How does water current velocity affect invertebrate community and leaf-litter breakdown in a physicochemically stable freshwater ecosystem? An experimental study in two nearby reaches (erosional vs. depositional) of the Vera Spring (Central Italy)

The decomposition of allochthonous dead organic matter is a key process for the metabolism and functioning of stream and spring ecosystems. The litter breakdown process is influenced by several abiotic and biotic factors. Among abiotic parameters, the role of current velocity and physical abrasion was poorly investigated. Field studies gave contrasting results, mainly because of the covariation and the interaction of current velocity with other biotic/abiotic variables. For these reasons, we assessed leaf-litter breakdown and the structure of crenic assemblages in two nearby reaches (erosional vs. depositional) of a physicochemically stable rheocrene spring. The two zones investigated were characterized by similar environmental conditions, but water current velocity was about four times greater in the erosional reach. We found substantial differences in the structure and functional organization of crenic assemblages. Overall taxa richness and density were higher in the depositional reach, while diversity and abundance of Ephemeroptera, Plecoptera and Trichoptera were taller in the erosional zone. Shredders were more abundant in the erosional zone, and scrapers were more represented in the slow current sector of the spring. We also demonstrated that water flow may promote a faster decomposition of leaf detritus in the spring erosional reach mainly through indirect effects: higher richness and abundance of shredder detritivores. Our results indicate that water current velocity may have a key role in affecting both spring assemblage composition and ecosystem processes.