Interdecadal change in the upper water column environment and spring diatom community structure in the Japan Sea: an early summer hypothesis

Abstract

Diatom community structure in the offshore Tsushima Current area of the Japan Sea was studied from 1972 to 1999 in relation to temporal variability of the upper water environment. We observed a distinct change in the community structure in spring during the 1980s, the period after the 1976/77 climate regime shift which has been reported to occur in the vast areas of the North Pacific. Chlorophyll a (chl a) concentration and chl a per cell were markedly low, and summer species includ- ing Pseudonitzschia spp. dominated the diatom community in spring during the 1980s. Mixed layer phosphate concentrations during the 1980s were lower in spring compared to the1970s and 1990s, suggesting that nutrient depletion to levels limiting diatom growth might occur early. This change seemed subsequently to cause a shift in the dominant diatom species from those adapted to eutrophic conditions to those adapted to oligotrophic conditions. Density profiles between the surface and 300 m showed the thickness of the surface Tsushima Current water and the cold subsurface water decreasing and increasing, respectively, from the late 1970s to the late 1980s. In addition, spring solar radiation increased during the 1980s. These conditions indicate intensified stratification of the upper water column. Increases in the phosphate gradient between the surface and subsurface layers sug- gested that the intensified stratification reduced nutrient supply to the surface and may be responsi- ble for early formation of the summer-like oligotrophic conditions. Based on these results, we propose the 'early summer hypothesis' as the cause of the apparent decline of the spring phytoplankton bio- mass in the Japan Sea during the 1980s.