Short-Term Response of Chlorophyll a Concentration Due to Intense Wind and Freshwater Peak Episodes in Estuaries: The Case of Fangar Bay (Ebro Delta)

Estuaries and coastal bays are areas of large spatio-temporal variability in physical and biological variables due to environmental factors such as local wind, light availability, freshwater inputs or tides. This study focuses on the effect of strong wind events and freshwater peaks on short-term chlorophyll a (Chl a) concentration distribution in the small-scale and microtidal, Fangar Bay (Ebro Delta, northwestern Mediterranean). The hydrodynamics of this bay are primarily driven by local wind episodes modulated by stratification in the water column. Results based on field-campaign observations and Sentinel-2 images revealed that intense wind episodes from both NW (offshore) and NE-E (onshore) caused an increase in the concentration of surface Chl a. The mechanisms responsible were horizontal mixing and the bottom resuspension (also linked to the breakage of the stratification) that presumably resuspended Chl a containing biomass (i.e., micropyhtobentos) and/or incorporated nutrients into the water column. On the other hand, sea-breeze was not capable of breaking up the stratification, so the chlorophyll a concentration did not change significantly during these episodes. It was concluded that the mixing produced by the strong winds favoured an accumulation of Chl a concentration, while the stratification that causes a positive estuarine circulation reduced this accumulation. However, the spatial-temporal variability of the Chl a concentration in small-scale estuaries and coastal bays is quite complex due to the many factors involved and deserve further intensive field campaigns and additional numerical modelling efforts.info:eu-repo/semantics/publishedVersio

Use of a hydrodynamic model for the management of water renovation in a coastal system

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.info:eu-repo/semantics/publishedVersio

Assessing Nature Based Solutions for Managing Coastal Bays

Aquest recurs no està disponible en accés obert per política de l’editorial. This item isn't open access because of publisher's policyThis work investigates the hydrodynamic response of two small and shallow bays located in the Ebro Delta, in order to solve the problems affecting mussel farms, such as the high temperatures reached in the summer period (>28 °C), causing the death of these bivalves, and the limited water renewal that causes anoxia within the bay. Nature Based Solutions are described focusing on two aspects: one, modifying the flow of these drainage channels, increasing the arrival of freshwater to the bays; and two, opening a gap in the sand bars so that there is a flow of sea water in and out through the innermost zone, which is where both bays have greater problems of water renewal. The impact of those solutions simulated with the Regional Oceans Modeling System (ROMS), consist of a set of nested domains that use CMEMS-IBI data for the initial and open boundary conditions (coarser domain). To validate the numerical model, data from in situ campaigns have been used for one full year (2014) for Alfacs Bay, and for the summer period (July-August 2017) for Fangar Bay. In addition, the Sea Surface Temperature (SST) is also studied as an indicator to reduce the mortality problems of mussels, but the results obtained do not reduce this variable. What the results do show is a decrease in residence time in both bays for both experiments, with the increase in flow being more viable than the increase in sand bar breakage.info:eu-repo/semantics/publishedVersio