Effects of burial on leaf litter quality,microbial conditioning and palatability to three shredder taxa

Summary 1. Heterotrophic microorganisms are crucial for mineralising leaf litter and rendering it more palatable to leaf‐shredding invertebrates. A substantial part of leaf litter entering running waters may be buried in the streambed and thus be exposed to the constraining conditions prevailing in the hyporheic zone. The fate of this buried organic matter and particularly the role of microbial conditioning in this habitat remain largely unexplored. 2. The aim of this study was to determine how the location of leaf litter within the streambed (i.e. at the surface or buried), as well as the leaf litter burial history, may affect the leaf‐associated aquatic hyphomycete communities and therefore leaf consumption by invertebrate detritivores. We tested the hypotheses that (i) burial of leaf litter would result in lower decomposition rates associated with changes in microbial assemblages compared with leaf litter at the surface and (ii) altered microbial conditioning of buried leaf litter would lead to decreased quality and palatability to their consumers, translating into lower growth rates of detritivores. 3. These hypotheses were tested experimentally in a second‐order stream where leaf‐associated microbial communities, as well as leaf litter decomposition rates, elemental composition and toughness, were compared across controlled treatments differing by their location within the streambed. We examined the effects of the diverse conditioning treatments on decaying leaf palatability to consumers through feeding trials on three shredder taxa including a freshwater amphipod, of which we also determined the growth rate. 4. Microbial leaf litter decomposition, fungal biomass and sporulation rates were reduced when leaf litter was buried in the hyporheic zone. While the total species richness of fungal assemblages was similar among treatments, the composition of fungal assemblages was affected by leaf litter burial in sediment. 5. Leaf litter burial markedly affected the food quality (especially P content) of leaf material, probably due to the changes in microbial conditioning. Leaf litter palatability to shredders was highest for leaves exposed at the sediment surface and tended to be negatively related to leaf litter toughness and C/P ratio. In addition, burial of leaf litter led to lower amphipod growth rates, which were positively correlated with leaf litter P content. 6. These results emphasise the importance of leaf colonisation by aquatic fungi in the hyporheic zone of headwater streams, where fungal conditioning of leaf litter appears particularly critical for nutrient and energy transfer to higher trophic levels.     

Changes in microphytobenthos fluorescence over a tidal cycle: implications for sampling designs

The breakdown rate of leaf litter buried inside river sediments (i.e., in the hyporheic zone) remains poorly known. The burial of large bags (15 × 15 cm) used in the benthic layer generates disturbances of the vertical connectivity with surface water, and thus affects the breakdown rate. We performed field and laboratory experiments to test the suitability of two leaf litter containers: small litter bags (5 × 4 cm) and perforated stainless steel cylinders (6 cm long, 1.6 cm in diameter), both introduced inside sediments using mobile mini-piezometers (1 m long, 1.7 cm or 3 cm in diameter). We compared the two containers for (i) the hydrological exchanges toward leaf litter, (ii) the impact of a benthic shredder on leaf breakdown, and (iii) the response of leaf litter breakdown to changes in sediment characteristics. The two methods give similar patterns of breakdown rates with stream sediment characteristics, but the use of perforated stainless steel cylinders provides an artificial empty volume in which invertebrates can over degrade leaf litter. Small plastic bags do not alter physico-chemical conditions around leaf litter and give lower values of breakdown rates in the hyporheic zone.     

