Mean water level setup/setdown in the inlet-lagoon system induced by tidal action—a case study of Xincun Inlet,Hainan Island in China

With the tides propagating from the open sea to the lagoon, the mean water level （MWL） in the inlet and lagoon becomes different from that at the open sea, and a setup/setdown is generated. The change of MWL （setup/setdown） in the system imposes a great impact on regulating the development of tidal marshes, on determining the long-term water level for harbor maintenance, on the planning for the water front development with the flood control for the possible inundation, and on the interpretation of the historical sea level change when using tidal marsh peat deposits in the lagoon as the indicator for open sea＇ s sea level. In this case study on the mechanisms which control the setup/setdown in Xincun Inlet, Hainan in China, the 2-D barotropic mode of Eulerian - Lagrangian CIRCulation （ELCIRC） model was utilized. After model calibration and verification, a series of numerical experiments were conducted to examine the effects of bottom friction and advection terms, wetting and drying of intertidal areas, bathymetry and boundary conditions on the setup/setdown in the system. The modeling results show that setup occurs over the inlet and lagoon areas with an order of one tenth of the tide range at the entrance. The larger the bottom friction is, a larger setup is generated. Without the advection term, the setup is reduced clue to a decrease of water level gradient to compensate for the disappearance of the advection term. Even without overtides, a setup can still be developed in the system. Sea level rise and dredging in the inlet and tidal channel can cause a decrease of setup in the system, whereas shoaling of the system can increase the setup. The uniqueness of the Xincun Inlet with respect to MWL change is that there is no evident setdown in the inlet, which can be attributed to the complex geometry and bathymetry associated with the inlet system.

Morphology and distribution of the marine diatom Azpeitia africana(Janisch ex A. Schmidt) G. Fryxell & T. P. Watkins in the South China Sea

The purpose of the present paper is to study the morphological structure and variability of Azpeitia africana and to determine its geographical distribution in the surface sediments of the South China Sea(SCS). Sediment samples were collected with grabs or box corers in one cruise in 2001 and two cruises in 2007. The sampling stations were located between 3°56.61′–20°59.37′N and 108°30.68′–116°46.70′E,where the water depth ranged from 72 m to 4 238 m. The diatom was observed by phase contrast microscopy and scanning electron microscopy. Microscopical observation showed that A. africana had circular valves with the areolar lines radiating from the eccentric ring. The central rimoportula had an external tube recessed on the edge of a central ring. The marginal rimoportulae were not evenly spaced, and they were positioned more closely together in one quadrant than in the others. Azpeitia africana is the most abundant diatom species in the southern region of the SCS, and accounted for 0.9%–5.6% of all diatom species in the Xisha Islands area. Average cell density of A. africana was 1.1×10~5 valves/g. The percentage abundance of A.africana was low(0%–2.5%) in the northern regions of the SCS and the Sunda Shelf, and it was not detected in the northwestern continental shelf(shallow water area) and northern Kalimantan Island shelf. Our results suggested that A. africana is a typical warm water species and that it could be used as an indicator of the warm Paci?c Ocean water, including the Kuroshio Current, ?owing into the SCS.

Diatom assemblages in surface sediments from the South China Sea as environmental indicators

We studied diatom distribution from 62 samples from the uppermost 1 cm of sedimem in the South China Sea （SCS）, using grabs or box corers in three cruises between 2001-2007. Fifty six genera, 256 species and their varieties were identified. Dominating species included Coscinodiscus africanus, Coscinodiscus nodulifer, Cyclotella stylorum, Hemidiscus cuneiformis, Melosira sulcata, Nitzschia marina, Roperia tesselata, Thalassionema nitzschioides, Thalassiosira excentrica, and Thalassiothrix longissima. Most surface sediments in the SCS were rich ill diatoms, except for a few coarse samples. Average diatom abundance in the study area was 104 607 valve/g. In terms of the abundance, ecology, and spatial distribution, seven diatom zones （Zones 1-7） were recognized. Zone 1 （northern continental shelf） is affected by warm currents, SCS northern branch of the Kuroshio, and northern coastal currents; Zone 2 （northwestern continental shelf） is affected by intense coastal currents; Zone 3 （Xisha Islands sea area） is a bathyal environment with transitional water masses; Zone 4 （sea basin） is a bathyal-to-deep sea with stable and uniform central water masses in a semi-enclosed marginal sea; Zone 5 （Nansba Islands marine area） is a pelagic environment with relatively high surface temperature; Zone 6 （northern Sunda Shelf） is a tropical shelf environment; and Zone 7 （northern Kalimantan Island shelf area） is affected by warm waters from the Indian Ocean and coastal waters. The data indicate that these diatom zones are closely related to topography, hydrodynamics, temperature, nutrients and especially the salinity. Better understanding of the relationship between diatom distribution and the oceanographic factors would help in the reconstruction of the SCS in the past.

Coral cover as a proxy of disturbance:A case study of the biodiversity of the hermatypic corals in Yongxing Island, Xisha Islands in the South China Sea

The Yongxing Island is the biggest island of Xisha Islands in the middle part of the South China Sea. It occupies 1.8 square kilometers of land area and has permanent residence on it. There are only a few papers on the coral community of the Xisha Islands, reporting the species composition, structure and zonations of the hermatypic coral community in 1970s. The present study describes the hermatypic coral community based on the quadrat survey after almost 30 years in Yongxing Island in August 2002. It was the first time to present the percent cover data of live corals of Yongxing Island via a systematic scheme of quantitative quadrat sampling. The average total percent cover of the live coral is 68.4%. We found the total percent cover (TPC) could be an idea proxy of the overall disturbance regime impinging on the coral community. TPC can be used as an integrated measure of disturbance to coral reef. Using this surrogate, the best fit relation between TPC of the live corals and species richness, species diversity, or the species evenness is the unimodal second-order polymorphic parabola equation. This result is expected by the intermediate disturbance hypothesis (IDH). From the fitted parabola equations, we calculated the best TPCs corresponding to the maximal species richness, the highest species diversity, or the most evenness respectively. They are surprisingly stabilized between 55% and 58%. Coralcover is the basic parameter widely available for most observation, research or monitoring programs. The good qualitative characteristics of percent cover provide powerful tool for the experimental, theoretical and modeling studies of coral reef in response to the disturbance.