Winter dominance of Ceratium hirundinella in a southern north-temperate reservoir

The 1-year seasonal distribution of Ceratium hirundinella was studied in a southern Spanish reservoir. This species grew from June 1988 to April 1989 and showed the highest density in winter, coinciding with the lowest water temperature values and with the period of water circulation. These seasonal dynamics do not accord with the typical description of C. hirundinella in temperate systems as a phytoplankton species of stratified and warm waters. We compared the factors that influenced C. hirundinella development in this system with those operating at different latitudes and concluded that the light and temperature conditions would allow C. hirundinella to grow throughout the year in this reservoir. However, the occurrence of this potential growth is probably regulated by ecological factors, such as the presence of hypolimnetic anoxic conditions and the hydraulic regime of the reservoir.