Effect of leaf litter characteristics on leaf conditioning and on consumption by Gammarus pulex

Terrestrial leaf litter inputs provide an essential energy source for many freshwater organisms. Processing of leaf litter involves several physicochemical and biological factors but always starts with the colonisation of leaves by aquatic hyphomycetes. Here, we document changes occurring on leaf tissues of three tree species, alder, hornbeam and oak, with contrasted leaf properties. Changes in the mechanical properties of leaves and in fungal growth were followed at 10, 25, 35, 45 and 55days of immersion in natural winter conditions. We hypothesised that fungal growth will be faster and mechanical characteristics will decrease more rapidly in softer, nutrient-rich, than in tougher leaves. In addition, we tested whether the consumption rate of conditioned leaves by the amphipod shredder Gammarus pulex was correlated with the mechanical properties of leaves and their ergosterol content (as a proxy for fungal biomass). Leaf toughness decreased (with different rates for the 3 litter species), whereas the leaf thickness remained stable. The mechanical properties of alder and hornbeam leaves changed similarly, in contrast to oak leaves. Ergosterol contents increased over time more rapidly for alder and hornbeam than for oak leaves for which the fungal growth was delayed. Ergosterol content, leaf toughness and leaf consumption rates by G.pulex were significantly correlated. Leaf consumption rates by G.pulex were higher for soft than for tough leaves. Our results suggest that the consumption of different types of leaves may be delayed according to the time required to acquire conditioning sufficient for shredder consumption. The presence of tough leaves in riparian vegetation may constitute a reservoir of trophic resources available for aquatic organisms at the end of the winter, when soft leaves have been entirely consumed. The diversity of riparian vegetation may hence contribute to sustaining the availability of food resources for adjacent aquatic ecosystems.