Dragonfly (Odonata) Diversity Patterns in Mixohaline Coastal Wetlands

Salinity is a limiting factor for many invertebrates, especially for Odonata which are typically associated with freshwater ecosystems. In Europe, 15 Odonata species inhabit brackish wetlands and only few detailed data on their tolerance towards salinity are available. We investigated Odonata fauna in 11 sampling stations situated in three estuarine areas (northern Adriatic coastline) which differed in salinity conditions (freshwater- polyhaline habitats) in order to assess affinity of Odonata species to brackish habitats and to describe their distribution pattern in coastal wetlands. Adults, exuviae (the remains of the exoskeleton after the last larval instar), and the main chemical and physical water parameters were sampled every 2 weeks for 1 year in each station. In total, 25 species were detected and 56% of them were able to complete their life cycle in brackish water environments. Our results showed that freshwater and oligohaline ponds were the most favorable for dragonflies, with an overall higher species richness. There was a high species turnover along the salinity gradient, with a strong differentiation among the communities along the gradient. Considering the exuviae, we observed a high specificity with respect to the habitat conditions (seven species exclusive of freshwater sites and six of oligohaline ones, respectively). Among the adults, four species were found exclusively in freshwater habitats and no species seemed to be strictly connected with oligohaline habitats. Coastal wetlands are composed by a mosaic of different habitats especially when freshwater and seawater are close together, supporting many Odonata species with different tolerance toward salinity conditions. They also provide useful insights for conservation and management actions.