Upper reaches Minho Estuary are narrow and have very short residence time. At mouth the estuary becomes wider and shallower increasing light availability in the water column. Tidal prism, daily river discharge and mean volume are numbers of the same order of magnitude. As a consequence the residence time in the estuary is very low (1 day) and there is no time for phytoplankton blooms to develop. In some areas with good light and nutrients conditions, the phytoplankton experiments some growth, but has no possibility to develop bloom. In the overall balance the system is productive. The estuary produces and export biomass, consuming a small part of nutrients discharging by rivers and WWTP. Primary production in the estuary is residence time limited thus changes in the flow regime will, cause changes over the biological processes even more, if nutrients availability is kept maintained.

A 50% reduction in agriculture nitrate or the elimination of and organic matter from WWTP has negligible effects on phytoplankton production.