Combined effects of seawater temperature and nutrient condition on growth and survival of juvenile sporophytes of the kelp Undaria pinnatifida (Laminariales; Phaeophyta) cultivated in northern Honshu, Japan

Recently, withering of farmed juvenile sporophytes of the kelp Undaria pinnatifida (Harvey) Suringar has led to reduced production of this species in northern Japan, possibly because of the high water temperature and low nutrient concentration in cultivation areas. This problem may be solved by introducing parental plants with greater tolerance to high temperature and low nutrient conditions. We examined the combined effects of various temperatures (15, 20, 24, and 27 °C) and nutrient availabilities (seawater enriched with 25 % PESI medium and non-enriched seawater) on the growth and survival of cultivated juvenile sporophytes (1-2 cm) collected from Matsushima Bay, Miyagi Prefecture in northern Japan and Naruto, Tokushima Prefecture in southern Japan. The relative growth rates of juvenile sporophytes from both locations were significantly greater at 15 and 20 °C than at 24 and 27 °C. The juvenile sporophytes cultured in enriched medium had significantly higher relative growth rates than those cultivated in non-enriched seawater. Dead juveniles were observed in non-enriched seawater at all temperatures and the survival percentage decreased with increasing seawater temperatures. Compared to the juvenile sporophytes from Matsushima Bay, those from Naruto showed greater tolerance to high temperature even under the low nutrient condition. These results suggest that the withering of juvenile sporophytes is caused by the combined effects of low nutrient availability and high temperature. A potential solution to this problem is to introduce ecotypes with greater tolerance to high temperature and low nutrient conditions from a warmer region of Japan.