Where Do You Fly From? Assessing Aerial Dispersal in Temporary Rivers Using Stable Isotopes

Abstract

1. Dispersal is a crucial ecological process that facilitates gene flow and species exchange within ecosystems. In temporary rivers (TRs), dispersal is a key driver of community assembly because it determines the ability of the organisms to adapt to seasonal drying and maintain stable populations. In particular, disconnected pools (DPs) in TRs function as critical ecological refuges during dry periods, preserving local and regional biodiversity and acting as sources for aerial macroinvertebrate dispersal. Aquatic macroinvertebrate traits have been used as a proxy to estimate dispersal. However, studies measuring the actual dispersal of aquatic macroinvertebrates in TRs are very limited. 2. In this study, we investigated macroinvertebrate dispersal in TRs with DPs remaining during the non-flowing phase by applying isotopic enrichment in the DPs and using mesocosms to assess dispersal. Our final aim was to assess the ability of aquatic macroinvertebrates to aerially disperse in TRs with DPs. The experiment was conducted in a headwater Mediterranean TR stream in northeast Spain, characterised by flow intermittence during summer. We used eight uncolonized mesocosms installed along the dry stream channel, both upstream and downstream from DPs. Labelled ammonium chloride (15NH4Cl) was employed as an isotopic tracer to mark DPs and track macroinvertebrate dispersal. 3. We first assessed the effectiveness of 15N isotopic tracers in tracking aquatic macroinvertebrate aerial dispersal. Then, we examined dispersal distance upstream and downstream of the DPs, predicting that community similarity between mesocosms and DPs would decrease with increasing geographic distance, highlighting the key role of spatial proximity in shaping community structure. 4. Isotopic enrichment increased over time in DPs, with a proportion of enriched individuals varying with life cycle stages (e.g., larvae or adults). Enriched Cloeon sp. and Chironomini larvae were the most abundant taxa captured in both DPs and mesocosms. Enriched aquatic obligate taxa (e.g., Mollusca, water mites) were also present in the mesocosms. There was a gradual decrease in enriched isotopic samples both upstream and downstream from DPs. Community similarity analysis revealed significant differences between DPs and mesocosms, while the upstream and downstream reaches clustered together. Additionally, weak positive correlations were observed between environmental and geographic distances and community composition. 5. Our findings contribute to the understanding of aquatic macroinvertebrate dispersal mechanisms through the use of isotopic enrichment techniques, pioneering their use in TRs and successfully detecting isotopic marking in subsequent generations. We emphasise the importance of maintaining DPs in TR networks to host colonisers that will facilitate community recovery during and after drying, either through active aerial dispersal of aquatic insect adults or passive aerial dispersal through zoochory. Considering future drying scenarios, the conservation (and potential promotion) of DPs as refuges should be prioritised to avoid local and regional biodiversity loss in TRs.