The Role of Early Fungal Colonizers in Leaf‐Litter Decomposition in Portuguese Streams Impacted by Agricultural Runoff

We conducted a transplant experiment between two streams in NW Portugal impacted by agricultural runoff, mainly differing in phosphate concentration, to determine whether fungi on decomposing leaves would adapt to the new environment or would be replaced by fungi of the recipient stream. The most nutrient enriched stream had lower fungal diversity but faster leaf decomposition. Leaf transplantation did not alter fungal activity or species dominance. Multidimensional scaling ordination of fungal communities, from DNA fingerprint or conidial production, revealed that transplanted communities resembled more those of the original stream than the recipient stream. Results suggest that early fungal colonizers will determine the development and activity of fungal communities on decomposing leaves in streams impacted by agricultural practices. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Exotic riparian vegetation lowers fungal diversity but not leaf decomposition in Portuguese streams

1. We characterised the fungal communities of eight streams in Portugal, four bordered by native deciduous forest and four bordered by pure stands of Eucalyptus globulus .
2. Aquatic hyphomycete species richness and evenness, but not numbers of water-borne conidia, of aquatic hyphomycetes were significantly lower in eucalypt bordered streams.
3. Multivariate analyses subdivided the fungal communities into two distinct groups corresponding to riparian vegetation.
4. Despite these differences in the dominant decomposer community, decay rates of eucalypt leaves (accounting for ≥98% of naturally occurring leaves in eucalypt bordered streams, absent in native forest) and chestnut leaves (occurring naturally in native forests) did not differ between the two groups of streams.     

Fungal and Bacterial Colonisation of Salix pedicellataLeaves Decaying in Permanent and Intermittent Streams in Eastern Morocco

Leaf degradation was investigated at four sites in the Zegzel/Cherraa river system (Oueds). Two sites (Upper Zegzel and Lower Zegzel) carry water throughout the year and two (Upstream and Downstream Cherraa) are dry for 5–7 months each year. The dynamics of leaf weight loss and microorganisms associated with Salix pedicellata leaves decaying at the four sites were compared for the first time during the same period over this permanent and intermittent system. Overall decay rates of leaves were significantly higher in the permanent Zegzel stream sections (k = 0.0094 d^–1 upstream and 0.0056 downstream) than in the intermittently flowing Cherraa sites (k = 0.0046 upstream, and 0.0036 downstream). In the latter, decay was much slower during dry than during wet periods (Upstream Cherraa: k = 0.0028 and 0.0446, respectively; Downstream Cherraa, k = 0.0008 and 0.0357, respectively). Similar gradients from permanent to intermittent sites were observed in numbers of bacteria per unit area or per weight of decaying leaves (direct counts by epifluorescence microscopy), in numbers of fungal species and in sporulation rates, from leaves recovered at the four sites. Ten hyphomycete species were new for Morocco.     

Diversity and activity of aquatic fungi under low oxygen conditions

1. The objective was to test whether a decrease in oxygen concentration in streams affects the diversity and activity of aquatic hyphomycetes and consequently leaf litter decomposition. 2. Senescent leaves of Alnus glutinosa were immersed for 7 days in a reference stream, for fungal colonization, and then incubated for 18 days in microcosms at five oxygen concentrations (4%, 26%, 54%, 76% and 94% saturation). Leaf decomposition (as loss of leaf toughness), fungal diversity, reproduction (as spore production) and biomass (ergosterol content) were determined. 3. Leaf toughness decreased by 70% in leaves exposed to the highest O₂ concentration, whereas the decrease was substantially less (from 25% to 45%) in treatments with lower O₂. Fungal biomass decreased from 99 to 12 mg fungi g⁻¹ ash‐free dry mass on exposure to 94% and 4% O₂ respectively. Sporulation was strongly inhibited by reduction of dissolved O₂ in water (3.1 × 10⁴ versus 1.3 × 10³ spores per microcosms) for 94% and 4% saturation respectively. 4. A total of 20 species of aquatic hyphomycetes were identified on leaves exposed to 94% O₂, whereas only 12 species were found in the treatment with 4% O₂ saturation. Multidimensional scaling revealed that fungal assemblages exposed to 4% O₂ were separated from all the others. Articulospora tetracladia, Cylindrocarpon sp. and Flagellospora curta were the dominant species in microcosms with 4% O₂, while Flagellospora curvula and Anguillospora filiformis were dominant at higher O₂ concentrations. 5. Overall results suggest that the functional role of aquatic hyphomycetes as decomposers of leaf litter is limited when the concentration of dissolved oxygen in streams is low.     

