How does drought alter the synthesis and cross-ecosystem transfer of long-chain polyunsaturated fatty acids (LC-PUFA) from riverine to adjacent terrestrial ecosystems?

Adult aquatic insects emerging from freshwater systems fertilize surrounding watersheds, supporting riparian predators such as spiders, lizards, birds, and bats. Research on these cross-ecosystem subsidies has largely focused on the quantity of insects transported, often overlooking nutritional aspects—particularly omega-3 long-chain polyunsaturated fatty acids (ω3 LC-PUFAs), which are rare in terrestrial ecosystems but critical to vertebrate growth, immune function, and reproduction. Considering that global change stressors like altered light and temperature regimes will likely alter ω3 LC-PUFA production, the effects of river drying on their synthesis and transfer remain poorly understood. We reviewed existing research on how drought affects the production and export of ω3 LC-PUFAs from riverine to terrestrial ecosystems and identified key knowledge gaps. We also identified challenges and opportunities for future research. Key mechanisms include reductions in surface water and habitat connectivity linked to changes in algal communities and the base of production; reductions in hydroperiod length that constrain stream insect life cycles; behavioral responses of invertebrates to drying that prevent them from grazing diatoms; and changes in emergence phenology that could decouple peak insect emergence from the timing of riparian predator needs. To illustrate some of these mechanisms, we present preliminary data analyzing ω3 LC-PUFA profiles and compound-specific stable isotopes in immature and emergent aquatic insects from an intermittent watershed in Pinnacles National Park, California. As droughts increase in frequency and severity under climate change, incorporating nutrient quality into models of cross-boundary resource flows will be key to understanding and mitigating climate-driven ecosystem change.