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In the past, two physical mechanisms, baroclinic instability (BI) and strong asymmetric tidal mixing (SATM) during the spring tidal period, were proposed for the offshore detachment of the low-salinity plume over the inner shelf of the East China Sea (ECS). These two mechanisms were re-examined using both observations and a fully three-dimensional (3-D), high-resolution, unstructured-grid, free-surface, primitive-equation, Finite-Volume Community Ocean Model (FVCOM). The observed currents and salinities showed that the plume was characterized by a two-layer system, in which the upper layer is mainly driven by the river discharge-induced buoyancy flow and the lower layer is predominantly controlled by tidal mixing and rectification. The SATM mechanism was based on the model run without calibration against observed currents and salinity around the plume region, so that it should be applied with caution to a realistic condition observed on the inner shelf of the ECS. The BI mechanism was derived under a condition without consideration of tidal mixing. Although BI could still occur along the frontal zone when tides were included, it was unable to produce a single, large, detached low-salinity lens observed on the inner shelf of the ECS. The process-oriented model experiment results suggest that for a given river discharge and realistic tidal flow, the spatially non-uniform southwesterly surface wind during the southeast monsoon-dominant summer could increase frontal spatial variability and thus produce a significant offshore detachment of low-salinity water on the inner shelf of East China Sea. [ABSTRACT FROM AUTHOR]

Coastal ecosystems are strongly influenced by terrestrial inputs of freshwater, sediments, and nutrients, particularly in a megariver estuary of the Changjiang River. A remarkable increase in nutrient loading from the Changjiang River to the shelf has been observed over the period from 1999 to 2016 and turned the region into a high eutrophication condition. The Finite‐Volume Community Ocean Model and the European Regional Seas Ecosystem Model were coupled to assess the impact of the nutrient loading on the interannual variability of nutrients and phytoplankton. The model was first validated via observational data, and then dynamical analysis were conducted. Singular vector decomposition analysis indicated that the rapid change of local ecosystem was highly correlated with the change in river nutrient contributions. The Changjiang estuarine ecosystem was phosphate limited. The phosphate exhibited local variation, while the abundant nitrate from the river was diluted by the low‐nitrate oceanic water. The suspended sediment was significantly correlated with phytoplankton but not with nutrients. The ratio of diatom biomass to dinoflagellate biomass respected a rapid response to strong oscillations in the river nutrient input. High diatom primary production occurred near the sediment front, whereas the dinoflagellate bloom extended significantly offshore. The spring diatom and dinoflagellate blooms had major peaks in the empirical orthogonal function Modes 1 and 2, and the autumn bloom is characterized by secondary peaks from Mode 2 in the autumn. Plain Language Summary: There was an increase in the nutrient input into the Changjiang Estuary with an increased use of agricultural fertilizer as food demand rises globally. Further, there will be additional changes to river and estuary fluxes due to global anthropogenic activities in the future. This study presented the results of a novel coupled physical‐biogeochemical model which was designed to examine the seasonal and interannual variability of nutrients and phytoplankton dynamics in the Changjiang Estuary over the period of 1999–2016. The variations of nutrient source from the Changjiang Estuary in the last 18 years had been identified. The model demonstrated that this estuary ecosystem had a rapid response to the changes in riverine nutrients. The total nutrient concentrations, as well as their fluxes of the system, had similar variation patterns with river discharge, indicating that the river was the principal source of nutrients for the nearshore and offshore regions in that area. The phytoplankton population in terms of the ratio between diatoms and dinoflagellates responded quickly to changes in river nutrient fluxes. Key Points: Physical and biogeochemical models have been coupled to study pelagic planktonic ecosystem variability in a river estuary systemConsequences of changing nutrient fluxes over the last 18 years into the estuary on nutrient biogeochemistry and plankton were studiedPelagic ecosystem responded rapidly to river discharge and nutrient flux but exhibited different patterns for nutrients and phytoplankton [ABSTRACT FROM AUTHOR]

Abstract: A dike–groyne module is developed and implemented into the unstructured-grid, three-dimensional primitive equation finite-volume coastal ocean model (FVCOM) for the study of the hydrodynamics around human-made construction in the coastal area. The unstructured-grid finite-volume flux discrete algorithm makes this module capable of realistically including narrow-width dikes and groynes with free exchange in the upper column and solid blocking in the lower column in a terrain-following coordinate system. This algorithm used in the module is validated for idealized cases with emerged and/or submerged dikes and a coastal seawall where either analytical solutions or laboratory experiments are available for comparison. As an example, this module is applied to the Changjiang Estuary where a dike–groyne structure was constructed in the Deep Waterway channel in the inner shelf of the East China Sea (ECS). Driven by the same forcing under given initial and boundary conditions, a comparison was made for model-predicted flow and salinity via observations between dike–groyne and bed-conforming slope algorithms. The results show that with realistic resolution of water transport above and below the dike–groyne structures, the new method provides more accurate results. FVCOM with this MPI-architecture parallelized dike–groyne module provides a new tool for ocean engineering and inundation applications in coastal regions with dike, seawall and/or dam structures. [Copyright &y& Elsevier]

High-turbidity water is a common feature in the estuary and inner shelf. Sediment suspension functions as a modulator that directly influences the interactions among nutrients, phytoplankton and other related ecosystem variables. A physical-biological coupling model system was applied to examine the impact of sediment front on interactions among on suspended sediment, vertical mixing, nutrients and phytoplankton over the inner shelf off the high-turbidity, phosphate-limited Changjiang Estuary. The physical model was the Finite-Volume Community Ocean Model (FVCOM) and the biological model was the European Regional Seas Ecosystem Model (ERSEM). Results revealed that in the nearshore region the growth of phytoplankton over the spring-summer seasons was limited by suspended sediments and intensified vertical mixing during the autumn-winter seasons extended the sediment-induced suppression extended offshore to restrict the phytoplankton growth over the shelf. Nutrients were diluted by spreading of freshwater discharge and significantly decreased off the suspended sediment front due to the depletion by the offshore phytoplankton growth. The simulation results showed that although the diatom phytoplankton dominated the Chlorophyll a (Chl- a) concentration, the non-diatom group had a more contribution to the biomass. The relatively high phytoplankton biomass was found over the offshore deep underwater valley area as results of remote advection by the Taiwan Warm Current and weak turbulent mixing. • Physical-biogeochemical model was developed to study sediment-ecosystem interaction. • Model revealed high nutrients within the front and dense Chl a out of the front. • Chl a was mainly from diatom, however non-diatom group contributed more on biomass. [ABSTRACT FROM AUTHOR]