Interactions between fauna and sediment control the breakdown of plant matter in river sediments

1. A substantial portion of particulate organic matter (POM) is stored in the sediment of rivers and streams. Leaf litter breakdown as an ecosystem process mediated by microorganisms and invertebrates is well documented in surface waters. In contrast, this process and especially the implication for invertebrates in subsurface environments remain poorly studied. 2. In the hyporheic zone, sediment grain size distribution exerts a strong influence on hydrodynamics and habitability for invertebrates. We expected that the influence of shredders on organic matter breakdown in river sediments would be influenced strongly by the physical structure of the interstitial habitat. 3. To test this hypothesis, the influence of gammarids (shredders commonly encountered in the hyporheos) on degradation of buried leaf litter was measured in experimental systems (slow filtration columns). We manipulated the structure of the sedimentary habitat by addition of sand to a gravel‐based sediment column to reproduce three conditions of accessible pore volume. Ten gammarids were introduced in columns together with litter bags containing alder leaves at a depth of 8 cm in sediment. Leaves were collected after 28 days to determine leaf mass loss and associated microbial activity (fungal biomass, bacterial abundance and glucosidase, xylosidase and aminopeptidase activities). 4. As predicted, the consumption of buried leaf litter by shredders was strongly influenced by the sediment structure. Effective porosity of 35% and 25% allowed the access to buried leaf litter for gammarids, whereas a lower porosity (12%) did not. As a consequence, leaf litter breakdown rates in columns with 35% and 25% effective porosity were twice as high as in the 12% condition. Microbial activity was poorly stimulated by gammarids, suggesting a low microbial contribution to leaf mass loss and a direct effect of gammarids through feeding activity. 5. Our results show that breakdown of POM in subsurface waters depends on the accessibility of food patches to shredders.     

Small stonefly predators affect microbenthic and meiobenthic communities in stream leaf packs

相似文献   

Effects of buried leaf litter and vertical hydrologic exchange on hyporheic water chemistry and fauna in a gravel-bed river in northern New South Wales, Australia

1. Large amounts of coarse particulate organic matter (CPOM) are buried in the sand and gravel beds of many rivers during spates. The effects of these patchily distributed resources on hyporheic invertebrates and water chemistry are poorly understood. Buried CPOM may provide local ‘hot-spots’ of food for hyporheic detritivores and their predators, alter nutrient supply to nearby sediment biofilms, and generate habitat for some invertebrates. 2. To examine potential short-term effects on hyporheic water chemistry, nutrient concentrations and invertebrate assemblage composition, leaf packs were buried in downwelling (surface water infiltrating the hyporheic zone) and upwelling (hyporheic water emerging to the surface) zones at two sites along a gravel-bed river in northern New South Wales. At one site, pits were excavated to simulate leaf burial (procedural control) and plastic ‘leaves’ were buried to test whether invertebrates might respond to leaves as refuges rather than food. Hyporheic CPOM, sediment size fractions, and interstitial silt content were also quantified at these sites. 3. Dry weights of naturally buried CPOM (leaf litter and wood fragments) varied substantially (0.6–71.7 g L^–1 sediment). Amounts of CPOM did not differ between up- vs. downwelling zones or between sites. Hyporheic dissolved oxygen saturation was generally high (> 75%), and was lower in upwelling zones. The hyporheos was dominated taxonomically by water mites (≈ 20 species), whereas small oligochaetes were most abundant (40% of total abundance). Tiny instars of elmid beetle larvae and leptophlebiid mayfly nymphs were also common. Before experimental manipulation, faunal composition differed between up- and downwelling zones. In upwelling zones, bathynellaceans and blind peracarids were found, whereas small individuals of the surface benthos were common in samples from downwelling zones. This validated stratification of the experiment across zones of hydrologic exchange. 4. Twenty days after leaf burial, there was no effect of the treatments at either site on changes in most variables, including mean numbers of taxa and individuals per sample. Similarly, changes in faunal composition of the hyporheos in the treatments paralleled those in the controls except for a weak response in the buried leaves treatment in the upwelling zone at site 1. Artificially buried leaf litter does not seem to influence hyporheic water chemistry or fauna at these two sites. It is probable that naturally buried leaf litter is swiftly processed soon after entrainment and that repeating this experiment immediately after a flood may yield different results.     

