Autumnal input of leaf litter is a pivotal energy source in most headwater streams. In temporary streams, however, water stress may lead to a seasonal shift in leaf abscission. Leaves accumulate at the surface of the dry streambed or in residual pools and are subject to physicochemical preconditioning before decomposition starts after flow recovery. In this study, we experimentally tested the effect of photodegradation on sunlit streambeds and anaerobic fermentation in anoxic pools on leaf decomposition during the subsequent flowing phase. To mimic field preconditioning, we exposed Populus tremula leaves to UV-VIS irradiation and wet-anoxic conditions in the laboratory. Subsequently, we quantified leaf mass loss of preconditioned leaves and the associated decomposer community in five low-order temporary streams using coarse and fine mesh litter bags. On average, mass loss after approximately 45 days was 4 and 7% lower when leaves were preconditioned by irradiation and anoxic conditions, respectively. We found a lower chemical quality and lower ergosterol content (a proxy for living fungal biomass) in leaves from the anoxic preconditioning, but no effects on macroinvertebrate assemblages were detected for any preconditioning treatment. Overall, results from this study suggest a reduced processing efficiency of organic matter in temporary streams due to preconditioning during intermittence of flow leading to reduced substrate quality and repressed decomposer activity. These preconditioning effects may become more relevant in the future given the expected worldwide increase in the geographical extent of intermittent flow as a consequence of global change.

ABSTRACT1. The search for simple and effective descriptors of biological ecosystem components is a major challenge of monitoring aquatic ecosystem health.2. The relevance of body-size-related descriptors of benthic invertebrate guilds in monitoring the health of transitional aquatic ecosystems is discussed. The rationale is that macroinvertebrate body size relates body-size-abundance distributions to disturbance pressures through individual energetics, population dynamics, interspecific interactions and species coexistence responses.3. Body size is generally easy to measure and amenable to intercalibration procedures, it is comparable across taxa, guilds and sites, and, as a community feature, it is expected to vary on disturbance gradients, according to energetic and ecological constraints.4. The mechanistic relevance of individual body size as a community feature, through coexistence relationships, still requires field and laboratory tests; standard methods to analyse body-sizeabundance distributions are not yet fully developed.5. Field experiments on coastal lagoons and freshwater ecosystems of southern Italy, which were designed to test the relevance of body-size-related constraints on the organization of detritus-based benthic guilds, are reviewed.6. Study cases emphasized a number of interesting features of body size and related descriptors, that support their relevance as benthic invertebrates descriptors of ecosystem health: (a) body-sizeabundance distributions are consistently less variable than taxonomic composition; (b) the width of body-size-abundance distribution is mainly due to the interspecific component; (c) the descriptors of body-size-abundance distributions seem to respond on environmental gradients and generally covary with species density, richness and diversity, on which most of the monitoring programmes actually rely.

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ABSTRACT1. Leaf litter decomposition rates in aquatic ecosystems are known to be related to many abiotic and biotic factors.2. Field experiments were carried out during spring 2005 in 16 ecosystems, each with four sampling sites, using the litter bag technique to investigate the influence of abiotic factors on patterns of reed litter breakdown in different physiographic, hydrological and physico-chemical gradients occurring in transitional water ecosystems in the Eastern Mediterranean and Black Sea.3. Significant differences in leaf litter decomposition were observed among the studied ecosystems along univariate gradients of tidal range, water temperature, salinity and sinuosity index.4. Overall, 71% of variance in the litter breakdown rate was explained by the hydrological, physico-chemical and physiographic components. Specifically, tidal range, salinity and sinuosity index are among the key factors in the most commonly used typological schemes for classifying transitional water ecosystems (i.e. Confinement Concept and Venice System), due to their influence on abundance and distribution of benthic macroinvertebrates and other guilds.5. The patterns observed at the regional scale of the study suggest that certain key abiotic factors are likely to play a major role as drivers of plant detritus decomposition processes, through their influence on the overall metabolism of microorganisms and benthic macroinvertebrates.

Although habitat size is known to influence both structural and functional properties of ecosystems, there have been few attempts to assess the influence of habitat size on ecosystem processes. Here we investigated the relationships between leaf litter decomposition and ecosystem surface area, macroinvertebrates and physico-chemical factors in five freshwater springs located in Huntingdon County (Pennsylvania, U.S.A.). Leaves of Ulmus americana L. were used to study leaf litter breakdown with the litter-bag technique. Field work was carried out at one sampling station per spring, each with eight replicates per sampling time (3, 20, 40 days), from April to May 2004. American elm leaves decomposed at different rates in the different springs, varying inversely with the spring area. The leaf bags were colonized by 16 taxa of benthic macrofauna, amongst which scrapers and shredders were the most common guild. Macroinvertebrate species richness co-varied with spring area, but not with other physico-chemical variables.Moreover, a significant inverse relationship was observed between American elm leaf decay rate and taxonomic richness. In the studied springs, habitat area was an ecosystem feature indirectly affecting detritus processing by influencing the structure of the detrital food web within the systems.