A combined field and modelling study to understand the modulation of phytoplankton variability by physical forcing in Alfacs Bay (NW Mediterranean)

Abstract

Alfacs Bay (Ebre delta) is a shallow microtidal estuary whose high productivity (compared to the adjacent oligotrophic Mediterranean waters) sustains a rich ecosystem and allows a valuable aquaculture activity. The appropriate management of the natural resources in the area requires a thorough understanding of the spatio-temporal variability of phytoplankton. Here we will summarize our main findings arisen from the field studies conducted between 2007 and 2012 combined with modeling approaches addressed to study the ecosystem of Alfacs Bay. Synoptic cruises were performed in different periods of the year in order to characterize the physico-chemical (water temperature and salinity, nutrient concentrations) and biological (phytoplankton biomass and community structure) parameters. Deployed sensors provided the continuous record of water temperature, salinity and velocities at a fixed central station. These data and concurrent meteorological data (wind intensity) were used to implement and validate a semi-implicit Three-Dimensional Circulation Model (Si3D) in the bay. The overall study showed the existence of a preferential phytoplankton accumulation area in the inner NE side of the Bay. Nutrient supply, mainly through the irrigation channels discharging into the northern coast, fuels phytoplankton biomass development, but does not explain the patchy distribution over the year. This is better understood by the physical forcing established by the wind conditions and freshwater input which control the stratification and water circulation patterns in Alfacs bay. Using the 3D hydrodynamic model, we simulated the particle distribution dynamics under different hydrodynamic scenarios. The simulation experiments suggest that the estuarine circulation dynamics operating in the bay would foster the observed phytoplankton confinement in the inner NE area in Alfacs bay