Restoration of two small Mediterranean lagoons: The dynamics of submerged macrophytes and factors that affect the success of revegetation

摘要

Two small, shallow lagoons newly created from former rice fields in the Mediterranean coast (Albufera de Valencia Natural Park, Spain) were restored by planting several species of submerged macrophytes (Myriophyllum spicatum, Ceratophyllum demersum, Potamogeton spp., Zannichellia peltata and Ranunculus peltatus) in 2008. Charophytes also appeared spontaneously. M. spicatum was the species that dominated both lagoons and almost completely covered their surface. M. spicatum reached a high biomass and displayed a seasonal pattern, declining during the cold season and reappearing in the spring. No submerged vegetation was observed in the lagoons from 2011 onward. The major purpose of the present study was to elucidate the factors that caused the total disappearance and prevented the regeneration of submerged vegetation cover in both lagoons. Such factors may include negative biotic relationships such as those related to the presence of fish and crayfish, waterfowl grazing, phytoplankton and epiphytic development that causes shading and abiotic factors such as water nutrient concentration. Three different types (treatments) of 1-m~2 exclosures were set up and planted with macrophyte cultures that formed 3 different assemblages: Chara hispida alone, a mixture of C. vulgaris, C. baltica and Nitella hyalina and a mixture of higher plants (M. spicatum, C. demersum and Potamogeton pectinatus). The "Open" type of exclosure permitted the entrance of fish, crayfish, and birds. The "Lateral" exclosure prevented the entry of fish. The "Closed" exclosure also prevented bird grazing. To determine the potential of the seed sediment bank to regenerate submerged vegetation, we investigated lagoon sediments (i) to quantify seeds and (ii) to examine seed germination. The results indicate that there is not a sufficiently large reservoir of phanerogam seeds in the lagoon sediments to recover vegetation by germination. The "Closed" exclosures allowed the growth of all species in spite of the intense development of periphyton on macrophyte tissues and filamentous algae, which reduced light availability by up to 45%. The "Lateral" exclosures had a lower development of macrophytes. In the "Open" exclosures, almost no macrophytes were detected 3 weeks after planting. Water chemistry (P and N content) did not limit the development of the tested macrophyte species but did appear to negatively affect charophytes in one of the lagoons. The deleterious effect of exotic fauna (particularly carps and crayfish) in combination with the high herbivory pressure by waterfowl, primarily during the periods when the rice fields surrounding the lagoons were dry, were the major factors that acted in a negative synergistic manner to prevent the recolonisation of the vegetation currently in the lagoons. A procedure to eradicate exotic fish and crayfish is required prior to any type of restoration effort involving the plantation of submerged macrophytes. More areas with permanent submerged vegetation in the Natural Park are required to decrease herbivory pressure.