An important fraction of the world’s streams and rivers are subjected to dry periods that cause occasional disruption of flow (Larned et al. 2010). The resulting flow intermittency involves contraction and fragmentation of running waters and temporary loss of aquatic habitat (Lake 2003). Climate change is an important factor likely to alter stream flow in the future, exacerbating both the spatial and temporal extent of intermittency in different climatic zones around the world (Milly, Dunne & Vecchia 2005). Particularly susceptible are Mediterranean streams, where climate change is projected not only to increase the extent of intermittency (Milly, Dunne & Vecchia 2005; IPCC 2007), but also to induce shifts from currently perennial to intermittent flow.
Changes in flow regimes have direct impacts on aquatic communities and processes (e.g. Boulton 2003; Lake 2003) and can also affect the density, composition and dominance patterns of riparian vegetation (Poff & Zimmerman 2010). Such changes in the structure of riparian communities affect both the amount and quality of leaf litter delivered to stream channels. The potential repercussions for stream communities and ecosystem processes are large, as allochthonous litter is a main source of carbon and nutrients for stream food webs and metabolism (Wallace et al. 1997; Webster 2007; Tank et al. 2010). Key in this respect is the utilization of leaf litter by decomposers and the resultant litter decomposition.
If riparian vegetation changes, the question arises to what extent litter species identity and diversity will affect stream communities and processes. The role of diversity in sustaining ecosystem processes in general has received great attention in recent years (Loreau et al. 2001; Hooper et al. 2005). However, although this includes experiments assessing diversity effects on decomposition (Gartner & Cardon 2004; Hättenschwiler, Tiunov & Scheu 2005; Kominoski et al. 2010; Lecerf & Richardson 2010), the significance of litter diversity for decomposition has not been resolved (Gessner et al. 2010). In part, this is because mechanisms behind the effects of litter diversity observed in various studies have rarely been specifically examined (but see McArthur et al. 1994; Schimel & Hättenschwiler 2007).
Diversity effects are particularly likely to arise when the species within a community contrast in their functional traits (Hillebrand & Matthiessen 2009; Gessner et al. 2010). In the case of litter decomposition, relevant litter traits include nutritional quality and properties that influence microenvironmental conditions afforded to decomposers. The significance of litter traits for microenvironmental conditions is unclear, but it could be tested and be particularly relevant in intermittent streams. As stream channels are drying, microenvironmental conditions in litter packs retained in the channels fundamentally change, leading to strong declines in decomposition rates (Herbst & Reice 1982; Maamri et al. 1998; Langhans & Tockner 2006; Leberfinger, Bohman & Herrmann 2010). However, different litter species differ greatly in their water-holding capacity (Dirks et al. 2010). Therefore, in litter mixtures subject to desiccation, the presence of species with high water-holding capacity might slow the loss of moisture in adjacent species with low water-holding capacity (Wardle et al. 2003). This would extend the window of decomposer activity during dry periods and enhance survival of microbes until flow resumes (Langhans & Tockner 2006), notwithstanding the fact that some aquatic decomposers can occur in terrestrial environments as well (Sanders & Webster 1978; Sridhar & Bärlocher 1993). Litter-mixing effects on decomposition of individual species can thus arise.
Particularly important drivers of litter decomposition in streams are a group of microfungi known as aquatic hyphomycetes (Gessner et al. 2007; Krauss et al. 2011). Their role is twofold: They directly degrade leaf litter, and they stimulate litter consumption by detritivores. This stimulation occurs because fungi produce biomass rich in nutrients and because they enzymatically change the physical and chemical properties of decomposing litter, thereby enhancing litter quality and palatability for consumers (Gessner et al. 2007). Unlike invertebrates, which also can be important for litter decomposition in streams (Graça 2001; Hieber & Gessner 2002), fungi are intimately associated with their substrate. As a consequence, they cannot evade desiccation when stream flow recedes and water levels drop. This makes fungal decomposers particularly vulnerable to desiccation stress accompanying stream intermittency.
This study aimed at assessing litter mixture effects on decomposition under simulated stream intermittency. By determining litter mass loss, fungal biomass and sporulation activity in mixed- and single-species litter under different drying regimes in a field experiment, we tested whether (i) aquatic fungi and litter decomposition are affected by drying resulting from stream intermittency; (ii) litter mixing alleviates effects on fungal biomass and activity induced by desiccation stress; and (iii) any effect of litter mixing, desiccation and the interaction of both factors on fungi translates into effects on litter decomposition. Effects on decomposers and decomposition during dry periods might extend well beyond the time when flow resumes because microbial decomposers and litter-consuming invertebrates might have to recolonize the litter, resume growth or both. Therefore, assessments of drying effects need to consider both immediate and propagated effects.