Colonization,Species Composition,and Conidial Production of Aquatic Hyphomycetes on Chirpine Needle Litter in a Freshwater Kumaun Himalayan Stream

Aquatic hyphomycetes colonizing the submerged chirpine (pinus roxburghii SARG .) needle litter in a high altitude, Kumaun Himalayan stream were studied. 15 species belonging to different genera of aquatic Hyphomycetes have been recognized as the colonizers of chirpine needle litter. Clavariopsis aquatica, Heliscus lugdunensis, Lunulospora cymbiformis, Triscelophorus acuminatus and T. monosporus were found with a high frequency of occurrence. The conidial production was highest in Flagellospora penicillioides, however, Campylospora chaetocladia, L. cymbiformis and T. acuminatus had less number of conidia per unit area of pine needles. The chirpine needle litter decomposition in the freshwater habitat is also discussed.     

Leaf‐litter colonisation and breakdown in relation to stream typology: insights from Mediterranean low‐order streams

1. Scant information is available on leaf breakdown in streams of arid and semiarid regions, including the Mediterranean, where environmental heterogeneity can be high and the relationship between stream characteristics and leaf breakdown is poorly known. We tested the hypotheses that differences in leaf breakdown metrics would be substantially higher between mountain and lowland Mediterranean streams than among streams within each subregion and that variability among streams would be substantially higher in the lowlands, because permanent reaches in the semiarid lowland streams are rare and isolated. 2. We compared leaf breakdown and associated dynamics of nutrients, fungi and invertebrates in low‐order Mediterranean streams draining sub‐humid forests in the Sierra Nevada Mountains and nearby semiarid lowlands of south‐eastern Spain. Streams differed between the two subregions mainly in water ion content, temperature and riparian tree cover. We detected higher environmental heterogeneity among streams within the lowlands compared to the Sierra Nevada mountain range. In the lowlands, breakdown coefficients (k) of alder leaves spanned almost the entire range reported for this species from temperate streams, overlapping with less variable breakdown coefficients in the Sierra Nevada. 3. The high variability of k values among the lowland sites appeared to be caused primarily by variability in the composition and abundance of a few leaf‐consuming invertebrate taxa, particularly the snail Melanopsis praemorsa. Fungal and nutrient dynamics were less variable among sites within each subregion. 4. These results indicate that the critical condition for stream functional assessment of well‐constrained breakdown rates, or related metrics, could be met at reference sites within homogenous bio‐geo‐climatic regions such as the Sierra Nevada. By contrast, in heterogeneous areas such as the semiarid lowland streams, natural variability of breakdown rates can greatly exceed the magnitude of effects expected in response to anthropogenic disturbances.     

Effects of Zn,Fe and Mn on Leaf Litter Breakdown by Aquatic Fungi: a Microcosm Study

We investigated the effects of heavy metals on leaf litter decomposition in streams. Leaves were immersed (10 days) at a reference (R) and a metal‐impacted (I) site and exposed in microcosms with increased Zn, Mn or Fe content, and to stream water from site R or I. Fungal biomass was higher in microcosms with leaves colonized at I and water from R. Fungal sporulation was higher in microcosms with leaves and water from R. Concentrations of 4.9, 9.6 and 5 ppm of Zn, Mn and Fe decrease fungal sporulation. The number of fungal species (spore counts and DGGE fingerprints) was lower in leaves colonized at site I. Cluster analyses of DGGE showed that Fe was the metal that most altered the structure of fungal community. Our results suggest that metal pollution affect leaf‐associated fungi depending on metal identity and concentration, and effects appear to be less pronounced in metal‐adapted communities. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two microcrustaceans affect microbial and macroinvertebrate‐driven litter breakdown

