Interactive effects of nutrient availability and temperature on growth and survival of different size classes of <i>Saccharina japonica</i> (Laminariales, Phaeophyceae)

Abstract

:In northern Japan, the production of Saccharina japonica exhibits marked annual fluctuation due to changes in seawater temperature and nutrient availability during winter and spring. To better understand this phenomenon, we examined the combined effects of temperature (5°C, 10°C, 15°C and 20°C) and nutrient availability (seawater enriched with 25% Provasoli's enriched seawater vs nonenriched seawater) on photosynthesis, growth and survival and nitrogen and chlorophyll a contents of juvenile sporophytes (2–3 cm) and larger size class sporophytes (100–150 cm) from southern Hokkaido. Both juvenile sporophytes and discs from 100–150-cm-class sporophytes cultured in enriched seawater showed significantly higher growth rates and photosynthetic activities than those cultured in nonenriched seawater. This difference likely was due to the significantly higher nitrogen and chlorophyll a contents of thalli grown in enriched seawater. Significant effects of temperature on growth and photosynthesis were also detected. A significant interaction between temperature and nutrient availability for relative growth rate (RGR) was detected. The positive effect of elevated nutrient availability on RGRs was magnified by an increase in temperature from 5°C to 15°C for juvenile sporophytes and from 5°C to 10°C for 100–150-cm-class sporophytes; whereas, the negative effect of elevated temperature from 10°C to 15°C on RGRs of 100–150-cm-class sporophytes was antagonised by a reduction in nutrient availability. In contrast to the 100% survival rate in enriched seawater, dead juvenile sporophytes and discs from 100–150-cm-class sporophytes were found at 15°C and 20°C in nonenriched seawater, and the survival rates decreased with increasing temperature. Therefore, the growth and survival of sporophytes of S. japonica are significantly affected by nutrient availability and temperature, which may lead to marked fluctuations in annual production.