EFFECT OF SHORT-TERM VARIATION IN IRRADIANCE ON LIGHT-HARVESTING AND PHOTOSYNTHESIS OF THE MARINE DIATOM SKELETONEMA-COSTATUM - A LABORATORY STUDY SIMULATING VERTICAL MIXING

A laboratory study was conducted into the physiology of Skeletonema costatum grown under a simple sinusoidal and a fluctuating light regime. The latter simulated a light regime similar to that which could result from the vertical mixing caused by Langmuir circulation. It was shown that the culture simulating vertical mixing reacted by decreasing the photosynthetic unit (PSU) size and increasing the number of PSUs, and hence optimized the rate of maximal photosynthesis at high, saturating irradiances. This culture also showed some change in photosynthetic parameters during the fight period, which was especially pronounced during the shift from a low to a higher irradiance. The effect of this on estimates of primary production in a water column is discussed. Further, it is speculated that the assimilation number is regulated by the maximum light intensity experienced during the day rather than the total daily tight dose, because only the culture submitted to a fluctuating light regime showed a real change in the maximum rate of photosynthesis (P(max)B) upon transfer to higher tight levels.