Blade-order-dependent radiocarbon variability in brown seaweed (Undaria pinnatifida) reflected a cold Oyashio water intrusion event in an embayment of the Sanriku coast, northeastern Japan

Abstract:

Radiocarbon in dissolved inorganic carbon (DIC) in seawater varies greatly, both geographically and with depth. This “reservoir effect” is thought to be reflected in the radiocarbon content (∆¹⁴C) of marine organisms, via DIC fixation by primary producers and subsequent trophic transfer. The ∆¹⁴C of marine organismal soft tissues might thus provide unique information about their habitats, diets, migration and other environmental histories. However, the effectiveness of this approach has yet to be extensively explored, with data on ∆¹⁴C variability in soft tissues of marine organisms being markedly limited. Here we examined whether ∆¹⁴C values of individual pinnate blades (leaf-like structures) of brown seaweed (Undaria pinnatifida) reflect the ∆¹⁴C of DIC in the water current prevailing at the time of blade formation. The study was conducted in Otsuchi Bay located in the Sanriku coastal region, northeastern Japan, where ¹⁴C-depleted cold Oyashio current and warm Tsugaru current (high ∆¹⁴C) converge, affecting the physiology and growth of marine organisms growing there. U. pinnatifida individuals cultured in the bay (length of saprophytes, 140–215 cm) were harvested in April 2014 and ∆¹⁴C of blades were determined by accelerator mass spectrometry. Younger blades formed after the Oyashio water intrusion had significantly lower ∆¹⁴C values compared to older blades formed before the event. The ∆¹⁴C values of younger and older blades were generally consistent with the ∆¹⁴C of DIC in Oyashio (-60.5 ‰) and Tsugaru (24.9 ‰) waters, respectively. Thus, despite possible turnover of organic carbon in seaweed soft tissues, blade-order-dependent ∆¹⁴C variability appeared to strongly reflect the Oyashio intrusion event (radiocarbon shift) in the bay.