Contribution of fungi and bacteria to leaf litter decomposition in a polluted river

The contribution of fungi and bacteria to the decomposition of alder leaves was examined at two reference and two polluted sites in the Ave River (northwestern Portugal). Leaf mass loss, microbial production from incorporation rates of radiolabeled compounds into biomolecules, fungal biomass from ergosterol concentration, sporulation rates, and diversity of aquatic hyphomycetes associated with decomposing leaves were determined. The concentrations of organic nutrients and of inorganic nitrogen and phosphorus in the stream water was elevated and increased at downstream sites. Leaf decomposition rates were high (0.013 day(-1) < k < 0.042 day(-1)), and the highest value was estimated at the most downstream polluted site, where maximum values of microbial production and fungal biomass and sporulation were found. The slowest decomposition occurred at the other polluted site, where, along with the nutrient enrichment, the lowest current velocity and dissolved-oxygen concentration in water were observed. At this site, fungal production, biomass, and sporulation were depressed, suggesting that stimulation of fungal activity by increased nutrient concentrations might be offset by other factors. Although bacterial production was higher at polluted sites, fungi accounted for more than 94% of the total microbial net production. Fungal yield coefficients varied from 10.2 to 13.6%, while those of bacteria were less than 1%. The contribution of fungi to overall leaf carbon loss (29.0 to 38.8%) greatly exceeded that of bacteria (4.2 to 13.9%).     

Contribution of Fungi and Bacteria to Leaf Litter Decomposition in a Polluted River

The contribution of fungi and bacteria to the decomposition of alder leaves was examined at two reference and two polluted sites in the Ave River (northwestern Portugal). Leaf mass loss, microbial production from incorporation rates of radiolabeled compounds into biomolecules, fungal biomass from ergosterol concentration, sporulation rates, and diversity of aquatic hyphomycetes associated with decomposing leaves were determined. The concentrations of organic nutrients and of inorganic nitrogen and phosphorus in the stream water was elevated and increased at downstream sites. Leaf decomposition rates were high (0.013 day⁻¹ < k < 0.042 day⁻¹), and the highest value was estimated at the most downstream polluted site, where maximum values of microbial production and fungal biomass and sporulation were found. The slowest decomposition occurred at the other polluted site, where, along with the nutrient enrichment, the lowest current velocity and dissolved-oxygen concentration in water were observed. At this site, fungal production, biomass, and sporulation were depressed, suggesting that stimulation of fungal activity by increased nutrient concentrations might be offset by other factors. Although bacterial production was higher at polluted sites, fungi accounted for more than 94% of the total microbial net production. Fungal yield coefficients varied from 10.2 to 13.6%, while those of bacteria were less than 1%. The contribution of fungi to overall leaf carbon loss (29.0 to 38.8%) greatly exceeded that of bacteria (4.2 to 13.9%).     

Copper and zinc mixtures induce shifts in microbial communities and reduce leaf litter decomposition in streams

1. To assess the impact of metal mixtures on microbial decomposition of leaf litter, we exposed leaves previously immersed in a stream to environmentally realistic concentrations of copper (Cu) and zinc (Zn) (three levels), alone and in all possible combinations. The response of the microbial community was monitored after 10, 25 and 40 days of metal exposure by examining leaf mass loss, fungal and bacterial biomass, fungal reproduction and fungal and bacterial diversity.
2. Analysis of microbial diversity, assessed by denaturing gradient gel electrophoresis and identification of fungal spores, indicated that metal exposure altered the structure of fungal and bacterial communities on decomposing leaves.
3. Exposure to metal mixtures or to the highest Cu concentration significantly reduced leaf decomposition rates and fungal reproduction, but not fungal biomass. Bacterial biomass was strongly inhibited by all metal treatments.
4. The effects of Cu and Zn mixtures on microbial decomposition of leaf litter were mostly additive, because observed effects did not differ from those expected as the sum of single metal effects. However, antagonistic effects on bacterial biomass were found in all metal combinations and on fungal reproduction in metal combinations with the highest Cu concentrations, particularly at longer exposure times.     

