Interactions between physical forcing, water circulation and phytoplankton dynamics in a microtidal estuary

Abstract

This thesis focuses on the interactions between water circulation dynamics, nutrient availability and the structure of the phytoplanktonic community from a mesoscale point of view. To accomplish this aim we studied the micro-tidal estuaries of Alfacs and Fangar, located in the Ebre river Delta, in the Northwestern Mediterranean. The work was developed along three main lines. First, the analysis of the variability of a 14-year long time series of the phytoplankton and environmental data showed a strong seasonality of the phytoplankton communities of both bays, characterized by an autumn diatom assemblage and a winter group of dinoflagellates in Alfacs, and a flagellate-dominated group in winter, and a mixed group of dinoflagellates and diatoms in late spring and summer in Fangar. The phytoplankton dynamics and composition was different in the two bays, a finding that could be attributed to a lower residence time of the water in Fangar, which has a smaller volume than Alfacs but receives comparable inputs of freshwater. No relevant temporal trends were detected in phytoplankton abundance or composition. Second, scale considerations and a three-dimensional circulation model of Alfacs revealed that the principal forcing factors are freshwater inflow and wind, while the tide is not relevant regarding water transport or mixing. The freshwater input is important at a seasonal scale, and is responsible for the stratified situation usually found in Alfacs. The wind is particularly important at time scales of a few days and, above a certain threshold, wind events can mix the estuary and break the stratification. Based on the strength of the stratification relative to the wind speed, as expressed by a Richardson number, and the direction of the wind, three scenarios regarding the coupling between wind forcing and hydrodynamics have been defined, demonstrating the importance of wind in controlling the exchange of water with the exterior. Finally, the third line of research approached the budget of major nutrients (nitrogen N and phosphorus P) in Alfacs. A zero-dimensional ecosystem model incorporating phytoplankton, zooplankton and the concentrations of various (organic and inorganic) N and P pools suggested that the inputs of dissolved organic phosphorus (DOP) through the discharge channels are a crucial source of phosphorus to the system, explaining the draw-down of nitrogen during the summer and the observed high primary production. Two non-exclusive mechanisms could explain DOP availability for phytoplankton: direct uptake and remineralisation to dissolved inorganic phosphorus. Input of phosphorus from sediment resuspension could be important at short time scales, but did not seem to be a substantial contribution to the total P budget from a seasonal perspective. In conclusion, this thesis shows how the physical forcing can influence the composition, timing and abundance of the phytoplanktonic community in two bays that can be taken as an example of microtidal estuarine water bodies