Within the illuminated water column, there is at a given time for any particular species in the phytoplankton population a particular depth at which the light intensity is optimal for photosynthesis: high enough to give the maximum rate without causing photoinhibition. This optimal depth will vary with solar altitude during the day. The ability to move within the water column to whatever depth suits it best would clearly be of advantage to an alga; many groups have this capability.
The possession of flagella is common in all the algal divisions other than the Rhodophyta, Phaeophyta, Bacillariophyceae and Cyanophyta. Vertical migration by flagellated cells has been studied most among the dinoflagellates. In some cases the behaviour pattern is for the cells to migrate downwards away from the surface as the light becomes more intense, towards noon, and then to ascend again during the remainder of the day and the night.124,1360 Migration appears to be in part phototactic, but can be positive or negative within any given species, the direction of migration being determined by the light intensity at the water surface. For example, Blasco (1978) found that off the coast of Baja California, USA, the marine dinoflagellate Ceratium furca migrated towards the sea surface at 07:00 h when the solar irradiance was 280 Wm~2, but away from the sea surface at 12:00 h when the irradiance was 900 Wm~2.
Not all dinoflagellate species show a regular up-and-down diurnal migration but instead appear to use their powers of locomotion to move themselves to, and remain at, a particular depth. Heaney and Talling (1980) found that in Esthwaite Water, England, in the spring and summer, Ceratium hirundinella showed surface avoidance but not vertical migration. The dino-flagellate cells were found in maximum density at the depth 3 to 4 m, at which irradiance was reduced to about 10% of that penetrating the surface.
Patterns of dinoflagellate movement can be influenced by the availability of nitrate in the water. Cullen and Horrigan (1981) studied the movement of the marine species Gymnodinium splendens in a 2 m deep laboratory tank under a 12 h dark/12h light regime. When nitrate was present throughout the container, the cells spent most of the day near the surface and most of the night at greater depth: they started swimming upwards before the end of the dark period and downwards before the end of the light period. When the nitrate in the water became depleted, the light-saturated photosynthetic rate per unit biomass decreased, and the cells formed a layer at a depth corresponding to the light level at which photosynthesis was saturated. The behaviour pattern of this species was also observed in the sea off the
California coast. During the night (01:10 h) the cells were at 18 m depth, in the region of the nitrocline (the layer in which nitrate concentration is rising from the low value in the upper waters to the high value of deep water). By 10:50 h in the morning the cells had moved up into nitrate-depleted water at 14 m, a depth at which the light intensity was sufficient to saturate photosynthesis. It therefore seems that G. splendens moves down at night to a depth where it can accumulate nitrogen and then up during the day to a depth where, although nitrate is lacking, it can photosynthesize at its maximum rate. Its behaviour may be chemotactic as well as phototactic.
Figueroa et al. (1998) studied the diel vertical migration of a number of flagellated phytoplanktonic species in a stable, well-stratified estuary, the Ria de Vigo, NW Spain. Each species had its own migration pattern. Mesodinium rubrum, which is a marine planktonic ciliate protozoan harbouring a reduced cryptophyte endosymbiont, was highly active: it was present in maximum concentration at the surface during the day, and was able to move down through the pycnocline during the night. A euglenoid flagellate, Eutreptiella sp., aggregated at the surface at noon, and migrated down through the pycnocline at night. Cells of the dinoflagel-late, Ceratiumfurca, were present throughout the water column during the day, with a tendency to form a subsurface maximum at about 2 m depth in the afternoon, but were located below the pycnocline at dusk. Another dinoflagellate species, Dinophysis acuminata, aggregated at the surface at noon, and in the night migrated downward but was unable to cross the pycnocline and so remained mainly above the density gradient.
In lakes, a common (but not universal) flagellate behaviour pattern is for the cells to congregate in the surface water by day and migrate below the thermocline at night. Such behaviour has been described, for example, in the cryptomonad flagellate Cryptomonas marssonii in a Finnish brown-water lake,1165 and in the colonial green flagellate Volvox in Lake Cahora Bassa (Mozambique).1264 In both cases the adaptive significance was considered to be nutrient retrieval (in this case phosphate - generally the limiting nutrient in inland waters) from deep water at night, followed by a return to the illuminated, but nutrient-depleted, upper layer to photosynthesize during the day.
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