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Phytoplankton primary production in coastal bays and estuaries is influenced by multiple physical variables, such as wind, tides, freshwater inputs or light availability. In a short-term perspective these factors may influence the composition of biological variables such as phytoplankton biomass, as well as the amount of nutrients within the waterbody. Observations in Fangar Bay, a small, shallow, stratified and micro-tidal bay in the Ebro Delta (NW Mediterranean Sea), have shown that during wind episodes the biological variables undergo sudden variations in terms of concentration and distribution within the bay. The Regional Ocean Model System (ROMS) coupled with a nitrogen-based nutrient, phytoplankton, zooplankton, and detritus (NPZD) model has been applied to understand this spatio-temporal variability of phytoplankton biomass in Fangar Bay. Idealised simulations prove that during weak wind events (< 6 ms- 1), the stratification is maintained and therefore there is not dynamic connection between surface and bottom layers, penalizing phytoplankton growth in the whole water column. Conversely, during intense wind events (> 10 ms-1) water column mixing occurs, homogenising the concentration of nutrients throughout the column, and increasing phytoplankton biomass in the bottom layers. In addition, shifts in the wind direction generate different phytoplankton biomass distributions within the bay, in accordance with the dispersion of freshwater plumes from existing irrigation canals. Thus, the numerical results prove the influence of the freshwater plume evolution on the phytoplankton biomass distribution, which is consistent with remote sensing observations. The complexity of the wind-driven circulation due to the bathymetric characteristics and the modulation of the stratification implies that the phytoplankton biomass differs depending on the prevailing wind direction, leading to sharp Chl a gradients and complex patterns. [ABSTRACT FROM AUTHOR]

In this study we investigate the variability in near-bottom turbidity in Alfacs Bay (in the northwestern Mediterranean Sea). The bay is characterized by a micro-tidal environment and seiching activity that may lead to flow velocities of more than 50 cm s -1. A set of current meters and optical sensors were mounted near the sea bottom to acquire synchronous hydrodynamic and optical information from the water column. The time-series observations showed an evident relation between seiche activity and sediment resuspension events. The observations of turbidity peaks are consistent with the node–antinode location of the fundamental and first resonance periods of the bay. The implementation of a coupled wave–current numerical model shows strong spatial variability of the potential resuspension locations. Strong wind events are also a mechanism responsible for the resuspension of fine sediment within the bay. This is confirmed by suspended sediment concentration maps derived from Sentinel-2 satellite imagery. We suggest that the sequence of resuspension events plays an important role in the suspended sediment concentration, meaning that the occurrence of sediment resuspension events may increase the suspended sediment in subsequent events. The suspended sediment events likely affect the ecological status of the bay and the sedimentary process over a long-term period. [ABSTRACT FROM AUTHOR]

In this contribution we investigate the hydrodynamic response in Alfacs Bay (Ebro Delta, NW Mediterranean Sea) to different anthropogenic modifications in freshwater flows and inner bay–open sea connections. The fresh water coming from rice field irrigation contains nutrients and pesticides and therefore affects in multiple ways the productivity and water quality of the bay. The application of a nested oceanographic circulation modelling suite within the bay provides objective information to solve water quality problems that are becoming more acute due to temperature and phytoplankton concentration peaks during the summer period when seawater may exceed 28 ∘ C, leading to high rates of mussel mortality and therefore a significant impact on the local economy. The effects of different management "solutions" (like a connection channel between the inner bay and open sea) are hydrodynamically modelled in order to diminish residence times (e -flushing time) and water temperatures. The modelling system, based on the Regional Ocean Modeling System (ROMS), consists of a set of nested domains using data from CMEMS-IBI for the initial and open boundary conditions (coarser domain). One full year (2014) of simulation is used to validate the results, showing low errors with sea surface temperature (SST) and good agreement with surface currents. Finally, a set of twin numerical experiments during the summer period (when the water temperature reaches 28 ∘ C) is used to analyse the effects of proposed nature-based interventions. Although these actions modify water temperature in the water column, the decrease in SST is not enough to avoid high temperatures during some days and prevent eventual mussel mortality during summer in the shallowest regions. However, the proposed management actions reveal their effectiveness in diminishing water residence times along the entire bay, thus preventing the inner areas from having poor water renewal and the corresponding ecological problems. [ABSTRACT FROM AUTHOR]

