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

Abstract

Summary 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 55 days 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.

Effect of Climate-Related Change in Vegetation on Leaf Litter Consumption and Energy Storage by Gammarus pulex from Continental or Mediterranean Populations

“…In the winter, disks from each leaf species were stream-conditioned in the same water body by the same pool of hyphomycete species under natural conditions, according to the description of Foucreau et al [27]. As this study monitored the consumption rate of leaf litter by shredders, leaves had to be conditioned for long enough so that gammarids consumed enough leaves to obtain a consumption rate.…”

“…As this study monitored the consumption rate of leaf litter by shredders, leaves had to be conditioned for long enough so that gammarids consumed enough leaves to obtain a consumption rate. Due to initial differences in leaf toughness and in the conditioning rate, alder leaves were conditioned for 10 d, whereas hornbeam leaves were conditioned for 25 d, and oak leaves for 35 d [27]. Before and at the end of the conditioning, the biomechanical characteristics and leaf thickness were measured according to Dehedin et al [44] and Foucreau et al [27].…”

“…In addition, the shredder leaf consumption rate is influenced by leaf characteristics [24], [25]. Therefore, the potential replacement of temperate vegetation with Mediterranean species [20] showing different biomechanical and/or chemical characteristics (tannin composition, wax components) might significantly alter ecosystem function [26], [27].…”

“…Each pair comprised one species adapted to the northern continental climate from the North of the Rhône River Valley and one species adapted to the southern Mediterranean climate from the South of the Rhône River Valley. As leaf toughness, which is related to the leaf mechanical structure, significantly modifies shredder feeding activity, we studied two pairs of leaf species: two species of hornbeam with soft leaves and two species of oak leaves with tough leaves [27]. A fifth species, present in both the southern and northern regions of Rhône Valley, with soft leaves was also studied.…”

The relative importance of diet-related and waterborne effects of copper for a leaf-shredding invertebrate

“…Leaves of Alnus glutinosa (black alder), a species that features comparably low C:N ratios of <20 (e.g., Foucreau et al, 2013) and is decomposed in freshwater systems relatively fast (e.g., Schindler and Gessner, 2009), were collected in October 2012 near Landau, Germany (49 11 0 N; 8 05 0 E) and stored at À20 C until further processing. Following the procedure described by Zubrod et al (2011), leaves were deployed for 14 days in fine-mesh bags in the Rodenbach, Germany (49 33 0 N, 8 02 0 E), upstream of any agricultural activity, settlement, and wastewater inlet, to establish a natural microbial community.…”