The role of eutrophication reduction of two small man-made Mediterranean lagoons in the context of a broader remediation system: Effects on water quality and plankton contribution

Abstract

In order to meet the requirements of the European Union Water Framework Directive for the Albufera de Valencia (AV) Natural Park, in 2009, several areas of free water surface constructed wetlands (FWSCWs) planted with emergent vegetation and two small shallow lagoons planted with submerged macrophytes were created over a 40-ha area formerly occupied by rice fields. This area is currently a reserve known as Tancat de la Pipa. The dual goal of this programme was to improve the quality of the hyper-eutrophicated waters of the AV lagoon, the largest littoral lagoon in the Iberian Peninsula, and to restore former lost habitats to increase the biodiversity of the area. The lagoons were mainly fed with the outflows of the FWSCWs. The submerged macrophytes covered both lagoons and sustained a diverse waterfowl community. In this study, the nutrient reduction between the inflows and the outflows of the lagoons was evaluated, along with the change in the composition and biomass of plankton (i.e., phytoplankton and zooplankton), over the first four years of operation of the two lagoons. The nutrient reduction contribution of the lagoons to the overall Tancat de la Pipa system was also quantified. The results indicated that 42% of the total suspended solids, 30% of the total nitrogen and 49% of the total phosphorus were retained in the lagoons. An increase in the zooplankton and phytoplankton richness was observed in the outflows of both lagoons. These aquatic systems served as a site for zooplankton production and large cladocerans played a significant role in water clearing by consuming microalgae. Submerged vegetation was also critical for reducing nutrient loads, although macrophytes disappeared after the second year. Despite the macrophyte loss, the lagoons continued to act as nutrient sinks. Both lagoons facilitated over 30% mass removal of TP, TSS and Chl a and over 20% mass removal of DIN and TN for the overall Tancat de la Pipa system. Therefore, these lagoons represent a new sustainable aquatic ecosystem for improving water quality and promoting biological development. We recommend the configuration of this ecosystem as a complement to FWSCWs for managing eutrophicated waters in protected sites.