Effects of Zinc on Leaf Decomposition by Fungi in Streams: Studies in Microcosms

The effect of zinc on leaf decomposition by aquatic fungi was studied in microcosms. Alder leaf disks were precolonized for 15 days at the source of the Este River and exposed to different zinc concentrations during 25 days. Leaf mass loss, fungal biomass (based on ergosterol concentration), fungal production (rates of [1-¹⁴C]acetate incorporation into ergosterol), sporulation rates, and species richness of aquatic hyphomycetes were determined. At the source of the Este River decomposition of alder leaves was fast and 50% of the initial mass was lost in 25 days. A total of 18 aquatic hyphomycete species were recorded during 42 days of leaf immersion. Articulospora tetracladia was the dominant species, followed by Lunulospora curvula and two unidentified species with sigmoid conidia. Cluster analysis suggested that zinc concentration and exposure time affected the structure of aquatic hyphomycete assemblages, even though richness had not been severely affected. Both zinc concentration and exposure time significantly affected leaf mass loss, fungal production and sporulation, but not fungal biomass. Zinc exposure reduced leaf mass loss, inhibited fungal production and affected fungal reproduction by either stimulating or inhibiting sporulation rates. The results of this work suggested zinc pollution might depress leaf decomposition in streams due to changes in the structure and activity of aquatic fungi.     

Functional stability of stream-dwelling microbial decomposers exposed to copper and zinc stress

1. We investigated the resistance of aquatic microbial decomposers to Cu and Zn stress and their ability to recover after release from metal exposure, by examining leaf mass loss, fungal reproduction and microbial biomass and diversity.
2. Alder leaves, colonised in a reference stream, were exposed in microcosms to copper (Cu) or zinc (Zn), alone or in mixtures, with metals added together or sequentially (at day 0 or after 10 days). After 20 days, half of the microcosms were released from metals.
3. Leaf mass loss and fungal reproduction were reduced in most metal treatments, and the structure of fungal and bacterial communities was altered as indicated by identification of conidia and DNA fingerprinting based on denaturing gradient gel electrophoresis. Metals reduced the biomass of bacteria, but not that of fungi.
4. After release from metal stress, the structure of fungal communities became similar to that of control, and a recovery of microbial activity seemed to occur as shown by the lack of differences in leaf mass loss, bacterial biomass and fungal reproduction between control and metal treatments.     

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.     

Fungal communities on decaying leaves in streams: a comparison of two leaf species

Decomposition of leaf litter is a microbial mediated process that helps to transfer energy and nutrients from leaves to higher trophic levels in woodland streams. Generally, aquatic hyphomycetes are viewed as the major fungal group responsible for leaf litter decomposition. In this study, traditional microscopic examination (based on identification of released conidia) and phylogenetic analysis of 18S rRNA genes from cultivated fungi were used to compare fungal community composition on decomposing leaves of two species (sugar maple and white oak) from a NE Ohio stream. No significant differences were found in sporulation rates between maple and oak leaves and both had similar species diversity. From the 18S rRNA gene sequence data, identification was achieved for 12 isolates and taxonomic affiliation of 12 of the remaining 14 isolates could be obtained. A neighbor-joining tree (with bootstrap values) was constructed to examine the taxonomic distribution of the isolates relative to sequences of known operational taxonomic units (OTUs). Surprisingly, only 2 of the isolates obtained were aquatic hyphomycetes based on phylogenetic analysis. Overall, there were no differences between the two leaf types and a higher diversity was observed via culturing and subsequent 18S rRNA gene sequencing than by conidia staining. These differences resulted from the fact that traditional microscopy provides estimates of aquatic hyphomycete diversity while the other approach revealed the presence of both aquatic hyphomycete and non-aquatic hyphomycete taxa. The presence of this broad array of taxa suggests that the role of aquatic hyphomycetes relative to other fungi be re-evaluated. Even though the functional role of these non-aquatic hyphomycetes taxa is unknown, their presence and diversity demonstrates the need to delve further into fungal community structure on decomposing leaves.     

