Changes in abundances of Alexandrium tamarense resting cysts after the tsunami caused by the Great East Japan Earthquake in Funka Bay, Hokkaido, Japan

Highlights

• We compared abundances of the toxic Alexandrium cysts before and after the 2011 huge tsunami.

• The cysts abundance significantly increased throughout a sampling bay after the tsunami.

• After the tsunami, the large toxic A. tamarense bloom occurred throughout the bay in 2011.

• Bottom-mixing effect by the tsunami was suggested to cause the accumulation of the cysts.

Abstract

The 2011 Great East Japan Earthquake and the subsequent huge tsunami greatly affected both human activity and the coastal marine ecosystem along the Pacific coast of Japan. The tsunami also reached Funka Bay in northern Japan and caused serious damage to the scallop cultures there, and this tsunami was believed to have affected the coastal environments in the bay. Therefore, we investigated the changes in the spatial abundance and distribution of the toxic dinoflagellates Alexandrium tamarense cysts before the tsunami (August 2010) and after the tsunami (May 2011, August 2011, May 2012 and August 2012) in the bay. Further, monthly sampling was conducted after the tsunami to identify seasonal changes of Alexandrium catenella/tamarense cysts and vegetative cells. Significant increases were observed in the populations of A. catenella/tamarense cysts, comparing the abundances before the tsunami (in August 2010; 70 ± 61 cysts g⁻¹ wet sediment) to those just after it (in May 2011; 108 ± 84 cysts g⁻¹ wet sediment), and both A. tamarense bloom (a maximum density was 1.3 × 10³ cells L⁻¹) and PSP (Paralytic Shellfish Poisoning) toxin contamination of scallops (9.4 mouse unit g⁻¹ was recorded) occurred in the bay. Seasonal sampling also revealed that the encystment of A. tamarense and the supply of the cysts to bottom sediments did not occur in the bay from September to April. These results strongly suggested that the mixing of the bottom sediments by the tsunami caused the accumulation of the toxic A. tamarense cysts in the surface of bottom sediment through the process of redeposition in Funka Bay. Moreover, this cyst deposition may have contributed to the toxic bloom formation as a seed population in the spring of 2011.

Introduction

Tsunamis have affected human activity and coastal environments throughout the world. The Great East Japan Earthquake and the subsequent huge tsunami that occurred on March 11, 2011 caused catastrophic damage to human activity, buildings, and fisheries facilities in the Pacific coast of Japan, especially in the Tohoku region (Ministry of Agriculture Forestry Fisheries, 2011). This tsunami also caused many physical changes to coastal environments, and these changes have greatly influenced the coastal marine ecosystem (e.g., Akimoto et al., 2012, Tanaka, 2012, Takami et al., 2013).

The relationship between tsunamis and PSP (Paralytic Shellfish Poisoning) incidence had been empirically suspected by Japanese scientists (Fukuyo, personal communication) because the first mass PSP incident was reported with one human death at Ofunato Bay, Tohoku, Japan in 1961, just after the 1960 Chilean tsunami and earthquake (Kawabata et al., 1962). Toxic species of the genus Alexandrium form mandatory resting cells called ‘cysts’ and have a resting stage in the bottom sediments as part of their life cycle (Dale, 1977). Furthermore, it is known that the cysts resting on bottom sediments play an important role in initiating bloom formation in the water column (Anderson et al., 2012). Based on this background, some studies about distribution of the toxic Alexandrium catenella/tamarense cysts and vegetative cells were conducted along the Sanriku Coast of northeastern Japan after the 2011 tsunami. Kamiyama et al. (2014) reported the change of spatial distribution of the toxic A. catenella/tamarense cysts and the increase of cyst density in Sendai Bay, comparing sediment samples to those collected from the bay in 1995. Nishitani et al. (2012) also observed the bloom formation of A. catenella and A. tamarense in Kesennuma Bay and discovered an area with abundant A. catenella/tamarense cyst deposition in the most inner part of the bay. Similarly, Yamada et al. (2012) reported the abnormally large bloom occurrences of A. tamarense in Ofunato bay during the spring of 2011 and 2012 (Fig. 1, Table 1). They considered that the bottom-mixing effect of the tsunami led to the vertical and horizontal accumulation of the viable but buried Alexandrium cysts on the surface of bottom sediments. Thus, the accumulated cysts were able to germinate simultaneously and initiate the large bloom observed after the tsunami. In detail, the strong bottom friction by the tsunami in shallow areas (Bernard and Robinson, 2009) was believed to cause bottom sediments to mix deeply and stir up these depositing particles, including Alexandrium cysts, into the water column. Then, in the redeposition process, Alexandrium cysts settled more slowly than other abiotic and accumulated in the surface layer. As a result, the spatial distribution and abundance of Alexandrium resting cysts in the surface of the bottom sediments changed after the tsunami.

Funka Bay is semi-enclosed and is located along the Pacific coast of northern Japan, with the mouth of the bay facing southeastward, an area of ca. 2.5 × 10³ km² and a mean water depth of ca. 59 m (Fig. 1). In the bay, about 8–10 × 10⁴ tons of scallop (Mizuhopecten yessoensis) have been harvested every year with a suspension method of aquaculture (Ministry of Agriculture Forestry Fisheries, 2007–2010), and the occurrence of the toxic Alexandrium tamarense and the toxin contamination of cultured scallops have mainly impacted the scallop harvesting during spring and summer (Shimada et al., 1996, Kudo et al., 2005). The huge 2011 tsunami attacked the bay (164 cm wave height in the Mori Port and 92 cm in the Muroran Port; Fig. 1; Japan Meteorological Agency, 2011) and damaged most of the facilities for scallop aquaculture (Ministry of Agriculture Forestry Fisheries, 2011). Thus, although the greatest terrestrial damage of the 2011 tsunami did not occur around the Funka Bay when compared to the Tohoku region, the coastal environments near the bay were considered to be disrupted by the tsunami.

As stated above, the 2011 tsunami was suspected to affect the distribution of the toxic Alexandrium cysts and the occurrence of the vegetative cells in Funka Bay. This study is aimed to evaluate the influence of the tsunami on the population dynamics of Alexandrium species.