Danshuei River Research Articles

Uncertainty assessment for three-dimensional hydrodynamic and fecal coliform modeling in the Danshuei River estuarine system: The influence of first-order parametric decay reaction

Modeling fecal contamination in water bodies is of importance for microbiological risk assessment and management. This study investigated the transport of fecal coliform (e.g., up to 2.1 × 106 CFU/100 ml at the Zhongshan Bridge due to the main point source from the Xinhai Bridge) in the Danshuei River estuarine system, Taiwan with the main focus on assessing model uncertainty due to three relevant parameters for the microbial decay process. First, a 3D hydrodynamic-fecal coliform model (i.e., SCHISM-FC) was developed and rigorously validated against the available data of water level, velocity, salinity, suspended sediment and fecal coliform measured in 2019. Subsequently, the variation ranges of decay reaction parameters were considered from several previous studies and properly determined using the Monte Carlo simulations. Our analysis showed that the constant ratio of solar radiation (α) as well as the settling velocity (vs) had the normally-distributed variations while the attachment fraction of fecal coliform bacteria (Fp) was best fitted by the Weibull distribution. The modeled fecal coliform concentrations near the upstream (or downstream) stations were less sensitive to those parameter variations (see the smallest width of confidence interval about 1660 CFU/100 ml at the Zhongzheng Bridge station) due to the dominant effects of inflow discharge (or tides). On the other hand, for the middle parts of Danshuei River where complicated hydrodynamic circulation and decay reaction occurred, the variations of parameters led to much larger uncertainty in modeled fecal coliform concentration (see a wider confidence interval about 117,000 CFU/100 ml at the Bailing Bridge station). Overall, more detailed information revealed in this study would be helpful while the environmental authority needs to develop a proper strategy for water quality assessment and management. Owing to the uncertain decay parameters, for instance, the modeled fecal coliform impacts at Bailing Bridge over the study period showed a 25 % difference between the lowest and highest concentrations at several moments. For the detection of pollution occurrence, the highest to lowest probabilities for a required fecal coliform concentration (e.g., 260,000 CFU/100 ml over the environmental regulation) at Bailing Bridge was possibly greater than three.

Molybdenum Isotopic Fingerprints in Taiwan Rivers: Natural versus Anthropogenic Sources

Two rivers from Taiwan with different source inputs, the Danshuei (polluted) and Liwu (non-polluted), were selected to study the behavior of molybdenum (Mo) isotopes during weathering and riverine transport. In the Danshuei River, δ98/95Mo ranges from 0.83‰ to 1.50‰ (wet season) and 0.54‰ to 1.25‰ (dry season). With a few exceptions, δ98/95Mo in the Danshuei River is lighter during the wet season, while, in contrast, heavier in the dry season. In the Liwu River, δ98/95Mo varies from 0.54‰ to 1.30‰ and gets heavier along the mainstream. Using the MixSIAR model, three Mo sources are identified in the Danshuei River: seawater intrusion, rock–water interaction, and anthropogenic inputs. Seawater intrusion can explain the heavy δ98/95Mo downstream signal during the wet season contributing 14–39% from the MixSIAR model. However, the lighter δ98/95Mo signal during the dry season is most likely due to anthropogenic inputs in the middle and lower reaches of the Danshuei River contributing 75–98%. In the Liwu River, dissolved Mo isotopes correlate with SO42−/Na and (Sr/Na) ×1000 ratio, suggesting that pyrite oxidation coupled with carbonate weathering governs the heavy δ98/95Mo signature, with sequestration of light δ98/95Mo into secondary mineral phases in bedload sediments. Furthermore, these results have important implications for riverine Mo sources to the ocean, controlled by anthropogenic activity and weathering processes.

