Abstract
The radiocarbon content (∆14C) 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) ∆14C and cold subarctic Oyashio Current (low ∆14C-DIC) flow into the bay seasonally. We hypothesized that ∆14C values in U. pinnatifida pinnae (leaflet-like structures arising sequentially along the vertical axis of the blade) reflect ∆14C-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 ∆14C-DIC, from 15.7 ± 8.6‰ to − 57.1 ± 20.7‰. The ∆14C of the pinnae reflected ∆14C-DIC, as pinnae that formed in early March had significantly higher ∆14C (3.0 ± 5.4‰) than those that formed in early April (− 23.6 ± 7.5‰). These results indicate that the ∆14C of U. pinnatifida pinnae can serve as a record of Oyashio intrusion into the bay during algal growth.
Highlights
The radiocarbon content (∆14C) of dissolved inorganic carbon (DIC) in surface seawater is influenced by upwelling because there is less 14C in deeper water than in surface water (Key 2001)
We demonstrated that ∆14C in U. pinnatifida pinnae reflected a shift in exposure from a warm current to the cold Oyashio Current during algal growth
The large body size and rapid growth of U. pinnatifida, which produces a pair of pinnae every 1 or 2 days (Saitoh et al 1999; Yoshikawa et al 2001; Gao et al 2013a), allow us to use its position-dependent variability in ∆14C as a unique tool to reconstruct oceanographic conditions experienced by macroalgae during growth
Summary
The radiocarbon content (∆14C) of dissolved inorganic carbon (DIC) in surface seawater is influenced by upwelling because there is less 14C in deeper water than in surface water (Key 2001). In the coastal upwelling region of California, there was a decrease in ∆14C-DIC from 42 to 26‰ during active upwelling in 2006 (Santos et al 2011). Undaria pinnatifida is extensively cultivated in this region, and previous studies have examined the nutrient sources, photosynthetic rates, and other physiological parameters of this kelp species (e.g., Yoshikawa et al 2001; Carvalho et al 2009; Gao et al 2013b; Endo et al 2017, 2018). Because the Oyashio Current originates from high latitudes of the North Pacific, where 14C-depleted deep water is upwelled, Oyashio waters have substantially less 14C (∆14C-DIC: − 60 to − 10‰) than the Tsugaru Warm Current waters and Kuroshio waters (∆14C-DIC: 20–40‰);
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