Rapoport’s Rule, the Ecotone Concept, and Salinity Gradient Predict the Distribution of Benthic Foraminifera in a Southeastern Pacific Estuary

This study explores the biogeographic processes shaping the distribution of benthic foraminifera along a salinity gradient in the Contaco Estuary, southeastern Pacific, Chile. The primary aim was to evaluate the applicability of key ecological paradigms—Rapoport’s rule, the mid-domain effect, ecotones, and source–sink dynamics—to unicellular eukaryotes in estuarine environments. A 1550 m longitudinal transect, sampled at 50 m intervals, revealed a pronounced salinity-driven pattern in species richness and diversity, with calcareous taxa dominating euhaline zones and agglutinated taxa thriving in brackish and freshwater areas. Source–sink dynamics were not supported, as beta diversity analyses identified turnover as the dominant driver, highlighting species replacement along the salinity gradient. Evidence of a longitudinal Rapoport effect was observed, with broader distribution ranges in low-salinity environments, reflecting adaptations to suboptimal conditions. Contrary to predictions, the mid-domain effect was not supported, as foraminiferal richness showed a monotonic decline. These findings extend macroecological principles to microbial communities, emphasizing deterministic processes in shaping estuarine diversity. This research provides a robust framework for understanding biodiversity patterns in dynamic ecosystems, offering valuable insights for conservation and ecological monitoring.