Wave–current interactions (WCIs) are investigated. The study area is located at the northern margin of the Ebro shelf (northwestern Mediterranean Sea), where episodes of strong cross-shelf wind (wind jets) occur. The aim of this study is to validate the implemented coupled system and investigate the impact of WCIs on the hydrodynamics of a wind-jet region. The Coupled Ocean–Atmosphere–Wave–Sediment Transport (COAWST) modeling system, which uses the Regional Ocean Model System (ROMS) and Simulating WAves Nearshore (SWAN) models, is used in a high-resolution domain (350 m). Results from uncoupled numerical models are compared with a two-way coupling simulation. The results do not show substantial differences in the water current field between the coupled and the uncoupled runs. The main effect observed when the models are coupled is in the water column stratification, due to the turbulent kinetic energy injection and the enhanced surface stress, leading to a larger mixed-layer depth. Regarding the effects on the wave fields, the comparison between the coupled and the uncoupled runs shows that, when the models are coupled, the agreement of the modeled wave period significantly improves and the wave energy (and thus the significant wave height) decreases when the current flows in the same direction as the waves propagate. [ABSTRACT FROM AUTHOR]

In this contribution we investigate the hydrodynamic response in Alfacs Bay (Delta Ebro, NW Mediterranean Sea) to different anthropogenic modifications in freshwater flows and inner bay-open sea connections. The fresh water, coming from rice field irrigation, contains nutrients and pesticides and therefore affects in multiple ways the productivity and water quality of the bay. The application of a nested oceanographic circulation modelling suite within the bay provides objective information to solve water quality problems that are becoming more acute due to temperature and phytoplankton concentration peaks during the summer period, when sea water may exceed 28 °C leading high rates of mussels mortality and therefore a significant impact on the local economy. The effects of different management solutions (like a connection channel between the inner bay and open sea) are hydro-dynamically modelled in order to diminish residence times (e-flushing time) and water temperatures. The modelling system, based on the Regional Ocean Modelling System (ROMS), consists in a set of nested domains using data from CMEMS-IBI for the initial and open boundary conditions (coarser domain). One full year (2014) of simulation is used to validate the results showing low errors with SST and good agreement with surface currents. Finally, a set of twin numerical experiments during the summer period (when water temperature reaches 28 °C) are used to analyse the effects of proposed nature-based interventions. Although these actions modify water temperature in the water column, the decrease in SST is not high enough to avoid high temperatures during some days preventing eventual mussel mortality during summer in the shallowest regions. However, the proposed management actions reveal their effectiveness in diminishing water residence times along the entire bay, thus preventing the inner areas to have low renovation and the corresponding ecological problems. [ABSTRACT FROM AUTHOR]

In this contribution we investigate the origin of the variability in near-bottom turbidity observations in the Alfacs Bay (NW Mediterranean Sea). This bay is characterized by a micro-tidal environment and a relevant seiching activity which may lead to flow velocities of more than 50 cm s-1. A set of current meters and optical sensors mounted near the sea bottom were used to acquire synchronous hydrodynamic and optical information of the water column. The time-series observations showed an evident relation between seiche activity and sediment resuspension events. The observations of turbidity peaks are consistent with the node/anti-node location for the fundamental and first resonance periods of the bay. The implementation of a coupled wave-current numerical model shows a strong spatial variability of the potential resuspension locations. Strong wind events are also a mechanism responsible of the resuspension of fine sediment within the bay. This is confirmed using retrieval of suspended sediment concentration from Sentinel-2 data. We suggest that the sequence of resuspension events plays a relevant role in SSC, in such a way that previous sediment resuspension events may influence the increase of suspended sediment in subsequents events. The suspended sediment events likely affect the ecological status of the Bay and the sedimentary process at long-term period. [ABSTRACT FROM AUTHOR]

Wave-Current Interactions (WCIs) are investigated. The study area is located at the northern margin of the Ebro Shelf (northwestern Mediterranean Sea), where episodes of strong cross-shelf wind (wind jets) occur. The aim of this study is to validate the implemented coupled system and investigate the impact of WCIs on the hydrodynamics of a wind-jet region. The Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) modeling system, which use Regional Ocean Model System (ROMS) and Simulating WAves Nearshore (SWAN) models, is used in a high-resolution domain (350 m). Results from uncoupled numerical models are compared with a two-way coupling simulation. The results do not show substantial differences in the water current field between the coupled and the uncoupled runs. The main effect observed when the waves are considered is in the water column stratification, due to the turbulent kinetic energy injection and the enhanced surface stress, leading a larger mixed-layer depth. Additionally, when the water currents are considered, the agreement of the modeled wave period significantly improves and the wave energy (and thus the significant wave height) decreases when the current flows in the same direction as the waves propagate. [ABSTRACT FROM AUTHOR]