Warming off southwestern Japan linked to distributional shifts of subtidal canopy-forming seaweeds

Abstract

To assess distributional shifts of species in response to recent warming, historical distribution records are the most requisite information. The surface seawater temperature (SST) of Kochi Prefecture, southwestern Japan on the western North Pacific, has significantly risen, being warmed by the Kuroshio Current. Past distributional records of subtidal canopy-forming seaweeds (Laminariales and Fucales) exist at about 10-year intervals from the 1970s, along with detailed SST datasets at several sites along Kochi's >700 km coastline. In order to provide a clear picture of distributional shifts of coastal marine organisms in response to warming SST, we observed the present distribution of seaweeds and analyzed the SST datasets to estimate spatiotemporal SST trends in this coastal region. We present a large increase of 0.3°C/decade in the annual mean SST of this area over the past 40 years. Furthermore, a comparison of the previous and present distributions clearly showed the contraction of temperate species' distributional ranges and expansion of tropical species' distributional ranges in the seaweeds. Although the main temperate kelp Ecklonia (Laminariales) had expanded their distribution during periods of cooler SST, they subsequently declined as the SST warmed. Notably, the warmest SST of the 1997-98 El Niño Southern Oscillation event was the most likely cause of a widespread destruction of the kelp populations; no recovery was found even in the present survey at the formerly habitable sites where warm SSTs have been maintained. Temperate Sargassum spp. (Fucales) that dominated widely in the 1970s also declined in accordance with recent warming SSTs. In contrast, the tropical species, S. ilicifolium, has gradually expanded its distribution to become the most conspicuously dominant among the present observations. Thermal gradients, mainly driven by the warming Kuroshio Current, are presented as an explanation for the successive changes in both temperate and tropical species' distributions.

Keywords: ENSO; Ecklonia; Kuroshio Current; Sargassum; climate change; distributional shifts; ocean warming; sea surface temperature; seaweeds.

Figure 1

Map of observation region in Kochi, southwestern Japan. Thin arrows show observation periods. Thick and medium arrows show the main axis of the warm Kuroshio Current and the long-term dominant surface current inflow to Tosa Bay, respectively (Kuroda et al. 2008). Enclosed names show SST measurement locations.

Figure 2

Annual mean SSTs recorded from 1970 to 2009 at four measurement locations in Kochi.

Figure 3

SST trends calculated for 5-year periods at four locations in each season [April to June as spring, (a); July to September as summer, (b); October to December as autumn, (c); and January to March as winter, (d)] and annual means (e). Error bars show ± 1SE.

Figure 4

Sargassum ilicifolium. This tropical species is the most dominant, and has expanded its distributional ranges in the study area.

Figure 5

Distributional shifts of temperate Ecklonia populations. Distribution of 1970s, 1980s, 1990s and 2000s are shown in (a), (b), (c) and (d) respectively. Environment agency (1994) is the report of the Nature Conservation Bureau of the Environment Agency of Japan and the Marine Park center of Japan.

Figure 6

Distributional shifts of main Sargassum spp. populations. Distribution of 1970s, 1980s, 1990s and 2000s are shown in (a), (b), (c) and (d) respectively. Environment agency (1994) is the report of the Nature Conservation Bureau of the Environment Agency of Japan and the Marine Park center of Japan.