Growth and survival of juvenile sporophytes of the kelp Ecklonia cava in response to different nitrogen and temperature regimes

Abstract

Deforestation of the kelp Ecklonia cava has been widely reported in Japan, and may be due to physiological stress caused by warm seawater temperatures and low nitrogen availability. To test this hypothesis, we examined the effects of temperature (20, 26, 28, and 30 °C) and nitrogen conditions (seawater enriched with 25 % PESI vs. non-enriched seawater) on photosynthesis, growth and survival, and nitrogen and chlorophyll a content of juvenile sporophytes (3–4 cm) along the coast of the Izu Peninsula, Japan. The juvenile sporophytes cultured in enriched medium showed significantly greater photosynthetic activity and relative growth rates than those cultured in non-enriched seawater at 20–28 °C, likely because of the much higher nitrogen and chlorophyll a content in the enriched medium. A significant effect of temperature on photosynthesis and relative growth rates was also detected under both nitrogen conditions. However, in contrast to the excellent survival exhibited in the enriched medium, dead juveniles were observed in non-enriched seawater at all temperatures, and survival rates decreased with increasing temperature. These results reveal that growth and survival of E. cava juvenile sporophytes are negatively affected by low nitrogen availability and high seawater temperature, which may result in deforestation of this kelp.