Litter decomposition in a temperate and a tropical stream: the effects of species mixing,litter quality and shredders

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Elevated temperature may intensify the positive effects of nutrients on microbial decomposition in streams

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Effects of land use on the structure and function of leaf‐litter microbial communities in boreal streams

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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.     

Leaf litter identity alters the timing of lotic nutrient dynamics

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

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Do leaf-litter decomposers control biofilm primary production and benthic algal community structure in forest streams? Insights from an outdoor mesocosm experiment

• 1. Forested headwater streams are generally considered to be light-limited ecosystems where primary production is reduced, and the main source of energy and nutrients is composed of allochthonous detritus. We hypothesised that in these ecosystems, the development of primary producers might also be limited by (1) competition for nutrients with leaf-litter decomposers (e.g. bacteria and fungi), and (2) leaf-litter leachates or allelopathic compounds produced by aquatic fungi.
• 2. To test these hypotheses, a 48-day mesocosm experiment was performed in 12 artificial streams containing stream water inoculated with epilithic biofilm suspensions collected from a forested headwater stream. Three different treatments were applied: control without leaf litter (C), microbially conditioned leaf litter added at the beginning of the experiment and left to decompose throughout the experiment (L), or leaf litter renewed three times during the experiment (RL).
• 3. We predicted that (1) the presence of litter, through microbial nutrient immobilisation and allelopathy, would reduce primary production and that (2) this effect would be amplified by litter renewal. We also predicted that nutrient competition would mean that (3) leaf-litter decomposers will alter primary producer community composition and physiology. These predictions were tested by analysing biofilm development, physiology, stoichiometry, and benthic algal community structure. To distinguish between the effects of nutrient immobilisation and allelopathy, the biofilm responses to leaf-litter leachates collected after different microbial conditioning durations were also measured in a parallel laboratory experiment.
• 4. Contrary to our expectations, by day 28, primary producer growth was higher in the mesocosms containing leaf litter (L and RL) despite the rapid decrease in dissolved nutrients when leaf litter was present. After 48 days, the lowest phototrophic biofilm development was observed when leaf litter was renewed (RL), whereas phototrophic biofilm development was similar in the C and L treatments. Biofilm stoichiometry indicated that this effect was most probably related to greater nitrogen limitation in the RL treatment. The presence of leaf litter also affected primary producers' photophysiology, which could be attributed to changes in taxonomic composition and to physiological adjustments of primary producers.
• 5. Laboratory measurements showed that despite a strong inhibition of primary producer growth by unconditioned leaf-litter leachates, microbially conditioned leaf litter had either low or no effects on the development of primary producers.
• 6. These results reveal that leaf-litter decomposers can have both positive and negative effects on primary producers underlining the need to consider microbial interactions when investigating the functioning of forested headwater streams.

     

Calcium concentration in leaf litter affects the abundance and survival of crustaceans in streams draining warm–temperate forests

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Potential effects of leaf litter on water quality in urban watersheds

Leaf litter plays a critical role in regulating ecological functions in headwater forest streams, whereas the effects of leaves on water quality in urbanized streams are not fully understood. This study examined the potential importance of leaf litter for the release and transformations of organic carbon and nutrients in urban streams, and compared the effects with other types of natural organic substrates (periphyton and stream sediment). Nutrients and organic carbon were leached from senescent leaves of 6 tree species in the laboratory with deionized water, and maximal releases, leaching rate constants, composition and bioavailability of the leached dissolved organic carbon (DOC) were determined. Stream substrates (leaf debris, rocks with periphyton, and sediment) were seasonally collected from urban and forest reference streams of the NSF Baltimore Long-term Ecological Research Site and incubated with overlying stream water to estimate areal fluxes of DOC and nitrogen. Leaf litter leaching showed large ranges in maximal releases of DOC (7.0–131 mg g^?1), dissolved organic nitrogen (DON; 0.07–1.39 mg g^?1) and total dissolved phosphorus (TDP; 0.14–0.70 mg g^?1) among tree species. DOC leaching rate constants, carbon to nitrogen ratios, and DOC bioavailability were all correlated with organic matter quality indicated by fluorescence spectroscopy. Results from substrate incubation experiments showed far higher DOC and DON release and nitrate retention with leaf debris than with sediment, or rocks with periphyton. DOC release from leaf debris was positively correlated with stream nitrate retention at residential and urban sites, with the highest values observed during the fall and lowest during the summer. This study suggests the potential importance of leaf litter quantity and quality on fostering DOC and nutrient release and transformations in urban streams. It also suggests that species-specific impacts of leaves should be considered in riparian buffer and stream restoration strategies.     

