Modern dinoflagellate cysts in hypertrophic coastal waters of Tokyo Bay, Japan

JOURNAL OF PLANKTON RESEARCH j VOLUME 25 j NUMBER 12 j PAGES 1461-1470 j 2003 Modern dinoflagellate cysts in hypertrophic coastal waters of Tokyo Bay, Japan KAZUMI MATSUOKA LINDA B. JOYCE 1 2 YUICHI KOTANI 0 YUKIHIKO MATSUYAMA 0 0 FISHERIES RESEARCH AGENCY NATIONAL RESEARCH INSTITUTE OF FISHERIES AND ENVIRONMENT OF INLAND SEA 1 HERIOT-WATT UNIVERSITY (ORKNEY CAMPUS) 2 PRESENT ADDRESS: MARINE AND COASTAL MANAGEMENT A survey of dinofl agellate resting cysts in surface sediment samples was carried out in Tokyo Bay, Japan, to document their horizontal distribution. At least 21 different cyst types were found. Dominant cyst types allowed the recognition of assemblages which form three different dinofl agellate cyst communities: the innermost part of the Bay, the central area and the mouth area. In all stations in Tokyo Bay, heterotrophic dinofl agellate cysts always occupied more than half of the cyst populations. Cysts of Polykrikos schwartzii/kofoidii are the most abundant heterotrophic species. These assemblages may refl ect highly nutrient-enriched (hypertrophic) and turbulent water conditions. Among the cyst types found were probable ellipsoidal cysts of Alexandrium tamarense. This is the first record of toxic Alexandrium species cysts in Tokyo Bay sediments. - I N T R O D U C T I O N Many marine dinoflagellates include species that cause red tides, shellfish poisoning and other harmful events, and some of them have a resting cyst stage as part of their life history. Those harmful events associated with dinoflagellate species are mainly caused by the motile biflagellated stage in the life history. Cysts are important in studies of dinoflagellate ecology and biogeography (Wall et al., 1977). More than 80 species of marine dinoflagellates are now known to produce cysts as part of their life history (Matsuoka and Fukuyo, 2000). Many qualitative or semi-quantitative studies of cyst assemblages in coastal areas have been conducted in the past. Studies carried out in the Pacific include surveys from New Zealand (Baldwin, 1987), Tasmania (Bolch and Hallegraeff, 1990), South Korea (Lee and Matsuoka, 1994) and Japan (Matsuoka, 1985, 1987). No such survey, however, has ever been conducted in Tokyo Bay. Matsuoka reported preliminary data of only dinoflagellate cyst concentration and species number in the sediments of Tokyo Bay without a discussion of the species diversity of modern dinoflagellate cysts in surface sediments around the Japanese Islands (Matsuoka, 1992). A more advanced study on dinoflagellate cysts in Tokyo Bay was later carried out by Matsuoka (Matsuoka, 1999). That study was primarily to investigate the process of eutrophication using fossilized dinoflagellate cysts in the sediments; however, the study was restricted to the Yokohama Port area of Tokyo Bay. The present study was conducted over the whole of the Bay. Surveys of cyst assemblages in sediments can give an early warning of the presence of toxic species in a given area. The present study describes the species composition, abundance and distribution of recent marine dinoflagellate cysts from Tokyo Bay with the first occurrence of possible cysts of toxic Alexandrium tamarense, and discusses the environmental significance of the heterotrophic cyst-dominated assemblage. M E T H O D Study area Tokyo Bay is a semi-enclosed bay located in the centre of the south-east coast of Honshu, the main island of Japan. The Bay is open to the western region of the North Pacific via the narrow Uraga Strait and covers an area of 960 km3 (Kawabe, 1998). The Bay supports the greatest concentration of human population of perhaps any water body in the world (Kawabe, 1998). Tokyo Bay has long been subject to a variety of human activities, mainly since the 1600s, and has received substantial anthropogenic inpacts on both the shoreline and the water body itself (Kawabe, 1998). Sampling Sediments were collected with a KK type corer at 35 stations in Tokyo Bay (Figure 1), by the R/V Shirafuji-Maru, belonging to the National Research Institute of Fisheries and Environment of Inland Sea of Fisheries Agency of Japan, at the end of August, 1999. A transect line was placed from the innermost area of the Bay to the mouth of the Bay to investigate the horizontal change in dinoflagellate cysts. For this purpose, samples from 13 stations were selected and analysed. The samples were made by mixing the upper 5 cm of the sediments and then kept in dark cool conditions until analysis. Sample preparation and microscopy The original samples were divided into two parts: one for measuring water content to show dinoflagellate cyst concentration per gram dry weight (DW) of sediment and the other for dinoflagellate cyst analysis in wet condition. The samples for dinoflagellate cyst analysis were treated using the method suggested by Matsuoka and Fukuyo (Matsuoka and Fukuyo, 2000). An 2 ml wet sample from each station was placed in a polyethylene beaker and then treated to remove c (...truncated)