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Response of Aquatic Leaf Associated Microbial Communities to Elevated Leachate DOC: A Microcosm Study

In Central Europe climate change will increase summer droughts, which cause both, premature leaf fall and fragmentation of small streams during summer and early autumn. As a consequence dissolved organic carbon (DOC) leached from leaves will be dispersed into pools with long water residence time. A microcosm experiment was performed to test the effect of high concentrations of leachate DOC and the relative importance of labile and refractory leachate compounds on leaf associated microbial parameters. In microcosms leaf discs colonized in a stream were exposed to high concentrations of either leaf leachate, glucose or tannic acid. Leaf associated respiration, fungal sporulation, leaf mass loss and fungal biomass (ergosterol) were measured during a 3 weeks experimental period and compared to control without DOC amendment. The results imply that depending on source and composition elevated leachate DOC may have variable effects on microbial mediated litter decomposition. Our findings suggest reduced microbial decomposition rates in pools of fragmented streams receiving premature leaf fall. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diversity and functions of leaf-decaying fungi in human-altered streams

1. Stream conditions have been evaluated using leaf breakdown, and aquatic hyphomycetes are a diverse group of fungal decomposers which contribute to this process. 2. In field surveys of three pairs of impact‐control stream sites we assessed the effect of eutrophication, mine pollution and modification of riparian vegetation on alder leaf breakdown rate in coarse and fine mesh bags and on mycelial biomass, spore production and species diversity of leaf‐colonizing fungi. 3. In addition, we gathered published information on the response of leaf‐colonizing fungi to these three types of perturbations. We conducted a meta‐analysis of 23 published papers to look for consistent patterns across studies and to determine the relevance of four fungal‐based metrics (microbial breakdown rate, maximum spore production, maximum mycelial biomass and total species richness) to detect stream impairment. 4. In our field surveys, leaf breakdown rates in coarse mesh bags were lower at impact than at paired control sites regardless of perturbation type. A similar trend was observed for leaf breakdown rates in fine mesh bags. Mycelial biomass and spore production were higher in the eutrophied stream than in the control stream. Spore production was depressed in the mine polluted stream, while it was slightly enhanced in the stream affected by forestry. Fungal diversity tended to be lower at impact than at paired control sites, though the mean and cumulative species richness values were often inconsistent. 5. Results of the meta‐analysis confirmed that mine pollution reduces fungal diversity and performance. Eutrophication was not found to affect microbial breakdown rate, maximum spore production and maximum mycelial biomass in a predictable manner because both positive and negative effects were reported in the literature. However, fungal species richness was consistently reduced in eutrophied streams. Modification of riparian vegetation had at most a small stimulating effect on maximum spore production. Among the four fungal‐based metrics included in the meta‐analysis, maximum spore production emerged as the most sensitive indicator of human impact on streams. 6. Taken together, our findings indicate that human activities can affect the diversity and functions of aquatic hyphomycetes in streams. We also show that leaf breakdown rate and simple fungal‐based metrics, such as spore production, are relevant to assess stream condition.     

<Emphasis Type="Italic">Encentrum (Parencentrum) walterkostei</Emphasis> n. sp., a new dicranophorid rotifer (Rotatoria: Monogononta) from the high alpine zone of the Central Alps (Austria)

Leaf litter processing rates and fungal biomass on leaf detritus were compared in four streams of different water chemistry. The streams drained catchments underlain by different bedrock types and varied in mean pH from 4.3 to 7.5 and in mean alkalinity from 0.0 to 35.8 mg CaCO₃ l^–1. Processing rates were fastest in WS3 and WS4, which had a pH of 6.0; slowest in SFR, which had a pH of 4.3; and intermediate in HSR which had a pH of 7.5. Fungal biomass as measured by the fungal sterol, ergosterol, was similar in WS3, WS4, and HSR but was much lower in SFR. These results suggest that reduced processing rates in SFR were associated in part with reduced fungal biomass on the leaves, whereas reduced processing rates in HSR were not related to differences in fungal biomass on the leaves.The Unit is jointly sponsored by the U.S. Fish and Wildlife Service, the West Virginia Division of Natural Resources, West Virginia University, and the Wildlife Management Institute.     