Effects of Environmental Factors on Suspended Sediment Plumes in the Continental Shelf Out of Danshuei River Estuary

The effects of environmental factors on suspended sediment plumes in the continental shelf out of the Danshuei River estuary were numerically investigated using an unstructured-grid three-dimensional hydrodynamic model (SCHISM) together with a suspended sediment (SS) module. The coupled model (SCHISM-SS) was calibrated and validated against the in situ measurement data in 2016. Consistent with the observation results, the model simulations satisfactorily reproduced the water levels, velocities, salinities, and suspended sediment concentrations. The model was then applied to explore the role of various environmental factors in the dynamics of suspended sediment plumes from the estuary to the adjacent coastal seas. These factors include tidal forcing, salinity, river discharge, and wind stress. Analysis and comparisons of different scenario results indicated that the suspended sediment plume was greatly affected by tides, e.g., a longer plume distance resulted from a larger flux under tidal motions. A higher sediment concentration in the plume in the offshore area was also found during the neap tide, relative to that observed during the spring tide. In addition, salinity affects the movement of density currents and the spread of the sediment plume, i.e., the plume distance is longer due to the residual circulation when a salinity difference is present. Further, an extreme river flow could occur during typhoon periods and would discharge a greater water volume into the coastal region, causing the suspended sediment plume to expand from the near shore. Finally, the directions of prevailing winds can slightly influence the sediment plumes.

Flood–Ebb and Discharge Variations in Observed Salinity and Suspended Sediment in a Mesotidal Estuary

To explore the spatial and temporal variations in salinity and suspended-sediment concentration in the Danshuei River estuary of northern Taiwan, two intensive field surveys were conducted in July 2016 and 2019 to assign high- and low-flow conditions, respectively. According to the analysis of tidal characteristics, the duration during ebb tide was longer than that during flood tide, while the maximum ebb discharge was higher than the maximum flood discharge, causing the occurrence of tidal asymmetry during ebb and flood tides. The barotropic forcing dominated during high flow, resulting in lower salinity and a shorter distance of saltwater intrusion. Based on the analyzed results using stratification indices, most of the time was spent in the state of partial mixing at the Guandu Bridge and good mixing at the Taipei Bridge during high flow, while most of the time was spent in the states of partial mixing and good mixing at both Guandu Bridge and Taipei Bridge during low flow. More stratification occurred during high flow at high slack tide compared to that during low flow. The freshwater discharges from upriver reaches controlled the suspended-sediment concentration (SSC) in tidal estuaries. The higher SSC appeared downstream of the tidal estuary at ebb tide during high flow. Observations also revealed that there was an estuarine turbidity maximum at the bottom layer of Guandu Bridge.

Distribution of Geochemical Species of P, Fe and Mn in Surface Sediments in the Eutrophic Estuary, Northern Taiwan

The Danshuei River Estuary (DRE) in northern Taiwan is a seriously eutrophic estuary due to the domestic effluent discharge. Surface sediment samples were collected from the DRE to study the concentrations and spatial distributions of different fractions of phosphorus through the five-step sequential extraction method which chemically divides the sedimentary P into five fractions: PSORB, PCDB, PCFA, PDET, and PORG. The Fe and Mn contents in the extracted solution were also determined. The total organic carbon (TOC) and grain size in sediment samples were analyzed as well. The sedimentary total P (TP) concentrations ranged within 537–1310 mg/kg and mostly exceeded 800 mg/kg, suggesting that the DRE sediments were moderately polluted by phosphorus. The PCDB was the dominant fraction of P, averagely contributing 58% of TP, followed by PDET 31%. The contributions of the PSORB and PCFA fractions to the TP were relatively minor. Two fractions, FeCDB and FeORG, of sedimentary Fe equally shared approximately 70% of total Fe, followed by FeDET with 22%. The contribution of different fractions of sedimentary Mn followed the sequence: MnCDB (36%) > MnCFA (29%) > MnORG (14.7%) > MnDET (14.5%) > MnSORB (5.3%). The sedimentary P, Fe, and Mn within the DRE are easily mobilized because they were mainly present in the reducible fraction. The concentrations of sedimentary TP positively correlated with the TOC contents and inversely negatively correlated with grain size, suggesting that the TOC and grain size play the crucial roles in influencing the distribution of sedimentary P within the DRE. Finally, the Fe(III) (hydro)oxides seems to play an important carriers to adsorb dissolved P because PCDB positively correlated with FeCDB.

