China Sea Off Research Articles

Amblypomacentrus vietnamicus sp. n., a new damselfish from the South China Sea (Teleostei: Perciformes: Pomacentridae)

A new species of Amblypomacentrus is described from the South China Sea off Vietnam. It differs from congeners in scale and gill-raker counts, slightly protruding snout, and coloration. This is the first record of this genus from the mainland Asia.

Variabilities of particulate flux and 210Pb in the southern East China Sea and western South Okinawa Trough

Abstract Variabilities of the particulate mass flux and associated 210 Pb activity were examined on samples collected from time-series sediment traps deployed in the continental slope area of the East China Sea off northeast Taiwan and in the western South Okinawa Trough (SOT) as a part of the KEEP program. The particulate flux decreases laterally from the continental slope toward the trough, increases vertically toward the bottom at most of the mooring sites, and shows fairly synchronous temporal variations among the traps in each mooring for most of the deployments. The temporal variation of 210 Pb is generally smaller than that of the particulate flux in terms of relative change as expressed in standard deviation of the mean of each trap. In the lower slope area, the mean particulate flux varies between 9.5 and 79 g/m 2 /d, and the mean 210 Pb varies between about 44 and 81 dpm/g. In the trough, the mean particulate flux has the smallest values, from 0.06 to 5.7 g/m 2 /d, while the mean 210 Pb has the highest, from 82 to 192 dpm/g. The particulate flux decreases while the 210 Pb activity increases from the slope toward the trough. In the trough, 210 Pb increases with depth while the particulate flux may show a mid-depth maximum of a nepheloid layer. Although the 210 Pb activity increases from the slope toward the trough the 210 Pb flux decreases toward the trough because of much greater particulate flux in the slope area. Within the trough the 210 Pb flux as determined from the traps is comparable to that determined from the underlying sediments, but on the lower slope, the 210 Pb flux determined from the traps is much higher than that obtained from the underlying sediments. Thus a large portion of the particulates and the associated 210 Pb may have been transported laterally from elsewhere to the slope area in transit without depositing onto the underlying sediments. Plots of the mean 210 Pb flux versus the mean mass flux for each of the traps show positive linear correlation with four distinctive slopes, each representing a mean 210 Pb activity of a particular area or a group of traps. In the Mien-Hua canyon the mean activity is about 40 dpm/g; in the SOT the mean value is about 130 dpm/g; on the lower slope the mean values fall in-between those of the canyon and the trough. In the middle traps where the mass flux is higher in a nepheloid layer the mean activity is about 47 dpm/g, while the remaining traps yield a mean of about 65 dpm/g. Taking the mean activity of the canyon and that of the trough as the two end-member values, the slope with a nepheloid layer is a mixture of about 90% canyon and 10% trough materials, indicating the canyon as the major particulate provider for the nepheloid layer. The slope without a nepheloid layer is a mixture of about 70% canyon and 30% trough materials.

Carbon flux during the last interglacial cycle in the inner continental shelf of the South China Sea off Hong Kong

Information obtained from previous studies has been used to study carbon flux during the last interglacial cycle in the siliciclastics-dominated inner continental shelf of the South China Sea off Hong Kong. The information included grab sampling surveys, many kilometres of high-resolution reflection seismic profiles and numerous boreholes. During the high sea-level stand of the Holocene and the last interglacial period, the inner shelf of Hong Kong was a net carbon sink. This is particularly so in the present day mainly through the discharge of anthropogenic sewage. The offshore dredging of mud and the offshore mining of sand and gravel deposits have nevertheless led to the release of methane and other gases either trapped beneath Holocene deposits or originating from the bacterial breakdown of plant matter within Pleistocene deposits contributing greenhouse gases. During the last glacial period, significant quantities of carbon dioxide formed by the dissolution of carbonates through acid-sulphate development on the subaerially marine deposits of last interglacial age, and biogenic methane formed by the bacterial breakdown of fossilized plant matter in the underlying deposits was released into the atmosphere from the exposed shelf. Due mainly to the poor drainage conditions, the exposed shelf was unlikely to support a dense vegetation cover causing a lowering of carbon storage through terrestrial plants. The role of greenhouse gases generated, trapped and escaping from the present-day continental shelves during the last interglacial cycle requires consideration in the estimation of global carbon flux. Reflection seismic profiling is useful as a method for mapping the present-day shelves to permit the identification of areas affected by acoustic turbidity. For the interval between the Holocene and the last interglacial, the geochemical characterization of sediment extracts is needed for the estimation of carbon flux and for obtaining information on past vegetation history. This necessitates the study of outer shelf sediments from water depths between about 130 and 200 m below the present-day sea level. The release of greenhouse gases from the exposed shelves during the last glacial period is suggested to provide a triggering mechanism for switching the earth into the present interglacial mode.

