Climatology and nutrient fluxes in the Tagus estuary: A coupled model application

Abstract

Rivers are responsible for the main transport of nutrients from land to estuaries and coastal areas. Understanding the fate of these anthropogenic inputs and their impacts on estuaries and adjacent marine ecosystems is essential for the protection of coastal waters. Here, the 3D MOHID hydrodynamic model coupled with a biogeochemical model was used to determine seasonal nutrient fluxes between areas of the Tagus estuary. For this purpose, the estuary was divided into boxes, which allowed to evaluate nutrients (and phytoplankton) exchanges between areas, to identify source and sink zones, to estimate the amount of nutrients exported to the coastal zone adjacent to the estuary, and to understand the recycling capacity of this estuarine system. Globally, for all nutrients and seasons, the highest net fluxes were observed in the main and central body of the estuary. The differences observed in the flux budget for each box and for all nutrients are mostly related to variations in the seasonal abundance of phytoplankton, which is much higher in Spring/Summer than in the rest of the year due to the higher seawater temperature and irradiance. During these seasons the high temperatures also promoted the fast mineralization of organic matter along the water column and in the sediment. For all seasons, an export of nutrients and phytoplankton from upstream to downstream was observer at the surface, and from the most confined areas of the southern margin into the central body and main channel of the estuary, whilst the opposite was observed in the bottom layers. Nevertheless, surface fluxes were always higher than those recorded at depth, indicating that the estuary exported nutrients to the coast, all year round. For 2020 it was estimated that 52%, 88% and 65% of the ammonium, nitrate, and phosphate, respectively, that entered the estuary from both rivers and wastewater treatment plants were exported to the adjacent coast. The amount of nutrients exported to the coastal area is closely related to river discharge, being much higher in flood years than in dry years, particularly for nitrate.