Synergistic effects of water temperature and dissolved nutrients on litter decomposition and associated fungi

In woodland streams, the decomposition of allochthonous organic matter constitutes a fundamental ecosystem process, where aquatic hyphomycetes play a pivotal role. It is therefore greatly affected by water temperature and nutrient concentrations. The individual effects of these factors on the decomposition of litter have been studied previously. However, in the climate warming scenario predicted for this century, water temperature and nutrient concentrations are expected to increase simultaneously, and their combined effects on litter decomposition and associated biological activity remains unevaluated. In this study, we addressed the individual and combined effects of water temperature (three levels) and nutrient concentrations (two levels) on the decomposition of alder leaves and associated aquatic hyphomycetes in microcosms. Decomposition rates across treatments varied between 0.0041 day^?1 at 5 °C and low nutrient level and 0.0100 day^?1 at 15 °C and high nutrient level. The stimulation of biological variables at high nutrients and temperatures indicates that nutrient enrichment of streams might have a higher stimulatory effect on fungal performance and decomposition rates under a warming scenario than at present. The stimulation of fungal biomass and sporulation with increasing temperature at both nutrient levels shows that increases in water temperature might enhance fungal growth and reproduction in both oligotrophic and eutrophic streams. The stimulation of fungal respiration and litter decomposition with increasing temperature at high nutrients indicates that stimulation of carbon mineralization will probably occur at eutrophied streams, while oligotrophic conditions seem to be ‘protected’ from warming. All biological variables were stimulated when both factors increased, as a result of synergistic interactions between factors. Increased water temperature and nutrient level also affected the structure of aquatic hyphomycete assemblages. It is plausible that if water quality of presently eutrophied streams is improved, the potential stimulatory effects of future increases in water temperature on aquatic biota and processes might be mitigated.     

Whole-stream nitrate addition affects litter decomposition and associated fungi but not invertebrates

We assessed the effect of whole-stream nitrate enrichment on decomposition of three substrates differing in nutrient quality (alder and oak leaves and balsa veneers) and associated fungi and invertebrates. During the 3-month nitrate enrichment of a headwater stream in central Portugal, litter was incubated in the reference site (mean NO₃-N 82 μg l⁻¹) and four enriched sites along the nitrate gradient (214–983 μg NO₃-N l⁻¹). A similar decomposition experiment was also carried out in the same sites at ambient nutrient conditions the following year (33–104 μg NO₃-N l⁻¹). Decomposition rates and sporulation of aquatic hyphomycetes associated with litter were determined in both experiments, whereas N and P content of litter, associated fungal biomass and invertebrates were followed only during the nitrate addition experiment. Nitrate enrichment stimulated decomposition of oak leaves and balsa veneers, fungal biomass accrual on alder leaves and balsa veneers and sporulation of aquatic hyphomycetes on all substrates. Nitrate concentration in stream water showed a strong asymptotic relationship (Michaelis–Menten-type saturation model) with temperature-adjusted decomposition rates and percentage initial litter mass converted into aquatic hyphomycete conidia for all substrates. Fungal communities did not differ significantly among sites but some species showed substrate preferences. Nevertheless, certain species were sensitive to nitrogen concentration in water by increasing or decreasing their sporulation rate accordingly. N and P content of litter and abundances or richness of litter-associated invertebrates were not affected by nitrate addition. It appears that microbial nitrogen demands can be met at relatively low levels of dissolved nitrate, suggesting that even minor increases in nitrogen in streams due to, e.g., anthropogenic eutrophication may lead to significant shifts in microbial dynamics and ecosystem functioning. Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Microbial growth and detritus transformations during decomposition of leaf litter in a stream

