Water temperature characteristics and the maintenance of Sargassum piluliferum (Fucales, Phaeophyceae) in Namako-ike Lake, Kami-Koshiki-shima Island, Kagoshima Prefecture, Japan

抄録

In the southern Japanese coastal area affected by the Kuroshio Current, seaweed beds consisting of brown macroalgae has been reduced owing to increases in water temperature and damage from grazing by herbivorous animals. Kami-Koshiki-shima Island, located in western part of Kagoshima Prefecture, contains landlocked brackishwater lagoons, and Sargassum piluliferum is a dominant species in Namako-ike Lake, located in one of these lagoons. In this study, seawater temperature in Namako-ike Lake and neighboring waters was measured in 2014 in order to elucidate the factors contributing to the maintenance of the S. piluliferum populations. The annual average seawater temperature in 2014 was 19.5 ℃ in Namako-ike Lake, with the highest water temperature recorded in July, 2014 at 35.6 ℃ and the lowest water temperature recorded in March, 2014 at 6.6 ℃. These results suggest that S. piluliferum has tolerance to high water temperature in Namako-ike Lake, which is far higher than in surrounding sea areas. The lack of herbivorous animals in the lake is suggested to be the main factor contributing to the maintenance of the S. piluliferum population rather than an increase in water temperature. In order to maintain the S. piluliferum population, it is necessary to continue monitoring the organisms growing in Namako-ike Lake in cooperation with local residents.