Mondego estuary is a narrow estuary where the tidal prism, the daily river discharge and the mean volume are numbers of the same order of magnitude. Has a consequence the residence time in the estuary is very short (2 to 4 days) and there is no time for blooms of phytoplankton to develop. The residual flow is quite uniform, from the river to the sea.

The results of the model show that the phytoplankton growth is limited by the residence time. The phytoplankton produced in the estuary is exported before having the time to generate a bloom. This happens both in the northern and southern channels, although in the southern channel residence time is longer due to the artificial closure of the upper connection with the northern channel that gives it “a bay like” behaviour. The southern channel is a net importer of particulate organic matter from the northern channel, which is deposited and mineralised on the bottom, enriching the sediments. This behaviour of the southern channel is a consequence of the smaller transient and residual velocities and is at the origin of the eutrophication by macro-algae development. Several scenarios of reduction of nutrients discharge were tested and it was verified that no modification of the estuary behaviour must be expected. A test of the consequences of opening the upper communication between the channels was performed and it was verified that in this case the velocities increase and that a seaward residual flow is produced. From the tests it was concluded that the resolution of the eutrophication symptoms in southern channel by macro-algae development must pass by re-opening the communication between the channels that was closed artificially with the purpose of enhancing the self cleaning of the sediments in the navigation channel in the northern branch of the estuary.