Replacement of Ulva ohnoi in the type locality under rapid ocean warming in southwestern Japan
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23RD INTERNATIONAL SEAWEED SYMPOSIUM, JEJU
Replacement of Ulva ohnoi in the type locality under rapid ocean warming in southwestern Japan Masanori Hiraoka 1
&
Kouki Tanaka 1 & Tomohito Yamasaki 2 & Osamu Miura 3
Received: 18 July 2019 / Revised and accepted: 24 October 2019 # Springer Nature B.V. 2019
Abstract 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 markerbased 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. Keywords Green tide . Species identification . Tropicalization . Tosa Bay . Chlorophyta . Ulva ohnoi . Ulva reticulata
Introduction Tosa Bay, southwestern Japan, western North Pacific, is strongly influenced by warm water that is constantly carried from tropical regions by the Kuroshio Current. The surface seawater temperature (SST) in this bay has significantly risen because the SST warming rate over the path of the Kuroshio Current has been shown to be two to three times faster than the global mean SST warming rate (Wu et al. 2012). The annual mean SST increase from 1970 to 2009 was 0.3 °C per decade at both ends of Tosa Bay near the main axis of the Kuroshio Current (Tanaka et al. 2012). Therefore, ecosystems are significantly changing in this region. Tosa Bay has been regarded Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10811-019-01974-8) contains supplementary material, which is available to authorized users. * Masanori Hiraoka [email protected] 1
Usa Marine Biological Institute, Kochi University, Kochi, Japan
2
Faculty of Science and Technology, Kochi University, Kochi, Japan
3
Faculty of Agriculture and Marine Science, Kochi University, Kochi, Japan
as a dramatic example of a tropicalization phase shift, where benthic communities dominated by temperate kelp Ecklonia in the early 1990s were replaced by reef-building corals by 2013 (Vergés et al. 2014). In accordance with the recently warming SSTs, temperate Sargassum spp. that were widely distributed in the 1970s have also declined, while a tropical Sargassum species has expanded its distribution to become the most conspicuously dominant species in Tosa Bay (Tanaka et al. 2012). Accordingly, a negative effect on fish communiti
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