Oceanographic Processes on Freshwater-Dominated Continental Shelves II
Presiding: J H Muelbert, Fundacao Universidade Federal do Rio Grande; E J Campos, Insituto Oceanografico da Universidade de Sao Paulo
OS22A-01 10:30h
Numerical Modeling of the Impact of the River Runoff on the Formation of the Anoxia in the Kangjin Bay, South Sea, Korea
Introduction The Kangjin Bay, located in the central part of the South Sea of Korea, is interconnected to neighboring bays and the South Sea through the three narrow channels. The geometry is very complex and the depth is less than 20 m in general. The width at the narrowest site of the Noryang Channel is about 500 m, and the narrowest width of the Changson Channel is about 300 m. The bay receives the Sumjin and Namgang River runoff from the north. The averaged annual discharge of the Sumjin River is about 120 cms. The main objectives of this study to understand the circulation pattern in the Kangjin Bay by tidal current and the impact of the river runoff from the Sumjin River on the ecosystem in the Bay. The Kangjin Bay has been a renowned aqua-culturing bed for several important shellfishes such as large arc shell (Scapharca broughtonii) and short-necked clam(Tapes philippinarum) et al. The water quality, current and meteorological conditions are continuously monitored with very high sampling resolution (10 min.) throughout the year and is published on the Internet web pages (http://oceaninfo.co.kr/kangjin). Detailed technical information for the realtime monitoring system can be referred to Ro et al. (2004). Numerical Solution The tidal circulation of the Kangjin Bay was simulated based on ECOM-3D by forcing the tidal elevations at the open boundaries. The model domain is consisted of the 265*320 grid points with the horizontal grid size is 200 m and 11 vertical sigma levels. Under the CFL condition, the external time step is 10 sec and the internal time step is 300 sec. The drying-flooding procedure was taken into account for treating the area of intertidal flats in the Kangjin bay. Model Results Results of the model simulations will be presented to show the tidal current distributions at flood and ebb phases. The tidal currents in the Kangjin Bay are rather complicated due to its physical settings such as complicated geometry, bottom topography and three open boundaries. Because of the three open channels, the current distribution varies remarkably depending on the location inside the Bay. The tidal residual current patterns show the eddy structures developed near the mouth of the three channels which would have significant impact on the local water quality and ecosystem. Implications for the local ecosystem The circulation is driven by the tidal current most of the year except in the summer season when river runoffs from inflowing two rivers affect the salinity distribution and thereby density current is generated. One of the critical conditions in the Bay is dissolved oxygen contents which exhibit hypoxic conditions in the summer season (June, July and August) when water temperature is very high above 25 deg C and stratification is strong which inhibit the vertical supply of oxygen from the air. It is also considered that the critical bottom shear stress plays an important role in the hypoxic condition by resuspending the organic-rich bottom sediment into the water column which then consumes the oxygen. This study will illucidate the one of the mechnanism which will cause the hypoxic condition by the strong pycnocline formed by the river runoffs in the summer monsoon. The massive death rates of the large arc sheel in the summer might be attributed by the hypoxia formed by the river runoff.
http://oceaninfo.co.kr/kangin/
OS22A-02 10:45h
Oceanographic Processes in the Region of Fluvial Influence of the Itajai-Acu River
The oceanographic processes in the region of fluvial influence (ROFI) of the Itajai-Acu river were assessed through fourteen monthly surveys from November 2002 until December 2003. The main objective of this study was to investigate the effects of the fluvial riverine in the physical, biogeochemical and biological processes in the ROFI. Twenty eight sampling stations were positioned in five transects oriented in a radial shape from the estuarine mouth, with further two stations in the estuary. The transects length was about of 5-km each, reaching the isobath of 25 m. CTD, turbidity and dissolved oxygen profiles were acquired in all stations. Chlorophyll-a were sampled at surface in all stations. Water samples for determination of suspended particulate matter, dissolved inorganic nutrients, chlorophyll-a, phytoplankton and bacterioplankton were sampled in ten stations at surface and near bottom levels. The water samples stations were taken in selected stations to cover all studied area, where was also sampled zooplankton with a ring net. Current speed and direction were acquired with an ADCP moored about 1.5 km off the estuarine mouth at 10 m deep, from November 2002 until March 2003. The river discharge was below the average during most of the sampling period. Although, low salinity water was a permanent feature in the vicinity of the estuarine mouth. The estuarine plume dispersion was allways to north-northeast, what was observed by surface low salinity and higher concentrations of dissolved inorganic nutrients and chlorophyll-a. Below the surface layer, in all ROFI, the water column was dominated by salinity about of 33, named as Coastal Waters. Oceanic waters (salinity higher than 35) were observed only during the summer, with presence of Tropical Water and South Atlantic Central Water. Currents off the estuarine mouth presented strong low frequency signal, with dominance of longshore orientation. Despite of the limitation of the near surface ADCP data, it was possible to observe the plume effects, inducing offshore currents.
