Distributional shifts in size structure of phytoplankton community

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Hisatomo Waga1, Toru Hirawake2, Amane Fujiwara3, Shigeto Nishino3, Takashi Kikuchi3, Koji Suzuki4 and Shitaro Takao5, (1)Hokkaido University, Graduate School of Fisheries Sciences, Hakodate, Japan, (2)Hokkaido University, Faculty of Fisheries Sciences, Hakodate, Japan, (3)JAMSTEC Japan Agency for Marine-Earth Science and Technology, Kanagawa, Japan, (4)Hokkaido University, Sapporo, Japan, (5)Hokkaido University, Faculty of Environmental Earth Science, Sapporo, Japan
Increased understanding on how marine species shift their distribution is required for effective conservation of fishery resources under climate change. Previous studies have often predicted distributional shifts of fish using satellite derived sea surface temperature (SST). However, SST may not fully represent the changes in species distribution through food web structure and as such this remains an open issue due to lack of ecological perspective on energy transfer process in the earlier studies. One of the most important factors in ecosystem is composition of phytoplankton community, and its size structure determines energy flow efficiency from base to higher trophic levels. To elucidate spatiotemporal variation in phytoplankton size structure, chlorophyll-a size distribution (CSD) algorithm was developed using spectral variance of phytoplankton absorption coefficient through principal component analysis. Slope of CSD (CSD slope) indicates size structure of phytoplankton community where, strong and weak magnitudes of CSD slope indicate smaller and larger phytoplankton structure, respectively. Shifts in CSD slope and SST were derived as the ratio of temporal trend over the 12-year period (2003-2014) to 2-dimensional spatial gradient and the resulting global median velocity of CSD slope and SST were 0.361 and 0.733 km year-1, respectively. In addition, the velocity of CSD slope monotonically increases with increasing latitude, while relatively complex latitudinal pattern for SST emerged. Moreover, angle of shifts suggest that species are required to shift their distribution toward not limited to simple pole-ward migration, and some regions exhibit opposite direction between the velocity of CSD slope and SST. These findings further imply that combined phytoplankton size structure and SST may contribute for more accurate prediction of species distribution shifts relative to existing studies which only considering variations in thermal niches.