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.
Highlights
According to the Intergovernmental Panel on Climate Change, 4th report (IPCC 2007), the average Northern Hemisphere temperature during the second half of the 20th century was very likely higher than during any other 50-year period in the last 500 years, and the global climate has warmed by approximately 0.74°C in the last 100 years (1906–2005)
On the eastern coast of the North Pacific, the giant kelp, Macrocystis pyrifera forest decreased during a period of warming surface seawater temperature (SST) and recovered during a subsequent SST cooling (Edwards 2004; Ladah and Zertuche-Gonzalez 2004)
In central Japan’s Shizuoka, on the western coast of the North Pacific, a severe decline of temperate Ecklonia populations correlated with warming SST in the coastal area with the approach of the Kuroshio Current alongshore (Kawajiri et al 1981)
Summary
According to the Intergovernmental Panel on Climate Change, 4th report (IPCC 2007), the average Northern Hemisphere temperature during the second half of the 20th century was very likely higher than during any other 50-year period in the last 500 years, and the global climate has warmed by approximately 0.74°C in the last 100 years (1906–2005). Significant changes in biological systems are occurring on all continents and in most oceans due to global climate change (Rosenzweig et al 2008). The most basic prediction of ecological responses to increasing temperatures is that species’ distributions, influenced by the climatic regime, should shift toward the poles or higher altitudes (Walther et al 2002; Harley et al 2006). Following these predictions, distributional shifts of a wide range of terrestrial taxa have been well documented (Hickling et al 2006; Parmesan 2006).
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