Temporal occurrence of Ceratium hirundinella in Spanish reservoirs

Ceratium hirundinella has traditionally been characterised as a species that thrives in warm waters and in stratified conditions. In our study, however, we found that the temporal occurrence of C. hirundinella in Spanish reservoirs greatly differs from that typically described in temperate zone aquatic systems. We analysed the temporal occurrence of C. hirundinella populations as well as physical and chemical variables in one hundred Spanish reservoirs. C. hirundinella was present in most (74%) of the reservoirs. In 78% of the reservoirs with C. hirundinella occurrence, the species was present during winter time and in 70% it was present during all four seasons.C. hirundinella was very commonly present in Spanish reservoirs in winter time despite the mixing conditions and lower temperatures and light availability. The presence of the species was positively related to water ionic content (HCO₃^–, SO₄^2-, Ca²⁺, Mg²⁺). We conclude that C. hirundinella temporal occurrence in southern north-temperate systems greatly differs from the seasonality typically described for the temperate zone and could be regulated by different factors than those operating in the northern north-temperate zone.