Deep-sea in Sagami Bay
Organization
Department of Marine Biodiversity Research, Japan Agency for Marine-Earth Science and Technology
Research Theme
Biodiversity and environment at deep-sea in Sagami Bay
Site Outline
1. Background
Sagami Bay, Central Japan, is located at the northern end of the Izu-Ogasawara (Bonin) Islands and forms a semicircle that opens southward to the Kuroshio region of the western North Pacific (Fig. 1). The area of Sagami Bay is approximately 2,450 km2. The mouth of the Bay, located between the Izu and Boso peninsulas, is about 60 km wide and the northward extent of the Bay is about 55 km. The mouth of Sagami Bay is divided into the Oshima-East and Oshima-West channels by Oshima Island. Maximum depth in the Oshima-West channel is less than 500 m, while that in the Oshima-East channel is 1,900 m owing to the Sagami Trough, which extends southward from the center of the Bay.
Current regimes in the Bay are affected by the state of the Kuroshio Current, which flows eastward over the Shichito-Iwojima Ridge and fluctuates either onshore or offshore over the ridge. Nutrient salt concentrations are at their highest in Sagami Bay from February to April and start decreasing with the spring phytoplankton bloom. Further blooms occur during July/August, when nutrient-rich river water or water from Tokyo Bay flows into the upper layers.
Deep-sea is one of important ecological units on Earth. Deep-sea ecosystems should play important functions among global biogeochemical cycles, as most of particulate or dissolved organic carbon deposited on and stored in deep in sediments after consuming by benthic organisms at sediment-water interface. However, very limited knowledge is accumulated in terms of roles of deep-sea ecosystems on Earth. We should monitor deep-sea floor ecosystems through long-term deep-sea observations at fixed or mobile monitoring stations, in particular at deep-sea floor.
Sagami Trough is tectonically active, located at the junction of two tectonic plates, the Philippine Sea Plate and the North American Plate. Several chemosynthesis- based communities including methane seep communities (Fig. 2) and whale fall community (Fig. 3) are distributed in the bay. The central part of the bay is distal part of deep-sea fan of the Tokyo Bay Canyon and consists of organic-rich muddy substrate. Photosynthesis-based deep-sea benthic community flourishes at the deep-sea floor of the central bay where we keep another deep-sea station, named OBB2.
The horizontal biomass profile for macroplankton mirrors the topology of the continental shelf; the highest biomass (wet weight) occurs at the mouth of Tokyo Bay, along the western edge of the Miura Peninsula, and in the area east of the Izu Peninsula below Manazuru. The maximum wet weight of macroplankton as sampled by a Toku B net between 1967 and 1982 in the upper 150 m was found to occur in May at 130 g m-3 with a second smaller peak at 80 g m-3 in September. Because of the variety of water masses found in the Bay and its varied topography, species diversity is high. For example, 39 species of euphausiids were identified over a 12-month sampling period and more than 19 species of chaetognaths. Recent surveys with submersibles have identified 50 pelagic cnidarian forms and 22 forms of ctenophores in only two dives using the Shinkai 2000 submersible.
2. Target Ecosystems and Areas
2-1. Deep-sea chemosynthesis-based ecosystems
2-1-1. Methane Seep Ecosystem（Fig. 1, 2）
Off Hatsushima Island site. 35o00.14’-00.94’N, 139o13.37’-13.48’E, Depth 800-1200m.
2-1-2. Whale Fall Ecosystem（Fig. 1, 3）
North-east Off Hatsushima Island site. 35o04.94’N, 139o13.37’-12.98’E, Depth 918m.
2-2. Deep-sea photosynthesis-based ecosystems
2-2-1. Midwater Ecosystem
Above the Off Hatsushima Island site. Depth surface to 1100 m.
2-2-2. Benthic Ecosystem（Fig. 1, 4）
Central Site (OBB2), 35o00.83’N, 139o21.64’E, Depth 1455m
3. Long-term Observation
There is a long-term deep-sea observatory (1177m in the depth) at the Off Hatsushima Island site（Fig. 5）. The station assembles CTD, ADCP and TV cameras and deep-sea cable connectors. Observatory is supported by land-based electric supply. Since 1993, JAMSTEC continuously monitors both environmental changes and benthic biological activities at the permanent observatory. We could pile up time-series video records and environmental dataset. Through these dataset we can trace environmental changes at continental slope regions and also watch how deep-sea benthic organisms actively dwell at deep-sea floor in relation to environmental changes.
Fig.1. Monitoring points at deep-sea in Sagami Bay. •Off Hatsushima Island site(methane seep ecosystem) and North-east Off Hatsushima Island site(whale fall ecosystem), •Central site(OBB2).
Fig.2. Chemosynthesis-based community at Off Hatsushima site.
Fig.3. Whale fall community at North-east Off Hatsushima Island site.
Fig.4. Intensive observation at fixed monitoring point OBB2, the centoral area in the bay.
Fig.5. Long-term deep-sea observatory at the Off Hatsushima Island site.
Contact
- Address : Department of Marine Biodiversity Research, JAMSTEC, 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061, Japan
- Name : Katsunori Fujikura
- Phone : 046-867-9555
- Fax : 046-867-9525
- E-mail : fujikura@jamstec.go.jp