Position-dependent radiocarbon content of the macroalgae Undaria pinnatifida as an indicator of oceanographic conditions during algal growth

摘要

The radiocarbon content (Delta C-14) of the large brown macroalga Undaria pinnatifida was investigated as an indicator of oceanographic conditions during algal growth in Otsuchi Bay on the Pacific coast of northeastern Japan. In this region, warm boundary currents characterized by high dissolved inorganic carbon (DIC) Delta C-14 and cold subarctic Oyashio Current (low Delta C-14-DIC) flow into the bay seasonally. We hypothesized that Delta C-14 values in U. pinnatifida pinnae (leaflet-like structures arising sequentially along the vertical axis of the blade) reflect Delta C-14-DIC values in bay water during photosynthetic production. During the U. pinnatifida cultivation period (November 2013-April 2014), the Oyashio intrusion occurred from early March to early April, accompanied by a decrease in bay water Delta C-14-DIC, from 15.7 +/- 8.6 parts per thousand to -57.1 +/- 20.7 parts per thousand. The Delta C-14 of the pinnae reflected Delta C-14-DIC, as pinnae that formed in early March had significantly higher Delta C-14 (3.0 +/- 5.4 parts per thousand) than those that formed in early April (-23.6 +/- 7.5 parts per thousand). These results indicate that the Delta C-14 of U. pinnatifida pinnae can serve as a record of Oyashio intrusion into the bay during algal growth.