Over the last few decades, the excessive growth of macroalgae and decline of seagrass beds, associated with increased eutrophication, has become a worldwide problem. It is known that submersed aquatic vegetation (SAV) offers stable habitats, allowing the continuous availability of food and protection against predators and contributing to biodiversity, sediment stability and water transparency when compared to areas covered by macroalgae mats. In the Mondego estuary (Portugal), several mitigation measures (nutrient-load reduction, seagrass-bed protection and freshwater-circulation enhancement) were implemented in 1998 in order to promote the recovery of the seagrass beds and the entire surrounding environment following a long period of eutrophication. Here the success of this restoration project is evaluated by comparing the water nutrient concentrations, the extent of seagrass cover and the dynamics of the bivalve Scrobicularia plana before and after the implementation of the management measures. During the period in which environmental quality declined, S. plana's adult abundance, total biomass and growth production also declined, parallel with the almost total disappearance of Zostera noltii. After the implementation of management measures, dissolved nutrients and green macroalgal blooms were much reduced, and seagrass beds started to recover. The S. plana population also responded positively, becoming more structured (including individuals of all age classes), with higher biomass and growth production.

For some decades, the Mondego estuary has been under severe ecological stress, mainly caused by eutrophication. The most visible effect was the occurrence of macroalgal blooms and the concomitant decrease of the area occupied by Zostera noltii beds. Since the end of 1998, mitigation measures were implemented in the estuary to promote the recovery of the seagrass beds and the entire surrounding environment. The present study offers a unique opportunity to evaluate the impact of disturbance and the success of the initial recovery process (before and after implementation of the management measures), over a 10-year period, having secondary production as the descriptor. Before the implementation of the mitigation measures, in parallel with the decrease of the Z. noltii beds, species richness, mean biomass and production also decreased, lowering the carrying capacity of the whole Mondego's south arm. Yet, after the introduction of management measures, the seagrass bed seemed to recover. Consequently, the biomass and production also increased substantially, for the whole intertidal area. Nevertheless, even after the mitigation measures implementation, natural-induced stressors, such as strong flood events induced a drastic reduction of annual production, not seen before the implementation of those measures. This shows that the resilience of the populations may have been lowered by a prior disturbance history (eutrophication) and consequent interactions of multiple stressors.

The Mondego estuary, a shallow warm-temperate intertidal system located on the west coast of Portugal, has for some decades been under severe ecological stress, mainly caused by eutrophication. Water circulation in this system was, until 1998, mainly dependent on tides and on the freshwater input of a small tributary artificially controlled by a sluice. After 1998, the sluice opening was effectively minimised to reduce the nutrient loading, and the system hydrodynamics improved due to engineering work in the upstream areas. The objective of the present study was to evaluate the effect of the mitigation measures implemented in 1998. Changes to the hydrodynamics of the system were assessed using precipitation and salinity data in relation to the concentrations of dissolved inorganic nutrients, as well as the linkage between dissolved N:P ratios and the biological parameters (phytoplankton chlorophyll a concentrations, green macroalgal biomass and seagrass biomass). Two distinctive periods were compared, over a ten year period: from January 1993 to January 1997 and from January 1999 until January 2003. The effective reduction in the dissolved N:P atomic ratio from 37.7 to 13.2 after 1998 is a result of lowered ammonia, but not the oxidised forms of nitrogen (nitrate plus nitrite), or increased concentrations of dissolved inorganic phosphorus. Results suggest that the phytoplankton is not nutrient limited, yet maximum and mean biomass of green macroalgae was reduced by one order of magnitude after the mitigation measures. This suggests that besides lowering the water residence time of the system, macroalgal growth became nitrogen limited. In parallel to these changes the seagrass-covered area and biomass of Zostera noltii showed signs of recovery.