226Ra evidence for the ecosystem shift over the past 40 years in the North Pacific Subtropical Gyre

Surface seawater was collected for Ra-226 measurement in the North Pacific Subtropical Gyre from July to October, 1999 and October to December, 2003. Combined with the historical data reported for this sea area, a declined trend of surface Ra-226 concentrations was observed since 1960s, indicating the ecosystem shift in response to global warming. On one side, the enhanced stratification of the upper water column resulting from global warming reduced the Ra-226 input from the depth, on the other, the temporal increase of biological production resulting from the climate-related ecosystem structure change strengthened the Ra-226 removal from the surface ocean. Both the physical and biological processes resulted in the decrease of surface Ra-226 concentrations in the North Pacific Subtropical Gyre. The temporal trend of surface Ra-226 concentrations was consistent with the trends of chlorophyll a, silicate, phosphate and primary production previously reported. This study provided Ra-226 evidence for the ecosystem shift under global change.