Unusual Distribution of Floating Seaweeds in the East China Sea in the Early Spring of 2012

J Appl Phycol DOI 10.1007/s10811-013-0152-y Unusual distribution of floating seaweeds in the East China Sea in the early spring of 2012 Teruhisa Komatsu & Shizuha Mizuno & Alabsi Natheer & Attachai Kantachumpoo & Kiyoshi Tanaka & Akihiko Morimoto & Sheng-Tai Hsiao & Eva A. Rothäusler & Hirotoshi Shishidou & Masakazu Aoki & Tetsuro Ajisaka Received: 2 June 2013 /Revised and accepted: 6 September 2013 # The Author(s) 2013. This article is published with open access at Springerlink.com Abstract Floating seaweeds play important ecological roles and massive strandings of seaweed rafts were observed on the in offshore waters. Recently, large amounts of rafting seaweed northern coast of Taiwan and on Tarama Island in the Ryukyu have been observed in the East China Sea. In early spring, Archipelago. Environmental data (wind, currents, and sea juveniles of commercially important fish such as yellowtail surface height) were compared among the surveys of 2010, accompany these seaweed rafts. Because the spatial distribu- 2011, and 2012. Two factors are speculated to have caused the tions of seaweed rafts in the spring are poorly understood, unusual distribution in 2012. First, a continuous strong north research cruises were undertaken to investigate them in 2010, wind produced an Ekman drift current that transported sea- 2011, and 2012. Floating seaweed samples collected from the weed southwestward to the continental shelf and eventually East China Sea during the three surveys contained only stranded seaweed rafts on the coast of Taiwan. Second, an Sargassum horneri. In 2010 and 2011, seaweed rafts were anticyclonic eddy covering northeast Taiwan and the distributed only in the continental shelf and the Kuroshio Kuroshio Current west of Taiwan generated a geostrophic Front because they had become trapped in the convergence current that crossed the Kuroshio Current and transported zone of the Kuroshio Front. However, in 2012, seaweed was the rafts to the Kuroshio Current and its outer waters. Such also distributed in the Kuroshio Current and its outer waters, unusual seaweed distributions may influence the distribution of fauna accompanying the rafts. * : : : : T. Komatsu: ( ) S. Mizuno A. Natheer A. Kantachumpoo K. Tanaka E. A. Rothäusler Keywords Floating seaweed . Sargassum horneri . 5-1-5, Kashiwanoha, Kashiwa 277-8564, Japan Phaeophyta . East China Sea . Kuroshio . Transport . e-mail: [email protected] Ekman drift current . SSH A. Morimoto Hydrospheric Atmospheric Research Center, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan Introduction S.<T. Hsiao Marine Fisheries Division, Fisheries Research Institute of Taiwan, Sargassum species grow on rocky coasts in the temperate 199, Hou-Ih Road, Keelung 20246, Taiwan zone of the northwestern Pacific coast, forming luxuriant forests in spring that become less dense by summer H. Shishidou Kagoshima Prefectural Fisheries Technology and Development (Komatsu et al. 1982). Their stems extend for several meters Center, Ibusuki, Kagoshima 891-0315, Japan in spring when most of these species mature. Sargassum forests have important influences on coastal marine environ- M. Aoki ments. For example, seaweeds affect the spatial and temporal Division of Applied Aquatic Bio-Science, Graduate School of Agricultural Science, Tohoku University, 1-1 Amamiya-machi, distribution of water temperature (Komatsu et al. 1982, 1994; Tsutsumidori, Aoba-ku, Sendai, Miyagi 981-8555, Japan Komatsu 1985), downward illumination via shading by their canopies (Komatsu 1989; Komatsu et al. 1990), pH (Komatsu T. Ajisaka and Kawai 1986), and dissolved oxygen content by their Division of Applied Biosciences, Graduate School of Agriculture, Kyoto University, Kitashirakawa Oiwakecho, Sakyoku, respiration and assimilation (Komatsu 1989). They can also Kyoto 606-8502, Japan slow water flow (Komatsu and Murakami 1994), and they J Appl Phycol play vital ecological roles as spawning, nursery, and feeding Yoshida (1963) reported that most Japanese floating sea- grounds for marine animals (e.g., Thiel and Gutow 2005;Abé weeds were found in nearshore coastal waters (within 20 km et al. 2012). from the shore) in the vicinity of oceanic fronts west or north Sargassum species contain many vesicles that are filled of Kyushu Island in the East China Sea (Fig. 1). Yoshida with gas for buoyancy and keep plants facing upward from (1963) also noted that floating seaweeds were most abundant the bottom (Hurka 1971). In spring, the drag force of waves from March to May and that their abundance became signif- and currents may become greater than the fixation force of icantly lower in waters south of the Osumi Peninsula and, elongating seaweeds, detaching them from the bottom especially, south of