IMPACT OF NATURAL AND ARTIFICIAL UVB RADIATION ON MOTILITY AND GROWTH-RATE OF MARINE DINOFLAGELLATES

The growth rates and motility of dinoflagellates were studied in the f ield in the presence or absence of UVB radiation, as well as in the la boratory under artificial radiation conditions. Photosynthetically act ive radiation (PAR, 400-700 nm) and UVB radiation showed large variati ons due to cloud cover and seasonal changes in natural daylight. In Sw edish coastal water, UVB radiation was attenuated to about 10% of surf ace irradiance at a depth of 120 cm. There was no significant differen ce in the motility of two strains of Prorocentrum minimum (Atlantic, L AC4LI; Kattegat, LAC6KA83) kept in the water at different depths (35 a nd 120 cm) for 4 h, with or without natural solar UV radiation, except for a day with high UVB irradiance (1.2 W m(-2)), which decreased the motility at a depth of 35 cm for the two species). Simulated in situ experiments with 2 h natural daylight, with and without natural UV rad iation (UVB, 1.6 W m(-2)), had a dramatic effect on the motility of Gy rodinium aureolum. Artificial UVB radiation from UV lamps (4 h, 2.72 k J m(-2) day(-1), biologically effective UVB radiation, UVBBE) in the l aboratory decreased the motility of Heterocapsa triquetra (LAC20) by 5 6% and the two strains of P. minimum (Atlantic, LAC4LI; Kattegat, LAC6 KA83) by 43% and 36% respectively; the growth was inhibited for all sp ecies, as well as for Amphidinium carterae (LAC1KA83), when organisms were exposed to more than 0.7 kJ m(-2) day(-1) of UVBBE radiation.