Effect of diurnal variations in natural irradiance on the apical length growth and light saturation of growth in five species of benthic macroalgae

Abstract

The diurnal variation in apical length growth rate of five benthic macroalgae from the Swedish West Coast (Fucus spiralis L., F. vesiculosus L., F. serratus L., Ascophyllum nodosum (L.) Le Jolis, and Chondrus crispus Stackhouse) was measured during 1.5 to 2.0-h intervals for 3 to 5 d in September 1984. All species showed a maximum in growth rate in the morning, followed by a continuous decline throughout the rest of the day (most pronounced in F. serratus, A. nodosum and C. crispus). By comparing the growth rates with the average irradiance during the same interval, saturation irradiances for length growth in natural daylight regime were estimated; 60 to 70 Wm-2 for F. spiralis and F. vesiculosus, 90 to 100 Wm-2 for A. nodosum, 30 to 40 Wm-2 for F. serratus and 10 to 20 Wm-2 for C. crispus. Average growth rates during the dark hours ranged from 33% of average day growth rates in A. nodosum to 63% in C. crispus, respectively.