Tidally induced upwelling off Yangtze River estuary and in Zhejiang coastal waters in summer

MASNUM wave-tide-circulation coupled numerical model (MASNUM coupled model, hereinafter) is developed based on the Princeton Ocean Model (POM). Both POM and MASNUM coupled model are applied in the numerical simulation of the upwelling off Yangtze River estuary and in Zhejiang coastal waters in summer. The upwelling mechanisms are analyzed from the viewpoint of tide, and a new mechanism is proposed. The study suggests that the tidally inducing mechanism of the upwelling includes two dynamic aspects: the barotropic and the baroclinic process. On the one hand, the residual currents induced by barotropic tides converge near the seabed, and upwelling is generated to maintain mass conservation. The climbing of the residual currents along the sea bottom slope also contributes to the upwelling. On the other hand, tidal mixing plays a very important role in inducing the upwelling in the baroclinic sea circumstances. Strong tidal mixing leads to conspicuous front in the coastal waters. The considerable horizontal density gradient across the front elicits a secondary circulation clinging to the tidal front, and the upwelling branch appears near the frontal zone. Numerical experiments are designed to determine the importance of tide in inducing the upwelling. The results indicate that tide is a key and dominant inducement of the upwelling. Experiments also show that coupling calculation of the four main tidal constituents(M-2, S-2, K-1, and O-1), rather than dealing with the single M-2 constituent, improves the modeling precision of the barotropic tide-induced upwelling.

Numerical simulation on the process of saltwater intrusion and its impact on the suspended sediment concentration in the Changjiang (Yangtze) estuary

To study the relationship between sediment transportation and saltwater intrusion in the Changjiang (Yangtze) estuary, a three-dimensional numerical model for temperature, salinity, velocity field, and suspended sediment concentration was established based on the ECOMSED model. Using this model, sediment transportation in the flood season of 2005 was simulated for the Changjiang estuary. A comparison between simulated results and observation data for the tidal level, flow velocity and direction, salinity and suspended sediment concentration indicated that they were consistent in overall. Based on model verification, the simulation of saltwater intrusion and its effect on sediment in the Changjiang estuary was analyzed in detail. The saltwater intrusion in the estuary including the formation, evolution, and disappearance of saltwater wedge and the induced vertical circulation were reproduced, and the crucial impact of the wedge on cohesive and non-cohesive suspended sediment distribution and transportation were successfully simulated. The result shows that near the salinity front, the simulated concentrations of both cohesive and non-cohesive suspended sediment at the surface layer had a strong relationship with the simulated velocity, especially when considering a 1-hour lag. However, in the bottom layer, there was no obvious correlation between them, because the saltwater wedge and its inducing vertical circulation may have resuspended loose sediment on the bed, thus forming a high-concentration area near the bottom even if the velocity near the bottom was very low during the transition phase from flood to ebb.

The main phosphorous sources in the Changjiang estuary

Analysis using historical data on the phosphate sources in Changjiang (Yangtze River) estuary show that phosphate was supplied equally from the east, south, west and north of the estuary. These sources include the Changjiang River, the Taiwan Warm Current (TWC), a cyclone-type eddy, and the 32A degrees N Upwelling, supplying different phosphates in different times, ways and intensities. The magnitude of their supplying phosphate concentration was related with the size in the order of the Changjiang River < the TWC < the 32A degrees N Upwelling < the cyclone-type eddy, and the duration of the supplying was: the Changjiang River > the TWC > the cyclone-type eddy > the 32A degrees N Upwelling. The four sources supplied a great deal of phosphate so that the phosphate concentration in the estuary was kept above 0.2 mu mol/L in previous years, satisfying the phytoplankton growth. The horizontal and vertical distribution of the phosphate concentration showed that near shallow marine areas at 122A degrees E/31A degrees N, the TWC in low nutrient concentration became an upwelling through sea bottom and brought up nutrients from sea bottom to marine surface. In addition, horizontal distribution of phosphate concentration was consistent with that of algae: Rhizosolenia robusta, Rhizosolenia calcaravis and Skeletonema, which showed that no matter during high water or low water of Changjiang River, these species brought by the TWC became predominant species. Therefore, the authors believe that the TWC flowed from south to north along the coast and played a role in deflecting the Changjiang River flow from the southern side.

