Biological contamination of macrobenthic communities in seminatural streams: implications for monitoring habitat quality in Isoëtes malinverniana stands.

Seminatural streams and irrigation canals can act as important refuges for freshwater biodiversity in intensively managed landscapes, but they are also highly vulnerable to habitat degradation and biological invasions. In the northwestern Po Valley (Northern Italy), this secondary hydrographic network hosts the last remaining populations of Isoëtes malinverniana, a critically endangered endemic quillwort that has undergone a severe decline in recent decades. Despite the ecological relevance of benthic macroinvertebrates as indicators of habitat conditions, their communities have never been investigated in relation to the distribution of this species. We characterized macrobenthic communities in nine seminatural streams, including current and historical sites of I. malinverniana occurrence and one recent reintroduction site. Macroinvertebrates were sampled in spring 2025 together with basic physicochemical variables. Community composition, diversity patterns, and the occurrence of alien taxa were compared among streams to explore their relationship with the presence of I. malinverniana. Benthic assemblages were dominated by insects, particularly Chironomidae, while mollusks and crustaceans contributed less to total abundance. Only four alien taxa were recorded, yet their abundance varied markedly among streams. Streams hosting extant populations of I. malinverniana showed low levels of alien macroinvertebrates and environmental conditions indicative of limited disturbance, whereas historical sites of species loss and the reintroduction site exhibited higher proportions of alien taxa and signs of habitat alteration. Although overall macroinvertebrate diversity was higher where I. malinverniana is absent, this pattern was largely driven by alien species. These results indicate that macrobenthic community composition, particularly the presence of alien taxa, reflects environmental gradients critical for the persistence of I. malinverniana. Integrating macrobenthic assessments into monitoring programs may therefore support conservation and management actions for this critically endangered species.