Semidiurnal and spring-neap variations in the Tagus estuary: application of a process-oriented hydro-biogeochemical model

Abstract

The main objectives of this work are the presentation of a hydro-biogeochemical model for the Tagus estuary and the study of the interaction between tides and river discharge. Special emphasis will be given to hydrographic, fine sediment dynamics and biogeochemical variations at two different time scales: the tidal cycle and the fortnight cycle. The numerical model validation was performed comparing harmonic analysis results of measured and model predicted sea surface height for 12 stations covering the whole estuary. For the M2 constituent the difference is less than 5% of the local amplitude for almost all the stations. Differences in phase are small, with an average value of 5º. The results show strong salinity and suspended sediments gradients which are advected up and downstream according to the tidal cycle. Moreover, the estuary is divided in three main regions: a marine, a mixing and a freshwater influence area. The suspended sediments and chlorophyll concentrations reveal strong fortnight time dependence near the estuary mouth. Higher concentrations of fine sediments are found during spring tides that on neaps, which induces a decrease (increase) in the chlorophyll concentration during spring (neap) tides. This study presents the first results of the Tagus estuary predictive model, which are consistent with observations and with previously knowledge about the estuary. Preliminary outcomes obtained with this high resolution numerical modeling system are very encouraging to pursuit the development of a numerical tool suitable to deliver products that can be used both in scientific endeavors and in estuarine management, in order to assess the ecological estuarine quality.