Invasive Macroalgae and Their Impact on Benthic Ecosystems in Ria Formosa: The Case of Caulerpa Prolifera and Rugulopteryx Okamurae

Abstract

The Ria Formosa coastal lagoon, a biodiverse and ecologically valuable coastal system in southern Portugal, is experiencing significant changes due to the invasion of the macroalgae species, namely Caulerpa prolifera and Rugulopteryx okamurae. This study evaluates the impact of these invasive macroalgae on benthic communities’ patches, in the Ria Formosa, near the Armona inlet. These patches are distinct spatially restricted areas on otherwise unvegetated, bare sediment bottom, exhibiting a range of different epi-macroinvertebrates and macroalgae. Field surveys conducted between 2014 and 2024 revealed a shift in flora composition, with the replacement of native seaweed by the invasive macroalgae. In previous years, the patches were dominated by a high density and biomass of Paracentrotus lividus sea urchins and ascidians. In 2024 surveys, 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 Caulerpa-invaded patches, particularly affecting arthropods, echinoderms, and chordates, when compared with patches invaded by R. okamurae. Non-metric multidimensional scaling, confirm the observed differences between R. okamurae and C. prolifera invaded patches. The invasion by R. okamurae, first reported in 2023 in the Algarve, complicates the ecological balance, potentially leading to species replacement and eventual biodiversity loss. Additionally, the disappearance of shell layers in theses patches, observed in previous years, raises concerns about habitat stability and resilience. The findings highlight the urgent need for continuous monitoring and management strategies to mitigate the ecological consequences of macroalgal invasions in the Ria Formosa.

Keywords: Bioinvasion, Effect of Invasive Species, Non-Indigenous Species, Benthic communities, Ria Formosa, Scientific diving