Effect of tidal regime on estuarine residence time spatial variation ANTÓNIO A. L. S. DUARTE 1 ; JOSÉ M. P. VIEIRA 1 1 Department of Civil Engineering 1 University of Minho 1 Campus de Gualtar, 4710-057 Braga PORTUGAL aduarte@civil.uminho.pt http://www.civil.uminho.pt Abstract: - The hydrology and the ecology of shallow estuaries are strongly influenced by the freshwater inflow and the adjacent open sea, due to tide and wind generated water exchange, creating salinity gradients, thermal stratification and assuring large transport of silt, organic material and inorganic nutrients into the estuarine waters. Nutrient enrichment is a key-factor for habitat degradation, leading to sensible structural changes in estuarine ecosystems with the consequent occurrence of episodic algal blooms. In the last two decades, the south arm of the Portuguese river Mondego estuary was stressed by an eutrophication process due to massive nutrient loading from urbanised areas and from intensively agricultural land runoff. The aim of this work is to calculate estuarine water residence time values, which are broadly recognised as important descriptors of estuarine circulation patterns and constitute key-parameters to assess estuarine eutrophication vulnerability. In fact, estuaries with nutrients residence time values, shorter than the algal cells doubling time, will inhibit algae blooms occurrence. The increase of estuarine flushing capacity can be seen as a management measure to mitigate or to invert eutrophication processes. In this work, the MONDEST model, a 2-DH water quality model, was developed and applied to calculate water residence time, at different simulated management scenarios. The results shows the effect of simulated tides on the spatial distribution of estuarine residence time values, which are related with the eutrophication gradients, observed in the Mondego estuary south arm during the last decades. This integrated model constitutes a powerful tool to support authorities’ decisions concerning the best water management practices and restoration measures for the environmental sustainable management of this complex ecosystem. Key-Words: - estuarine water management; numerical modelling; eutrophication; residence time; Mondego estuary. 1 Introduction Estuaries and coastal lagoons are commonly subjected to intensive anthropogenic stress due to massive pollutant loading from urbanised riverain areas. According to a strategic EU document [1], research and technological development play an important role in the implementation of the Water Framework Directive (WFD), in order to improve the knowledge about the pressures and ecological status of the aquatic ecosystems. Excessive organic carbon input associated with nutrient enrichment, leading to eutrophication of estuarine and coastal waters is widely recognized as a major worldwide threat [2; 3]. As a response to this, there has been an enormous increase in restoration plans for reversing habitat degradation, based on knowledge of the major processes which driven the observed ecological changes. Much progress has been made in understanding eutrophication processes and in constructing modelling frameworks useful for projecting the effectiveness of nutrient reduction strategies [4]. The influence of hydrodynamics must not be neglected on estuarine eutrophication vulnerability assessment, as flushing time affects the transport and the permanence of water and its constituents inside an estuary [5]. Estuarine water residence time (WRT) has a strong spatial and temporal variability, which is accentuated by exchanges between the estuary and the coastal ocean due to chaotic stirring at the mouth. So, the concept of a single WRT value per estuary, while convenient from both ecological and engineering viewpoints, is therefore shown to be an oversimplification [6]. Residence time (RT) values, related with the water constituents (conservatives or not) permanence inside an aquatic system, are broadly recognised as important descriptors of estuarine circulation patterns and, so, a convenient parameter representing the time scale of physical transport processes, often used for comparison with time scales of biogeochemical processes [7]. In fact, estuaries with nutrients residence time values, shorter than the algal cells doubling time, will inhibit algae blooms occurrence. ENERGY, ENVIRONMENT, ECOSYSTEMS, DEVELOPMENT and LANDSCAPE ARCHITECTURE ISSN: 1790-5095 240 ISBN: 978-960-474-125-0