Kazuo Iseki

Seasonality and composition of downward particulate fluxes at the continental shelf and Okinawa Trough in the East China Sea

Abstract As part of the Marginal Sea Flux Experiment in the West Pacific (MASFLEX) program, conventional sediment traps were deployed at three mooring sites on the East China Sea shelf for 1–2 days during 4 seasonal cruises. Time-series sediment traps also were deployed in the East China Sea, one at the northern part of the Okinawa Trough (Stn. SST 1) and the other to the north-west of the Okinawa Island (SST 2), for 1 year from March 1993 to February 1994. The settling particles were analyzed for total mass flux and the abundance of major biogenic constituents (organic carbon, nitrogen, inorganic carbon and opal). On the inner shelf, particulate fluxes were large and rapidly increased near the bottom throughout the year, probably due to resuspended bottom sediment and suspended particles from the Changjiang, forming a bottom turbid layer (BTL). On the central shelf, particulate fluxes in the BTL were extremely high in summer when Changjiang discharge peaks, possibly indicating a seasonal supply of the Changjiang particles to the central shelf. At the shelf edge, the fluxes in the BTL were significantly higher in autumn and winter than in spring and summer. At the Okinawa Trough stations (ca. 1100 m ), particulate fluxes at 600 m depth were extremely low but increased noticeably with depth (800 and 50 m above the bottom). The flux generally peaked from late autumn to early spring and was lowest during summer, with an unusually large peak in September at SST1. Considering that there is no strong seasonality of the primary productivity and phytoplankton biomass in the surface layers at the Okinawa Trough, we hypothesize that the seasonality of particle fluxes at the deep waters of the Trough is linked to seasonal events in shelf waters. Downwelling and seaward bottom flow induced by the northeast monsoon in winter favor the offshore transport of particles in the BTL, while in summer a transverse circulation pattern may inhibit it (Hu, 1994; Yanagi et al., 1996). Near-bottom transport may be a key process for shelf-to-deep sea export of biogenic/lithogenic particles. As a result, the continental slope and the Okinawa Trough may play an important role as a carbon sink for the East China Sea.

Ocean Color Satellite Imagery and Shipboard Measurements of Chlorophyll a and Suspended Particulate Matter Distribution in the East China Sea

Satellite-derived ocean color data of Coastal Zone Color Scanner (CZCS) on board the Nimbus-7 and Ocean Color and Temperature Scanner (OCTS) on board the Advanced Earth Observing Satellite (ADEOS) are jointly used with historical in situ data to examine seasonal and spatial distributions of chlorophyll a (Chl-a) and suspended particulate matter (SPM) concentrations in the East China Sea. Ocean color imagery showed that Chl-a concentrations on the continental shelf were higher than those of the Kuroshio area throughout the year. Satellite-derived Chl-a concentrations are generally in good accordance with historical in situ values during spring through autumn (although no shipboard in situ measurement was conducted at nearshore areas). In contrast, ocean color imagery in winter indicated high Chl-a concentrations (4–10 mg m−3) on the continental shelf where bottom depth was less than 50 m when surface water was turbid (2–72 g m−3 of SPM at surface), while historical in situ values were usually less than 1 mg m−3. This suggests that resuspended bottom sediment due to wind-driven mixing and winter cooling is responsible for the noticeable overestimation of satellite-derived Chl-a concentrations. The algorithm for ocean color needs to be improved urgently for turbid water.

Variation in biomass and species composition of epiphytic community on the different aged leaves of Zostera marina (Chlorophyta)

The development of the epiphytic community on the leaves of Zostera marina was followed by comparing variations in epiphytic organic carbon, chlorophyll a, cell abundance, and species composition between the younger and older leaves. Biomass of epiphytic community on Z. marina depended on leaf age; particulate organic carbon and cell abundance on the older leaves were 21 and 192 times higher than those on the youngest leaves, respectively. The abundant taxa of epiphytic community were Leptolyngbia sp. (Cyanobacteria), Cocconeis scutellum, Campylopyxis garkeana, and Gomphonemataceae (Bacillariophyceae), and the change of species composition of epiphytic community was independent on the leaf age. Significant relation between CHLa and POC showed organic matter on the leaves was consisted of algal cells. The amount of epiphytic POC was well correlated with diatom cell abundance (P<0.1), but not with total cell abundance. The prevalence of diatoms instead of Cyanobacteria on the leaves was shown by both chemical and taxonomic analyses. Low POC/DW ratios of epiphytic community on the older leaves indicated that inorganic suspended particles were more likely to adhere.

周防灘における光環境，クロロフィルa 及び濁度の季節・海域変動

2007年の5月から11月にかけて，ほぼ毎月，周防灘全域の全水柱（ここでは，海表面から海底直上20cmまで）における水温，塩分，光量子，クロロフィルα（以下Chl.α），濁度の鉛直分布及び透明度の海域分布を調査し，アサリ生産との関連性について検討した。その結果，海底高濁度層が灘全体に見られ，特に灘南西部の浅海域（水深10m程度）では，観測期間を通じて沖合域よりも濁度は高く，Chl.αも高濃度だった。これらの浅海域を含めて，灘西部の水深20m程度以浅の海域では，一般に海表面から海底までの全水柱が有光層になっていた。周防灘のアサリ漁場は，干潟から水深5m程度までの海域にあるため，底生濾過捕食者のアサリにとって，餌料供給の点では沖合域よりは好適条件を備えていると考えられた。また，Chl.α濃度は，鉛直混合の開始初期の10月に周防灘全体で最も高く，時期的にはアサリの秋季産卵の数週間前であると考えられた。これらのことから，周防灘のアサリ生産は，地域的・季節的な環境条件の違いとそれに伴う植物プランクトンの変動に関連があることが明らかとなった。