Different Mercury Species Partitioning and Distribution in the Water and Sediment of a Eutrophic Estuary in Northern Taiwan

The total Hg and methyl Hg in waters and sediments, as well as particulate total Hg (PTHg), were analyzed to study their distribution and partitioning in the Danshuei River Estuary (DRE), northern Taiwan. TOC and grain size were also determined in the sediment samples. The dissolved total Hg (DTHg) in waters ranged from 24.0 to 45.8 ng/L. The dissolved methyl Hg (DMeHg) concentrations contributed 0.6–30.4% of the DTHg pool, with the higher percentage appearing in the upper estuary. The DMeHg concentration positively correlated with the Chl.a within the estuary, suggesting that phytoplankton plays an important role in influencing the DMeHg concentration. The partitioning results indicated that DTHg chiefly dominates the THg (DTH + PTHg) pool, especially at a salinity of >15 psu region. The value of partition coefficient, log(KD), was within a range of 3.54 to 4.68, and the value linearly decreased with increasing salinity. The sediment total Hg (STHg) concentrations ranged from 80 to 379 ng/g, and most data exceeded the NOAA guidelines value (ERL < 150 ng/g), indicating that the DRE is contaminated with Hg. The STHg concentrations inversely and positively correlated with the grain size and TOC content, respectively, suggesting that sediment Hg distributions are strongly influenced by the both parameters.

Zooplankton Fluctuations in the Surface Waters of the Estuary of a Large Subtropical Urban River

The Danshuei River has a third largest catchment area and third longest in Taiwan. It flows through the capital, Taipei, and more than six million people live within its catchment area. Its estuary is characterized by a highly variable chemical and physical environment that is affected by the interaction of inland freshwater runoff with wastewater, and toward the coast is also influenced by the China Coastal Current and the Kuroshio Current. By collecting zooplankton bimonthly in 2014 from the surface layer (0–2 m depth) at five sites in the estuary, we were able to demonstrate that the composition of the zooplankton, and particularly its copepod fraction, varied significantly among sampling stations and months, thereby revealing seasonal succession. Fourteen higher taxa or other categories of zooplankter were identified, with the following being most common taxa: Decapoda, Copepoda (including Calanoida, Cyclopoida, and Harpacticoida), and “other larvae.” The Copepoda comprised 44 taxa (including eight only identified to genus) belonging to 3 orders, 17 families, and 29 genera, the five most abundant of which wereBestiolinan. sp. (undescribed),Corycaeusspp.,Parvocalanus crassirostris,Acartiasp., andParacalanus parvus. The highest and lowest copepod abundances were recorded in July (2557.88 inds. m–3) and January (1.3 inds. m–3), respectively. Observed changes in abundance of many kinds of copepod appeared to be significantly related to changes in physico-chemical parameters (e.g., salinity, temperature, pH, and dissolved oxygen concentration). Cluster analysis confirmed the existence of distinct copepod communities, each characterized by a preference for a different set of environmental conditions. Our comprehensive literature review of the copepod biodiversity of Taiwan’s major rivers for comparison with similar data compiled for other estuaries in the world, the first time such a review has been compiled, shows that 32 copepod taxa have been recorded from the brackish and freshwater parts of the Danshuei River. They represent 58.2% of the total number of brackish- and freshwater copepod species in Taiwan, and five of them have so far only been recorded in the Danshuei River: the calanoidsAcartiella sinensisandPseudodiaptomus forbesi, the cyclopoidsOithona fragilisandOithona simplex, and the harpacticoidTachidius(Tachidius)discipes.