Future increase in temperature may stimulate litter decomposition in temperate mountain streams: evidence from a stream manipulation experiment

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Continuous ammonium enrichment of a woodland stream: uptake kinetics, leaf decomposition, and nitrification

• 1 In order to test for nitrogen limitation and examine ammonium uptake by stream sediments, ammonium hydroxide was added continuously at concentrations averaging 100 μg1^-1 for 70 days to a second- order reach of Walker Branch, an undisturbed woodland stream in Tennessee.
• 2 Ammonium uptake during the first 4h of addition corresponded to adsorption kinetics rather than to first-order uptake or to Michaelis- Menten kinetics. However, the calculated adsorption partition coefficient was two to four orders of magnitude greater than values reported for physical adsorption of ammonium, suggesting that the uptake was largely biotic.
• 3 Mass balance indicated that the uptake of ammonium from the water could be accounted for by increased nitrogen content in benthic organic detritus. Nitrification, inferred from longitudinal gradients in NO₃, began soon after enrichment and increased dramatically near the end of the experiment.
• 4 Both ammonium and nitrate concentrations dropped quickly to near background levels when input ceased, indicating little desorption or nitrification of excess nitrogen stored in the reach.
• 5 There was no evidence of nitrogen limitation as measured by weight loss, oxygen consumption, phosphorus content, and macroinvertebrate density of red oak leaf packs, or by chlorophyll content and aufwuchs biomass on plexiglass slides. A continuous phosphorus enrichment 1 year earlier had demonstrated phosphorus limitation in Walker Branch.

     

Preconditioning of leaves by solar radiation and anoxia affects microbial colonisation and rate of leaf mass loss in an intermittent stream

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Spatial heterogeneity in water velocity drives leaf litter dynamics in streams

1. Stream hydro-morphology refers to the heterogeneous distribution of hydrologic conditions that occur above a complex benthic surface such as a streambed.
2. We hypothesised that hydro-morphological conditions will influence the retention, re-distribution, and microbial-driven decomposition of leaf litter inputs in stream ecosystems because each process varies with overlying water velocity.
3. We tested this hypothesis using: (1) the spatial distribution of water velocity within a stream reach as a surrogate of stream hydro-morphology; (2) leaf tracer (i.e. Ginkgo biloba L.) additions with serial recovery to examine the relationship between benthic retention and overlying velocity; and (3) measurements of leaf litter decomposition (i.e. Alnus glutinosa [L.] Gaertn.) under different water velocity conditions.
4. Results demonstrate that water velocity exerts a significant influence on the retention and re-distribution of leaf litter inputs within the reach. The observed range of water velocity (from c. 0 to 0.92 cm/s) also strongly influences the range of leaf litter decomposition rates (0.0076–0.0222/day).
5. Our findings illustrate that water velocity influences leaf litter dynamics in streams by controlling leaf litter transport, retention and re-distribution as well as how leaves decompose within recipient stream reaches. Ultimately, the results show that the efficiency of leaf litter inputs in supporting stream ecosystem function is dependent on the hydro-morphological characteristics of the receiving stream ecosystems.

     

Consumption of leaf detritus by a stream shredder: Influence of tree species and nutrient status

Four species of riparian vegetation (alder, birch, willow and poplar) were fertilized with nitrogen, phosphorus, nitrogen + phosphorus, or no fertilizer (control). The resulting leaf detritus (leached but not microbially colonized) was offered to a stream shredder, Hydatophylax variabilis (Trichoptera: Limnephilidae). In one experiment, shredder consumption of leaf detritus from different nutrient treatments (within tree species) was compared, and in a second experiment, consumption of different tree species (within nutrient treatments) was compared. Larvae preferred leaf detritus from nitrogen + phosphorus treatments (except in poplar where nitrogen treatment was preferred). Alder was preferred over other tree species for all treatments. Chemical and physical analyses of leaf litter showed differences between tree species and nutrient treatments in nutrient content, tannins and leaf toughness. Leaf consumption by larvae was positively associated with nitrogen content and negatively associated with condensed tannin content. Species composition and nutrient status of riparian vegetation may strongly influence detrital food webs in streams.     