Nutrient characteristics in the Yangtze River Estuary and the adjacent East China Sea before and after impoundment of the Three Gorges Dam

From November 2002 to 2006, five cruises were undertaken in the Yangtze River Estuary and the adjacent East China Sea to compare the nutrient concentrations, ratios and potential nutrient limitation of phytoplankton growth before and after impoundment (June 2003) of the Three Gorges Dam (TGD). Concentrations of dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP) and total nitrogen (TN) exhibited an increasing trend from 2002 to 2006. In contrast, total phosphorus (TP) concentration exhibited a decreasing trend. The mean concentrations of DIN, SRP, and TN in the total study area increased from 21.4 mu M, 0.9 mu M, and 41.8 mu M in 2002 to 37.5 mu M, 1.3 mu M. and 82.2 mu M in 2006, respectively. while TP decreased from 2.1 mu M to 1.7 mu M. The concentration of dissolved reactive silica (DRSi) had no major fluctuations and the differences were not significant. The mean concentration of DRSi in the total study area ranged from 52.5 to 92.3 mu M. The Si:N ratio decreased significantly from 2.7 in 2002 to 1.3 in 2006, while TN: TP ratio increased from 22.1 to 80.3. The area of potential P limitation of phytoplankton growth expanded after 2003 and potential Si limitation appeared in 2005 and 2006. Potential P limitation mainly occurred in an area of salinity less than 30 after 2003, while potential Si limitation occurred where the salinity was greater than 30. By comparison with historical data, the concentrations of nitrate and SRP in this upper estuary during November 1980-2006 increased obviously after impoundment of TGD but DRSi decreased. Meanwhile, the ratios of N:P, Si:N and Si:P decreased obviously. (c) 2009 Elsevier B.V. All rights reserved.

The nitrogen isotopic composition of dissolved nitrate in the Yangtze River (Changjiang) estuary, China

The nitrogen isotopic composition of dissolved nitrate (delta N-15-NO3-) in surface water of the Yangtze River estuary was determined in four seasons of 2006. delta N-15-NO3- ranged from 0.4 parts per thousand to 6.5 parts per thousand and varied with seasons and geographic regions, reflecting the dynamics of nitrogen cycling in the estuarine ecosystem. delta N-15-NO3- was markedly lower in February than in other seasons and exhibited conservative mixing, which was probably attributed to the NO3- being sourced from the atmospheric deposition and agricultural fertilizer. In the upper estuary, the influence of riverine inputs was important during all surveys. in the turbidity maximum zone, nitrification was found with nitrate depleted in N-15 in May, whereas denitrification resulting in heavy delta N-15-NO3- played an important role in August. More enriched delta N-15-NO3- values coinciding with losses of nitrate concentrations based on the conservative mixing model were found in the adjacent marine area in May, and may reflect obvious phytoplankton assimilation of dissolved nitrate. In this manner, delta N-15-NO3- may be a sensitive indicator of nitrogen sources and biogeochemical processing existing in this estuary in conjunction with the variations of dissolved nitrate and other environmental factors. (C) 2009 Elsevier Ltd. All rights reserved.

