Recent fluctuations in distribution and biomass of cold and warm temperature species of Laminarialean algae at Cape Ohma, northern Honshu, Japan

摘要

The Cape Ohma region of Shimokita Peninsula, the northernmost point of Honshu Island, Japan, is subject to both the warm Tsugara Current and the cold Kurile Current. As a result, the Laminarialean flora includes both cold temperature species (Laminaria japonica Areschoug, Kjellmaniella crassifolia Miyabe and Costaria costata (C. Agardh) Saunders) and warm temperature species (Undariapeterseniana (Kjellman) Okamura, Ecklonia stolonifera Okamura), as well as Undaria pinnatifida (Yendo) Okamura, which is distributed in both waters. The frequency of occurrence (as a measure of distribution) and the biomass of these species were recorded in June 1976 (at 50 points in depths between 8-30 m), July 1988 (192 points, 2.5-25 m) and July 2001 (78 points, 2.5-25 m). Comparison of these data revealed a decrease in cold temperature species and an increase in warm temperature species from 1976 or 1988 to 2001. Long-term data of seawater temperature measured at 5 m depth near the study site showed that mean temperatures in the middle of winter (late January to February) in 1989-2000 were 0.9-1.1 deg C higher than those in 1980-1988. Higher seawater temperatures in the last decade appear to have affected the frequency of occurrence and biomass of the Laminarialean species along the coasts of Cape Ohma. This result supported our previous conclusion that 1 deg C higher mean seawater temperature in late January caused a decrease in the biomass of L. japonica (by ca. 64%) along the same coast.