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- Sea urchins’ patches in Ria Formosa coastal lagoon altered under invasion pressure of Caulerpa prolifera and Rugulopteryx okamuraePublication . Hartmann, Tim Felix; P. Moreira Ribeiro, Ines; Ramos Duarte, Duarte Nuno; Gamito, SofiaThis study investigates specific epibenthic community in sea urchin patches from Ria Formosa lagoon, near the Armona Inlet in southern Portugal. These patches are concentrated in distinct spatially isolated areas surrounded by unvegetated, bare sediment bottom and support diverse organisms, dominated by a high density and biomass of Paracentrotus lividus and ascidians. Field surveys conducted between 2014 and 2023 revealed a shift in flora composition, with the replacement of native seagrasses and seaweed by the invasive macroalgae Caulerpa pro lifera. The main aim of this study was to evaluate the effects of this invasive alga in the epibenthic communities of the patches. However, in 2024 surveys, another invasive macroalgae, Rugulopteryx okamurae, was detected in the patches. Sphaerechinus granularis was the dominant sea urchin species in biomass, in R. okamurae patches. In the patches invaded by Caulerpa prolifera, sea urchins were generally absent, as were ascidians’ species. Faunal analyses indicate a significant decline in macroinvertebrate taxa richness, abundance and biomass within Cau lerpa-invaded patches, when compared with patches invaded by R. okamurae. Non-metric multidimensional scaling, confirm the observed differences between the two invaded patches. The invasion by R. okamurae complicates the ecological balance, potentially leading to species replacement and eventual biodiversity loss. Additionally, the disappearance of shell layers in these patches, observed in previous years, raises concerns about habitat stability and resilience. The findings highlight the urgent need for continuous monitoring and man agement strategies to mitigate the ecological consequences of macroalgal invasions in the Ria Formosa.