Seasonal variation in the diel vertical distribution of the migratory alga Cryptomonas marssonii (Cryptophyceae) in a small,highly humic lake

The diel vertical distribution patterns of a migratory alga Cryptomonas marssonii in a small, steeply stratified humic lake were investigated during a summer season (10 diurnal experiments between May and September) using a close-interval Blakar-type sampler. The results indicate that the cells were phototactic; they were typically concentrated at the surface or subsurface during daylight, while in darkness the highest densities were recorded in deeper water, usually near the upper limit of anoxia. During a dense blue-green bloom in August the cells of C. marssonii were also concentrated by day into the same water layer, where oxygen was depleted. However, the cells seemed to avoid totally anoxic water. Because the vertical distribution pattern of C. marssonii had special diurnal and seasonal characteristics, care is needed when designing a sampling programme for a phytoplankton population dominated by this species.