Effect of increased seawater temperature on biomass, growth, and maturation of Saccharina japonica near its southern limit in northern Japan

Recent studies on global warming report that increases in seawater temperature cause coastal ecosystem changes, including a shift in marine algal distribution in temperate regions. To assess the influence of increased seawater temperature on ecophysiological responsiveness and potential shifts in abundance and distribution of kelp, we investigated the seasonal changes in standing biomass, growth, and maturation of natural populations of Saccharina japonica by taking monthly measurements in 2005, 2006, and 2007 in Matsushima Bay, northern Honshu, Japan, which is the southernmost range of this species. Compared to kelps collected in 2005, a sharp decline in standing biomass, later sporophyte occurrence, early growth, greater erosion, and failed sporophyte reproduction were found in kelps collected in 2006, which was a year characterized by relatively high seawater temperature. The population of S. japonica was absent in 2007, when the seawater temperature was higher than that in 2005 and 2006. These results suggest that the seawater temperature in Matsushima Bay is unfavorable for the development and reproduction of this cold-water kelp. If seawater temperatures in this bay remain high over long time periods because of continuous global warming, the southern limit of distribution of S. japonica likely will shift further north.