Hydrobiologia 161: 89-98 (1988) R. I. Jones and V. Ilmavirta (eds.). Flagellates in Freshwater Ecosystems 89 © Kluwer Academic Publishers Seasonal variation in the diel vertical distribution of the migratory alga Cryptomonas marssonii (Cryptophyceae) in a small, highly humic lake U. Smolander & L. Arvola* Lammi BiologicalStation, University of Helsinki, SF-16900 Lammi, Finland, *Correspondence: Dr. L. Arvola, Lammi Biological Station, University of Helsinki, SF-16900 Lammi, Finland Key words: phytoplankton, vertical migration, Cryptomonas, humic lake Abstract The diel vertical distribution patterns of a migratory alga Cryptomonasmarssonii 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 typical- ly concentrated at the surface or subsurface during daylight, while in darkness the highest densities were record- ed 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. How- ever, 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. Introduction Cryptomonads seem to be very abundant in Finnish lakes (Heinonen, 1980; K. Ilmavirta et al., 1984). Es- pecially in small lakes with brown water colour and reduced turbulent mixing, these algae undertake dis- tinct diurnal vertical migrations (cf. Ilmavirta, 1974; Arvola, 1984; Arvola et al., 1986). Although some experimental results indicate that cryptomonads can retrieve nutrients, at least phosphate (Salonen et al., 1984) from deeper water layers, we still have a poor understanding of the factors regulating vertical migration of cryptomonads in humic waters. For ex- ample, it is not known if the migration behaviour of these algae has some seasonal peculiarities or not, and whether the migrations are related to the physi- cal and chemical properties of the lake. This work describes the migration patterns of a Cryptomonas species (tentatively referred to C. marssonii Mars.) in a small, highly humic lake dur- ing a summer season. The main purpose was to iden- tify the special features of the behaviour of this spe- cies at different times of the growing season. Material and methods Material for this study was collected in summer 1983 from a small Finnish forest lake, Lake Nimeton (Fig. 1). The lake has a very dark water colour be- cause of the high load of dissolved organic matter from the catchment. The main morphometric and physico-chemical properties of the lake are given in Table 1. Samples for the phytoplankton analyses were col- lected fortnightly between May and September. Al- together 10 diurnal experiments were made during this growing season. Algal samples were obtained ev- ery 4 hours during a 24 h period. Samples were taken from the shore (Fig. 2) using a close-interval syringe