Application of stable isotopic analyses for fish host–parasite systems: an evaluation tool for parasite-mediated material flow in aquatic ecosystems

Parasites potentially have important roles in aquatic ecosystems, although relatively little is known about their contributions to the complexity of food web structure. In this study, stable carbon and nitrogen isotope analyses (δ¹³C and δ¹⁵N) were applied for fish host–parasite systems in a shallow swamp–lake ecosystem to assess the validity of stable isotope technics to reveal the parasite-mediated trophic linkages in the food web. Forty host–parasite pairs, including seven parasite taxa (cestodes, trematodes, crustaceans, and hirudinids), found on six host fish species (cyprinids and percids) were examined. The parasites showed unusual δ¹⁵N fractionation, with an overall average of − 1.9‰, suggesting the intake of ¹⁴N-rich ammonia for amino acid synthesis and/or selective absorption of ¹⁵N-depleted amino acids in the host fluid. The isotopic signatures of fish hosts and their parasites were positively correlated, suggesting the absorption and transfer of host-derived nutrients during infection. A δ¹³C-based isotope mixing model showed that each host fish species exhibited unique dependencies on POM, land-derived organic matter, and macrophytes, suggesting the host-specific trophic niche of the associated parasites in the lake–swamp food web. These emphasized that parasites are potential pathways of material and energy flows in aquatic ecosystems, contributing substantially to the food web complexity. Stable isotope analyses are the useful tools to elucidate the host–parasite trophic linkages, and case-specific isotopic fractionation factors are the mandatory information for a better understanding of the parasite-mediated material flow in ecosystems.