Climate change leads to summer low flow conditions and premature litter input in lowland streams in Central Europe. This may cause a sudden reduction of flow and fragmentation into isolated pools of permanently flowing streams, with a simultaneous increase in the food supply for detrivores during summer months. We performed a mesocosm study to investigate shredder and microbial mediated litter decomposition under these conditions. Leaf litter was placed in a lowland stream with a natural flow regime (reference) and in a stream mesocosm with significant flow reduction (FR) and a representative density of macroinvertebrates and detritus. Physicochemical parameters, leaf mass loss, macroinvertebrate abundance and biomass, leaf-associated respiration, fungal sporulation, and biomass were measured at regular intervals for 6 weeks. Coarse and fine-mesh bags were used to include or exclude macroinvertebrate shredders. In the coarse-mesh bags, leaf mass loss was significantly lower in the FR system than in the reference regime. In the fine-mesh bags, leaf respiration, fungal sporulation, and biomass but not leaf mass losses were substantially lower with flow reduction.Chironomid larvae (Micropsectra spp.) appeared to effectively fragment leaf litter in fine-mesh bags. In the FR system, leaf respiration was higher in the coarsethan in the fine-mesh bags. Our results suggest that, in temperate lowland streams, premature litter input during or after a sudden fragmentation into isolated pools and a reduction of stream flow reduces direct shredder-mediated litter decomposition, but shredders may indirectly influence the decomposition process.

Climate change models for Central Europe predict hydrological drought with fragmentation into pools during periods of high litter input in numerous lowland streams, presumably affecting in-stream leaf decay processes. To investigate this assumption, we measured physicochemical parameters, macroinvertebrate colonization, microbial activity, and decay rates of exposed leaves during and after a supraseasonal drought in a German lowland stream. Microbial activity, shredder colonization and leaf decay rates during fragmentation were low, presumably caused by drought-related environmental conditions. Microbial activity and temperature-corrected decay rates increased after the flow resumption but not leaf mass loss and shredder colonization. During both periods, exposed leaves appeared physically unaffected suggesting strongly reduced shredder-mediated leaf decay despite shredder presence. Our results indicate that hydrological drought can affect organisms and processes in temperate lowland streams even after flow resumption, and should be considered in climate change scenarios. IntroductionThe decomposition of allochthonous organic matter, such as leaf litter from riparian trees, is a major ecosystem-level process in streams running through forested watersheds (WEB- STER and BENFIELD, 1986;ABELHO, 2001). Leaves are processed by a complex interaction of several abiotic and biotic processes, such as physical leaching, mechanical abrasion, microbial degradation, and consumption by macro-invertebrates (leaf shredders). Aquatic hyphomycetes play a predominant role in microbial leaf decay (HIEBER and GESSNER, 2002;PASCOAL and CÁSSIO, 2004), as they rapidly colonize submerged leaves, degrade plant cell polymers (CHAMIER, 1985) and increase the palatability of leaves for shredders (GRAÇA and CANHOTO, 2006). Fine organic particles and dissolved compounds resulting from leaf decay serve as energy sources for various consumers (BENFIELD, 1996). Hence, understanding the responses of in-stream leaf decay processes to climate change is central to interpreting and predicting the impacts of climate change on stream ecosystems.Higher temperatures, locally reduced precipitation and increased frequency and duration of extreme events (e.g., seasonal and supra-seasonal hydrological droughts) are expect-* Corresponding author 634 J. SCHLIEF and M. MUTZ

No abstract

Leaf Associated Microbial Activities in a Stream Affected byAcid Mine Drainage key words: leaf litter breakdown, acid mine drainage, respiration rate, ochre deposition, iron plaques AbstractMicrobial activity was assessed on birch leaves and plastic strips during 140 days of exposure at three sites in an acidic stream of the Lusatian post-mining landscape, Germany. The sites differed in their degrees of ochre deposition and acidification. The aim of the study was (1) to follow the microbial activities during leaf colonization, (2) to compare the effect of different environmental conditions on leaf associated microbial activities, and (3) to test the microbial availability of leaf litter in acidic mining waters. The activity peaked after 49 days and subsequently decreased gradually at all sites. A formation of iron plaques on leaf surfaces influenced associated microbial activity. It seemed that these plaques inhibit the microbial availability of leaf litter and serve as a microbial habitat by itself. IntroductionAcid mine drainage is a persistent effect of human resource usage and affect many aquatic ecosystems throughout the world (NOIKE et al., 1983;MCKNIGHT and FEDER, 1984;SCHULTZE and GELLER, 1996). The acid drainage is generated by the oxidative weathering of sulphidecontaining minerals, which often lies in or around coal seams. Streams affected by acid mine drainage typically have low pH values, high concentrations of sulphate and iron, and abundant deposition of precipitated metal oxides, predominantly ochre (iron oxyhydroxide), that are covering the streambed (ENDER and LESSMANN, 2000). The complex interaction of all these stressors can lead to severe alterations of the biological structure and ecosystem processes. Consequently, the examination of ecosystem processes, like leaf litter breakdown, is one proposed measure to test the integrity of such affected streams (GESSNER and CHAU- VET, 2002). Leaf litter breakdown is a well-documented process, in which the interplay of abiotic and biotic subprocesses leads to a disintegration of leaves (WEBSTER and BENFIELD, 1986;GESSNER et al., 1999). A number of recent investigations revealed that the leaf litter breakdown process in acidic mining streams is slow and primarily microbial mediated, due to the absence of macroinvertebrate "shredders" (SIEFERT and MUTZ, 2000;NIYOGI et al., 2001 and. The authors assume that, like in circumneutral aquatic environments, microorganisms exploit leaf material through enzymatic cleavage of plant polymers in acidic mining waters. If this is true, one can predict higher microbial activities on natural leaf litter than on an artificial material that can only serve as a holdfast for microorganisms. Leaf associated respiration rates are a suitable measure to assess microbial activity and were already applied by GROOM and HILDREW (1989) and NIYOGI et al., (2001) and (2002) in acidic streams. These studies, however, gave no information about the temporal dynamics of leaf associated microbial activity.Internat. Rev. Hydrobiol.

Both the absence of leaf shredding macroinvertebrates and low microbial activity are of major importance in determining slow and incomplete leaf decay in extremely acidic (pH<3.5) mining streams. These streams are affected by a heavy ochre deposition causing the formation of massive iron plaques on leaf surfaces that hinder microbial exploitation. An investigation was carried out to determine whether iron plaques and leaf conditioning status (acid conditioned with and without iron plaques, neutral conditioned, unconditioned) affect the feeding preference of the shredder Gammarus pulex (L.). Leaf respiration rates and fungal biomass (ergosterol contents) were measured to determine microbial colonization. Neutral conditioned leaves had significantly higher microbial colonization than acid conditioned leaves with iron plaques. Notwithstanding, leaves of both conditioning types were consumed at high rates by G. pulex. The microbial colonization had no influence on feeding preference in the experiment. It is presumed that iron adsorbed organic material caused the high palatability of leaves with iron plaques. The results indicate that the large deposits of leaves coated with iron plaques will be available to the stream food web when water quality will be restored to neutral as planed in scenarios for the future development of mining streams.