Dissolved and particulate nitrogen species partitioning and distribution in the Danshuei River estuary, northern Taiwan

Danshuei River Estuary (DRE) total and inorganic nitrogen in the dissolved (TDN, DIN) and particulate (TPN, PIN) phases were analyzed to study their distribution and partitioning. The carbon contents in particles were also analyzed. The upper estuary contained higher ammonium concentration (304–557 μM), leading to TDN completely dominating (>95%) the total N (TDN + TPN) pool within the DRE. Ammonium played a crucial role in controlling the speciation variation of DIN and partitioning between dissolved and particulate phases. Nitrification seemed to occur in the salinity >30 region where elevated percentages of nitrite and nitrate were observed. PON dominated the particulate N and contributed an average of 62% of the TPN pool. A constant organic C/N ratio (6.55) was observed in particles, indicating that POM was mainly from phytoplankton detritus. The N distribution coefficient values, log(KD), ranged from 3 to 4, suggesting that the affinity of DIN for particles was weak.

Benthic meiofauna with emphasis on nematode assemblage response to environmental variation in the intertidal zone of the Danshuei River estuary, northwest Taiwan

AbstractThis study aims to determine whether the benthic meiofaunal assemblages on the intertidal zone of Danshuei River estuary (DRE) in Taiwan vary from the estuary to the upper stream, and in response to different habitats. A total of six major taxa (Nematoda, Copepoda, Ostracoda, Polychaeta, Oligochaeta and Gastrotricha) of meiofauna and 44 nematode genera were recorded at 12 sampling stations in DRE in October, 2013. Meiofauna densities ranged between 4.67 ± 2.08 and 294.00 ± 180.40 ind./10 cm2, the average was 162.31 ± 84.36 ind./10 cm2. Nematodes were the dominant group (relative abundance, RA: 89.27%) in the meiofaunal community at all stations, with its assemblage in DRE dominated by nonselective deposit feeders (55.14%). There were striking differences in nematode assemblages between mangroves and mangroves with reed habitats. Pearson's correlation analysis showed that the dominances of the genera Anoplostoma, Metacylicolaimus and Deontostoma were significantly negative while Terschellingia correlated positively with salinity. These genera showed either decreasing or increasing trends at stations from the estuary to the upper stream. The dominances of the genera Megadesmolaimus and Theristus were significantly positively correlated with the proportion of sand and negatively correlated with the proportion of silt and clay. Our results indicate that salinity and sediment type influenced nematode assemblages in the DRE.

Dissolved and particulate phosphorus species partitioning and distribution in the Danshuei River Estuary, Northern Taiwan

Different phosphorus fractions, including total dissolved P (TDP), dissolved inorganic P (DIP), total particulate P (TPP), and particulate inorganic P (PIP), were analyzed in the Danshuei River Estuary (DRE), northern Taiwan to study the P partitioning within the estuary. Relatively higher concentrations of TDP (4.3–12.4 μM) and TPP (2.3–8.7 μM) were generally found in the upper estuary, salinity <5 region, during the four surveys. The DIP concentration generally dominated the total P pool (TDP + TPP) within the estuary. However, dissolved organic P (DOP) became the important fraction in the salinity >25 region, probably attributed to phytoplankton production because the higher DOP concentration was generally accompanied with a higher Chl. a concentration. The TPP concentrations generally correlated well with the total particulate Fe and Mn concentrations, suggesting that particulate Fe and Mn played crucial roles influencing the P distribution within the DRE.

Occurrence and distribution of pharmaceutical compounds in the Danshuei River Estuary and the Northern Taiwan Strait

Ten pharmaceutically active compounds (PhACs) were determined in northern Taiwan estuarine waters and Taiwan Strait (TS) seawater. The ecological risk of these PhACs was assessed using risk quotient (RQ), which is the ratio of the measured maximum concentration to the predicted no-effect concentration. Six PhACs were detected within the estuarine waters. Caffeine concentration (130–718 ng l−1) was the highest among the analyzed PhACs. The distribution of PhACs in the Danshuei River Estuary generally exhibited addition behavior, except that caffeine showed conservative behavior. Carbamazepine, gemfibrozil, caffeine, and ketoprofen were detected in TS seawaters. Their concentrations follow the sequence: gemfibrozil > ketoprofen > caffeine > carbamazepine. The caffeine concentrations in TS seawaters were 2–3 orders of magnitude lower than those in Danshuei estuarine waters. With few exceptions for caffeine, erythromycin, and sulfadiazine posing low risk in some estuarine waters, most of the RQ values were <0.01, suggesting no adverse effects on aquatic organisms.

