Nutrient Distributions in the Highly Turbid Chikugo River Estuary and the Difference Between the Early Spring and The Summer of 2010

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Particle Distributions and Transport Processes in the East China Sea

MASFLEXプロジェクトの一環として, 東シナ海のPN線を中心とした陸棚, 陸棚斜面, 沖縄舟状海盆, 及び隣接外洋域において粒状懸濁物, 濁度の分布を調べると共に, セディメントトラップ, 濁度計, 流速計の短期・長期係留実験を実施した. その結果, クロロフィル, 生物起源粒状珪素等の分布から陸棚における植物プランクトンの春季増殖, 及び長江沖の高生産力域が明瞭に示された。一方, 陸起源粒状珪素, 濁度等の分布から, 夏季・秋季の陸棚における海底高濁度層の顕著な発達と冬季鉛直混合による内部陸棚域での活発な再懸濁過程が明らかとなった. 沖縄舟状海盆の中・深層における粒子フラックスは, 陸棚上の季節風変動パターン, 台風等との関連が示峻され, 陸棚斜面域における粒子フラックスは内部潮汐の影響を大きく受けていることが示された. これらのことから, 沖縄舟状海盆の中・深層における粒子フラックスは, 海盆表層からの鉛直輸送よりは主に陸棚からの水平輸送によるものと推察された. また, 陸棚域における粒子フラックスと基礎生産量との比較から, 基礎生産量の5～20%程度が沈降粒子として有光層下に運ばれていることが明らかとなった.We have conducted a long- and short-term particulate flux studies using sediment traps together with turbidity and current meters on the shelf, slope, Okinawa Trough in the East China Sea and the adjacent Pacific Ocean. Temporal and spatial variations of suspended matter and turbidity were also studied. The distribution of chlorophyll a and particulate biogenic silica (BSiO_2) showed a phytoplankton spring bloom on the shelf and a high productive area off Changjiang River. On the other hand, the distribution of particulate lithogenic silica (LSiO_2) and turbidity revealed a well-developed bottom turbid layer on the shelf in summer and autumn, and resuspended bottom sediments of the entire water column on the inner shelf, particularly in winter mixing period. The variability of particle fluxes at the deep waters of the Okinawa Trough was seasonally coupled to local climate and oceanographic events such as monsoon wind and typhoon. The variability of particle fluxes on the slope was also influenced by the short-term events such as internal tide. The observation showed that resuspension and near-bottom transport of the sedimented particles from the shelf to the Okinawa Trough may be key processes of cross-shelf transport of biogenic and lithogenic particles. Further, 5-20% of annual primary production was transported into deep layers, indicating that the most of primary production is utilized within the euphotic zone

Horizontal and vertical variations in survival rates of juvenile bivalves, Anadara kagoshimensis (Tokunaga, 1906), Ruditapes philippinarum (Adams & Reeve, 1850) and Atrina sp., experimentally caged in the inner area of the Sea of Ariake, Japan

Abstract In the inner area of the Sea of Ariake, rearing experiments were conducted to examine how the survival rates of juveniles of three bivalve species (Anadara kagoshimensis, Ruditapes philippinarum, Atrina sp.) vary among sea areas and heights above the seabed. In three runs of the experiment, replicate samples of the three bivalves were kept in trays set at various heights above the seabed (range 0–40 cm) at one to six stations for 15–16 days. Results were as follows. (1) Survival rates varied among heights and stations more markedly for Atrina sp. than for A. kagoshimensis and R. philippinarum; (2) Atrina sp. always showed higher survival rates at greater heights above the seabed, whereas the other species did not always show this pattern. (3) Survival rates of Atrina sp. showed a different station-to-station variation pattern between experimental runs. (4) In statistical model selection, the spatial variations in the survival rate were not associated for either A. kagoshimensis or R. philippinarum with any of the environmental factors recorded (density of suspended solids, density of chlorophyll a, water temperature). In contrast, for Atrina sp. they were strongly and negatively associated with the spatial variations in density of suspended solids

Unique Dispersal of the Changjiang-Diluted Water Plume in the East China Sea Revealed from Satellite Monitoring of Colored Dissolved Organic Matter (CDOM)

The optical properties of colored dissolved organic matter (CDOM) in the Changjiang (Yangtze River) plume water were investigated during the summer of 2009 and 2010. The absorption coefficient of CDOM at 325 nm (aCDOM) increased inversely with decreasing sea-surface salinity (SSS), implying that aCDOM can be used as a natural tracer of Changjiang-diluted water (CDW). This aCDOM vs. SSS relationship, however, differed between 2009 and 2010. For mapping the CDW plume, the aCDOM was retrieved from an ocean-color satellite. Values of SSS were also derived from the satellite-retrieved aCDOM using field-based SSS vs. aCDOM relationships. Satellite observations revealed the temporary variable eastward extension of a high aCDOM, low SSS CDW plume in the central East China Sea (ECS) during the summer. The CDW plume during the summer of 2010 extended southeastward from the mouth of the Changjiang (Yangtze River) to almost 26°N. Moreover, the branch of this southeastward CDW plume was seen to extend northeastward to the eastern ECS where the Kuroshio Current flows northeastward along the shelf break. Subsequently, this branch was distributed around southern Kyushu. Satellite observations revealed this unique dispersal of the CDW plume, which illustrates that the aCDOM and SSS from ocean-color satellite data are useful for monitoring the dispersal of this river-water plume

The long-term freshening and nutrient increases in summer surface water in the northern East China Sea in relation to Changjiang discharge variation

Using historical in situ data sets, we investigated long-term variations in summer surface salinity and dissolved inorganic nitrogen concentration (DIN) in the northern East China Sea (ECS). From 1950 to 2002, there were interannual and decadal variations in sea surface salinity, as well as a long-term decrease. Along with the Changjiang discharge, wind fields and surface circulations appeared to play roles in determining the interannual salinity variation. Although we attributed the long-term decrease in surface salinity to a long-term increase in the Changjiang discharge, the high salinity between 1978 and 1988 appeared to be associated with an increase in the Kuroshio Current transport. The increase in DIN from 1971 to 2001 appeared to be caused by a combination of changes in Changjiang discharge and an increase in nitrogen fertilizer use in China. The long-term increase in DIN modified the nutrient composition, implying that biological production in the northern ECS changed from a DIN-limiting to a phosphorous-limiting. We should pay attention to changes in the northern ECS ecosystem in future decades because the volume of the Changjiang discharge, as well as its physical and chemical properties, may change dramatically because of the continuous anthropogenic perturbations and climate change