Eutrophication and hydrodynamic confinement shape establishment patterns of the invasive Atlantic blue crab in a hypersaline coastal lagoon

Coastal lagoons are highly vulnerable to biological invasions due to their environmental heterogeneity, hydrodynamic confinement and increasing anthropogenic disturbance. Despite the rapid expansion of the invasive Atlantic blue crab ( Callinectes sapidus ) across Mediterranean transitional ecosystems, the processes shaping its establishment remain poorly understood. Here, we investigated the long-term dynamics of C. sapidus in the hypersaline Mar Menor coastal lagoon (SE Spain), a eutrophication-stressed system affected by recurrent dystrophic crises and mass mortality events. Using seasonal monitoring data collected between 2018 and 2024 across shallow waters, we analysed spatio-temporal patterns in occurrence, abundance and population structure in relation to hydrodynamic confinement, connectivity and eutrophication-driven disturbances. Our results revealed a progressive but markedly non-linear trajectory, with overall increases in occurrence and abundance interrupted by abrupt declines during major dystrophic events. Hydrodynamic confinement strongly shaped population dynamics: the least confined sectors consistently supported higher recruitment and abundance increases. Shallow sectors associated with a semi-natural inlet supported the strongest evidence of population establishment, being dominated by early life stages and functioning as key recruitment and settlement areas. Size-frequency distributions revealed recurrent recruitment pulses, mainly during winter, and progressive cohort transitions, indicating sustained establishment. In contrast, the most confined sectors were dominated by larger individuals and showed limited recurrent recruitment. Overall, our findings highlight the key role of shallow habitats, hydrodynamic connectivity and disturbance regimes in shaping the establishment patterns of invasive crabs in Mediterranean coastal lagoons. They further suggest that eutrophication may act as a non-linear modulator of invasion, causing short-term population declines while potentially facilitating subsequent recolonisation and demographic expansion.