Assessment of climate change impacts on large brown seaweed (Undaria pinnatifida) growth in the Sanriku coastal area, Japan

To assess the future impact of climate change on the large brown seaweed Undaria pinnatifida, sporophyte growth was projected in the Sanriku coastal area on the Pacific coast of northeastern Japan, which is one of the main Undaria cultivation areas in the country. We first developed a growth model for Undaria sporophytes based on environmental conditions measured by mooring observations at an Undaria farm in the central part of the Sanriku coastal area. The growth model was run under the projected environmental conditions by climate models under Representative Concentration Pathway (RCP) scenarios to project sporophyte growth in the 2010s and the 2090s. The dates of start and end of cultivation in the growth model were determined as being when the nitrate concentration exceeded 20 µg L⁻¹ and decreased below 20 µg L⁻¹, respectively. The projected growth at the mooring observation indicated that, under several projected environmental conditions, there will be no significant difference in total length between the 2090s and the 2010s. However, in an alternative projection under RCP2.6 showed that the total length in the 2090s would be significantly longer than that in the 2010s. A significant change was projected in the delay of the start date and forward shift of the end date of cultivation under RCP8.5, suggesting that the cultivation schedule will change significantly from the current schedule. Projections of future Undaria growth throughout the Sanriku coastal area revealed that a significant decrease in total length will occur south of 39°N under RCP8.5.