Interactive effects of elevated summer temperature, nutrient availability, and irradiance on growth and chemical compositions of juvenile kelp, Eisenia bicyclis

Previous experiments with a factorial design have revealed the effects of several environmental factors on species performance and their interactions, which indicate synergistic or antagonistic effects. Temperature, nutrient availability, and irradiance are well-known environmental factors that affect the growth and chemical composition of brown algae. However, relatively few studies have tested their combined effect on brown algal growth and chemical composition using a three-way factorial design. We conducted a culture experiment to test the combined effects of elevated summer temperatures (23 and 26 degrees C), irradiance (180 and 30 mol photon m(-2) s(-1)), and nutrient availability (enriched and non-enriched seawater) on four relative growth rates (RGRs; based on wet weight, blade width, length, and area) and three chemical compositions (including carbon, nitrogen, and phlorotannin content) in juvenile sporophytes of the kelp Eisenia bicyclis. RGR based on blade width was the most sensitive to abiotic factors among all RGRs. A significant interaction between temperature and nutrient availability on this RGR suggested that the negative effect of elevated temperature was antagonized by a reduction in nutrient availability. Similarly, the positive effect of elevated irradiance on carbon content was synergized by reduced nutrient availability. Moreover, the negative effect of increased irradiance on nitrogen content was antagonized by elevated temperature in nutrient-enriched treatments, but not in non-enriched treatments. The content of carbon-based phlorotannins increased with reduced nutrient availability but not with elevated irradiance. These results suggest that these abiotic factors have complex interactions on the growth and chemical composition of this species.