Limnological characteristics and zooplankton community structure of Mediterranean coastal lagoons undergoing restoration

Résumé

Between 1999 and 2003, a restoration Life project was developed in La Pletera salt marshes, affected by an urbanisation plan, and in the Ter Vell lagoon, a highly eutrophic lagoon (Baix Ter Wetlands, NE Iberian Peninsula). The aim of this thesis is to establish the natural functioning of both ecosystems, analyze their environmental problematic and also to evaluate the effects of the restoration measures. In La Pletera salt marshes, the role of the hydrological regime in the nutrient and zooplankton composition and dynamics was analysed in five brackish lagoons. Three of them were created in the framework of the restoration project as new refuges for an endangered fish species (A. iberus). The hydrology was determined by a prolonged period of confinement without water inputs, irregularly interrupted by sudden flooding events. While the dynamics of inorganic nitrogen depended on the water inputs variability, dynamics of phosphorus, total nitrogen and organic matter was related more to the cumulative mechanisms and internal recycling during the confinement periods. The zooplankton community structure was analysed by means of the combination of taxon- and size-based approaches. Whereas the community size structure was more affected by trophic interactions (predation and competition), the taxonomic structure appeared to be more sensitive to abiotic factors (nutrients). The fast growth of the size population of A. iberus in the new lagoons suggested that they had provided a suitable refuge for this species, at least in the short-term. Restoration actions in the Ter Vell lagoon were focused on the improvement of the water quality by means of (1) wetlands construction to reduce nutrient inputs and (2) sediment dredging. Simultaneously, but independent of the restoration activities, water management in agriculture drastically reduced the freshwater inflow to the lagoon and, therefore, changed the hydrological regime. The short-term effects of this hydrological change on the limnological characteristics and the zooplankton of the lagoon were analysed. Before the change, the hydrology was artificial since the freshwater flooding period was prolonged due to the high agricultural demand. As a result, the lagoon showed a high water turnover rate and high nutrient concentrations. After the change, hydrology was climate-dependent, with scarce water inputs and prolonged confinement periods. Then, the nutrient composition and dynamics tended to be more similar to those observed in the salt marsh lagoons. However, zooplankton did not tend to resemble to that of the salt marsh lagoons. The ecological status of the Ter Vell lagoon was improved after the hydrological change.