SUMMARY. 1. Despite the widely accepted importance of bacteria and fungi in degrading detritus in aquatic ecosystems there is still very little quantitative information on the abundance and dynamics of these microorganisms. Using epifluorescent microscopy, we measured the biomass of bacteria and fungi during decomposition of three types of leaf detritus. Bacterial production was determined from the rate of incorporation of ³H-thymidine into DNA.
2. The transformation of leaf carbon into dissolved organic carbon and fine particulate organic carbon was followed in order to compare the amounts of leaf material that were converted into these 'end-products' of decomposition versus the amount converted into microbial biomass.
3. The amount of microbial carbon in the leaf-detritus complex never exceeded 5.2% of the total carbon, and fungal biomass was always much greater than bacterial biomass. Despite the greater standing stock of fungi, the rapid turnover of bacteria (doubling about once per day) implies that their role in degrading leaf litter or as a food source for detritivores might be as great as for fungi.
4. Removal of microbial biomass from leaf litter may occur as release of fungal spores and consumption or shedding of bacterial biomass. Fungal spores can be a significant part of the fine particulate organic carbon released from leaf detritus and potentially represent an important food resource for filter-feeding organisms.     

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

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Effect of Inorganic Nutrients on Relative Contributions of Fungi and Bacteria to Carbon Flow from Submerged Decomposing Leaf Litter

The relative contributions of fungi and bacteria to carbon flow from submerged decaying plant litter at different levels of inorganic nutrients (N and P) were studied. We estimated leaf mass loss, fungal and bacterial biomass and production, and microbial respiration and constructed partial carbon budgets for red maple leaf disks precolonized in a stream and then incubated in laboratory microcosms at two levels of nutrients. Patterns of carbon flow for leaf disks colonized with the full microbial assemblage were compared with those colonized by bacteria but in which fungi were greatly reduced by placing leaf disks in colonization chambers sealed with membrane filters to exclude aquatic hyphomycete conidia but not bacterial cells. On leaves colonized by the full microbial assemblage, elevated nutrient concentrations stimulated fungi and bacteria to a similar degree. Peak fungal and bacterial biomass increased by factors of 3.9 and 4.0; cumulative production was 3.9 and 5.1 times higher in the high nutrient in comparison with the low nutrient treatment, respectively. Fungi dominated the total microbial biomass (98.4 to 99.8%) and cumulative production (97.3 and 96.5%), and the fungal yield coefficient exceeded that of bacteria by a factor of 36 and 27 in low- and high-nutrient treatments, respectively. Consequently, the dominant role of fungi in leaf decomposition did not change as a result of nutrient manipulation. Carbon budgets indicated that 8% of leaf carbon loss in the low-nutrient treatment and 17% in the high-nutrient treatment were channeled to microbial (essentially fungal) production. Nutrient enrichment had a positive effect on rate of leaf decomposition only in microcosms with full microbial assemblages. In treatments where fungal colonization was reduced, cumulative bacterial production did not change significantly at either nutrient level and leaf decomposition rate was negatively affected (high nutrients), suggesting that bacterial participation in carbon flow from decaying leaf litter is low regardless of the presence of fungi and nutrient availability. Moreover, 1.5 and 2.3 times higher yield coefficients of bacteria in the reduced fungal treatments at low and high nutrients, respectively (percentage of leaf carbon loss channeled to bacterial production), suggest that bacteria are subjected to strong competition with fungi for resources available in leaf litter.     

Contribution of fungal biomass to the growth of the shredder, Pycnopsyche gentilis (Trichoptera: Limnephilidae)

1.  1. It has been accepted that aquatic hyphomycetes colonising submerged leaves increase the nutritional value of leaf detritus and suggested that fungal biomass plays a greater role in the growth of shredders than leaf tissue itself. However, it is not clear what proportion of the nutritional needs of shredders is met by fungal biomass.
2.  We fed Pycnopsyche gentilis larvae with tulip poplar ( Liriodendron tulipifera ) leaf discs colonised by the aquatic hyphomycete, Anguillospora filiformis , which had been radiolabelled to quantify the contribution of fungal carbon to the growth of the shredder at different larval developmental stages. Instantaneous growth rates of larvae on this diet were also estimated.
3.  When provided with fungal-colonised leaves (14–16% fungal biomass), the third and the fifth instar larvae of P. gentilis grew at the rates of 0.061 and 0.034 day⁻¹, respectively, but on a diet of sterile leaves, both larval instars lost weight. The incorporation rates of fungal carbon were 31.6 μg C mg⁻¹ AFDM day⁻¹, accounting for 100% of the daily growth rate of the third instar larvae and 8.6 μg C mg⁻¹ AFDM day⁻¹, accounting for 50% of the daily growth rate of the fifth instar larvae.
4.  These results suggest that leaf material colonised by A. filiformis is a high quality food resource for P. gentilis larvae, and that fungal biomass can contribute significantly to the growth of these larvae. Differences in feeding behaviour and digestive physiology may explain the significantly greater assimilation of fungal biomass by the earlier instar than the final instar. To satisfy their nutritional needs the fifth instar larvae would have to assimilate detrital mass that may have been modified by fungal exoenzymes.     

