Disrupting the environmental continuum: a Neotropical fluviokarst cave system

Neotropical fluviokarst systems are ecologically complex, yet the dynamics of benthic communities within their subterranean segments remain poorly understood. This study investigated how benthic macroinvertebrate communities vary along an epigean-hypogean-epigean gradient in a Neotropical fluviokarst system. Based on the River Continuum Concept, we hypothesized that surface (epigean) segments would exhibit a similar faunal composition, whereas cave (hypogean) environments would differ due to environmental filtering. A total of 744 individuals representing 52 invertebrate taxa were sampled from upstream, cave, and downstream segments of the Peruaçu River. Species richness was significantly lower in the cave than in the surface reaches, and faunal similarity across segments was below 50%, with greater similarity observed between upstream and downstream sites. Community composition differed markedly between epigean and hypogean habitats, as revealed by analysis of similarities (ANOSIM: R = 0.45, p = 0.02). Dissolved oxygen emerged as the strongest predictor of faunal composition, explaining 35% of the variation, while species richness was best explained by a combination of dissolved oxygen and pH. These results indicate that caves act as environmental filters, disrupting longitudinal ecological continuity by reducing species richness and altering community structure, likely due to limited organic inputs and stable yet extreme physicochemical conditions. This study highlights the critical ecological role of cave segments in structuring aquatic biodiversity and reinforces their conservation importance within fluviokarst systems.