Leaf litter decomposition and microbial activity in nutrient-enriched and unaltered reaches of a headwater stream

SUMMARY 1. Decomposition of red maple ( Acer rubrum ) and rhododendron ( Rhododendron maximum ) leaves and activity of associated microorganisms were compared in two reaches of a headwater stream in Coweeta Hydrologic Laboratory, NC, U.S.A. The downstream reach was enriched with ammonium, nitrate, and phosphate whereas the upstream reach was not altered.
2. Decomposition rate, microbial respiration, fungal and bacterial biomass, and the sporulation rate of aquatic hyphomycetes associated with decomposing leaf material were significantly higher for both leaf types in the nutrient-enriched reach. Species richness and community structure of aquatic hyphomycetes also exhibited considerable changes with an increase in the number of fungal codominants in the nutrient-enriched reach.
3. Fungal biomass was one to two orders of magnitude greater than bacterial biomass in both reaches. Changes in microbial respiration rate corresponded to those in fungal biomass and sporulation, suggesting a primary role of fungi in leaf decomposition.
4. Nutrient enrichment increased microbial activity, the proportion of leaf carbon channelled through the microbial compartment and the decomposition rate of leaf litter.     

Response of Aquatic Hyphomycete Communities to Changes in Heavy Metal Exposure

Decomposition of Alnus glutinosa (alder) leaves was studied in a severely (site H4) and a moderately (site H8) heavy metal polluted stream in the former copper shale mining district of Mansfeld, Central Germany. Leaves at H8 had reduced fungal diversity and spore production but a high exponential decay rate (k = 0.065). No further mass loss of leaves occurred at H4 after 4–6 weeks, and fungal diversity and spore production were lower than in H8. Decay and sporulation rates gradually increased to values of H8 control leaves in leaves preincubated in H4 and then transferred to H8. These increases correlated with the invasion of transplanted leaves by Tetracladium marchalianum and Tricladium angulatum. In the reverse transplant experiment (H8 to H4), mass loss appeared to stop immediately. Sporulation rates also declined, but remained consistently above levels in H4 control leaves. Leaves precolonized in the laboratory by one of three aquatic hyphomycete species exhibited increased decay rates in both streams. Sporulation rates on these leaves were greater than those of control leaves in H4, but smaller than those of control leaves in H8. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Similar breakdown rates and benthic macroinvertebrate assemblages on native and Eucalyptus globulus leaf litter in Californian streams

