Seasonal changes of photosynthetic activity of a brown algaEcklonia cava Kjellman

Abstract

Samples of a brown algaEcklonia cava Kjellman from a depth of about 5 m in the subtidal zone of Nabeta Bay, Shimoda, were used to investigate photosynthesis in relation to light intensity, temperature, age and season. Photosynthesis and respiration were measured by gas-volumeters with discs (4.1 cm2) of the bladelet. The lightsaturation of photosynthesis at 20 C or atin situ temperatures occurred mostly at about 10 klux. Almost no photoinhibition of photosynthesis was observed within the light intensity range up to 30 klux. The optimum temperature for photosynthesis was around 25 C throughout a year, being slightly higher in summer and lower in winter. The light-saturated net photosynthetic rate at 20 C of the grown-up bladelets began to increase in late autumn, reached the maxima (48 μl O2 cm−2 hr−1, 16 μl O2 mg (d.w.)−1 hr−1, 2.3 μl O2 μg (chl.a)−1 hr−1) in winter, and declined to reach the minima (30 μl O2 cm−2 hr−1, 5.0 μl O2 mg(d.w.)−1 hr−1, 1.0 μl O2 μg(chl.a)−1 hr−1) in summer or early autumn. Young bladelets showed almost the same seasonal changes in the photosynthetic rates. The photosynthetic rates of the basal part were significantly lower than those of the apical part of a bladelet. The respiratory rate of the grown-up bladelet increased almost linearly with temperature in the range of 5–30 C. No marked change in the respiratory rate at 20 C was observed throughout a year.