Vertical migration of a dinoflagellate in a column-shaped enclosure in Lake Tovel (Adamello-Brenta Natural Park, Italian Alps)

摘要

Efficient flagellar motion coupled with phototactic capacity provides dinoflagellates with a great ability to migrate vertically throughout the water column for great depths (e.g. Lieberman et al. 1994, Graham & Wilcox 2000). The diel vertical migration (DVM) enables the exploitation of both high light in the near-surface water and nutrient resources from within or below the thermocline (Whittington et al. 2000). DVM gives many advantages: reduced losses due to sinking, hydraulic wash-out and grazing, avoidance of nutrient limitation by obtaining access to deeper reserves, avoidance of surface high light intensities (Whittington et al. 2000) and the possibility to become dominant in the phytoplankton community during stratified conditions (Liebermann et al. 1994). Many authors suggest that motility depends upon an endogenous circadian rhythm, not dependent upon the light-dark cycles, which, however, serve to confer phase to the system (e.g. Levandowski & Kaneta 1987, Taylor 1990, Park et al. 2001). For others, on the contrary, DVM is primarily induced by environmental conditions. According to Tilzer (1973), migration is primarily light induced. For Levandowski & Kaneta (1987) the rhythm would be an alternation between negative and positive geotaxis. Experimental studies using laboratory cultures (e.g. Heaney & Eppley 1981) have variously indicated the importance of gradients of light and temperature, the availability of nitrogen, and population age in modifying patterns of migration. For Kamykowski & Yamaza-ki (1997) taxis-directed orientation is based on positive phototaxis during daylight and positive geotaxis at night, simulating a circadian rhythm set by the daylight cycle.