Fine-mesh Bags Research Articles

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

Summary Seasonal flow intermittence in streams often coincides with early leaf abscission of riparian vegetation due to water stress. When accumulated on dry stream beds or in remaining pools, leaves are exposed to solar radiation or fermentation processes, respectively. However, very little information exists on how these preconditioning processes could affect leaf decomposition when stream flow has recovered. We simulated natural preconditioning of leaves by irradiation with UV‐VIS fluorescent lamps and incubation under anoxic conditions. Mass loss rates of preconditioned leaf litter from deciduous trees (Alnus glutinosa, Fraxinus excelsior, Populus tremula and Quercus petreae) were quantified in a temporary stream during base flow conditions. Coarse and fine mesh litter bags were used to study the effect of benthic macroinvertebrates and microorganisms on leaf mass loss. Preconditioning reduced the concentration of macronutrients such as P, K and Mg and increased the relative cellulose content of the leaves. Preconditioning changed the fungal community structure (analysed by DGGE) depending on leaf species and sampling date. Preconditioning in anoxic conditions also suppressed fungal decomposer biomass (measured as ergosterol) by 42% resulting in 33% lower mass loss rates in fine mesh bags. In contrast, mass loss rates were not affected by preconditioning when macroinvertebrate decomposers had access to the leaf litter. In streams exhibiting seasonal flow intermittence, preconditioning will influence organic carbon dynamics towards lower rates of microbially mediated turnover and towards poorer quality of downstream‐transported material.

Effects of retention site on breakdown of organic matter in a mountain stream

Summary Organic matter inputs to streams can be retained in contrasting sites, from small leaf packs on top of cobbles to thick organic deposits trapped by wood jams. Differences in environmental conditions and in stability among sites can affect the biological communities, the quality of organic matter and its use by consumers. We measured the accumulation and composition of coarse organic matter and the breakdown of alder leaves on the surface of the streambed, inside gravel bars and in thick litter deposits trapped by log jams. We also monitored fungal sporulation rates and macroinvertebrate diversity and density in these retention sites. The amount of organic matter in these locations differed significantly among sites and ranged from 118 g m−2 in gravel to 11 562 g m−2 in jams. The biomass of shredders also differed significantly among retention sites, being highest in jams (1440 mg m−2) and lowest in gravel (86 mg m−2). Breakdown of alder leaves in fine-mesh bags did not differ among retention sites. In coarse-mesh bags, leaf breakdown rate was significantly lower in gravel bars than in surface sites or jams. Despite large differences in amount and quality of organic matter between surface sites and jams, alder leaves broke down at the same rate. Density of invertebrates in bags and fungal sporulation rates tended to be highest in surface sites and lowest in gravel bars. The contribution of microbes to breakdown was 67% in gravel, 52% in surface and only 28% in jams. Overall, the differences among sites in leaf breakdown were correlated with invertebrate density. Our results showed large differences in the amount and quality of organic matter accumulated, in biological communities and in the use of organic matter among retention sites of a single stream reach. This spatial variability should be taken into account in studies of ecosystem functioning related to organic matter.

Hybrid seine for full fish community collections

Seines are simple and effective fish collection gears, but the net mesh size influences how well the catch represents the fish communities. We designed and tested a hybrid seine with a dual-mesh bag (1/4″ and 1/8″) and compared the fish assemblage collected by each mesh. The fine-mesh net retained three times as many fish and collected more species (as many as eight), including representatives of several rare species, than did the coarser mesh. The dual-mesh bag permitted us to compare both sizes and species retained by each layer and to develop species-specific abundance correction factors, which allowed comparison of catches with the coarse-mesh seine used for earlier collections. The results indicate that a hybrid seine with coarse-mesh wings and a fine-mesh bag would enhance future studies of fish communities, especially when small-bodied fishes or early life stages are the research focus.