Leaf litter degradation in the wave impact zone of a pre-alpine lake

Contrary to streams, decomposition processes of terrestrial leaf litter are still poorly understood in lakes. Here, we examined the decomposition of two leaf species, beech (Fagus sylvatica) and poplar (Populus nigra ‘italica’) in the littoral zone of a large pre-alpine lake at a wave exposed site. We focussed on the shredding impact of benthic invertebrates in a field experiment and on the effects of wave-induced disturbances under field and mesocosm conditions. In contrast to our expectations, benthic shredders did not reveal an important role in leaf processing under the conditions of the field experiment (early spring time, wave impact zone). Strong wave turbulence during storm events significantly reduced leaf mass, FPOM and invertebrate densities at field conditions. Several reasons can explain the low importance of shredders in our field study: (a) phenology of the shredder species, (b) feeding preferences and alternative food sources for gammarids, (c) generally low abundance of the native gammarid species due to the recent occurrence of an invasive predator, (d) disturbance of shredder activity due to high wave impact and (e) relatively low food value of the offered leaves. We suggest that leaf litter decomposition in lakes occurs in specific process domains, which largely depend on the hydraulic characteristics and on water-level fluctuations.     

Aquatic Hyphomycete Species Are Screened by the Hyporheic Zone of Woodland Streams

Aquatic hyphomycetes strongly contribute to organic matter dynamics in streams, but their abilities to colonize leaf litter buried in streambed sediments remain unexplored. Here, we conducted field and laboratory experiments (slow-filtration columns and stream-simulating microcosms) to test the following hypotheses: (i) that the hyporheic habitat acting as a physical sieve for spores filters out unsuccessful strategists from a potential species pool, (ii) that decreased pore size in sediments reduces species dispersal efficiency in the interstitial water, and (iii) that the physicochemical conditions prevailing in the hyporheic habitat will influence fungal community structure. Our field study showed that spore abundance and species diversity were consistently reduced in the interstitial water compared with surface water within three differing streams. Significant differences occurred among aquatic hyphomycetes, with dispersal efficiency of filiform-spore species being much higher than those with compact or branched/tetraradiate spores. This pattern was remarkably consistent with those found in laboratory experiments that tested the influence of sediment pore size on spore dispersal in microcosms. Furthermore, leaves inoculated in a stream and incubated in slow-filtration columns exhibited a fungal assemblage dominated by only two species, while five species were codominant on leaves from the stream-simulating microcosms. Results of this study highlight that the hyporheic zone exerts two types of selection pressure on the aquatic hyphomycete community, a physiological stress and a physical screening of the benthic spore pool, both leading to drastic changes in the structure of fungal community.     

Consequences of the colonisation of leaves by fungi and oomycetes for leaf consumption by a gammarid shredder

1. Leaf litter breakdown by shredders in the field is affected by leaf toughness, nutritional value and the presence of secondary compounds such as polyphenols. However, experiments involving the use of single fungal strains have not supported the assumption that leaf parameters determine food selection by shredders perhaps because of a failure to test for high consumption prior to isolation of fungal strains, overrepresentation of hyphomycetes or the potential effects of accompanying bacteria. In this study, we used bacteria‐free, actively growing fungi and oomycetes isolated from conditioned leaf litter for which a shredder had already shown high consumption rates. 2. Black alder (Alnus glutinosa) leaf litter was exposed to the littoral zone of Lake Constance in autumn, and subsamples were analysed for leaf parameters and consumption by Gammarus roeselii under standard conditions at regular intervals. On dates with a high consumption rate of the exposed leaves, 14 single strains of fungi and oomycetes were isolated, freed of bacteria and grown on autoclaved leaves. 3. Six of eight measured leaf parameters of exposed leaves were significantly correlated with Gammarus consumption rates, with high colinearity among leaf parameters hampering the identification of causal relations between leaf parameters and feeding activity. 4. When single strains of fungi and oomycetes were grown on autoclaved leaf litter, toughness of colonised leaves was always lower than in the control and the content of protein, N and P were increased. There were pronounced strain‐specific effects on leaf parameters. Consumption rates also differed significantly, with nine of fourteen isolates consumed at higher rates than controls and none proving to be a deterrent. Protein and polyphenol content were significantly correlated with consumption rates. Oomycete‐colonised leaves were consumed at similar rates but were of lower food quality than fungi‐colonised leaves. 5. We argue that direct strain‐specific attractant or repellent effects of fungi and oomycetes on consumption by G. roeselii are not important. However, we found indirect strain‐specific role operating via effects on leaf parameters.     

