Adjacent Coastal Sea Research Articles

Numerical determination of residence time and age in a partially mixed estuary using three-dimensional hydrodynamic model

This paper documents a numerical modeling study to calculate the residence time and age of dissolved substances in a partially mixed estuary. A three-dimensional, time-dependent hydrodynamic model was established and applied to the Danshuei River estuarine system and adjacent coastal sea in Taiwan. The model showed good agreement with observations of surface elevation, tidal currents and salinity made in 2002. The model was then applied to calculate the residence time and age distribution response to different freshwater discharges with and without density-induced circulations in the Danshuei River estuarine system. Regression analysis of model results reveals that an exponential equation can be used to correlate the residence time to change of freshwater input. The simulated results show it takes approximately 10, 4.5, and 3 days, respectively, for a water parcel that has entered the headwaters of the estuary to be transported out of the estuary under low, mean, and high flow conditions with density-induced circulation. The calculated age with density-induced circulation is less than that without density-induced circulation. The age of the surface layer is less than that at the bottom layer. Overall the study shows that freshwater discharges are the important factors in controlling the transport of dissolved substances in the Danshuei River estuarine system.

Modeling residence time response to freshwater discharge in a mesotidal estuary, Taiwan

A three-dimensional, time-dependent hydrodynamic model, SELFE, was performed and applied to the Danshuei River estuarine system and adjacent coastal sea in Taiwan. The model forcing functions consist of tidal elevations along the open boundary and freshwater flows from the main stem and tributaries in the Danshuei River system. The model was calibrated and verified with measured water surface elevation, tidal current, residual current, and salinity in 2000 and 2001, respectively. The overall model simulation results are in qualitative agreement with the available field data. The validated model was then used to probe the salinity response to the pulse of high freshwater discharge and residence time in the Danshuei River estuary. The modeling results show that salinity recovers to normal conditions in about 14 days at the Kuan-Du Bridge, 22 days at the Taipei Bridge, and 30 days at both Hwa-Jian Bridge and Pa-Ling Bridge. Regression analysis of model results reveals that an exponential regression equation can be used to correlate the estuarine residence time to change of freshwater input. A short residence time exhibits in the Danshuei River estuary. The simulated results provide useful information for understanding the transport processes and can be employed to estimate dissimilative capacity in the estuary.

Biogeochemistry of the Dumai River estuary, Sumatra, Indonesia, a tropical black‐water river

The biogeochemistry of the Dumai River estuary in eastern Sumatra, Indonesia, was studied in order to obtain information on the sources, transformation, and fate of organic matter. Between October and December 2003, water, total suspended matter (TSM), and sediments were sampled along a salinity gradient during four campaigns, and plants and soils were collected from the catchment. Water samples were analyzed for dissolved inorganic nutrients and dissolved organic carbon (DOC). The concentrations of organic carbon (Corg) and total nitrogen (N) and the stable carbon (δ13Corg) and nitrogen (δ15N) isotope distributions were determined in TSM, sediments, plants, and soils. The pH as well as the concentrations of dissolved inorganic nutrients and TSM were very low in the river and increased toward the sea. A maximum DOC concentration of 5,050 µmol L−1 was measured in the river, and concentrations decreased toward the sea. Low‐gradient relief and a dense vegetation cover, and hence little weathering and erosion, appear to be responsible for low river loads of dissolved nutrients and TSM in this black‐water river. Leaching from extensive peat soils in its catchment may account for the high DOC content of the Dumai River. Peat swamps drained by numerous small rivers are estimated to cover 3.3 X 104 km2 in eastern Sumatra, suggesting that leaching of DOC may be a significant source of carbon to the adjacent coastal seas. A comparison with “normal” rivers shows that black‐water rivers can export similar amounts of DOC from catchments that are orders of magnitude smaller. Thus, export from small black‐water rivers may be quantitatively more significant for the global DOC input into the ocean than previously thought.

Modelling the impact of wind stress and river discharge on Danshuei River plume

A three-dimensional, time-dependent, baroclinic, hydrodynamic and salinity model, UnTRIM, was performed and applied to the Danshuei River estuarine system and adjacent coastal sea in northern Taiwan. The model forcing functions consist of tidal elevations along the open boundaries and freshwater inflows from the main stream and major tributaries in the Danshuei River estuarine system. The bottom friction coefficient was adjusted to achieve model calibration and verification in model simulations of barotropic and baroclinic flows. The turbulent diffusivities were ascertained through comparison of simulated salinity time series with observations. The model simulation results are in qualitative agreement with the available field data. The validated model was then used to investigate the influence of wind stress and freshwater discharge on Dasnhuei River plume. As the absence of wind stress, the anticyclonic circulation is prevailed along the north to west coast. The model results reveal when winds are downwelling-favorable, the surface low-salinity waters are flushed out and move to southwest coast. Conversely, large amounts of low-salinity water flushed out the Danshuei River mouth during upwelling-favorable winds, as the buoyancy-driven circulation is reversed. Wind stress and freshwater discharge are shown to control the plume structure.

