Ecological differentiation across an epigean–hypogean gradient in the karst amphipod Niphargus pannonicus (Crustacea: Amphipoda)

Karstic streams exhibit strong environmental gradients over short spatial scales, providing natural settings for investigating ecological differentiation in subterranean organisms. We examined demographic structure, trophic ecology, stoichiometric imbalances, and nutrient fluxes of the stygobitic amphipod Niphargus pannonicus across epigean, intermediary, and deep sectors of a Romanian karst stream. Stream water chemistry remained relatively stable among sectors, whereas trophic resources differed markedly in composition and nutritional quality. Population density was highest in the guano enriched intermediary sector and lowest in the deep cave zone. Female size at maturity increased along the epigean–hypogean gradient, while body condition and fecundity declined. Stable isotope analyses revealed significant differences in trophic interactions, with the intermediary sector displaying the broadest isotopic niche and lowest trophic position, whereas the deep sector exhibited narrower niches and a greater reliance on cannibalism. Bayesian mixing models indicated marked spatial variation in assimilated resources, with guano dominating diets in the intermediary sector and animal prey contributing substantially in the epigean sector. Isotopic similarities among juvenile groups from subterranean sectors suggested potential trophic connectivity within the cave stream and highlighted the intermediary zone as an important trophic hotspot. Resource stoichiometry varied strongly among sectors, with elemental imbalances consistently lowest in the guano enriched microhabitat, while epigean and deep sectors exhibited contrasting nutrient constraints. These differences were reflected in annual ingestion fluxes of organic matter, carbon, nitrogen, and phosphorus. Collectively, demographic, trophic, stoichiometric, and flux related patterns revealed substantial ecological differentiation associated with resource heterogeneity across the epigean–hypogean interface.