Microclimate within litter bags of different mesh size: Implications for the ‘arthropod effect’ on litter decomposition

Litter bags are often used to determine the impact of soil animals on litter decomposition rates through the use of varied mesh sizes that exclude soil animals on the basis of body size. However, concerns have been raised regarding confounding factors that can co-vary with mesh size (e.g., microclimatic differences and leaching losses) and that might affect mass loss from litter bags independent of soil animal effects.To explore if these factors may interfere with the interpretation of studies that have used litter bags of varying mesh sizes, we quantified the effects of mesh size (50 μm, 0.4 mm and 1.0 mm aperture) on litter mass loss in the absence of soil animals. Further we measured potential microclimate (temperatures, light transmission and moisture) differences that may co-vary with mesh size. In addition, we quantified litter mass losses through litter fragmentation and leaching.Litter mass loss in the absence of soil animals did not differ between mesh sizes, suggesting that the use of the different meshes does not generate confounding factors that directly influence mass loss rates in the absence of animals.Microclimatic differences between litter bags of different mesh sizes were minimal and revealed slightly warmer (0.7 °C) temperatures and easier water entry for the fine than the coarse mesh bags, but no effect on litter evaporation rates. Light transmittance was largest for the fine mesh (79%) compared to the medium (60%) and coarse mesh (73%). The impact of UV on litter mass loss was 1% higher in the coarse mesh. Loss of fragmented litter was considerably higher from the coarse mesh (93%) than the fine mesh bags (17%).In conclusion, different mesh sizes can be used to reliably quantify the role of soil animals in litter mass loss from litter bags. The greater litter mass loss from litter bags with coarser mesh sizes is however caused by the soil animals both promoting litter decomposition and greater litter fragmentation (with the undecomposed fragments then being lost from the bags). Additional approaches are needed to determine the relative importance of these two effects of soil animals on litter mass loss from litter bags.

Effects of litter diversity on decomposition and biological colonization of submerged litter in temperate and tropical streams

AbstractDetrital food webs of woodland streams depend on terrestrial litter input and, thus, are susceptible to changes in riparian cover. We assessed effects of litter species richness and quality on decomposition and associated biological communities in temperate deciduous forest and tropical rainforest streams. Three native litter species were incubated in each stream in all combinations (7 litter treatments, 3 richness levels) in coarse- (invertebrate access) and fine-mesh bags (no invertebrate access) and were sampled 5 times over 74 (temperate stream) or 94 d (tropical stream). Decomposition, and fungal biomass, sporulation, and species richness were measured for each treatment. Alnus glutinosa litter was incubated in both streams to assess effects of environmental and biological differences between streams on litter decomposition. Biological colonization (number of fungal species, fungal biomass) and activity (conidial production) were lower in the tropical than the temperate stream, despite its hi...

Field Release in Florida ofDiaphorencyrtus aligarhensis(Hymenoptera: Encyrtidae), an Endoparasitoid ofDiaphorina citri(Homoptera: Psyllidae), from Mainland China