Tanegashima Island and Yakushima Island (Yoshida 1963). The buoyancy provided by the vesicles (Fig. 1). Senta (1965) reported that floating seaweeds were not allows seaweeds to float after detachment, and seaweed rafts present in the oceanic front between continental shelf waters form in waters around Japan (Yoshida 1963) and in areas and the Kuroshio Current. Thus, since the 1960s, floating such as the Sargasso Sea. Although floating Sargassum seaweeds have been reported to be rare in the offshore waters species in the Sargasso Sea can complete their entire life of the East China Sea (Fig. 1). through vegetative reproduction in a floating state (Parr Recently, however, floating seaweeds were found on the 1939), floating Sargassum species in the East China Sea continental shelf along the East China Sea oceanic front would have originally grown on rocky coastal bottoms (between Kuroshio and continental shelf waters) between (Yoshida 1963). March and May (Konishi 2000; Komatsu et al. 2007). In offshore waters, floating seaweeds are an important While densities of seaweed rafts along transects increased habitat, providing a moving ecosystem for attaching or ac- southward in survey areas in both months, survey area in companying flora and fauna (e.g., Cho et al. 2001). Attached March was limited in the northern East China Sea or accompanying organisms disperse with the floating sea- (Komatsu et al. 2009). These floating seaweeds were com- weeds, which thus serve as means of dispersal for littoral posed of only one species, Sargassum horneri (Turner) C. animals such as intertidal organisms (e.g., Ingólfsson 1995). Agardh. Although it is now accepted that floating seaweeds Seaweed sampling and aquarium experiments in a controlled occur in this specific area, little is known about their environment demonstrated that the seaweed composition in- abundance and distribution. The Kagoshima Prefectural fluences the macrofaunal species composition and abundance Fisheries Technology and Development Center investigated (Vandendriessche et al. 2006). For example, samples domi- floating seaweeds, mainly by a visual survey, to determine the nated by Sargassum muticum (Yendo) Fensholt displayed open season for fisheries of yellowtail (Seriola quinqueradiata) higher densities but lower diversities of macrofaunal species juveniles (Kubo 2004, 2006) to be used as aquaculture seed- compared to samples dominated by Ascophyllum nodosum lings because cannibalism behavior of yellowtail during the (L.) Le Jolis and Fucus vesiculosus L. Because floating algae juvenile period (Anraku and Azeta 1965) prevents the industrial may be dispersed over great distances (Hinojosa et al. 2011), production of its seedlings and yellowtail aquaculture, of which they influence the biogeography and evolution of the fauna the production is the most important among the finfish aqua- accompanying them (Thiel and Haye 2006). Floating sea- cultures in Japan, depending on wild juveniles accompanying weeds continue to grow while afloat (Vandendriessche et al. seaweed rafts. However, efforts were concentrated in coastal 2007; Rothäusler et al. 2009, 2012;Talaetal.2013). waters, and the abundance of floating seaweeds in deeper Moreover, seaweeds also enlarge their own habitats by means waters has remained unstudied. of drifting rafts. For example, Hernández-Carmona et al. In February to March, yellowtail juveniles start to accom- (2006) reported that sporophytes of drifting Macrocystis pany floating seaweeds in the East China Sea (Yamamoto pyrifera (L.) C. Agardh remained fertile with a high germina- et al. 2007). It is necessary to characterize the spatial distri- tion success as long as sori were present (125 days). Rafting bution of floating seaweeds in the area. Thus, using research bull kelp (Durvillaea antarctica (Chamisso) Hariot) near vessels, three surveys of seaweed rafts in the East China Sea north central Chile was also found to be fertile (Tala et al. were conducted from February to March in 2010 to 2012. We 2013). S. muticum invaded France in the 1960s and has found unusual distributions of seaweed rafts consisting of S. colonized the North Sea Coast through floating seaweeds horneri in the Kuroshio Current and also in the outer region which can grow and become fertile (Rueness 1989). of the Kuroshio Current. Huge floating masses of seaweeds Abiotic and biotic factors limit the floating persistence were stranded on the north coast of Taiwan and on Tarama and dispersal potential of floating seaweeds (Rothäusler Island in the Ryukyu Archipelago in 2012 (Fig. 1). et al. 2011). The floating persistence of seaweeds is in- Hydrographical and meteorological data were used to exam- creased by lower water temperatures and decreased by high ine the unusual phenomenon whereby many

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