Assessing the effect of emergent vegetation in a surface-flow constructed wetland on eutrophication reversion and biodiversity enhancement

A free water-surface constructed wetland (F4-unit) was created in a Mediterranean protected site to treat eutrophic inflows to the Albufera de Valencia lagoon, and enhance biodiversity. In 2013, the F4-unit was divided into three subunits and planted with different compositions of emergent macrophytes (Phragmites australis, Iris pseudacorus, and a mixture of several macrophytes: F4-Reed, F4-Iris and F4-Mixed subunits). Two nine-month periods were monitored after planting, and water quality variables were measured in the inflows and outflows. Waterfowl occupation and plankton biomass were analyzed to evaluate their role in eutrophication reversion. The low removal efficiencies related to scarce vegetation development and wind disturbance in the first period, significantly improved in the second, particularly in the F4-Reed subunit (total cumulative mass removals of 438, 363 and 372 kg of nitrogen, 57, 30 and 48 kg of phosphorus and 7109, 573 and 2315 kg of suspended solids in the Reed, Iris and Mixed F4 subunits). This was due to the high continuous coverage (around 70%) achieved by reeds, but also by other species that spontaneously germinated from the seed bank (cattails, Paspalum sp). The constructed wetland significantly reduced the exported microalgal biomass, stimulated the potential mixotrophic phytoplankton groups, such as Euglenophyceae, and produced large quantities of zooplankton, particularly ostracods and cladocerans, which efficiently grazed on microalgae. The total cumulative mass removal of chlorophyll for the second period of operation represented 2.6 times more removal than in the first period. Almost 50% of the waterfowl species visiting the zone were of special conservation concern. Differences in vegetation structure between subunits drove a concrete selection of stopover sites for migratory, endangered shorebirds that had low impact on solid resuspension. Endangered, large and medium-sized waterfowl (higher impact on resuspension while foraging), more abundant in the post-breeding period, showed no preferences between subunits. This study supports the idea of recommending the configuration of these kinds of systems for managing eutrophicated waters in protected sites and enhancing waterfowl species of conservation concern.