Three‐dimensional hydrodynamic and salinity transport modelling of Danshuei River estuarine system and adjacent coastal sea, Taiwan

AbstractA three‐dimensional, time‐dependent hydrodynamic and salinity model was applied to the Danshuei River estuarine system and adjacent coastal sea in Taiwan. The model forcing functions consist of tidal elevations along the open boundary and freshwater flows from the main stem and tributaries in the Danshuei River system. The bottom roughness height was calibrated and verified with model simulation of barotropic flow, and the turbulent diffusivities were calibrated through comparison of time‐series of salinity distributions. The overall model verification was achieved with comparisons of residual current and salinity distribution. The model simulation results are in qualitative agreement with the available field data.The model was then used to investigate the tidal current, residual current, and salinity patterns under the low freshwater flow condition in the modelling domain. The results reveal that the extensive intrusion of saline water imposes a significant baroclinic forcing and induces a strong residual circulation in the estuary. The downriver net velocity in the upper layer increases seaward despite the enlargement of the river cross‐section in that direction. Strong residual circulation can be found near the Kuan‐Du station. This may be the result of the deep bathymetric features there. Copyright © 2007 John Wiley & Sons, Ltd.

Modeling the influence of river discharge on salt intrusion and residual circulation in Danshuei River estuary, Taiwan

A 3-D, time-dependent, baroclinic, hydrodynamic and salinity model was implemented and applied to the Danshuei River estuarine system and the adjacent coastal sea in Taiwan. The model forcing functions consist of tidal elevations along the open boundaries and freshwater inflows from the main stream and major tributaries in the Danshuei River estuarine system. The bottom friction coefficient was adjusted to achieve model calibration and verification in model simulations of barotropic and baroclinic flows. The turbulent diffusivities were ascertained through comparison of simulated salinity time series with observations. The model simulation results are in qualitative agreement with the available field data. The validated model was then used to investigate the influence of freshwater discharge on residual current and salinity intrusion under different freshwater inflow condition in the Danshuei River estuarine system. The model results reveal that the characteristic two-layered estuarine circulation prevails most of the time at Kuan-Du station near the river mouth. Comparing the estuarine circulation under low- and mean flow conditions, the circulation strengthens during low-flow period and its strength decreases at moderate river discharge. The river discharge is a dominating factor affecting the salinity intrusion in the estuarine system. A correlation between the distance of salt intrusion and freshwater discharge has been established allowing prediction of salt intrusion for different inflow conditions.

Evaluation of marine outfall with three-dimensional hydrodynamic and water quality modeling

Numerical models are often used to evaluate the potential impact of human alternation of natural water bodies and to help the design of the alternation to mitigate its impacts. In the past decade, three-dimensional hydrodynamic and reactive transport modeling has matured from a research subject to a practical analysis technology. This paper presents a practical study in which a three-dimensional hydrodynamic and water quality model [hydrodynamic eutrophication model (HEM-3D)] was applied to determine the optimal location for treated wastewater discharged from marine outfall system in the Keelung harbor and the adjacent coastal sea. First, model validation was conducted with respect to surface elevation, current, and water quality variables measured in the Keelung harbor station and its coastal sea. The overall performance of the model was in qualitative agreement with the available field data. The model was then used to evaluate several scenarios of the locations from marine outfall system. Based on model simulation results, a location at the northeast of Ho-Ping Island was recommended for adoption because the environmental impact is smaller than any other alternative.