Relevance of large litter bag burial for the study of leaf breakdown in the hyporheic zone

Particulate organic matter is the major source of energy for most low-order streams, but a large part of this litter is buried within bed sediment during floods and thus become poorly available for benthic food webs. The fate of this buried litter is little studied. In most cases, measures of breakdown rates consist of burying a known mass of litter within the stream sediment and following its breakdown over time. We tested this method using large litter bags (15 × 15 cm) and two field experiments. First, we used litter large bags filled with Alnus glutinosa leaves (buried at 20 cm depth with a shovel) in six stations within different land-use contexts and with different sediment grain sizes. Breakdown rates were surprisingly high (0.0011–0.0188 day⁻¹) and neither correlate with most of the physico-chemical characteristics measured in the interstitial habitats nor with the land-use around the stream. In contrast, the rates were negatively correlated with a decrease in oxygen concentrations between surface and buried bags and positively correlated with both the percentage of coarse particles (20–40 mm) in the sediment and benthic macro-invertebrate richness. These results suggest that the vertical exchanges with surface water in the hyporheic zone play a crucial role in litter breakdown. Second, an experimental modification of local sediment (removing fine particles with a shovel to increase vertical exchanges) highlighted the influence of grain size on water and oxygen exchanges, but had no effect on hyporheic breakdown rates. Burying large litter bags within sediments may thus not be a relevant method, especially in clogged conditions, due to changes induced through the burial process in the vertical connectivity between surface and interstitial habitats that modify organic matter processing.     

Fungal alteration of the elemental composition of leaf litter affects shredder feeding activity

相似文献   

Assessing the influence of wildfire on leaf decomposition and macroinvertebrate communities in boreal streams using mixed-species leaf packs

1. We investigated how compositional differences in riparian leaf litter derived from burned and undisturbed forests influenced leaf breakdown and macroinvertebrate communities using experimental mixed-species leaf packs in boreal headwater streams. Leaf pack mixtures simulating leaf litter from dominant riparian woody-stem species in burned and undisturbed riparian zones were incubated in two references and two fire-disturbed streams for 5 weeks prior to measuring temperature-corrected breakdown rates and macroinvertebrate community composition, richness, and functional metrics associated with decomposers such as shredder abundance and % shredders.
2. Leaf litter breakdown rates were higher and had greater variability in streams bordered by reference riparian forests than in streams where riparian forests had been burned during a wildfire. Streams bordered by fire disturbance showed significant effects of litter mixture on decomposition rates, observed as significantly higher decomposition rates of a fire-simulated leaf mixture compared to all other mixtures.
3. Variation among sites was higher than variation among litter mixtures, especially for macroinvertebrate community composition. In general, fire-simulated leaf mixtures had greater shredder abundances and proportions, but lower overall macroinvertebrate abundance; however, the shredder abundance trend was not consistent across all leaf mixtures at each stream.
4. These results show that disturbance-driven riparian forest condition and resulting composition of leaf subsidies to streams can influence aquatic invertebrate community composition and their function as decomposers. Therefore, if one of the primary goals of modern forest management is to emulate natural disturbance patterns, boreal forest managers should adapt silvicultural practices to promote leaf litter input that would arise post-fire, thereby supporting stream invertebrate communities and their function.