1. Eucalyptus globulus, a tree species planted worldwide in many riparian zones, has been reported to affect benthic macroinvertebrates negatively. Although there is no consensus about the effects of Eucalyptus on aquatic macrobenthos, its removal is sometimes proposed as a means of ecological restoration. 2. We combined the sampling of macroinvertebrates with measurement of the colonisation of leaf packs in mesh bags, to examine the effects of riparian Eucalyptus and its litter on benthic macroinvertebrates in three small streams in California, U.S.A. Each stream included one reach bordered by Eucalyptus (E‐site) and a second bordered by native vegetation (N‐site). 3. The macrobenthos was sampled and two sets of litter bags were deployed at each site: one set with Eucalyptus litter (Euc‐bags) and one with mixed native tree litter (Nat‐bags) containing Quercus, Umbellularia, Acer and Alnus. Bags were exposed for 28, 56 and 90 days and this experiment was repeated in the autumn, winter and spring to account for effects of changing stream flow and insect phenology. 4. Litter input (average dry mass: 950 g m^?2 year^?1 in E‐sites versus 669 g m^?2 year^?1 in N‐sites) was similar, although in‐stream litter composition differed between E‐ and N‐sites. Litter broke down at similar rates in Euc‐bags and Nat‐bags (0.0193 day^?1 versus 0.0134 day^?1), perhaps reflecting the refractory nature of some of the leaves of the native trees (Quercus agrifolia). 5. Summary metrics for macroinvertebrates (taxon richness, Shannon diversity, pollution tolerance index) did not differ significantly between the E and N sites, or between Euc‐bags and Nat‐bags. No effect of exposure time or site was detected by ordination of the taxa sampled. However, distinct seasonal ordination clusters were observed in winter, spring and autumn, and one of the three streams formed a separate cluster. 6. The presence of Eucalyptus was less important in explaining the taxonomic composition of the macrobenthos than either ‘season’ or ‘stream’. Similarly, these same two factors (but not litter species) also helped explain the variation in leaf breakdown. We conclude that patches of riparian Eucalyptus and its litter have little effect on stream macrobenthos in this region.     

Fungi are involved in the effects of litter mixtures on consumption by shredders

1. Decomposition of litter mixtures in both terrestrial and aquatic ecosystems often shows non‐additive diversity effects on decomposition rate, generally interpreted in streams as a result of the feeding activity of macroinvertebrates. The extent to which fungal assemblages on mixed litter may influence consumption by macroinvertebrates remains unknown. 2. We assessed the effect of litter mixing on all possible three‐species combinations drawn from four tree species (Alnus glutinosa, Betula pendula, Juglans regia and Quercus robur) on both fungal assemblages and the rate of litter consumption by a common shredder, Gammarus fossarum. After a 9‐week inoculation in a stream, batches of leaf discs were taken from all leaf species within litter mixture combinations. Ergosterol, an indicator of fungal biomass, and the composition of fungal assemblages, assessed from the conidia released, were determined, and incubated litter offered to G. fossarum in a laboratory‐feeding experiment. 3. Mixing leaf litter species enhanced both the Simpson’s index of the fungal assemblage and the consumption of litter by G. fossarum, but had no clear effect on mycelial biomass. Specifically, consumption rates of J. regia were consistently higher for mixed‐species litter packs than for single‐species litter. In contrast, the consumption rates of B. pendula were not affected by litter mixing, because of the occurrence of both positive and negative litter‐mixing effects in different litter species combinations that counteracted each other. 4. In some litter combinations, the greater development of some fungal species (e.g. Clavariopsis aquatica) as shown by higher sporulation rates coincided with increased leaf consumption, which may have resulted from feeding preferences by G. fossarum for these fungi. 5. Where litter mixture effects on decomposition rate are mediated via shredder feeding, this could be due to indirect effects of the fungal assemblage.     

Experimental shading alters leaf litter breakdown in streams of contrasting riparian canopy cover

1. Headwater stream ecosystems are primarily heterotrophic, with allochthonous organic matter being the dominant energy. However, sunlight indirectly influences ecosystem structure and functioning, affecting microbial and invertebrate consumers and, ultimately, leaf litter breakdown. We tested the effects of artificial shading on litter breakdown rates in an open‐canopy stream (high ambient light) and a closed‐canopy stream (low ambient light). We further examined the responses of invertebrate shredders and aquatic hyphomycetes to shading to disentangle the underlying effects of light availability on litter breakdown. 2. Litter breakdown was substantially slower for both fast‐decomposing (alder, Alnus glutinosa) and slow‐decomposing (beech, Fagus sylvatica) leaf litters in artificially shaded stream reaches relative to control (no artificial shading) reaches, regardless of stream type (open or closed canopy). 3. Shredder densities were higher on A. glutinosa than on F. sylvatica litter, and shading had a greater effect on reducing shredder densities associated with A. glutinosa than those associated with F. sylvatica litter in both stream types. Fungal biomass was also negatively affected by shading. Results suggest that the effects of light availability on litter breakdown rates are mediated by resource quality and consumer density. 4. Results from feeding experiments, where A. glutinosa litter incubated under ambient light or artificial shade was offered to the shredder Gammarus fossarum, suggest that experimental shading and riparian canopy openness influenced litter palatability interactively. Rates of litter consumption by G. fossarum were decreased by experimental shading in the open‐canopy stream only. 5. The results suggest that even small variations in light availability in streams can mediate substantial within‐stream heterogeneity in litter breakdown. This study provides further evidence that changes in riparian vegetation, and thus light availability, influence organic matter processing in heterotrophic stream ecosystems through multiple trophic levels.     

