Hiroto Tateishi1, Shin-ichiro Tanimae1, Dominic Franco C. Belleza1, Kaho Yamaha2, Gregory N. Nishihara3
1Graduate School of Fisheries and Environmental Sciences, Nagasaki University, Nagasaki, Japan
2Faculty of Fisheries, Nagasaki University, Nagasaki, Japan
3Organization for Marine Science and Technology, Institute for East China Sea Research, Nagasaki University, Nagasaki, Japan
In Nagasaki Prefecture, Japan, economically important large brown seaweeds such as Ecklonia cava subsp. kurome, Sargassum fusiforme, and Sargassum horneri, are rapidly declining. We designed a survey to elucidate the environmental factors that may be responsible for the decline of seaweed communities in this region. We were especially interested in comparing two sites within Arikawa Bay of Nakadori Island, Nagasaki, Japan that were about 3000 m apart. We recorded observations of seaweed species, water temperature, wave action, water depth, and seaweed benthic cover at Kujiramiyama and Naname from December 2021 to May 2022. The species composition of seaweeds during the Spring was distinctly different among the sites. Small brown seaweeds such as Dictyota dichotoma, Padina arborescens, Sargassum hemiphyllum and Sargassum alternato-pinnatum dominated in Kujiramiyama, whereas large brown seaweeds such as Undaria pinnatifida, Dictyopteris undulata, Sargassum horneri, and Sargassum fusiforme were dominant in Naname. A few individuals of Ecklonia cava subsp. kurome was also observed only at Naname. Among the environmental variables that were recorded, we noted that the winter minimum water temperatures were up to 1.0 ± 0.89 °C (mean ± sd) lower at Naname. Given the slow increase in mean seawater temperature in Arikawa Bay due to climate change, we suggest that relatively low water temperature allows the large brown seaweeds to persist in Naname. Our surveys continue, with the aim to confirm our hypothesis.