Oceanography in Japan
Online ISSN : 2186-3105
Print ISSN : 0916-8362
ISSN-L : 0916-8362
Volume 20 , Issue 1
Showing 1-3 articles out of 3 articles from the selected issue
Original Paper
  • Mitsuyasu Waku, Seiho Hashiguchi, Kiyo Kurita, Kenji Kaneko, Tomoki Mi ...
    Type: research-article
    2011 Volume 20 Issue 1 Pages 1-17
    Published: January 05, 2011
    Released: April 09, 2020
    JOURNALS FREE ACCESS

    Intensive oxygen deficiency at the borrow pit causes fatal impact to the surrounding benthic community. In order to numerically model this process, the vertical flux of sinking particles -the major oxygen consumer, was investigated using data recorded in sediment traps four times during the summer of 2007 at three stations in and around a borrow pit in Mikawa Bay. The vertical flux of particulate carbon (PC flux) varied widely from 0.35 to 15.3 g m-2 d-1 depending on the observation day. An elevated PC flux (9.48-15.3 g m-2 d-1 ) was observed at the bottom of a borrow pit (CDL: -6.9 m) and the bottom of an offshore station with fle same depth (CDL: -6.4 m) as the borrow pit in the third observation period conducted from 4 July to 6 July. The mass mortality of the macrobenthos community occurred in the sublittoral zone north of the borrow pit just before the third observation period due to the upwelling of oxygen deficient water from the borrow pit. A high-temperature, low-salinity and high-oxygen concentration water mass was expected to originate at the bottom of the sublittoral zone, and was observed temporarily at the bottom of the borrow pit during the third observation period. The contribution of the horizontally transported PC flux to the elevated PC flux was estimated on the basis of the observed PC flux and the concentration of particulate carbon in the water column. Calculation of the horizontally transported PC flux estimates it at 7-11 times greater than the vertically transported PC flux in certain station. Thus the elevated PC flux was considered to be caused by horizontal offshore transport of a huge amount of sediment accumulated on the floor of the sublittoral zone due to the mortality of the macrobenthos community. Our results suggest that further oxygen deficiency in the borrow pit could be accelerated by an elevated load of particulate organic matter tothe borrow pit owing to the mortality of the macrobenthos community in the sublittoral zone near the borrow pit.

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  • Natsumi Suga, Seiichiro Shibanuma, Toshiro Yamada, Naoyuki Higaki, Shi ...
    Type: research-article
    2011 Volume 20 Issue 1 Pages 19-36
    Published: January 05, 2011
    Released: April 09, 2020
    JOURNALS FREE ACCESS

    Monthly field observations and a 36-hour survey were conducted in the brackish lagoon of Hichirippu in the eastern part of Hokkaido, Japan (43°02'N, 145°00'E). The lagoon covers an area of 3.58 km2 and has a mean water depth of ca. 70 cm. It is inhabited by many animals and benthic plants (e.g. short-necked clam, swan, and the Japanese red-crowned crane), and is designated as a wetland under the Ramsar Convention. The rich natural environment of the lagoon, with a catch yield of fish, shellfish and seaweeds of about 90 ton/yr, should therefore be preserved as a fishery area. In Hichirippu lagoon, we investigated the spatial and temporal distribution of nutrients and physical properties. The daily volume of freshwater input was 0.9~8.0% of total volume of the lagoon, while the volume of water entering the lagoon on the rising tide per half tidal day was 34~59%. The N:P and Si:N ratios were nearly below 16 and higher than 1, respectively, indicating nitrogen limitation. Plots of nutrients vs. salinity suggested nitrate+nitrite supply from the adjacent sea (Pacific Ocean), while the origin of ammonium was neither the adjacent sea nor freshwater. The results of the 36-hour survey showed that tidal nitrate+nitrite influx and outflux was 4.3 and 3.1 kmol/half tidal day, respectively. It implicates 1.2 kmol/half tidal day was supplied to the lagoon. Tidal ammonium flux values are nearly conserved. This suggests that ammonium is mainly regenerated by clam excretion in summer. Previous studies generally have shown that the freshwater input plays an important role in controlling estuarine primary production. Our results suggest that in Hichirippu lagoon both the nutrient import from the adjacent sea and the processes of nutrient regeneration within the estuary have an impor tant effect on the primary production rather than the freshwater input.

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Contribution
  • - Result achieved through two-year activities and future perspectives -
    Naoko Namizaki, Hiroya Yamano, Rintaro Suzuki, Kenji Oohori, Hitoshi O ...
    Type: letter
    2011 Volume 20 Issue 1 Pages 37-46
    Published: January 05, 2011
    Released: April 09, 2020
    JOURNALS FREE ACCESS

    We launched Sango (Coral) Map Project that is one of the citizen monitoring programs in the ocean field in the International Year of the Reef of 2008. We developed a web-based system to collect information on coral occurrence from various people, in order to understand the current status of coral reefs of Japan. During two years, 148 people (divers, snorkelers, etc.) provided information on coral occurrence, and 367 data were collected. Collaboration with other activities such as nature tours and diving programs were generated, and the data contributed not only to validate national coral-reef distribution maps but also to publish a scientific paper. Citizen monitoring programs are one of the useful methods for the collaboration with various stakeholders. We should share successful cases and experiences of citizen monitoring programs in marine areas to encourage citizen to contribute to marine conservation.

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