Inter‐ and Intra‐Regional Variability of Leaf Litter Breakdown in Reference Headwater Streams of Northern Spain: Atlantic versus Mediterranean Streams

The main goal of this study was to examine the natural variability of alder (Alnus glutinosa (L.) Gaertn.) leaf processing and explore its potentiality as a functional indicator to assess the ecological status of Spanish headwater streams. Breakdown of leaf litter was studied during autumn‐winter in reference headwater streams of two regions of northern Spain: the Basque Country (on the Atlantic) and Catalonia (on the Mediterranean). Spring experiments were also carried out in the Atlantic region in order to study seasonal changes. Leaf mass loss rates were slightly higher in Catalonian streams. Temperature was not the main factor for explaining differences between the two regions. In the Atlantic one, however, water temperature determined the spatial and seasonal variability of leaf litter processing. Because of the high natural variability in breakdown rates, our results highlight the difficulty in detecting moderate impairments on stream functioning through the analysis of leaf litter decomposition if this process is not accompanied by the study of other factors affecting it. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnitude and variability of process rates in fungal diversity-litter decomposition relationships

There is compelling evidence that losses in plant diversity can alter ecosystem functioning, particularly by reducing primary production. However, impacts of biodiversity loss on decomposition, the complementary process in the carbon cycle, are highly uncertain. By manipulating fungal decomposer diversity in stream microcosm experiments we found that rates of litter decomposition and associated fungal spore production are unaffected by changes in decomposer diversity under benign and harsher environmental conditions. This result calls for caution when generalizing outcomes of biodiversity experiments across systems. In contrast to their magnitude, the variability of process rates among communities increased when species numbers were reduced. This was most likely caused by a portfolio effect (i.e. statistical averaging), with the uneven species distribution typical of natural communities tending to weaken that effect. Curbing species extinctions to maintain ecosystem functioning thus can be important even in situations where process rates are unaffected.     

Palatability of Leaves Conditioned in Streams Affected by Mine Drainage: A Feeding Experiment with Gammarus Pulex (L.)

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.     

Ecosystem Functioning and Community Structure as Indicators for Assessing Environmental Impacts: Leaf Processing and Macroinvertebrates in Atlantic Forest Streams

Programs for evaluating environmental impacts are often carried out with chemical analyses that are inadequate for evaluating ecological aspects of rivers. In recent times there has been a strong movement towards biomonitoring using indices based on the structure of communities of organisms. Less attention has been given to functional parameters (ecosystem level processes such as decomposition, primary production, nutrient cycling, etc.), despite recent work that demonstrates their applicability. The breakdown of leaf material in streams provides a system in which an aspect of ecosystem functioning (decomposition) can be measured along with the community structure of the fauna associated with the leaves. We measured rate of leaf processing and the associated macroinvertebrates in 9 streams in 3 categories of environmental impact: Reference (pristine forest), Intermediate (pasture land‐use with intact riparian vegetation) and Impacted (pasture land‐use with degraded banks and stream‐bed). Leaf processing was fastest in the least impacted, “Reference”, streams. The abundance and taxon diversity of the associated fauna were not different among categories of impact. Thus this aspect of ecosystem functioning was more sensitive to perturbation than was the structure of the community in this case. We suggest that leaf processing can be a cheap and indicative parameter for biological assessment and monitoring. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)