Migration behaviour of three cryptophytes in relation to environmental gradients: an experimental approach

The diurnal vertical migration (DVM) behaviour of three species of cryptophytes of differing size (Rhodomonas minuta var. nannoplanctica and two species of Cryptomonas) was examined in 4 m tall laboratory columns in relation to gradients of light, temperature and nutrients. Differences were found in the DVM behaviour of the three species in relation to these environmental variables. Both species of Cryptomonas were able to cross steep thermal gradients whereas Rhodomonas did not. All three species avoided high levels of irradiance, especially when the cells were phosphorus-depleted. The large species of Cryptomonas made use of vertically separated resources of light and phosphorus by migrating from the illuminated, but phosphorus-depleted, upper layer during daytime to the phosphorus-rich hypolimnion at night. The smaller species of Cryptomonas had a reverse pattern of DVM swimming upwards during darkness, indicating a strong negative geotaxis. Moreover, downward migration of this smaller Cryptomonas ceased abruptly at the thermocline giving rise to well developed mid-water maxima in contrast to the larger species. The migration speeds of the Cryptomonas species were up to 0·7 m h-1 and cells were able to move quickly in response to environmental changes in the water column. The results demonstrate the importance of both the physiological state of the cells and the external physical and chemical conditions, particularly irradiance, as factors regulating the DVM behaviour of cryptophytes.