LAUSR

Abstract Phytoplankton in floodplain lakes are frequently characterised as “small” algae but are rarely described with respect to size category (pico-, nano-, or microphytoplankton). Moreover, it is not clear which size category responds first to flood or which category total phytoplankton biomass is most related to. These questions are important in understanding the development of floodplain ecosystems following flooding and were evaluated on the dataset collected from a thermally stratified floodplain lake, Lake Sakadaš (Kopački Rit, Croatia), during spring flood conditions. Relationships of phytoplankton with hydrology and environmental conditions and contribution of each size fraction to total phytoplankton biomass were evaluated using path analysis. At the surface water layer, picophytoplankton developed first, followed by nanophytoplankton and microphytoplankton, suggesting temporal niche separation. Picophytoplankton increased in abundance just before every maxima of chlorophyll concentration, a pattern absent at the bottom water layer where picophytoplankton were a dominant group. The results suggest that analyses of the smallest phytoplankton size category are needed to better understand the controlling mechanisms of whole phytoplankton dynamics in floodplain lakes, with specific emphasis on smallest pico-sized component as “first-responders” to floodplain inundation.