Vertical distribution of phytoplankton in two mesotrophic lakes

daily changes in the vertical distribution of phytoplankton biomass were investigated. In both lakes, phytoplankton biomass in metalimnion were two or three times higher than in epilimnion. In Rog??no Lake, Planktothrix rubescens dominated in phytoplankton ( > 80% of biomass), whereas in Zag??bocze Lake ? Ceratium hirundinella prevailed ( > 90% of biomass). The biomass maxima of Pl. rubescens (at 6 m depth – 43.5 mg dm-3 and at 7 m depth - 24.4 mg dm-3) were always below the lower limit of the euphotic zone, i.e. at the depth where the light was < 1%, and the water temperature was < 10 °C. Large biomass of C. hirundinella was observed always in metalimnion (up to 43.6 mg dm-3 in July) at the lower limit of the euphotic zone (transparency of Sd = 3.0 m). In August, when transparency of Sd = 2.5 m, biomass of C. hirundinella varied significantly during the day in the two thermal layers. In epilimnion, the largest increase of biomass was observed in daylight hours and the decrease at night. The reverse situation was observed in metalimnion ? the decline in the daytime and the increase at night. The studies revealed that despite different mechanisms of motility (buoyancy or flagellar movement), vertical migrations of these species corresponded mainly to the changing light.