Preliminary insights into macroinvertebrate communities associated with floating islands in two rivers under distinct anthropogenic pressures

Floating islands are bioengineering structures vegetated with riparian plant species that replicate natural ecological functions in degraded aquatic environments. As Nature-Based Solutions, these structures create new microhabitats that can support biological recolonization, enhance ecological connectivity, and provide new opportunities for environmental monitoring. In the context of the Bio Ilhas project, pilot floating islands were installed in two fluvial systems under distinct anthropogenic pressures: the Fervença River, an urban and heavily modified watercourse, and the Côa River, located in an agricultural-natural landscape but with longitudinal connectivity disrupted by dams. The primary objective of this study is to compare the composition and structure of aquatic macroinvertebrate communities associated with floating islands and adjacent non-island areas in both rivers. Additionally, it aims to compare the species present in each system, identify taxa, and assess differences in richness, relative abundance, and functional feeding groups. To complement the biological analysis, available water-quality data (including physical and chemical parameters such as dissolved oxygen, nutrients, temperature, pH, and conductivity) were integrated to characterize the sampling sites and to contextualize macroinvertebrate sensitivity as bioindicators. Preliminary observations indicate that the macroinvertebrate communities in the Côa River were almost exclusively dominated by Astacidea (invasive crayfish). In contrast, the Fervença River exhibited greater taxonomic variability, primarily represented by organisms from the orders Crustacea, Mollusca, Hirudinea, and Odonata. This approach aims to determine whether floating islands support more diverse and sensitive communities than areas without islands, and to understand how different anthropogenic pressures, urban impacts in the Fervença, and hydromorphological alterations in the Côa influence patterns of colonization and ecological tolerance.