Journal of Oceanography, Vol. 61 (No. 3), pp. 379-388, 2005
Nobuaki Ohi1*, Hiroaki Saito2 and Satoru Taguchi1
1Laboratory of Biological Oceanography, Department of Environmental Engineering for Symbiosis, Faculty of Engineering, Soka University, Tangi-cho, Hachiouji 192-8577, Japan
2Tohoku National Fisheries Research Institute, Shinhama-cho, Shiogama 985-0001, Japan
(Received 31 October 2003; in revised form 14 July 2004; accepted 14 July 2004)
Abstract: Diel patterns in the Chlorophyll a specific absorption coefficient of surface picoplankton, a*pico (l) (m2·[mg Chlorophyll a]-1), were studied at 7 stations under daily cycle of in situ light condition in the western subarctic Pacific and Japan Sea. All the data were normalized by dividing the anomaly with daily averaged a*pico (l). Opposite diel patterns were observed for the normalized a*pico (443) and a*pico (675) with maximum toward dawn or dusk and minimum toward midday at 4 stations under low-irradiance (LI) conditions and vice versa at 3 stations under high-irradiance (HI) conditions. The absorption efficiency factors at red absorption peak, Qa (675), were determined by reconstruction with intracellular Chlorophyll a concentration and cell diameter. The normalized Qa (675) also showed diel pattern with maximum toward midday and minimum toward dawn or dusk under LI. The diel pattern in a*pico (675) and Qa (675) were primarily caused by changes in intracellular Chlorophyll a concentration due to photoadaptation under LI. The diel pattern in a*pico (443) was influenced by pigmentation, as recognized by blue to red ratio [a*pico (443)/a*pico (675)] under HI. This study proposed that the opposite diel pattern in a*pico (l) might occur for a wide range of algal species. The results presented here have important consequences for the interpretation of diel variations in optical properties observed in the open ocean.