The Asian citrus psyllid, Diaphorina citri Kuy awama (Hemiptera: Psyllidae), is a serious pest of citrus that vectors the phloem-limited non culturable bacterium Candidatus Liberibacter asiaticus Jagoueix, Bove and Gamier (Jagoueix et al. 1994). The resulting disease is commonly referred to as citrus greening or huanglongbing (HLB) (Bove 2006). The psyllid was first reported from Florida in 1998 and citrus greening in 2005 (Halbert 1998, 2005). Two species specific parasitic wasps, Tamarixia radiata (Waterston) (Hymenoptera: Eulophidae) and Diaphorencyrtus aligarhensis (Shafee, Alam and Argarwal) (Hymenoptera: Encyrtidae), were imported into Florida from Taiwan and Vietnam and released beginning in 1999 as biological con trol agents for D. citri (Hoy & Nguyen 1998). Tam arixia radiata has established in Florida (Qureshi et al. 2009), while D. aligarhensis has not. An additional population of D. aligarhensis was collected in Guangdong province, China and sent to Gainesville, Florida in 2006. A colony was initiated in the Maximum Security Room of the Florida Biological Control Laboratory, Florida De partment of Agriculture and Consumer Services, Division of Plant Industry. Parasitoids were held in quarantine through 20 generations before re lease permitting was complete and mass rearing began. Sixty-seven releases were made at 10 sites in 6 Florida counties (Alachua, Collier, DeSoto, Hendry, Orange and Polk) between 8/2007 and 10/2009, deploying a total of approximately 11,100 wasps (Table 1). Sites included commer cial and private citrus groves as well as Murraya paniculata hedges. Wasps were either tapped out of holding tubes directly onto emerging plant shoots infested with immature psyllids or were released into cages consisting of fine mesh bags tied around new shoots infested with nymphs. Shoots were carefully inspected and predators removed prior to tying cages closed. Mesh size al lowed parasitoid wasps to move freely in or out of the cage while preventing most predaceous in sects from entering. Released wasps successfully parasitized nymphs in the field and second generation wasps emerged. Evidence of this includes laboratory emergence of adult wasps from field clippings, D. aligarhensis parasitized mummies found on shoots covered with mesh cages, and several adult wasps sighted foraging on D. citri infested M. paniculata plants (Table 1). All citrus groves used for wasp release, including certified organic groves, used pesticide sprays dur ing the release period. The use of insecticides target ing D. citri likely reduced both psyllid and natural enemy populations (Qureshi & Stansly 2007). Mur raya paniculata release sites were not subjected to any form of insect control, although occasional plant trimming by the property owners removed much of the new soft growth that I). citri uses for feeding and oviposition, consequently leaving few imma ture hosts for D. aligarhensis reproduction. Adult psyllids were present at all locations throughout the release period although their incidence appeared to decline over the >2 year release period, likely from increased insecticide use to reduce the spread of citrus greening dis ease. During many trips to various locations for parasitoid release there was little to no host plant flush or immature psyllids present, particularly during late summer and fall. Tamarixia radiata wasps were present at all release sites utilized in this study. Through the combined effect of host feeding and parasitism, a single female T. radiata can kill nearly twice as many psyllids in her lifetime as can a female D. aligarhensis (500 vs. 280, respectively) (Chien 1995). Additionally, T. radiata has a shorter gen eration time and a higher reproductive rate (Skel ley & Hoy 2004). Furthermore, any hosts parasit ized by T. radiata before or within 5 d following parasitism by D. aligarhensis will result in T. radiata offspring (E. R. unpublished data). Thus, competition with T. radiata could present another impediment to establishment of D. aligarhensis. Coccinellid predators were seen at all release sites. Generalist predators can induce high lev els of psyllid mortality (Qureshi & Stansly 2009). Curinus coeruleus Mulsant, Cycloneda sanguinea (L.) and Harmonia axyridis Pallas readily con sumed D. citri nymphs parasitized by D. aligar hensis (E. R. unpublished data) causing wasp mortality.

Groundwater Biofilm Dynamics Grown In Situ Along a Nutrient Gradient

This paper describes the in situ response of groundwater biofilms in an alluvial gravel aquifer system on the Canterbury Plains, New Zealand. Biofilms were developed on aquifer gravel, encased in fine mesh bags and suspended in protective columns in monitoring wells for at least 20 weeks. Four sites were selected in the same groundwater system where previous analyses indicated a gradient of increasing nitrate down the hydraulic gradient from Sites 1 to 4. Measurements during the current study classified the groundwater as oligotrophic. Biofilm responses to the nutrient gradients were assessed using bioassays, with biomass determined using protein and cellular and nucleic acid staining and biofilm activity using enzyme assays for lipid, carbohydrate, phosphate metabolism, and cell viability. In general, biofilm activity decreased as nitrate levels increased from Sites 1 to 4, with the opposite relationship for carbon and phosphorus concentrations. These results showed that the groundwater system supported biofilm growth and that the upper catchment supported efficient and productive biofilms (high ratio of activity per unit biomass).