OS22A-03 11:00h
Mineralogical contents of bottom sediments collected on the southeastern South American continental shelf
The Plata River and the Patos-Mirim lagoon system discharge a considerable amount of fresh water onto the eastern South American continental shelf. This water, coming from a drainage basin which represent near 20% of the entire South American continent, brings to the sea a variety of suspended material which is ultimately deposited on the bottom, mainly in the shelf region. The study of mineralogical contents of continental borne constituents on the sea floor could reveal valuable information about changes in the environmental conditions in the drainage basin. In this paper we report results of a study of the mineralogical contents of bottom sediments sampled in the eastern South American shelf between 4°S and 25°S. The present work is the first of this kind in that region and is likely to provide valuable information for choosing sites for drilling core samples in future work. The results we obtained with X-Ray Diffraction and other analysis techniques show that the non-clay part of the sediment consists mainly of Quartz, Feldspar, Calcite and Hematite. In only one sample, Amphibole was detected with non-negligible concentration (~20%). Quartz abundance varied from about 20% to 30%, on the average. However, in three of the samples the concentration of this mineral was found in the order of 40%. The Feldspar concentration was found to be of 15-20%. Concentrations of the other two non-clay phases Calcite and Hematite, have some similarity. In the region of 37°S to 33°S, Calcite content varied from 15% to 20% and Hematite concentrations lied within 1-5%. From 29°S to 33°S, the concentration of Calcite was between 4-9% while for Hematite it was in the range of 0-2.5% only. The general concentration pattern of non-clay part was 50-70% for near-shore samples while for samples away from the shore, this value was of about 30-50%. It was also observed that, from 33°S to 28°S, the non-clay concentration presented a trend, decreasing northward, even near the shore line.
http://www.labmon.io.usp.br
OS22A-04 11:15h
Physical Influences on CDOM (Colored Dissolved Organic Matter) DIstribution in the Mississippi River Bight
Dissolved organic matter forms an important component of the carbon pool in river dominated coastal waters. The colored component (CDOM) represents a fraction that can be determined through optical measurements and through remote sensing. CDOM absorption spectra obtained from four seasonal field campaigns in the Mississippi River Bight are examined in relation to the river discharge, wind-induced coastal circulation and episodic events such as cold front passage. Preliminary results indicate that river discharge is the main factor that controls the large-scale regional spatial CDOM distribution, while wind-induced coastal circulation plays an important role in determining short-scale temporal CDOM variability. We also examine the performance of a previously developed regional empirical CDOM algorithm and present results of surface CDOM distribution derived from ocean color satellite sensors.
OS22A-05 11:30h
Analysis of the Spectral Backscattering Coefficient Variability on the Northeastern shelf of the Gulf of Mexico
The continental shelf of the northeastern Gulf of Mexico between 87.5 W and 88.5 W is an ideal place to study coastal processes. In this region, the shelf slopes gently down to depths of 100 m, and then increases rapidly to depths greater than a mile. The Naval Research Laboratory at Stennis Space Center in Mississippi is currently undertaking an intensive measurement and modeling program to determine the cross-shelf exchange processes and their relation to the optical parameters of this area. In this study, we report our efforts to quantify the variability of the spectral backscattering coefficient derived from SeaWiFS imagery via empirical orthogonal functions. We compare the most relevant modes with the spatial distribution of Eddy Kinetic Energy (EKE) computed by the Inter Americas Seas (IAS) model and in-situ measurements by acoustic Doppler current profilers deployed between May 2004 and May 2005. The results indicate that most of the backscattering variability is contained in areas north of 29.2N which coincides with the edge of the continental shelf (100 m depth). Sporadic increases in backscattering are observed as far south as 29.0 N and to the east of 88.1W. These increases can be explained by fluctuations in surface EKE.
OS22A-06 11:45h
Redistribution of riverine water along the continental shelves of the northern and western Gulf of Mexico
The seasonal variability of circulation on the continental shelves of the northern and western Gulf of Mexico is explained using observational data and a high-resolution numerical model. It is shown that the primarily wind driven circulation governs the salinity field on the shelves, by transporting low salinity water formed by river discharge along seasonally shifting pathways. When buoyant low salinity water is present over the outer continental shelf near the shelf edge, it is available for entrainment in currents associated with the mesosocale eddy field of the Gulf. The preferred locations for this cross-shelf export of fresh water shift seasonally as the shelf circulation transports low salinity water along shelf, and impact the upper ocean salinity field throughout the Gulf. A low salinity tongue is found in the monthly climatology in the Bay of Campeche during the winter months, and another extends southeastward from De Soto Canyon along the eastern edge of the Loop Current during the summer. A similar pattern can be seen in the monthly climatology chlorophyll concentration produced from remotely sensed ocean color data, suggesting that the biological properties of the surface waters of the Gulf may be controlled by this combined wind-driven shelf circulation and eddy interaction mechanism responsible for redistributing river discharged fresh water.