Diurnal vertical migration study during a winter bloom of Cryptophyceae in a floodplain pool

The diurnal vertical migration (DVM) of the phytoplankton community was studied in a small (270m 2 ), shallow (1.8m), and frozen floodplain pool. Steep vertical gradients were detected in O 2 , NH 4 + ‐N, NO 3 − ‐N and PO 4 3‐ ‐P profiles. The phytoplankton of the pool is characterized by year round dominance of Cryptophyceae: Cryptomonas curvata and Cryptomonas marssonii created more than 90% of total phytoplankton biomass during the time of study. DVM was studied by analysing vertical profiles six times during a 24 hour cycle; samples were taken at 20 cm intervals along the profiles. Koliella planktonica , the only non‐motile alga, showed a stable uniform distribution. Dinobryon divergens and Chrysococcus rufescens created a stable distinct concentration maximum at 40cm. Unlike these three species, both Cryptomonas spp. actively changed their position during the 24 hour cycle with distinctive evening downward and morning upward migration. The absence of filtering zooplankton excluded its influence on the patterns of phytoplankton DVM. The results showed the key role of PhAR (Photosynthetic active irradiance) in day ‐ positioning of Cryptomonas . Night‐time uptake of phosphate and/or ammonium at the bottom is probable, although direct evidence is lacking.