Enhancement of coccolithophorid blooms in the Bering Sea by recent environmental changes

Since 1997, ocean color satellite images have revealed large-scale blooms of the coccolithophorid Emiliania huxleyi in the eastern Bering Sea. The blooms are often sustained over several months and have caused ecosystem changes in the Arctic Ocean, as well as in the Bering Sea. We examined continental shelf sediment profiles of alkenone, a biomarker for E. huxleyi, covering the past similar to 70 years. The alkenone records suggest that large E. huxleyi blooms are a novel feature in the Bering Sea as they have occurred only since the late 1970s. Recent changes in alkenone content were closely related to the 1976-77 climatic regime shift in the North Pacific, implying that warming and freshening of Bering Sea waters promoted E. huxleyi blooms. The production rate of diatoms (total valves in sediment samples), the dominant primary producers in the Bering Sea, also increased during the past several decades. However, the ratio of alkenone content to total diatom valves in the sediments increased as E. huxleyi production increased, suggesting that the increase in the E. huxleyi production rate frequently exceeded the increase in the diatom production rate. Overall, our results indicate a possible subarctic region ecosystem shift driven by climate change.We are grateful to Captain Akamine and the crew of R/V Mirai for their help with sediment collection and water sampling during the MR00-06 and MR06-04 cruises. This paper benefited from important comments by Eric Sundquist, Editor in Chief and two anonymous reviewers. This work was supported by the Japan Agency for Marine-Earth Science and Technology and the Japan Society for the Promotion of Science, KAKENHI (B) 19310018 and (S) 22221003