Predicting richness effects on ecosystem function in natural communities: insights from high-elevation streams

. Despite the increased complexity of experimental and theoretical studies on the biodiversity-ecosystem functioning (B-EF) relationship, a major challenge is to demonstrate whether the observed importance of biodiversity in controlled experimental systems also persists in nature. Due to their structural simplicity and their low levels of human impacts, extreme species-poor ecosystems may provide new insights into B-EF relationships in natural systems. We address this issue using shredder invertebrate communities and organic matter decomposition rates in 24 high-altitude (3200-3900 m) Neotropical streams as a study model. We first assessed the effects of stream characteristics and shredder diversity and abundance on organic matter decomposition rates in coarse- and fine-mesh bags. We found the interaction term shredder richness x shredder abundance had the most significant impact on decomposition rates in the field, although water discharge may also play a role locally. We also examined the relative contribution of the three most abundant shredders on decomposition rates by manipulating shredder richness and community composition in a field experiment. Transgressive overyielding was detected among the three shredder species, indicating complementary resource use and/or facilitation. By integrating survey and experimental data in surface response analyses we found that observed B-EF patterns fit those predicted by a linear model that described litter decomposition rates as a function of increasing shredder richness and the relative abundance of the most efficient shredders. Finally, the validity of our approach was tested in a broader context by using two independent but comparable data sets from 49 French and Swedish streams showing more complex shredder community structure. Results revealed that richness and identity effects on decomposition rates were lost with increasing shredder community complexity. Our approach of combining experimental and empirical data with modeling in species-poor ecosystems may serve as an impetus for new B-EF studies. If theory can explain B-EF in low-diversity ecosystems, it may also have credibility in more complex ones.

Leaf Litter Decomposition of Three Riparian Tree Species and Associated Macroinvertebrates of Eswathu Oya, a Low Order Tropical Stream in Sri Lanka

AbstractLeaf decomposition, an important component of the organic matter dynamics in streams, has been widely examined in temperate regions but much less documented in tropical regions. We report here the first study of leaf decomposition in a Sri Lankan stream. The litterbag technique was used. Coarse (8 mm) and fine (100 µm) litterbags, that included or excluded macroinvertebrates respectively, were used to enclose leaves of three dominant riparian tree species: native Ochlandra stridula (bamboo), and the introduced Alstonia macrophylla and Hevea brasiliensis (rubber). A fourth set of litterbags contained a mixture of these three species. Leaf colonization by macroinvertebrates was highest in the early stages of the decomposition process on Hevea leaves but invertebrate densities declined later. The opposite colonization effect was observed on the native Ochlandra leaves: slow colonization with continuing low densities from the beginning to the end of the process. Decomposition of all three species was significantly faster in the coarse than in the fine mesh bags. Alstonia, which has the softest leaf tissue, was most rapidly decomposed while Ochlandra, with its tough structure, was the slowest. Among the invertebrates, insects were the most important leaf colonizing animals, with Diptera, Coleoptera and Trichoptera the most dominant. The invertebrate variety in the mixed bags was higher than in the single‐species leaf bags, where Chironomidae dominated the colonizing assemblages. This study has shown that toughness, indicated by the ‘specific weight of leaf tissue’, and the quality of the leaves was more important in determining breakdown rates than their origin. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The influence of macroinvertebrate shredders, leaf type and composition on litter breakdown in a Hong Kong stream