Investigation of the fecal coliform plumes induced by river discharge and wind stress using a three-dimensional model

A three-dimensional hydrodynamic, suspended sediment, and fecal coliform transport model was developed and applied to characterize the fecal coliform plume in the Danshuei River estuarine system and its adjacent coastal sea. The numerical model was calibrated and validated with time-series water levels and spatial distributions of suspended sediment and fecal coliform concentrations in 2012 and 2013, respectively. Three indices (i.e., mean absolute error, root-mean-square error, and skill score) were used to compare the predicted results and measured data for assessing the performance of the model. Quantitatively, there was good agreement between the simulation and the measurement results. The validated model was later applied to explore the influence of river discharge and wind stress on the fecal coliform plume in the Danshuei River coastal area. In the absence of wind stress, no cyclonic circulation occurs as a result of river discharge under flood conditions. However, anticyclone circulation occurs on the east coast of the Danshuei River mouth at low river discharge. The plume distances of fecal coliform from the Danshuei River mouth reach 17 km and 5 km under flood flow and Q50 flow conditions, respectively. In the absence of river discharge, the plume distance of fecal coliform is confined to the Danshuei River mouth. Consequently, the wind directions control the upwelling and downwelling plumes. If the river discharge and wind stress are included in the model simulations, the surface fecal coliform concentrations are flushed out of the Danshuei River mouth and move to the southwestern coast as a result of the wind’s favoring downwelling. Conversely, the fecal coliform concentrations are flushed out of the Danshuei River mouth and flow to the northeastern coast, with the wind favoring upwelling. Comparing simulated results with and without wind stresses, the plume distance/area of fecal coliform decreases with the inclusion of wind stress.

An Assessment of Direct Dissolved Inorganic Carbon Injection to the Coastal Region: A Model Result

The amount of carbon dioxide (CO2) in the atmosphere has increased in the past 60 years and the technology of carbon capture and storage (CCS) has recently been extensively studied. One of the strategies of CCS is to directly inject a high dissolved inorganic carbon (DIC) concentration (or high partial pressure of carbon dioxide, pCO2) solution into the ocean. However, the carbonate dynamics and air-sea gas exchange are usually neglected in a CCS strategy. This study assesses the effect of a DIC-solution injection by using a simple two end-member model to simulate the variation of pH, DIC, total alkalinity (TA) and pCO2 between the river and sea mixing process for the Danshuei River estuary and Hoping River in Taiwan. We observed that the DIC-solution injection can contribute to ocean acidification and can also lead the pCO2 value to change from being undersaturated to oversaturated (with respect to the atmospheric CO2 level). Our model result also showed that the maximum Revelle factors (Δ[CO2]/[CO2])/(Δ[DIC]/[DIC]) among varied pH values (6–9) and DIC concentrations (0.5–3.5 mmol kg−1) were between pH 8.3 and 8.5 in fresh water and were between 7.3 and 7.5 in waters with a salinity of 35, reflecting the changing efficiency of dissolving CO2 gas into the DIC solution and the varying stability of this desired DIC solution. Finally, we suggest this uncoupled Revelle factor between fresh and salty water should be considered in the (anthropogenic) carbonate chemical weathering on a decade to century scale.

Assessing the natural and anthropogenic influences on basin-wide fish species richness

