Lilia Serrano-Grijalva, Raul Ochoa-Hueso, Raquel Sánchez-Andrés, Santos Cirujano, Salvador Sánchez-Carrillo

Wetlands provide a great variety of environmental services to society, but they are currently globally threatened by human activities. We evaluated the effects of anthropogenic disturbances on the ecological quality of semiarid wetlands from central Spain (La Mancha Húmeda) through the natural abundance of isotopes (13C and 15N) of aquatic plants. We measured water quality and also compiled historical information about land-use and socioeconomic characteristics at local (100 m around the lagoon) and regional (sub-basin) scales. We then related this information to isotopic signatures of three types of aquatic plants: (i) charophytes, (ii) marginal aquatic macrophytes and (iii) vascular plants. Aquatic plants exposed to high levels of nitrogen showed very low δ13C values, consistent with negative physiological effects. Vascular aquatic plants were the group that best reflected the effects of nutrient enrichment in wetlands and lagoons through significant correlations between their δ15N values and total nitrogen and phosphorus concentrations in water. Demographic factors did not exert a clear influence on aquatic plant isotopic signatures, although we observed inverse correlations between the coverage of natural vegetation at regional scale and δ13C of marginal plants and δ15N of vascular plants. Furthermore, the isotopic signatures of Phragmites australis, present in 96% of the studied la-goons, were not significantly correlated with any of the environmental quality variables evaluated. Although δ13C signatures of Typha dominguensis and Cladium mariscus increased significantly due to changes in water quality, their narrow isotopic variability at the regional scale limits their use as a bioindicators of environmental changes in this wetland system. Finally, we propose the use of δ15N measured in the vascular plant Salicornia sp. as the most suitable bio-indicator of anthropogenic impacts in La Mancha Húmeda region, a highly emblematic system of semiarid Mediterranean wetlands that is unique in the Mediterranean region of Europe.