The contribution of shredders to litter breakdown in a tropical stream in Hong Kong was investigated by using coarse- and fine-mesh bags to include or exclude large macroinvertebrates. Three native species, plus a mixture of all of them, were used to examine the effects of leaf type and species composition on breakdown. A well-studied temperate species, Alnus glutinosa, was included to benchmark breakdown rates in Hong Kong with counterpart streams outside the tropics. Shredders constituted 13% of macroinvertebrates in coarse-mesh bags but were virtually absent from fine-mesh bags. However, abundance of macroinvertebrates (especially chironomids) in fine-mesh bags was almost twice that in coarse-mesh bags. Despite interspecific differences, litter breakdown rates were consistently faster in coarse-mesh bags, indicating a probable effect of shredders, although current velocity also influenced breakdown of one native species. Alnus glutinosa breakdown rates were within the range reported in temperate streams. The litter mixture broke down at a rate approximately equal to the mean of the constituent species and non-additive compositional effects (indicative of biodiversity-ecosystem functioning relationships) were not apparent. Litter breakdown in fine-mesh bags was rapid relative to rates reported from temperate streams, despite the exclusion of shredders. Evidently, non-shredding macroinvertebrates and microbes played a significant role in litter breakdown in Hong Kong.

Experimentally Simulated Global Warming and Nitrogen Enrichment Effects on Microbial Litter Decomposers in a Marsh

Atmospheric warming and increased nitrogen deposition can lead to changes of microbial communities with possible consequences for biogeochemical processes. We used an enclosure facility in a freshwater marsh to assess the effects on microbes associated with decomposing plant litter under conditions of simulated climate warming and pulsed nitrogen supply. Standard batches of litter were placed in coarse-mesh and fine-mesh bags and submerged in a series of heated, nitrogen-enriched, and control enclosures. They were retrieved later and analyzed for a range of microbial parameters. Fingerprinting profiles obtained by denaturing gradient gel electrophoresis (DGGE) indicated that simulated global warming induced a shift in bacterial community structure. In addition, warming reduced fungal biomass, whereas bacterial biomass was unaffected. The mesh size of the litter bags and sampling date also had an influence on bacterial community structure, with the apparent number of dominant genotypes increasing from spring to summer. Microbial respiration was unaffected by any treatment, and nitrogen enrichment had no clear effect on any of the microbial parameters considered. Overall, these results suggest that microbes associated with decomposing plant litter in nutrient-rich freshwater marshes are resistant to extra nitrogen supplies but are likely to respond to temperature increases projected for this century.

Riparian land use and the relationship between the benthos and litter decomposition in tropical montane streams

Summary1. Although stream–catchment interactions have been analysed in some detail in temperate environments, little is known about the effects of land‐use changes in the tropics. Here, we analyse differences in benthic communities (macroinvertebrates and fungi) under two contrasting land uses (mature secondary forest and pasture) in montane streams in north‐western Ecuador and their influence on the rates of litter processing.2. Between 2005 and 2006, we used a combination of coarse and fine mesh bags to study the relative contribution of macroinvertebrates and fungi to processing of two types of litter, Alnus acuminata and Inga spectabilis, in three‐first‐order streams running through mature secondary montane forests and adjacent downstream reaches running through pastures. At the same time, we characterised the assemblages of shreddering macroinvertebrates and fungi communities and the litter processing rates in stream reaches under both vegetation types.3. Litter processing rates attributable to invertebrate feeding (coarse mesh bags) were significantly slower in streams running through pastures. Nevertheless, shredder diversity and richness were similar between pasture and forest sections, while shredder abundance was significantly higher in forest streams (mainly Phylloicus sp. :Trichoptera). Fungal reproductive activity and litter processing rates were low (fine mesh bags) and did not differ significantly between pasture and forest stream reaches.4. Phylloicus sp. abundance was the best predictor of the percentage of litter remaining in coarse mesh bags across pasture and forest sites. Neither shredder diversity nor their species richness was a significant predictor of mass loss, as most of the decomposition was performed by a single keystone species. Although litter decomposition by microbial decomposers was low, fungal biomass (but not diversity) was the best variable explaining the percentage of litter remaining in fine mesh bags.5. Our data suggest that, in these Neotropical montane streams, land use can have a significant impact on the rates of critical ecosystem processes, such as litter decomposition. In this study, this effect was not mediated by a major shift in the structure of the benthos, but by a decrease in the abundance and relative representation of a single species whose life history makes it critical to litter processing.6. This study highlights the significant role that macroinvertebrate fauna can have in the processing of litter in Neotropical streams and the predominant role that single species can have in terms of controlling stream ecosystem‐level processes. Understanding the extent to which these patterns affect the long‐term and large‐scale functioning of stream ecosystems still needs further research and will become increasingly important in terms of managing lotic ecosystems in the context of rapid land‐use change.