Nutrient Dynamics in the Upwelling Area of Changjiang (Yangtze River) Estuary

Nutrient dynamics and its influence on the distribution of chlorophyll-a in the upwelling area of the Changjiang (Yangtze) River estuary were investigated in the spring (May) and summer (August) of 2004. In the spring, upwelling was apparent in the region of 122 degrees 20'-123 degrees 00' E, 31 degrees 00'-32 degrees 00' N and was associated with low temperature (16-21 degrees C), high salinity (24-33 practical salinity units [psu]), and low dissolved oxygen (2.5-6.0 mg L-1) in the upper 10 m of the water column. The spring upwelling increased the mixed-layer phosphate, nitrate, and silicate concentrations to roughly 1, 15, and 15 mu mol L-1, respectively, and improved the light transparency in the euphotic zone. This improvement in phytoplankton growing conditions was followed by an increase in chlorophyll-a concentrations. The summer upwelling was weaker and occurred over a smaller geographical area (122 degrees 20'-123 degrees 00' E, 31 degrees 15'-31 degrees 50' N). Strongly influenced by turbid Changjiang diluted water (CDW), it had little impact on the upper 10 m of the water column but instead increased nutrient concentrations at greater depths. The high concentration of particulates in the CDW reduced light transmission in the upper 10 m and, hence, limited phytoplankton growth throughout the water column. Chlorophyll-a concentrations in the summer upwelling area were roughly an order of magnitude lower than in the spring. Water clarity, as influenced by the CDW, appears to be the principal factor limiting the impact of upwelling on phytoplankton biomass in this area.

The environment effect on fish assemblage structure in waters adjacent to the Changjiang (Yangtze) River estuary (1998-2001)

We collected fish abundance data in the Changjiang (Yangtze River) estuary and adjacent waters in November 1998, May 1999, November 2000, and May 2001. Using the data, we evaluated the characteristics of the fish assemblages at each site and investigated the effect of several environmental factors. We used a multivariate analysis, including community ordination methods such as detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA), and two-way indicator species analysis (TWINSPAN). We analyzed the biological community structure and environmental factors to determine their spatial distributions, temporal dynamics, and seasonal variations. Among the fish species, five exceeded 5% of the total abundance: Harpodon nehereus (42.82%), Benthosema pterotum (13.85%), Setipinna taty (11.64%), Thryssa kammalensis (9.17%) and Apogonichthys lineatus (6.49%). These were separated into four ecological assemblages: hypsithermal-saline, hypsithermal-brackish, hypothermal-brackish, and hypothermal-saline. We evaluated the degree of influence of environmental factors on the fish community. Our analyses suggested that environmental factors including water depth, salinity, turbidity, transparency, nutrient, and suspended matter formed a synthetic spatial gradient between the coastal and pelagic areas. Ecological and environmental factors changed temporally from 1998 to 2001, and drove the fish community succession. The environmental factors driving the fish community structure included bottom temperature, water depth, bottom and surface pH, surface total phosphorous, and bottom dissolved oxygen. This investigation was completed before completion of the Three Gorges Dam; therefore the results of this study provide an important foundation for evaluating the influence of the human activities.

Biochemical compositions of two dominant bloom- forming species isolated from the Yangtze River Estuary in response to different nutrient conditions

Variations of cellular total lipid, total carbohydrate and total protein content of two dominant bloom-forming species (Skeletonema costatum and Prorocentrum donghaiense) isolated from the Yangtze River Estuary were examined under six different nutrient conditions in batch cultures. Daily samples were collected to estimate the cell growth, nutrient concentration and three biochemical compositions content during 7 days for S. costatum and the same sampling procedure was done every other day during 10 days for P. donghaiense. Results showed that for S. costatum, cellular total lipid content increased under phosphorus (P) limitation, but not for nitrogen (N) limitation; cellular carbohydrate were accumulated under both N and P limitation: cellular total protein content of low nutrient concentration treatments were significantly lower than that of high nutrient concentration treatments. For P. donghaiense, both cellular total lipid content and total carbohydrate content were greatly elevated as a result of N and P exhaustion, but cellular total protein content had no significant changes under nutrient limitation. In addition, the capability of accumulation of three biochemical constituents of P. donghaiense was much stronger than that of S. costatum. Pearson correlation showed that for both species, the biochemical composition of three constituents (lipid, carbohydrate and protein) had no significant relationship with extracellular N concentration, but had positive correlation with extracellular and intracellular P concentration. The capability of two species to accumulate cellular total lipid and carbohydrate under nutrient limitation may help them accommodate the fluctuating nutrient condition of the Yangtze River Estuary. The different responses of two species of cellular biochemical compositions content under different nutrient conditions may provide some evidence to explain the temporal characteristic of blooms Caused by two species in the Yangtze River Estuary. (C) 2008 Elsevier B.V. All rights reserved.

