Replacement of Ulva ohnoi in the type locality under rapid ocean warming in southwestern Japan

Tosa Bay in southwestern Japan is experiencing dramatic community phase shifts where temperate seaweed beds of brown algae have been tropicalized, a trend that has coincided with the most significant increases in surface seawater temperature in the world. Here, we report that blooms of green seaweed (genus Ulva) are also undergoing species replacement in this region, coincident with the tropicalization. Ulva green tides have been reported in Uranouchi Inlet, central Tosa Bay, since the late 1970s. The causative species was described as a new species, Ulva ohnoi, in 2004. However, the present investigation using a DNA marker-based identification technique showed that U. ohnoi did not form blooms but sparsely produced thallus fragments in the inlet during the period 2017-2018. Instead, the cosmopolitan species Ulva reticulata which has a distribution centered in more tropical waters was repeatedly found to dominate and form dense drifts in the inner end of the inlet.