Functional leaf traits and biodiversity effects on litter decomposition in a stream: comment

In a recent paper, Schindler and Gessner (2009) tested how functional diversity of leaf litter explains litter diversity effects on decomposition. Their study builds on a large body of experimental evidence that decomposi tion rates of litter species mixtures are not simply predicted by the component species decomposing in isolation (Gartner and Cardon 2004, Hattenschwiler and Gasser 2005, Kominoski et al. 2007, Lecerf et al. 2007). Such litter diversity effects are thought to be explained by the presence of heterogeneous litter (in terms of leaf degradability) in mixtures, causing deviation (positive or negative) in decomposition of mixtures as compared to the average single-species decomposition of the component litter species (Gartner and Cardon 2004, Hattenschwiler et al. 2005). Func tional diversity represents a valuable framework for testing this hypothesis. Schindler and Gessner (2009) is the first published paper that addresses how functional litter diversity influences in-stream litter decomposition. Their manipulative experiment, with homogeneous and heterogeneous mixtures of litter enclosed in coarse and fine-mesh bags, tested for decomposition rates of individual litter species from monospecific and mixed species assemblages. Based on species-specific decay rates expected from published and unpublished data, they split up a set of nine litter species spanning a broad range of decay rates into three functional leaf groups (slow-, medium-, or fast-decomposing species). Homo geneous mixtures consisted of three litter species taken from a single functional group, and heterogeneous mixtures consisted of one species from each of three functional groups. The planned comparison of homo geneous versus heterogeneous mixtures was used to test for the effect of functional litter diversity on litter

Aquatic fungi associated with decomposingFicussp. leaf litter in a neotropical stream

Aquatic fungi are important decomposers of plant litter in temperate streams. However, these microorganisms have been poorly studied in tropical streams. We assessed the dynamics of aquatic hyphomycetes during the decomposition of Ficus sp. (Moraceae) leaves in an intermittent lowland stream of northwestern Venezuela. Our goals were to: 1) determine the composition of aquatic hyphomycetes during leaf decomposition, 2) assess the effect of different mesh sizes (i.e., coarse vs fine mesh) on assemblage composition and activity, and 3) assess the role of habitat (i.e., pools vs riffles) on fungal assemblages. Coarse- and fine-mesh litter bags were incubated in a pool and a riffle for 31 d. For each habitat and treatment, we determined physicochemical variables, sporulation activity, and the structure and composition of fungal assemblages. Leaf litter decomposition rates ranged from 0.058/d in fine-mesh bags in the riffle to 0.031/d in coarse-mesh bags in the pool. Decomposition rates were significantly influenced by habitat type, with the highest decomposition rates found in riffle habitats. Sporulation rates were significantly different between habitats but not between mesh sizes. The highest sporulation rate (399 spores mg−1 leaf dry mass d−1) and fungal diversity (16 spp.) was found in coarse-mesh bags in the riffle. Twenty-one fungal species were identified during the study. Anguillospora longissima and Lunuluspora curvula contributed up to 75% of the conidia produced in the coarse- and fine-mesh bags in the riffle, whereas Filosporella aquatica and Lemonniera terrestris accounted for 65 to 75% of the conidia produced in the coarse- and fine-mesh bags in the pool. In ordination space based on species composition, fungal assemblages formed 2 clear groups that corresponded to habitat type. A temporal change in fungal species composition was observed in riffle samples. Early succession assemblages (2–14 d) were characterized by the presence of A. longissima, L. curvula, and F. aquatica, whereas late assemblages (24–31 d) were dominated by Pyramidospora fluminea, Pyramidospora casuarinae, and unidentified sp 4. Overall, habitat type (pool or riffle) influenced sporulation rates and assemblage composition of aquatic hyphomycetes. Our study suggests that environmental factors associated with hydrology (e.g., stream velocity, width, depth) regulate fungal composition and activity and, hence, leaf litter decomposition in this tropical stream.

