| The diurnal vertical migration (DVM) of a poisonous red tide dinoflagellate Alexandrium tamarense isolated from Hong Kong waters was studied from Sept, 1994 to April, 1995 in laboratory culture tubes (glass: 4 × 100 cm and 4 × 195 cm; plastic: 10.6 × 200 cm). In any experiment, A. tamarense was always kept at density of about 1000 cell/ml, and pre-cultured in the experimental culture tube for at least 48h. The present study involved four experiments. The first was a phototaxis experiment, wherein the vertical distributions of A. tamarense were investigated under different light conditions by varying the distance of the light source to the water surface. The second experiment addressed the A. tamarense vertical distribution during continuous 48h of darkness and of light. The third experiment was done in temperature of 12.2 - 14.9, 13.7 - 21.4, 18.0 - 25.6, 18.1 - 27.8 and 18.1 - 29.5°C. The last experiment was done under conditions of nitrate enrichment (148.8 μmol/L) or deficiency (5.6 μmol/L).
The results showed that A. tamarense exhibited strong phototaxis and clear diurnal vertical migration. The general pattern of DVM was that the cells started to migrate up just after light on and finished downward migration from ca. 2h before light off to ca. 1h after light off. The diurnal vertical migration, however, completely ceased from the second or third day under continuous dark or light conditions. There was significant relationship between water temperature and the DVM speed of A. tamarense. The diurnal vertical migration was remarkably accelerated when the water temperature was near its optimum growth temperature range (20.0 - 24.0°C) or when the temperature gradient throughout the water column was small, and ceased when the temperature was below 12.5°C. The upward DVM speed of A. tamarense was generally faster than the downward speed. In its optimum growth temperature range, the highest upward speed was 1.20 m/h, but the highest downward speed was only 0.70 m/h. With insufficient nitrogen supply (5.6 μmol/L), A. tamarense greatly decreased its DVM speed, swarmed in the subsurface during light-time and tended to migrate downwards earlier than usual.
These findings suggest that the DVM behavior of this alga is strongly controlled by phototaxis, but that the biological clock rhythm might have certain effects on it. When the water temperature drops within its optimum growth temperature range, the DVM speed of A. tamarense is the fastest. However, the physiological state of A. tamarenseA. tamarense may affect its behavior and DVM speed. |
