<p>Groundwater-dominated streams support distinct biota and have significant conservation value. They include some of the most vulnerable freshwater habitats and are increasingly threatened by the invasion of signal crayfish (<i>Pacifastacus leniusculus</i>), a major driver of biodiversity loss in freshwater systems. This study aimed to assess the density-dependent impacts of signal crayfish on macroinvertebrate and macrophyte community dynamics in the River Shep, a UK chalk spring-fed stream. We conducted a four-week enclosure-exclosure experiment with four replicated treatments: two set signal crayfish densities (6 and 28 crayfish m⁻²), a crayfish exclosure and an open water control. We evaluated their impacts on macroinvertebrate abundance, diversity, and community composition, as well as on macrophyte cover, diversity, and community composition. Macroinvertebrate diversity was significantly higher in the exclosure treatments compared to the control and high and low crayfish density enclosures. However, no measurable impact of signal crayfish was observed on macrophytes nor on invertebrate abundance and community structures. These findings indicate that higher densities of signal crayfish negatively impacted macroinvertebrate diversity, while macrophyte communities were unaffected, suggesting that the ecological responses of groundwater-fed streams to signal crayfish invasions may differ from the responses of other stream environments. This study provides critical insights into the density-dependent ecological consequences of invasive non-native species in groundwater-fed streams. Our findings underscore the need for targeted management strategies that acknowledge the distinct processes of different freshwater systems, rather than applying broad, generalised management schemes that overlook the unique interactions and sensitivities specific to these ecosystems.</p>