Theory predicts that the number of fish species increases with river size in natural free-flowing rivers, but the relationship is lost under intensive exploitation of water resources associated with dams and/or landscape developments. In this paper, we aim to identify orthomorphic issues that disrupt theoretical species patterns based on a multi-year, basin-wide assessment in the Danshuei River Watershed of Taiwan. We hypothesize that multiple human-induced modifications fragment habitat areas leading to decreases of local fish species richness. We integrally relate natural and anthropogenic influences on fish species richness by a multiple linear regression model that is driven by a combination of factors including river network structure controls, water quality alterations of habitat, and disruption of channel connectivity with major discontinuities in habitat caused by dams.We found that stream order is a major forcing factor representing natural influence on fish species richness. In addition to stream order, we identified dams, dissolved oxygen deficiency (DO), and excessive total phosphorus (TP) as major anthropogenic influences on the richness of fish species. Our results showed that anthropogenic influences were operating at various spatial scales that inherently regulate the physical, chemical, and biological condition of fish habitats. Moreover, our probability-based risk assessment revealed causes of species richness reduction and opportunities for mitigation. Risks of species richness reduction caused by dams were determined by the position of dams and the contribution of tributaries in the drainage network. Risks associated with TP and DO were higher in human-activity-intensified downstream reaches. Our methodology provides a structural framework for assessing changes in basin-wide fish species richness under the mixed natural and human-modified river network and habitat conditions. Based on our analysis results, we recommend that a focus on landscape and riverine habitats and maintaining long-term monitoring programs are crucial for effective watershed management and river conservation plans.

Origins of Particulate Organic Matter Determined from Nitrogen Isotopic Composition and C/N Ratio in the Highly Eutrophic Danshuei Estuary, Northern Taiwan

The Danshuei River flows through the heavily populated metropolitan area of Taipei and New Taipei cities, which causes remarkable additions of nutrient elements. In spite of the rather short residence time of water, the Danshuei estuary is distinctive for the very high ammonium concentration and extensive hypoxia in its lower reach. Because particulate organic matter (POM) is potentially the culprit of hypoxia, we investigate the isotopic characteristics of POM collected in February and July 2009 at a fixed station over four semidiurnal tidal cycles. By using nitrogen isotopic composition and C/N ratio of POM, we derive the relative contributions of POM from different sources. One potential source that combines dead and living phytoplankton, phytodetritus, has δ15N values that can be predicted by the δ15N of ammonium and the isotope effect during ammonium uptake; however, the isotope effect is concentration dependent. We employ a three-end-member mixing model based on δ15N and C/N ratio to calculate the fractional contributions from three major POM sources, i.e., phytodetritus, soil, and sediment. Sensitivity test was conducted for the derivations from both carbon and nitrogen basis. For February 2009 we found the three fractions (in terms of contribution to the particulate organic carbon) to be 45 ± 19, 10 ± 11 and 45 ± 13 %, respectively; for July 2009, 71 ± 18, 11 ± 10 and 18 ± 13 %, respectively. The results imply that phytodetritus is probably the major culprit for the hypoxic conditions in the estuary, especially, in summer.

Assessment of Climate Change Impacts on Water Quality in a Tidal Estuarine System Using a Three-Dimensional Model

Climate change is one of the key factors affecting the future quality and quantity of water in rivers and tidal estuaries. A coupled three-dimensional hydrodynamic and water quality model has been developed and applied to the Danshuei River estuarine system in northern Taiwan to predict the influences of climate change on water quality. The water quality model considers state variables including nitrogen, phosphorus, organic carbon, and phytoplankton as well as dissolved oxygen, and is driven by a three-dimensional hydrodynamic model. The hydrodynamic water quality model was validated with observational salinity distribution and water quality state variables. According to the analyses of statistical error, predictions of salinity, dissolved oxygen, and nutrients from the model simulation quantitatively agreed with the observed data. The validated model was then applied to predict water quality conditions as a result of projected climate change effects. The simulated results indicated that the dissolved oxygen concentration was projected to significantly decrease whereas nutrients will increase because of climate change. Moreover, the dissolved oxygen concentration was lower than 2 mg/L in the main stream of the Danshuei River estuary and failed to meet the water quality standard. An appropriate strategy for effective water quality management for tidal estuaries is needed given the projected persistent climate trends.

Assessment of the climate change impacts on fecal coliform contamination in a tidal estuarine system.