Distribution of moisture contents and thermoluminescence ages in an inner shelf borehole from the new Hong Kong International Airport site, China

Offshore boreholes and excavations available through coastal engineering projects in the siliciclastics-dominated inner shelf of the northern South China Sea off Hong Kong have permitted the reconstruction of Quaternary sea-level history dating back to almost 0.5 Ma. In this study, moisture content and thermoluminescence (TL) dating are utilized to help verify ages for marine and terrestrial sediments in a 39.52 m offshore borehole from the new Hong Kong International Airport site. The distribution of moisture contents in borehole samples indicated the presence of three palaeo-desiccated crusts formed by soil development when the marine deposits of Oxygen Isotope (OI) stage 5, OI stage 7 and OI stage 9 were sub-aerially exposed during the low sea-level stand(s) of OI stages 2–4, OI stage 6 and OI stage 8, respectively. Out of the six TL dated borehole samples, three from a terrestrial deposit horizon have yielded finite ages of about 250,000 yr BP suggestive of OI stage 8. In these samples, the TL signal of quartz grains was found to increase well beyond the 250 Gy saturation level with the relatively high radiation fluxes allowing for age determination for only three samples. The three remaining samples of pre-Holocene marine deposits yielded minimum ages each having reached TL saturation. The importance of the moisture content upon the final determination of these sediment ages and the collection and storage of samples for TL dating is clearly demonstrated.

Diatom preservation in an inner continental shelf borehole from the South China Sea

Diatom distribution in a 60-m long borehole from a siliciclastics-dominated ‘stable’ inner continental shelf in the South China Sea off Hong Kong was investigated. The Quaternary sediments contained evidence for five interglacial marine transgressions dating back to oxygen-isotope Stage 12, and yielded 53 genera and 186 taxa of diatoms. Intertidal species and nearshore shallow water species, including both planktonic and benthonic tropical and subtropical varieties, were dominant. Four diatom zones reflecting the shallow water environment have been recognized in the top 13.8 m of the borehole. Diatoms are missing in Stage 11 while Stage 7 and Stage 9 are very low in diversity and abundance compared to Stage 1 and Stage 5. The distribution pattern found is accounted for by the duration of time available for frustule dissolution through acid–sulphate soil development during Stages 2, 6, 8 and 10 when the shelf was subaerially exposed. Preservation of diatoms is best in Stage 1 because the deposits have never been subaerially exposed followed closely by Stage 5. Aging mainly through post-depositional dissolution is identified as the important controlling factor in determining diatom distribution in pre-Holocene deposits.

Cross-shelf and along-shelf nutrient fluxes derived from flow fields and chemical hydrography observed in the southern East China Sea off northern Taiwan

Cross-shelf exchange of nutrients (N, P and Si) off northeastern Taiwan and along-shelf transports just north of the Taiwan Strait was assessed using chemical hydrography and velocity fields observed in August 1994 (summer) and March 1997 (winter). The summer survey was conducted along the periphery of a triangle. The velocity field was determined by phase-averaged current velocities measured with ship-borne ADCP on two rounds separated by 3.5 cycles of the dominant M 2 tide. Nutrient distributions were derived from phase-averaged hydrographic data. Although the inflowing and outflowing volume transports were mismatched by 14% largely due to poorly estimated surface flow that carried little nutrients, the inputs and outputs of nutrients agreed within 2–5%. Such consistency lends support to the adopted approach for estimating nutrient fluxes. The winter survey, which provided observations on two transects, one along-shelf and one cross-shelf, was conducted using two ships separated by 1/2 cycle of the M 2 tide. Results from the two ships allowed determination of the phase-averaged flow fields and chemical hydrography. The two surveys indicated that the Kuroshio upwelling provided rather constant nutrient fluxes to the shelf, which were comparable to the total nutrient influxes from slope waters to the Mid and South Atlantic Bight and considerably larger than the riverine sources. Significant amounts of nutrients from the shelf leaked back into the surface layer of the Kuroshio, replenishing its impoverished nutrient reserves. Along-shelf nutrient fluxes from the Taiwan Strait changed drastically between seasons, ranging from less than half to more than double the Kuroshio inputs. In the light of the variability of the shelf environment, more observations are needed to better determine the mean condition of this transport.