Effects of increased water temperature on leaf litter quality and detritivore performance: a whole‐reach manipulative experiment

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Phagotrophic protists are a key component of microbial communities processing leaf litter under contrasting oxic conditions

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A global meta‐analysis of exotic versus native leaf decay in stream ecosystems

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Nutrient enhancement, food quality and community dynamics in a tropical rainforest stream

• 1 A mixed nutrient supplement was added to stream‐side experimental channels in a closed canopy rainforest in tropical Queensland over a 7‐month period, raising nitrogen and phosphorus concentrations well above ambient.
• 2 Nutrient supplements enhanced decomposition of some leaf species, but levels of chlorophyll a and fine particulate organic matter did not differ between treatment and control channels.
• 3 Shredding by Anisocentropus kirramus (Trichoptera) had a significant effect on leaf litter breakdown, but nutrient supplements did not enable A.kirramus to shred more material; however, biochemical analysis of A. kirramus showed that nutrient supplements enhanced the nutritional quality of the litter for this species.
• 4 Treatment channels contained 75% more invertebrates than control channels, but significant differences in abundance (all positive) were detected in only five of the 109 invertebrate species present. There was no change in species richness or evenness between treatment and control channels.
• 5 The limited community response indicates that (a) variables other than nutrients (e.g. light) restricted primary productivity, (b) low nutrient concentrations limit invertebrate production, (c) invertebrate community composition in this closed canopy, forest stream is resistant to nutrient enhancement, and (d) there was no direct relationship between productivity and diversity in this tropical stream.

     

Nutrient Uptake and Mineralization during Leaf Decay in Streams – a Model Simulation

We developed a stoichiometrically explicit computer model to examine how heterotrophic uptake of nutrients and microbial mineralization occurring during the decay of leaves in streams may be important in modifying nutrient concentrations. The simulations showed that microbial uptake can substantially decrease stream nutrient concentrations during the initial phases of decomposition, while mineralization may produce increases in concentrations during later stages of decomposition. The simulations also showed that initial nutrient content of the leaves can affect the stream nutrient concentration dynamics and determine whether nitrogen or phosphorus is the limiting nutrient. Finally, the simulations suggest a net retention (uptake > mineralization) of nutrients in headwater streams, which is balanced by export of particulate organic nutrients to downstream reaches. Published studies support the conclusion that uptake can substantially change stream nutrient concentrations. On the other hand, there is little published evidence that mineralization also affects nutrient concentrations. Also, there is little information on direct microbial utilization of nutrients contained in the decaying leaves themselves. Our results suggest several directions for research that will improve our understanding of the complex relationship between leaf decay and nutrient dynamics in streams. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of a forest disturbance on shredder production in southern Appalachian headwater streams

• 1 The effects of a forest disturbance were investigated by comparing production of leaf-shredding aquatic insects in three streams draining a mature hardwood forest and three streams draining an 11-year-old, cable-logged clearcut.
• 2 Reference streams contained significantly greater mean annual standing crop of leaf material and significantly more slow-processing leaf material than disturbed streams. Disturbed streams had a significantly higher mean annual standing crop of fast-processing leaf material than the reference streams.
• 3 Leaf-shredding cranefly (Tipula abdominalis), caddisfly (Pycnopsyche gentilis), and stonefly (Tallaperla maria) larvae comprised over 95% of shredder biomass in all streams. Total shredder production was significantly greater (P<0.05) in disturbed versus reference streams, but individual production rates were not significantly different between stream types.
• 4 Pycnopsyche gentilis larvae were present at higher densities and achieved significantly greater annual biomass in disturbed versus references streams, Biomass of P. gentilis was significantly correlated with the standing crop of fast-processing, early successional leaf material in samples, whereas biomass of other shredders was correlated significantly with medium or slow-processing leaf species characteristic of later stages of forest succession.