Climate change is one of the key factors affecting the future microbiological water quality in rivers and tidal estuaries. A coupled 3D hydrodynamic and fecal coliform transport model was developed and applied to the Danshuei River estuarine system for predicting the influences of climate change on microbiological water quality. The hydrodynamic and fecal coliform model was validated using observational salinity and fecal coliform distributions. According to the analyses of the statistical error, predictions of the salinity and the fecal coliform concentration from the model simulation quantitatively agreed with the observed data. The validated model was then applied to predict the fecal coliform contamination as a result of climate change, including the change of freshwater discharge and the sea level rise. We found that the reduction of freshwater discharge under climate change scenarios resulted in an increase in the fecal coliform concentration. The sea level rise would decrease fecal coliform distributions because both the water level and the water volume increased. A reduction in freshwater discharge has a negative impact on the fecal coliform concentration, whereas a rising sea level has a positive influence on the fecal coliform contamination. An appropriate strategy for the effective microbiological management in tidal estuaries is required to reveal the persistent trends of climate in the future.

Modeling investigation of asymmetric tidal mixing and residual circulation in a partially stratified estuary

A three-dimensional hydrodynamic model was applied to the Danshuei River estuarine system in northern Taiwan to investigate the influence of flood-ebb, spring-neap tidal cycles, and salinity distribution on tidal mixing, residual circulation, stratification, and tidal asymmetry. The model was validated using observational data collected in 2008. The results from the model agreed well with observations of water surface elevation, tidal currents, and salinity. It was found that the depth-averaged tidal current during flood tide is weaker with a shorter duration than that during ebb tide in the estuary, which was attributed to tidal asymmetry. Vertical profiles of salinity, flow, eddy diffusivity, and Richardson number also showed a marked asymmetry between flood and ebb tides. Bottom boundary stresses were higher during flood tides than during ebb tides, resulting in more mixing occurrence and consequently decreasing the Richardson numbers. The tidally averaged salinity was more stratified during neap tides than during spring tides because the presence of the stronger vertical diffusivity and turbulent kinetic energy during spring tides. The modeling results also confirmed that the residual circulation was stronger during neap tides than during spring tides.

Geomorphologic dynamics and maintenance following mudflat, creek and pond formation in an estuarine mangrove wetland

Mudflats, creeks and ponds serve as critical habitat for shorebirds and fish in subtropical estuarine wetlands. A wetland restoration project was launched to remove partial mangrove trees from a predefined area and form a mudflat patch. A creek-pond-combo (CPC) construction project was also conducted in the Shezi wetland area along the Danshuei River. Long-term measurements were performed to test for geomorphologic responses and related habitat changes. The dynamic topography revealed significant patterns of sediment deposition, with the highest deposition rates found in the CPC habitat area. The sediment trap efficiency of the CPC was roughly seven-fold higher than that of the mudflat area. The CPC area gradually evolved into a mudflat habitat area through processes of sediment deposition and was significantly reduced in size and more shallow after typhoon events. The shrinking phenomenon occurred prior to deposition and at a faster rate. CPC construction is recommended every two years, and the related maintenance costs are estimated to be approximately 1,500 US dollars per year. The findings of this study indicate that long-term investments should be made to maintain CPC areas in estuarine mangrove wetlands.

Tradeoffs between reducing flood risks and storing carbon stocks in mangroves

Mangrove habitats are important carbon(C) reserve sites. However, the overgrowth or overexpansion of mangroves may increase the risk of flooding, thus affecting human wellbeing. To decrease the flood risks, partial mangrove removal has been proposed as a managerial action, which would decrease the C stocks. Using the Danshuei River as a case study, the objective of this study was to determine the optimal removal area to allow the mangroves to meet the demands of reducing the loss of mangrove C stocks and adequately controlling the flood risks. Our results show that the ratios of the effective reduction in flood level (benefit) and the loss of mangrove C stocks (cost) were only higher under the condition of the removal of aboveground structures of mangrove trees than the ratios under the condition of the removal of both above- and belowground structures. The highest ratio of the effective reduction in flood level and the loss of mangrove C stocks occurred under the condition of removal of 20% of aboveground structures of mangrove trees, indicating the optimal removal area for mangrove management in the Danshuei River. This study provides a case study exhibiting the tradeoffs between ecosystem services in mangrove management.