Remarkably diverse and contrasting archaeal communities in a large arctic river and the coastal Arctic Ocean

Although the microbial biodiversity of arctic seas has received an increasing amount of attention, little is known about the microbial communities of its inflowing rivers. In this study we examined the molecular diversity of Archaea in the largest arctic river in North America, the Mackenzie River, and in the adjacent coastal Beaufort Sea (Canadian Arctic) during maximum open water conditions (October 2002). The Mackenzie River 16S rRNA clone libraries revealed a remark- ably diverse assemblage of archaeal sequences, with an estimated 286 phylotypes defined as sequences with 97% similarity. These grouped mainly within 2 phylogenetic clusters, and were related to sequences earlier retrieved from flooded soils and sediments previously named RC-V and LDS. The marine coastal libraries were of very different composition to those of river libraries and were dominated by Group II Euryarchaeota followed by Group I Crenarchaeota. These coastal assemblages had greater archaeal diversity (18 to 23 phylotypes) than previously reported for marine communities elsewhere, and differed from previously described central Arctic Ocean assemblages. This may reflect the heterogeneous mixture of organic substrates and particles available for micro- bial heterotrophy in arctic coastal waters and the use of an alternative primer pair (109f-915r) in this study. The coastal sequences grouped within typical marine clusters, and we therefore conclude that they were an active autochthonous community rather than one derived from the large inflowing river. These results underscore the rich microbial diversity in arctic rivers and their adjacent coastal marine ecosystems.

Migration pattern of shirauo Salangichthys microdon Bleeker, in the Ishikari River system and adjacent nearshore sea area, Japan, as estimated by otolith microchemistry analysis

: The purpose of the present paper was to reveal the migration pattern of shirauo Salangichthys microdon Bleeker, in the Ishikari River system and its adjacent coastal sea areas. To this end, the strontium–calcium (Sr:Ca) ratios of the otoliths of 98 individuals were examined. Profiles of the Sr:Ca ratios during the life history after hatching in the estuary of the Ishikari River fell into two different patterns. One was characterized by a high (slightly fluctuating) Sr:Ca ratio throughout the life history; the other was characterized by an initial high Sr:Ca ratio followed by a lower value later in life. The former pattern was considered to be that of estuary–sea residents and the latter that of upstream migrants. Most upstream migrants migrated upstream once, although several did it twice. Back-calculated body lengths during the upstream and seaward migrations of the upstream migrants varied widely from 6.4 mm to 39.8 mm and from 13.5 mm to 79.4 mm, respectively. These findings suggest that estuary–sea residents and upstream migrants co-occurred spatiotemporally, and suggest a wide variation in migration patterns in the shirauo's life history.

Reflectances in the Ganges and Brahmaputra rivers and in the adjacent coastal sea

The distribution patterns of reflectance were analysed from Landsat TM band 3, along the course of the Ganges and Brahmaputra rivers and their adjacent coastal sea. It was observed that during the high discharge period, reflectance values in the Ganges river are higher than that in the Brahmaputra. But in the low discharge period, it is vice versa. Reflectances varied significantly throughout the river courses but increased distinctly at their confluence zones. In the coastal sea, reflectance values decreased markedly with a narrow zone, which corresponded to the zone of spectral contrast as seen from band 3 image. Influence of river discharge in reflectance values is not distinct in the coastal sea.

The Estuary Plume Zone: Source or Sink for Land-derived Nutrient Discharges?

The results from three cruises surveying nutrient distributions in the coastal plume discharge of the Humber Estuary, U.K., were used to examine ways of quantifying the influence of estuarine plume zones on the transfer of land-derived material from rivers and estuaries to coastal seas. The data were examined in three ways. First, the ‘mixing curve’ procedure, conventionally applied in estuarine studies was found to be only partially useful. This procedure indicated near-conservative behaviour of nitrate and silicate in winter. For nitrate, silicate and phosphate at other times, highly scattered data from the plume region indicated significant but generally unresolvable sources and sinks. The Wash area affected the plume distribution of phosphate in winter but not at other times. A second method, mass balancing based on estimates of the rates of all nutrient-controlling processes within the plume was considered impractical using presently available data due to the high spatial heterogeneity of significant nutrient-controlling processes, particularly primary production (as evidenced by chlorophyll distribution) and chemical fluxes across the sediment-water interface (as evidenced by shipboard, box-core incubations of sediments from the plume zone). Third, estimates of nutrient fluxes emanating from either the river or the estuary mouth were compared with estimates of fluxes across the plume boundary based on computer modeling of residual (non-tidal) water transport within the plume. Without unlimited resources, this procedure is the most practical method of obtaining quantitative flux estimates for assessing the influence of estuarine plume zones on river/estuary discharges. Using this latter procedure, it is shown that, in winter, river-derived nutrients are conservatively exported through the plume to the coastal sea. In spring, the plume zone acts as a net sink for nutrients derived both from the estuary and from the adjacent coastal sea. That is, nutrients are being consumed at a faster rate than they are supplied by the river/estuary system to the plume region. In summer, the plume is a net sink for nitrate and phosphate and a net supplier of silicate.