An advanced characterization of the trophic niche of non-indigenous species (NIS) may provide useful information on their ecological impact on invaded communities. Here, we used carbon and nitrogen stable isotopes to estimate pairwise niche overlaps between non-indigenous and native consumers in the winter food web of Lake Trasimeno (central Italy). Overall, a relatively low pairwise overlap of isotopic niches was observed between NIS and native species. The only exception was the Louisiana crayfish Procambarus clarkii, which showed a relatively high and diffuse overlap with other native invertebrates. Our findings highlighted a high niche divergence between non-indigenous and native species in Lake Trasimeno, suggesting a potentially low degree of interspecific competition that may facilitate coexistence and, in turn, limit the strength of impacts. The divergent results obtained for the Louisiana crayfish indicate that additional control measures for this invasive species are needed to mitigate its impact on the Lake Trasimeno system.

There is a growing recognition that an advanced understanding of the trophic characteristics of an invasive consumer can provide important information on its ecological impact. In recent years, the blue swimming crab Portunus segnis, one of the earliest Lessepsian invaders, has considerably expanded its distribution range in the Mediterranean Sea, yet, its trophic habits in invaded areas remain scarcely investigated. In this study, we used carbon and nitrogen stable isotopes analysis (SIA) to determine the trophic position and isotopic niche of the crab compared with other representatives of the flora and fauna occurring in Elounda Bay (Crete). P. segnis showed a trophic position of 3.9, higher than the values determined by SIA or conventional gut content analysis in other areas of the Mediterranean Sea or in the native range. Crab specimens showed a high inter-individual variability in both δ¹³C and δ¹⁵N values; further analysis indicated negligible differences in the isotopic niche of adult males and females. Conversely, δ¹⁵N values were significantly related to the size of the specimens, ultimately suggesting an ontogenetic dietary shift. This study represents the first attempt to characterize the trophic habits of the blue swimming crab in the context of an invaded food web and may contribute to the implementation of long-term management strategies of control and mitigation of its ecological impact.

Open ocean carbonate chemistry is altered by the dissolution of atmospheric CO₂ in seawater. Up to 40% of anthropogenic CO₂ emissions have dissolved in the surface ocean since the pre-industrial era, driving changes in marine carbonate pools and promoting ocean acidification. Under open ocean conditions, Total Alkalinity (TA) generally relates with salinity and temperature due to the conservative nature of its constituents. In coastal areas, however, land-ocean interactions may greatly contribute to making TA far less predictable, since freshwater inputs can affect the chemistry of coastal water masses by increasing TA and Dissolved Inorganic Carbon (DIC) or, alternatively, lowering them through dilution. Here we analysed and compared coastal carbonate chemistry dynamics in selected coastal areas of Ireland, in order to assess whether rivers and their catchment geology can influence coastal water chemistry and to verify the occurrence of local ocean acidification processes. Data on TA, DIC, salinity, temperature, and nutrients (total nitrogen, phosphate, and silicate) collected during several surveys along the Irish coastline by The Marine Institute Foras Na Mara (MIFNM) from 2009 to 2018 were analysed against those available on the main watercourses feeding the selected coastal areas. The relationships among the different variables were scrutinized, also considering the geological characteristics of river catchments. Results showed a clear linkage between coastal water chemistry and local freshwater inputs, whereas changes in the geology of catchment areas exerted only a secondary influence. An in-depth scrutiny of the Galway Bay sector performed by comparing MIFNM data with those collected during an oceanographic cruise carried out in October 2017 further indicated remarkable effects of the interaction between river inputs and submarine groundwater discharges on seasonal variations in the water chemistry of the area. Future studies, integrating chemical data across the land-ocean boundary, may provide additional, useful information on the factors actually affecting the variation in water carbonates in coastal areas. Elucidating how land-ocean interactions influence the coastal carbonate system may ultimately provide a key for a more advanced understanding of the resilience of coastal areas to ocean acidification.