Effect of sudden flow reduction on the decomposition of alder leaves (Alnus glutinosa [L.] Gaertn.) in a temperate lowland stream: a mesocosm study

Climate change leads to summer low flow conditions and premature litter input in lowland streams in Central Europe. This may cause a sudden reduction of flow and fragmentation into isolated pools of permanently flowing streams, with a simultaneous increase in the food supply for detrivores during summer months. We performed a mesocosm study to investigate shredder and microbial mediated litter decomposition under these conditions. Leaf litter was placed in a lowland stream with a natural flow regime (reference) and in a stream mesocosm with significant flow reduction (FR) and a representative density of macroinvertebrates and detritus. Physicochemical parameters, leaf mass loss, macroinvertebrate abundance and biomass, leaf-associated respiration, fungal sporulation, and biomass were measured at regular intervals for 6 weeks. Coarse and fine-mesh bags were used to include or exclude macroinvertebrate shredders. In the coarse-mesh bags, leaf mass loss was significantly lower in the FR system than in the reference regime. In the fine-mesh bags, leaf respiration, fungal sporulation, and biomass but not leaf mass losses were substantially lower with flow reduction. Chironomid larvae (Micropsectra spp.) appeared to effectively fragment leaf litter in fine-mesh bags. In the FR system, leaf respiration was higher in the coarse-than in the fine-mesh bags. Our results suggest that, in temperate lowland streams, premature litter input during or after a sudden fragmentation into isolated pools and a reduction of stream flow reduces direct shredder-mediated litter decomposition, but shredders may indirectly influence the decomposition process.

Leaf Decomposition in a Mountain Stream in the Sultanate of Oman

AbstractDecomposition of Juglans regia leaves was studied in fine and coarse mesh bags in a permanent mountain stream in Oman. A rapid initial mass loss, attributed to leaching, was followed by a more gradual decline. Daily exponential decay rates (k) calculated over 32 days were 0.011 (fine mesh litter bags) and 0.014 (coarse mesh litter bags). The difference between bag types was not significant, suggesting limited impact of leaf‐shredding invertebrates. Ergosterol levels on leaves from fine mesh bags peaked at 0.3 mg g–1 AFDM after 16 days of stream exposure.During the experimental period, which followed the annual leaf fall, the concentration of aquatic hyphomycete conidia in the stream varied between 82 and 1362 l–1. Based on the morphology of conidia found in the water column or released from leaves, we identified 14 species of aquatic hyphomycetes. Tetracladium apiense was the most common taxon (62.2% of conidia in water column during the field experiment). Three other Tetracladium species contributed another 8%. Plating out leaf particles yielded common epiphytic taxa such as Alternaria sp., Aureobasidium pullulans and Phoma sp.The measured metrics of leaf decay in this desert stream fall within the range of values observed in temperate and tropical streams, with clear evidence for an early leaching phase, and no evidence of a strong impact of leaf shredders. The community of aquatic hyphomycetes appears impoverished. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of shredder feeding and nutrients on fungal activity and community structure in headwater streams