Phytoplankton distributions and their relationship with the environment in the Changjiang Estuary, China

This study was carried out in the Changjiang Estuary from 19 to 26 May 2003. Based on the data collected from 29 stations, including two anchor stations, phytoplankton taxonomic composition, abundance, diurnal variability and spatial distribution were examined. Eighty-seven species, including 54 species of diatoms and 16 red tide causative species, were identified. Average diversity index (H') and evenness (J) values were 1.04 and 0.40, respectively. A bloom in abundance of certain phytoplankton species, especially Prorocentrum dentatum and Skeletoneina costatum, was thought to be the cause of the low diversity index and evenness values. Total phytoplankton abundance averaged 6.75 x 10(5) cells 1(-1), and was much higher than previous investigation carried out in the same month in 1986. Abundance increased seaward showing a distinct spatial difference, and the dominant species varied with salinity. Correlation between phosphorus and abundance further supported the former conclusion that phosphorus is the controlling factor in phytoplankton growth in the Changjiang Estuary where light is not limiting. Based on the relationship between DO, pH and abundance, it is likely that the bloom was caused by rapid in situ growth of phytoplankton with high nutrients and sufficient light. The data also indicated that the duration of the bloom was not long and

Spatial and temporal variations in pH and total alkalinity at the beginning of the rainy season in the Changjiang Estuary, China

The results of field observation carried out in May 2003 were used to examine pH and total alkalinity behaviors in the Changjiang Estuary. It was showed that PH and total alkalinity took on clear spatial variations in values with the minima in the low salinity region. Like salinity, transect distributions of PH and total alkalinity (TA) in a downriver direction had a sharp gradient each. These gradients appeared in such a sequence that the TA gradient was earlier than salinity and PH gradients, and the salinity gradient was earlier than the PH gradient. These distribution characteristics seemed to be strongly influenced by the mixing process of freshwater and seawater, for both PH and total alkalinity had significant linear relationships with salinity and temperature. For PH, phytoplankton activities also had a significant impact upon its spatial distribution. During a period of 48 h, PH and total alkalinity changed within wide ranges for every layer of the two anchor stations, namely, Stas 13 and 20, which were located at the mixed water mass and seawater mass, respectively. For both Stas 13 and 20, PH and TA fluctuation of every layer could be very wide during a 4 h period. As a whole, the data of the two anchor stations showed that neither variations in salinity and temperature nor phytoplankton activities were the main factors strongly influencing the total alkalinity temporal variability on a small time scale. The data of Sta. 20 implied that both salinity variation and phytoplankton activities had a significant influence on PH temporal variability, but the same conclusion could not be drawn from the data of Sta. 13.

Organochlorine micropollutants in the Jiulong River Estuary and Western Xiamen Sea, China

Organochlorine contaminants including 12 polychlorinated biphenyl (PCB) congeners and 18 insecticides were determined in water, pore water and sediments of the Jiulong River Estuary and Western Xiamen Sea, China. The results showed that the levels of the total PCBs ranged from non-detectable to 1500 ngl(-1) in water, from 209 to 3870 ngl(-1) in pore water, and from 2.78 to 14.8 ng g(-1) dry weight in sediments. Total organochlorine insecticide concentrations were from below the limit of detection to 2480 ngl(-1) in water, from 267 to 33400 ngl(-1) in pore water, and from 4.22 to 46.3 ng g(-1) dry weight in sediments. Concentrations of PCBs and insecticides in pore water were significantly higher than those in surface water, due to the high affinity of these hydrophobic compounds for sediment phase. The PCB congeners with the highest concentrations were CB153, CB180 and CB194, which together accounted for 68-87% of total PCBs in water, pore water and sediment. Among the hexachlorocyclohexane (HCH) compounds, beta-HCH was found to be a major isomer. Analysis of 1,1,1-trichloro-2,2-bis-chlorophenyl-ethane (DDT) and its metabolites showed that 1, 1-dichloro-2[o-chlorophenyl]-2[p-chlorophenyl]-ethylene (DDE) was dominant in the group. In comparison to a 1998 study in the Western Xiamen Sea, levels of organochlorines were enhanced due probably to recent inputs and changes in sediments. (c) 2004 Elsevier Ltd. All rights reserved.

