This report describes a numerical study of the trophic processes in the Tagus, Sado and Mondego Estuaries (Portugal), in order to evaluate their trophic levels and to identify the factors determining them. The study includes (i) a description of the hydrodynamics of each estuary and of the residence time of the water inside the estuary and inside each part of the estuary, (ii) a simulation of the estuary in the present conditions and (iii) simulations of several scenarios of reduction of nutrients, in order to evaluate the implications of different management scenarios.

The study puts into evidence the importance of the residence time for the ecological characteristics of the estuaries and shows that three ranges of residence time are important. If the residence time is smaller than the time required for developing a bloom, the nutrients discharged in the estuary will be exported without being consumed (this is the case of the Mondego estuary). The Sado estuary in on the other extreme, being the residence time long enough to allow the formation of a bloom and the regeneration of nutrients by mineralization of the organic matter generated. The Tagus Estuary is between this two ranges: the residence time is enough to generate a bloom, but their products are exported to the ocean as they are produced.

The study shows that in the Tagus estuary the trophic level is limited by light penetration due to the turbidity in the water column, which is associated to the resuspension of the fine sediments deposited in the intertidal areas by tidal currents and surface waves generated by the long fetch of local wind. As a consequence, a reduction of the nutrient loads discharged by the rivers or by the Urban Waste Water Treatment Plants has no consequences for the trophic activity in the estuary.

In the Sado estuary primary production is limited by nutrients and by the interaction between the phytoplankton and zooplankton and is modulated by the residence time inside the estuary and by the strong mixing between the different zones of the estuary. Although the residence time of the estuary is longer than one month, the residence time of the water inside each part of the estuary is of the order of one week, resulting into a strong mixing between zones of the estuary, with deposition and mineralization of the particulate organic matter in the shallow intertidal areas. The reduction of the nutrients load in the estuary does not modify the general behaviour of the estuary, maintaining the structure of a stronger bloom in spring followed by several smaller blooms. The amplitude of the peaks following the first one are however smaller, especially for the zooplankton.

In the Mondego estuary, the limiting factor of phytoplankton production is the residence time, which is not long enough to allow the growing of a bloom. In this estuary the concentration of nutrients is higher in the northern channel; however eutrophication symptoms are detected in the southern channel (growth of macro-algae). It was shown that this is a consequence of the hydrodynamical properties of this channel. In fact this channel is a net importer of particulate organic matter from the northern channel, which enriches the sediments creating conditions for macro-algae growth, which is also stimulated by the small values of the transient and residual velocities. The settling characteristics of the southern channel have been stimulated by the artificial closing of the upper connection between the two channels. This study shows that the modification of the trophic characteristics of the southern channel requires the reopening of the communication between the channels and can not be achieved by a realistic reduction of nutrients discharged by the rivers.

MARETEC