Département Ecologie aquatique

An increasing number of streams are becoming intermittent due to global warming and water abstraction. The transfer of omega-3 polyunsaturated fatty acids (PUFAs)—particularly eicosapentaenoic acid (EPA)—from aquatic to terrestrial systems is a key ecosystem service likely to be affected by increasing intermittency. Here, we investigated how flow intermittency affected EPA transfer to riparian spiders along 9 headwater streams of an alpine catchment (Val Roseg, Switzerland) with different degrees of intermittency. We used fatty acid analysis to test if there were differences in fatty acid content in spiders sampled along streams varying in degree of intermittency. In addition, we used compound-specific isotope analysis (²H-CSIA) to determine the origin of different fatty acids present in spiders. We found altered fatty acid profiles, characterized by lower content of the omega-3 fatty acids EPA and docosahexaenoic acid (DHA) in spiders collected along streams that dried periodically during multiple times of the year. In contrast, spiders from streams experiencing seasonal drying during winter did not show altered fatty acid profiles compared to spiders from perennial streams. δ²H values indicated that fatty acid sources also differed between streams varying in intermittency. Our results suggest that stream intermittency affects subsidy transfers from aquatic to riparian ecosystems, indicating far-reaching consequences for riparian communities in resource poor alpine environments, but potentially also lower elevation fluvial systems.

Ecosystems are interconnected, and ecological processes frequently transcend the physical boundaries that define them. Fluxes of energy, matter, and organisms not only form important ecosystem processes within but also between ecosystems. However, the role of biological drivers in simultaneously supporting multiple ecosystem processes at the interface between aquatic and terrestrial ecosystems (that is, aquatic-terrestrial ecosystem processes) remains poorly understood, both locally and across regions. To assess the relative importance of riparian forests, detritus consumers and leaf litter mixing on different ecosystem processes of freshwater detrital food webs, we used leaf litter bags to subsidise local terrestrial leaf litter to forested and non-forested headwater stream sites in a temperate and tropical region. We also manipulated macroinvertebrate access and the composition of leaf litter mixtures. We measured three key aquatic-terrestrial ecosystem processes: biomass accrual of aquatic fungi, nitrogen loss, and decomposition rates of local leaf litter. Across both temperate and tropical streams, ecosystem multifunctionality, that is, the simultaneous sustaining of these processes, was positively associated with macroinvertebrates and riparian forests but not with leaf litter mixing. Especially leaf litter nitrogen loss and decomposition rates were consistently higher when macroinvertebrates had access across all leaf litter species. Decomposition rates were also positively associated with the other ecosystem processes. These findings highlight consistent, cross-regional effects of riparian forests and macroinvertebrate detritivores on freshwater detrital food webs. In a rapidly changing world, understanding ecosystem processes in headwater streams demands a holistic view that transcends ecosystem borders and incorporates cross-ecosystem interactions.