Biomass distribution and zooplankton composition of the sound-scattering layer in the waters of southern East China Sea

Fuzzy classification was used to analyse hydroacoustic survey data (Sv, an acoustic volume backscattering strength index of biomass) and oceanographic data (temperature, salinity, water density, and chlorophyll a fluorescence) collected from the waters of the southern East China Sea off northeastern Taiwan. Four types of water masses were identified. Type A water mass corresponded to warm continental-shelf mixed water and had a high acoustic volume backscatter (Sv>59.37 dB). Type D corresponded to cold upwelling Kuroshio subsurface water with Sv<65.24 dB, i.e. about 6 dB lower than that of Type A. Type B and C waters were intermediate between Type A and D waters. Twenty-two IKMT (net) samples were taken, and their estimated biomasses showed a significant relationship to corresponding average Sv values. Copepoda were most dominant in the zooplankton community. Decapoda, appendicularia, and chaetognatha were abundant in Type A waters, and proportionally rarer in the other three types of water masses. Conversely, the percentage contribution made by ostracoda and gastropoda was greater in Type D waters than in Type A waters. The combination of IKMT net sampling and hydroacoustic measurements was found to be useful for assessing the biomass distribution and species composition of zooplankton. We also conclude that the fuzzy classification method used here can usefully be applied to hydroacoustic data for the classification and identification of water masses and zooplankton composition in the ocean, particularly in an upwelling area. 1999 International Council for the Exploration of the Sea

Neotectonic regime on the passive continental margin of the northern South China Sea

Between 1989 and 1994, more than 6600 km of reflection seismic profiles were obtained in the South China Sea off Hong Kong with the German research vessel Sonne during cruises SO-50B, SO-72A and SO-95. A seismo-stratigraphic interpretation of this data set leads to a new age assignment of the unconformity T 0 which we place within the Pleistocene. Both Neogene unconformities T 1 and T 0 are generated by uplift of the Dongsha Rise and truncation of their overlying strata. This uplift is caused by intrusion of magma into the upper crust. Our seismic profiles show plutons which have penetrated the sedimentary cover, whereby their original stratification in the contact zone is eliminated. These magmato–tectonic events may be correlated to the two main collision phases between Taiwan and the continental margin of East China 5–3 and 3–0 ma ago. The collisional events subsequent to the NNW to WNW drift of Taiwan transformed the compression into strike–slip movements along the continental margin of Southeastern China. The accompanying stress regime is transtensional, with subsidence of the cooling oceanic crust since the cessation of rifting and its consumption beneath the Manila Trench providing the extensional stress. The strike–slip movements remobilized many of the rift and drift faults providing pathways for magma ascent. The tectonic framework of the northern South China Sea is characterized by Miocene faults trending NE–SW. These faults are scarce but are distributed throughout the study area. Pliocene faults striking ENE–WSW to NE–SW are concentrated west of the Dongsha Islands and are mostly strike–slip in character. Recent faults are generally oriented NE–SW subparallel to the synrift faults. They result in part from local uplifts where they are normal in character, but strike–slip motion also occurs. Most of the faults involve the basement and represent reactivated zones of weakness of the rift and drift phases.

Reproduction and growth of Jack-knife shrimp, Haliporoides sibogae, off south-western Kyushu, Japan

Jack-knife shrimp Haliporoides sibogae (Decapoda, Penaeoidea, Solenoceridae) is the target species in a boat seine fishery in the East China Sea off south-western Kyushu, Japan. The reproduction and growth of this species were studied based on a large number of specimens which were sampled during 19 months in a commercial fishing ground. The body length at sexual maturity for females was estimated to be 100 mm (2 year old). The spawning season continues from September to April and peaks during late January and late March. Only the 3 year old group of females matured in the early spawning season, while the 2 and 3 year old groups matured in the primary spawning season. The growth patterns for both sexes were well modelled by fitting the Pauly and Gaschüktz equations to the mean body lengths at various ages. The growth rates were highest between summer and autumn and lowest during the primary spawning season. Females grow faster and attain a greater size earlier than males of the same age group. The H. sibogae population off south-western Kyushu has lower values of asymptotic size and higher values of growth coefficient in comparison to other local populations of this species, such as those in Kii Channel and eastern Australia.

