nutrient concentrations were observed. Our results will be important to understand how geometric shapes relate to the natural environment in lake ecosystems. Despite the well- known ecological value of phytoplankton morphology, phytoplankton shape structure has been only seldom considered in the literature compared to phytoplankton size structure.