     

Positive effect of shredders on microbial biomass and decomposition in stream microcosms

1. Animals play a major role in nutrient cycling via excretory processes. Although the positive indirect effects of grazers on periphytic algae are well understood, little is known about top‐down effects on decomposers of shredders living on leaf litter. 2. Nutrient cycling by shredders in oligotrophic forest streams may be important for the microbial‐detritus compartment at very small spatial scales (i.e. within the leaf packs in which shredders feed). We hypothesised that insect excretion may cause local nutrient enrichment, so that microorganism growth on leaves is stimulated. 3. We first tested the effect of increasing concentration of ammonium (+10, +20 and +40 μg NH₄⁺ L^?1) on fungal and bacterial biomass on leaf litter in a laboratory experiment. Then we performed two experiments to test the effect of the presence and feeding activity of shredder larvae. We used two species belonging to the trichopteran family Sericostomatidae: the Palaearctic Sericostoma vittatum and the Neotropical Myothrichia murina, to test the effect of these shredders on fungal and bacterial biomass and decomposition on leaves of Quercus robur and Nothofagus pumilio, respectively. All experiments were run in water with low ammonium concentrations (2.4 ± 0.34 to 14.47 ± 0.95 μg NH₄⁺ L^?1). 4. After 5 days of incubation, NH₄ concentrations were reduced to near‐ambient streamwater concentrations in all treatments with leaves. Fungal biomass was positively affected by increased ammonium concentration. On the other hand, bacteria abundance was similar in all treatments, both in terms of abundance (bacteria cells mg^?1 leaf DW) and biomass. However, there was a tendency towards larger mean cell size in treatments with 20 μg NH₄ L^?1. 5. In the experiment with S. vittatum, fungal biomass in the treatment with insects was more than twice that in the control after 15 days. Bacteria were not detected in treatments with insects, where hyphae were abundant, but they were abundant in treatments without larvae. In the decomposition experiment run with M. murina, leaf‐mass loss was significantly higher in treatments with larvae than in controls. 6. Our hypothesis of a positive effect of shredders on fungal biomass and decomposition was demonstrated. Insect excretion caused ammonium concentration to increase in the microcosms, contributing to microbial N uptake in leaf substrata, which resulted in structural and functional changes in community attributes. The positive effect of detritivores on microbes has been mostly neglected in stream nutrient‐cycling models; our findings suggest that this phenomenon may be of greater importance than expected in stream nutrient budgets.     

Riparian plant species loss alters trophic dynamics in detritus-based stream ecosystems

Riparian vegetation is closely connected to stream food webs through input of leaf detritus as a primary energy supply, and therefore, any alteration of plant diversity may influence aquatic ecosystem functioning. We measured leaf litter breakdown rate and associated biological parameters in mesh bags in eight headwater streams bordered either with mixed deciduous forest or with beech forest. The variety of leaf litter types in mixed forest results in higher food quality for large-particle invertebrate detritivores (‘shredders’) than in beech forest, which is dominated by a single leaf species of low quality. Breakdown rate of low quality (oak) leaf litter in coarse mesh bags was lower in beech forest streams than in mixed forest streams, a consequence of lower shredder biomass. In contrast, high quality (alder) leaf litter broke down at similar rates in both stream categories as a result of similar shredder biomass in coarse mesh bags. Microbial breakdown rate of oak and alder leaves, determined in fine mesh bags, did not differ between the stream categories. We found however aquatic hyphomycete species richness on leaf litter to positively co-vary with riparian plant species richness. Fungal species richness may enhance leaf litter breakdown rate through positive effects on resource quality for shredders. A feeding experiment established a positive relationship between fungal species richness per se and leaf litter consumption rate by an amphipod shredder (Gammarus fossarum). Our results show therefore that plant species richness may indirectly govern ecosystem functioning through complex trophic interactions. Integrating microbial diversity and trophic dynamics would considerably improve the prediction of the consequences of species loss.     

