Preconditioning effects of intermittent stream flow on leaf litter decomposition

Abstract

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.

Recent perspectives on temporary river ecology

“…As intermittence metrics (e.g., drying duration, drying frequency) appear to be relatively good predictors for some important ecosystem processes and biodiversity patterns (e.g., Dieter et al 2011;Corti et al 2011;Datry et al 2011, Fritz andFeminella 2011;Von Schiller et al 2011), we suggest that scientists accumulate more univariate intermittence-ecology relationships across a wide range of temporary rivers in order to test for transferability and ubiquity among catchments in these relationships. Also, most extant bioindicators have little chance to succeed in detecting anthropogenic influences on temporary rivers because of the overwhelming effects of flow intermittence on biotic communities.…”

“…Another study included here (Dieter et al 2011) demonstrates that leaf litter decomposition is strongly influenced by repeated cessations of flow in temporary rivers. They found that leaf litter collected from either the dry streambed or from remnant pools were preconditioned to decomposition by UV radiation or wet-anoxic conditions, respectively.…”

“…Invertebrate shredders and microbes were both influential in decomposing leaf material from intermittently submerged litterbags. The fact that leaf litter breakdown was detectable under permanently dry conditions could be explained by alternative mechanisms such as leaching and photodegradation (Dieter et al 2011). Datry et al (2011) found that that dry phase duration influenced litter decomposition rates primarily because of the negative effects of the dry phase on invertebrate shredders, and that this effect extended long after rewetting, as if there was a dry event ''memory''.…”

“…From this perspective, terrestrial organic matter that accumulates on streambeds during dry phases without being rapidly decomposed ) may play an important role in stream organic matter processing just after rewetting. Additionally, the consequences of low organic matter processing rates for aquatic foodwebs and ecosystem productivity deserves further investigation Dieter et al 2011). Biotic interactions (e.g., predation, competition, parasitism) among terrestrial and aquatic organisms are also still poorly-understood even though these interactions can be intense, obvious, and relatively easy to study during the drying process.…”

Non-perennial Mediterranean rivers in Europe: Status, pressures, and challenges for research and management

“…In NPRS, coarse particulate organic matter (CPOM) accumulates in standing pools and at the surface of dry habitats, and as the river dries, the primary agents of CPOM decomposition shift from leaching and processing by aquatic micro-organisms and invertebrates to photo-degradation by UV and processing by terrestrial micro-organisms and invertebrates (Corti et al, 2011;Dieter et al, 2011). The decomposition rate decreases according to drying duration and the progressive disappearance and inactivity of aquatic fungi and shredders (e.g.…”