Sources and transport of dissolved and particulate organic carbon in the Mississippi River estuary and adjacent coastal waters of the northern Gulf of Mexico

Dissolved organic carbon (DOC), stable carbon isotopic (delta(13)C) compositions of DOC and particulate organic carbon (POC), and elemental C/N ratios of POC were measured for samples collected from the lower Mississippi and Atchafalaya rivers and adjacent coastal waters in the northern Gulf of Mexico during the low flow season in June 2000 and high flow season in April 2001. These isotopic and C/N results combined with DOC measurements were used to assess the sources and transport of terrestrial organic matter from the Mississippi and Atchafalaya rivers to the coastal region in the northern Gulf of Mexico. delta(13)C values of both POC (-23.8parts per thousand to -26.8parts per thousand) and DOC (-25.0parts per thousand to -29.0parts per thousand) carried by the two rivers were more depleted than the values measured for the samples collected in the offshore waters. Strong seasonal variations in delta(13)C distributions were observed for both POC and DOC in the surface waters of the region. Fresh water discharge and horizontal mixing played important roles in the distribution and transport of terrestrial POC and DOC offshore. Our results indicate that both POC and DOC exhibited non-conservative behavior during the mixing especially in the mid-salinity range. Based on a simple two end-member mixing model, the comparison of the measured DOC-delta(13)C with the calculated conservative isotopic mixing curve indicated that there was a significant in situ production of marine-derived DOC in the mid- to high-salinity waters consistent with our in situ chlorophyll-a measurements. Our DOC-delta(13)C data suggest that a removal of terrestrial DOC mainly occurred in the high-salinity (>25) waters during the mixing. Our study indicates that the mid- to high- (10-30) salinity range was the most dynamic zone for organic carbon transport and cycling in the Mississippi River estuary. Variability in isotopic and elemental compositions along with variability in DOC and POC concentrations suggest that autochthonous production, bacterial utilization, and photo-oxidation could all play important roles in regulating and removing terrestrial DOC in the northern Gulf of Mexico and further study of these individual processes is warranted. (C) 2004 Elsevier B.V. All rights reserved.

Studies on the regional feature of organic carbon in sediments off the Huanghe River Estuary waters

Organic carbon (OC) in definitely small area sediments(according to marine dimension)off the Huanghe River Estuary is investigated in order to evaluate the feature of regional difference of physical and chemical properties in marginal sea sediments. The distributions of OC in sediments with natural grain size and the relationship with the pH, Eh,Es and Fe3+/Fe2+ are discussed. In addition,OC decomposition rates in surfacial/subsurfacial sediments are estimated. OC concentrations range from 0.26% to 1.8%(wt)in the study area. Significant differences in OC content and in horizontal distribution as well as various trends in surfacial/subsurfacial sediments exhibit the feature of regional difference remarkably in marginal sea sediments. The complicated distribution of OC in surface sediments is due to the influence of bacterial activity and abundance, bioturbation of benthos and physical disturbance. The OC decomposition rate constant in surfacial/subsurfacial sediments ranges from 0.0097 to 0.076 a(-1) and the relatively high values may be mainly related to bacteria that are mainly responsible for OC mineralization;meio-and macrofauna affect OC degradation both directly, through feeding on it, and indirectly through bioturbation and at the same time coarse sediments are also disadvantageous to OC preservation. In almost all the middle and bottom sediments the contents of OC decrease with the increase of deposition depth, which indicates that mineralization of OC in the middle and bottom sediments has occurred via processes like SO42- reduction and Fe-oxide reduction.