The shredding activity of gammarids facilitates the processing of organic matter by the subterranean amphipod Niphargus rhenorhodanensis

1. The functional feeding group approach has been widely used to describe the community structure of benthic invertebrates in relation to organic matter resources. Based on this functional framework, positive interactions between feeding groups (especially shredders and collector‐gatherers) were postulated in the River Continuum Concept. However, relationships with organic matter have been poorly documented for invertebrates living in the hyporheic zone. 2. We hypothesised that the common subterranean amphipod Niphargus rhenorhodanensis would feed on fine particulate organic matter (FPOM), which is more abundant than coarse particulate organic matter (CPOM) in hyporheic habitats, and should be favoured by the occurrence of shredders that produce FPOM from CPOM. 3. We used laboratory experiments to quantify leaf litter processing by N. rhenorhodanensis and a common shredder, the surface amphipod Gammarus roeselii. We estimated rates of feeding and assimilation (using nitrogen stable isotopes) of the two species separately and together to reveal any potential shredder–collector facilitation between them. 4. Measured leaf litter mass loss showed that N. rhenorhodanensis did not act as a shredder, unlike G. roeselii. Organic matter dynamics and ¹⁵N/¹⁴N ratios in tissues of niphargids indicated that N. rhenorhodanensis was a collector‐gatherer feeding preferentially on FPOM. We also found a positive influence of the gammarid shredders on the assimilation rate of N. rhenorhodanensis, which fed on FPOM produced by the shredders, supporting the hypothesis of a positive interaction between surface shredders and hyporheic collector‐gatherers.     

Stimulation of leaf litter decomposition and associated fungi and invertebrates by moderate eutrophication: implications for stream assessment

1. We investigated the effect of moderate eutrophication on leaf litter decomposition and associated invertebrates in five reference and five eutrophied streams in central Portugal. Fungal parameters and litter N and P dynamics were followed in one pair of streams. Benthic invertebrate parameters that are considered useful in bioassessment were estimated in all streams. Finally, we evaluated the utility of decomposition as a tool to assess stream ecosystem functional integrity. 2. Decomposition of alder and oak leaves in coarse mesh bags was on average 2.3–2.7× faster in eutrophied than in reference streams. This was attributed to stimulation of fungal activity (fungal biomass accrual and sporulation of aquatic hyphomycetes) by dissolved nutrients. These effects were more pronounced for oak litter (lower quality substrate) than alder. N content of leaf litter did not differ between stream types, while P accrual was higher in the eutrophied than in the reference stream. Total invertebrate abundances and richness associated with oak litter, but not with alder, were higher in eutrophied streams. 3. We found only positive correlations between stream nutrients (DIN and SRP) and leaf litter decomposition rates in both fine and coarse mesh bags, associated sporulation rates of aquatic hyphomycetes and, in some cases, total invertebrate abundances and richness. 4. Some metrics based on benthic invertebrate community data (e.g. % shredders, % shredder taxa) were significantly lower in eutrophied than in reference streams, whereas the IBMWP index that is specifically designed for the Iberian peninsula classified all 10 streams in the highest possible class as having ‘very good’ ecological conditions. 5. Leaf litter decomposition was sufficiently sensitive to respond to low levels of eutrophication and could be a useful functional measure to complement assessment programmes based on structural parameters.     

Leaf litter quality drives the feeding by invertebrate shredders in tropical streams

Amazon and Cerrado‐forested streams show natural fluctuations in leaf litter quantity along the time and space, suggesting a change on litter quality input. These natural fluctuations of leaf litter have repercussion on the organic matter cycling and consequently effects on leaf decomposition in forested streams. The effects of the quantity of leaf litter with contrasting traits on consumption by larvae of shredder insects from biomes with different organic matter dynamics have still been an understudied question. The Trichoptera Phylloicus spp. is a typical shredder in tropical headwater streams and keep an important role in leaf litter decomposition. Here, we assessed the consumption by shredder Phylloicus spp., from Amazonia and Cerrado biomes, on higher (Maprounea guianensis) and lower quality leaves (Inga laurina) in different proportions and quantities. Experiments were performed concomitantly in microcosms approaches, simulating Cerrado and Amazonian streams. Higher leaf consumption occurred in Cerrado microcosms. Litter quantity influenced negatively leaf consumption by shredders in Cerrado, in opposition to Amazonia, where consumption was not affected by leaf quantity. In both sites, we observed higher consumption by shredders in treatment with only M. guianensis and no difference between other treatments with mixture of leaves. In treatment with litter of I. laurina, we noted the use of substrate for case building (due to the higher leaf toughness), affecting the fragmentation process. Therefore, our results indicate that leaf litter quality drives the preference of consumption by Phylloicus larvae in Cerrado and Amazonia streams.     