Fossil red deer Cervus elaphus from the sea-floor of the East China Sea off Amakusa-shimoshima Island, southwestern Kyushu, Japan

Fossil red deer Cervus elaphus from the sea-floor of the East China Sea off Amakusa-shimoshima Island, southwestern Kyushu, Japan

Root production and belowground seagrass biomass

The root and rhizome biomass of the seagrass species present in 3 mixed and 2 monospecific meadows representative of different floras (Spanish Mediterranean, Mexican Caribbean, Kenyan coast, and the South China Sea off The Philippines) was examined to test for the existence of general patterns in the distribution of their biomass in the sediments, and to test a simple approach based on age determinations to estimate root production. The thickness of the roots was scaled to the thickness of the seagrass rhizomes (r = 0.92, p 100 and >200 g DW m(-2), respectively) for the mixed meadows examined; these belowground structures had a projected surface area often exceeding 1 m(2) m(-2) when roots and rhizomes were considered together, and they formed a dense web of root material comprising several hundred meters per square meter. Belowground biomass showed considerable vertical stratification within the sediments, with a tendency for the larger species to extend deeper into the sediments than smaller ones. This tendency for segregation should reduce the potential interspecific competition for sediment resources, which is likely to be greater in the uppermost layers, where the belowground biomass is more evenly distributed among species. The rate of adventitious root production on vertical shoots varied from species that produced a root on almost every node to species that produced 1 adventitious root for every 10 nodes. Root production-both on horizontal rhizomes and Vertical shoots-was substantial, with the combined root production approaching, or exceeding, 1000 g DW m(-2) yr(-1). The resulting root turnover was quite high, with most Values ranging between 2 and 10 yr(-1), indicative of a characteristic turnover time of months for the root compartment. The estimates of root production derived here often exceed those of rhizome production and reach Values comparable to leaf production, clearly demonstrating that root production is an important component (up to 50%) of total seagrass production. [KEYWORDS: seagrass; roots; rhizomes; biomass production Cymodocea-nodosa; syringodium-filiforme; thalassia-testudinum; posidonia-oceanica; growth-patterns; carbon; beds;decomposition; limitation; responses]

Distribution and diversity of recurrent groups of demersal fishes in the South China Sea off Viet Nam.

The distribution and diversity of demersal fish communities were described in the South China Sea off Viet Nam from December 1968 to July 1969. The first recurrent group including lizardfish as a principal member was located dispersedly along coastal front from winter to summer. Snapper which was a principal member of the second recurrent group coexisted with largeeyed bream at 40-60m depth in winter. However, snapper coexisted with grunt in the shallow region less than 40m depth in summer. Diversity was relatively high in southeastern area, and was low off the Mekong delta.

長崎県男女群島に来遊するブリの産卵群について

In the present paper are considered some characteristics of the shoals of the yellow-tail, Seriola quinqueradiata T. & S., which are migrating to Meshima Fishing Ground around the Danjo Islands in the East China Sea off Nagasaki, Japan. The author has based these characteristics on the close examination of the gonads of the fish caught in April and May, 1958. By the way, yellow-tails are then in season in that locality. The region around the Islands is oceanographically interesting to us for its high temperature with a narrow annual range and its high transparency ranging from 15 to 30m, as is seen from Table 1. The author can safely conjecture that the fish which migrate to the fishing ground just referred to between mid-April and early May are ready to spawn, judging from the fully ripe gonads they have, though most of the fish are medium-sized ones ranging from 70 to 85cm in fork length, or from 4.9 to 9.7kg in body weight. Their eggs spawned in the region may go up north carried by the Tsushima Current and grow up to be the resources in the Japan Sea. But larger fish more than 85cm in length, or over 9.7kg in weight, which are caught early in the season in the same fishing ground, are found to be ‘immature’, so far as the maturity grade of their gonads is concerned. They may migrate down to the south for spawning, and the greater part of the eggs spawned there may drift with the Kuroshio Current, adding finally to the resources in the Pacific. As for smaller fish ranging from 60 to 70cm in length, or from 3.4 to 4.9kg in weight, they do not spawn in the same fishing ground, but go up north after mid-May. Their eggs spawned there may bid fair to increase the resources in the Japan Sea. The body-size composition and maturity grade of the fish which come migrating to Meshima are diagrammatically shown in Fig. 2.