In stream detrital food webs, interactions occur between aquatic hyphomycetes associated with decomposing leaves and shredders consuming those leaves. However, few studies have examined how the feeding activity of shredders affects aquatic hyphomycetes. We examined the effect of shredder feeding on aquatic hyphomycete communities associated with submerged leaves in two southern Appalachian headwater streams in Coweeta Hydrologic Laboratory, North Carolina, USA. Coarse (allowing shredder access) and fine (preventing shredder access) mesh litter bags containing red maple (Acer rubrum) leaves were placed in the treatment stream (C54) which was enriched with nitrogen (N) and phosphorus (P), and in the reference stream (C53) and were retrieved monthly. Both shredder feeding and nutrient enrichment enhanced breakdown rates. The breakdown rates of leaves in coarse mesh bags in the reference stream (k = 0.0275) and fine mesh bags in the nutrient enriched stream (k = 0.0272) were not significantly different, suggesting that the higher fungal activity stimulated by nutrient enrichment could increase the relative contribution of fungi to leaf breakdown to the level similar to that of shredders in the reference stream. Macroinvertebrate abundance and biomass were higher in the litter bags submerged in the treatment stream. Fungal sporulation rates and biomass were higher in the treatment stream than in the reference stream, but neither fungal biomass nor sporulation rate was affected by shredder feeding in either stream. The enrichment with N and P altered fungal community composition more than shredder feeding. Species richness was higher in the nutrient enriched stream than in the reference stream, and fungal assemblages from fine and coarse mesh bag treatments within a stream were more similar to each other than the fungal assemblages from the same mesh bag treatments but from different streams.

Litter decomposition in earthworm-invaded northern hardwood forests: Role of invasion degree and litter chemistry

The effects of invasive earthworms on decomposition are little known, and the controls of their effect on decomposition may be different than those of microbes. Sugar maple—dominated forests previously devoid of earthworms in the western Great Lakes region (USA) exhibit different degrees of earthworm invasion, presenting a natural experiment to study its effects on litter decomposition. We hypothesized that litter decomposition would depend on the degree of earthworm invasion, the presence or absence of worms of differing sizes, and initial litter chemistry. We established an experiment using fine- and coarse-mesh litterbags (to allow access by different-sized worms) to study decomposition of 3 different litters in mixture under different degrees of earthworm invasion in 12 Minnesota sites. The effect of earthworm invasion degree on litter decomposition varied by identity of the litter, mesh size, and time of litter collection. Decomposition of Quercus rubra, the litter with the lowest initial calcium concentration and highest lignin:nitrogen, was not significantly influenced by earthworm invasion degree. In contrast, decomposition of Tilia americana, the litter with the highest calcium concentration and lowest lignin:nitrogen, was fastest in coarse-mesh litterbags. After 15 months the mass of the highest-quality litters was highest in the fine-mesh bags of heavily invaded plots, suggesting that microbially mediated decomposition slowed where earthworms had removed the forest floor.

Litter breakdown in a stream affected by drainage from a gold mine

The objective of the present study was to analyse the effects of drainage water from an abandoned gold mine on the breakdown of Alnus glutinosa leaves mediated by aquatic fungi and invertebrates. We analysed three sites (P = polluted, R = reference, and RP = intermediate) in the winters of 2002 and 2004. The sites had similar pH (6-7) but differed in the concentration of heavy metals (As, 0.4 mg L -1 ; Fe, 4.9 mg L -1 ; Mn and Zn, 1.2 mg L -1 in site P) and PO 4 3- (0.04 mg L -1 in site P). The breakdown of alder leaves was low, both in the absence (fine mesh bags) or presence (coarse mesh bags) of invertebrates. Sites did not differ (ANCOVA) in breakdown rates mediated by aquatic fungi (k = 0.0016 to 0.0080), but sporulation rates were significantly higher at site R in 2002 (up to 2488 spores mg AFDM -1 d -1 ). In the presence of shredding invertebrates, breakdown was significantly lower at the polluted sites in both years (k 0.0191), in spite of similar shredder biomass in all sites. The dominant shredder caddisfly Allogamus ligonifer discriminated between leaves exposed in sites R and RP in 2002 (consumption rates of 0.60 vs. 0.35 mg mg -1 AFDM day -1 ). In situ experiments revealed that shredders from site RP consumed leaves at lower rates than in site R (0.46 vs. 0.35 mg mg -1 AFDM day -1 ). Our results suggest that although pollution was not enough to suppress microbial growth and invertebrate survival, sub-lethal effects affected the ecosystem function, even when interannual variation was prominent.

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

Summary1. 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.