The rationale for evaluating the processes of estuarine waters in Portugal and their response to modifications of nutrient loads was designed having in mind the “Criteria for the Definition of Eutrophication in Marine/Coastal Waters”, proposed in the study carried out by ERM in 2000 on behalf of the European Commission. That study is divided into three main parts: (i) the scientific context of eutrophication, (ii) a tentative to review, assess and compare criteria used in some EU Member States and (iii) some suggestions of complementary criteria to define eutrophication in coastal areas.

In the scientific context part, some relevant statements can be used for demonstrate the benefit of assessing this problem also using models:

• Eutrophication is not a “black and white” process (page 1);

• Eutrophication is a process not a state. Many upwelling areas have always been eutrophic and are not experiencing eutrophication (page 4);

• Eutrophication is a slow but universal phenomenon that started centuries ago (page 6);

• Quantification of the eutrophication rate requires the knowledge of baseline information and its current or predicted status, but baseline information is the exception rather then the rule (page 4);

• Short-term measurements of chlorophyll provide less certain evidence for eutrophication. The evaluation of eutrophication based on chlorophyll measurements requires information on grazers too.

In the conclusion of the scientific context, it is stated that:

• Much monitoring data has low statistical power to resolve the questions that were asked [about eutrophication], despite the sometimes enormous effort that was put into gathering it. Some aspects of marine eutrophication may be unanswerable, particularly with regard to baseline data.

• The elaboration of the precautionary principle in the late 80’s was an attempt to resolve this question. The fact that marine eutrophication remains contentious implies that this attempt was not entirely successful.

• The relevant question is not whether eutrophication has occurred in marine waters, but whether unacceptable eutrophication has occurred.

• Expectation of a universal ecological index that provides a simple biometric approach for evaluating eutrophication may be unrealistic.

• When choosing criteria for assessing eutrophication, the relevant question is not “is the method perfect?”, but “is the method good enough for the purpose”. Techniques have to be judged in comparison with the strength and weakness of alternative methods rather then against a standard of perfection.

The considerations above are common to most systems. Identification and quantification of eutrophication rate in tidal systems is even more difficult due to the oscillating character of tidal flow. Models can be very useful tools in these systems.

Models aim to describe processes quantitatively and consequently they have the capability of putting into evidence the interaction between state variables in complex systems, as is the case of tidal estuaries.

In tidal estuaries, alternating flow enhances vertical and horizontal mixing and improves the interaction between nutrient load sources. Non-linear effects associated to the tidal flow can generate quite complex residual flows, which will be responsible for a wide range of residence times in the estuary. Residence time and mixing inside the estuary are major processes determining the fate of nutrients discharged in the estuary.

Circulation models are nowadays accept as powerful and economic tools to compute physical processes and are disseminate among coastal engineers and coastal system managers both for coastal works design and for management of navigation infrastructures. Validation of tidal circulation models is simple and economic since it can be based on historical data, which is available in major estuaries especially if commercial navigation is a relevant economic activity. In Portugal, this is the case in the main estuaries.

Processes describing primary production and nutrient recycling are nowadays sufficiently well known to build models based on a set of parameters short enough to be manageable, but large enough to be common to many coastal systems. Coupling primary production models to physical models describing tidal transport allows for the development of sophisticated modelling tools to support coastal areas management.

In this study, models are used to describe circulation and residence times of the water inside the estuary and to simulate primary production, nutrient recycling and estuary-sea exchanges. A detailed description of the model is provided here.