Fungal importance extends beyond litter decomposition in experimental early‐successional streams

Fungi are important decomposers of leaf litter in streams and may have knock‐on effects on other microbes and carbon cycling. To elucidate such potential effects, we designed an experiment in outdoor experimental channels simulating sand‐bottom streams in an early‐successional state. We hypothesized that the presence of fungi would enhance overall microbial activity, accompanied by shifts in the microbial communities associated not only with leaf litter but also with sediments. Fifteen experimental channels received sterile sandy sediment, minimal amounts of leaf litter, and one of four inocula containing either (i) fungi and bacteria, or (ii) bacteria only, or (iii) no microorganisms, or (iv) killed microorganisms. Subsequently, we let water from an early‐successional catchment circulate through the channels for 5 weeks. Whole‐stream metabolism and microbial respiration associated with leaf litter were higher in the channels inoculated with fungi, reflecting higher fungal activity on leaves. Bacterial communities on leaves were also significantly affected. Similarly, increases in net primary production, sediment microbial respiration and chlorophyll a content on the sediment surface were greatest in the channels receiving a fungal inoculum. These results point to a major role of fungal communities in stream ecosystems beyond the well‐established direct involvement in leaf litter decomposition.     

Stronger effects of litter origin on the processing of conifer than broadleaf leaves: A test of home‐field advantage of stream litter breakdown

相似文献   

Leaf litter processing rates in a Kenyan highland stream,the Njoro River

Mass loss of leaf litter from five common riparian trees in Kenya was determined using mesh bags over a 70-day period. The study was carried out in a river which previous research has shown to be apparently devoid of invertebrate shredders. There was a five fold difference between the most rapid and slowest decay rates, but all were within the fast range identified from temperate zone trees, demonstrating high rates of microbial activity. We conclude, therefore, that the low biomass of invertebrate shredders in Kenyan highland streams cannot be explained by poor quality of the leaf litter that enters them.     

Spatial Variability of Plant Litter Decomposition in Stream Networks: from Litter Bags to Watersheds

The decomposition of plant litter plays a fundamental role in the cycling of carbon and nutrients and is driven by complex interactions of biological and physical controls, yet little is known about its variability and controls across spatial scales. Here we address the indirect effects of riparian canopy cover on litter decomposition and decomposers and their variability within a set of hierarchical scales (watershed, stream segments and reaches) controlling for confounding factors that could co-vary with canopy cover (for example, temperature and nutrients), in high-altitude subtropical streams. Total, microbial and invertebrate-driven decomposition rates were approximately 1.4–6.6 times higher in closed-canopy than in open-canopy watersheds. Riparian canopy cover accounted for 62–69% of total variability of decomposition rates and indirectly (via light availability and litter inputs) promoted fungal facilitation of shredders through leaf litter conditioning. In contrast to what we expected, much of the spatial variability in the decomposition occurred at smaller scale (4–20% of total variability among reaches versus <1% among watersheds) and coincided with the greatest variability in shredder abundance and fungal biomass (70 and 17% among reaches, respectively). We conclude that riparian canopy cover may be an important control of natural variability of litter decomposition at the watershed scale through its effects on fungal decomposers and shredder consumption. We also provide evidence of higher reach and minor watershed variability of litter decomposition in stream networks. Our results point to the importance of identifying the sources of natural variability of decomposition and how they interact within and among spatial scales.