Organic Matter Dynamics in a Tropical Gallery Forest in a Grassland Landscape

The high biodiversity of tropical forest streams depends on the strong input of organic matter, yet the leaf litter decomposition dynamics in these streams are not well understood. We assessed how seasonal litterfall affects leaf litter breakdown, density and biomass of aquatic invertebrates, and the microbial biomass and sporulation of aquatic hyphomycetes in a South American grassland ‘vereda’ landscape. Although litter production in the riparian area was low, leaf litter breakdown was high compared with other South American systems, with maximum values coinciding with the rainy season. Fungal biomass in decomposing leaves was high, but spore densities in water and sporulation rates were very low. Invertebrates were not abundant in litter bags, suggesting they play a minor role in leaf litter decomposition. Chironomids accounted for ~70 percent of all invertebrates; only 10 percent of non‐Chironomidae invertebrates were shredders. Therefore, fungi appear to be the drivers of leaf litter decomposition. Our results show that despite low productivity and relatively fast litter decomposition, organic matter accumulated in the stream and riparian area. This pattern was attributed to the wet/dry cycles in which leaves falling in the flat riparian zone remain undecomposed (during the dry period) and are massively transported to the riverbed (rainy season).     

Natural UVR Does Not Affect Decomposition by Aquatic Hyphomycetes

We performed field and laboratory experiments to evaluate the effect of solar radiation (UVR and PAR) on leaf litter decomposition, fungal biomass and sporulation rates, in the Andean Patagonia, where high UVR levels are common. Leaves of Alnus glutinosa exposed to three treatments, normal radiation (PAR + UVR), protected from UVR and protected from total radiation (SHADE) by plastic films lost 31–37% of their mass. Leaves of Nothofagus pumilio lost 61–64% of their mass under the same conditions. For both leaf species, differences in mass losses among treatments were not statistically significant. Sporulation rates were significantly lower in the SHADE treatment. Fungal biomass accounted for 6.2 to 7.1% of leaf mass, without significant differences among treatments. In the laboratory, leaf discs of A. glutinosa colonized by single species of aquatic hyphomycetes (Articulospora tetracladia, Flagellospora curta or Lunulospora curvula) and exposed to or protected from UVR did not differ in mass loss and sporulation rates. Pure cultures of two fungal species grew at the same rates when exposed to light (PAR and PAR + UVR) or to the SHADE. In summary, we found no evidences that current high levels of UV radiation affect litter decomposition mediated by aquatic hyphomycetes. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Fungal endophyte‐infected leaf litter alters in‐stream microbial communities and negatively influences aquatic fungal sporulation

Endophytes are ubiquitous plant‐associated microbes and although they have the potential to alter the decomposition of infected leaf litter, this has not been well‐studied. The endophyte Rhytisma punctatum infects the leaves of Acer macrophyllum (bigleaf maple), causing the appearance of black ‘tar spots’ that persist in senesced leaves. Other foliar fungi also cause visible damage in healthy tissues of this host plant system including an unidentified bullseye‐shaped lesion, common in western Washington. Using three treatments of endophyte infection status in leaf tissue (R. punctatum‐infected, bullseye‐infected, lesion‐free), leaf litter discs were submerged in a third‐order temperate stream using mesh litter bags and harvested periodically over two months to determine the effects of litter treatment and incubation time on litter mass loss, fungal sporulation, and microbial community colonization. Litter containing symptomatic endophyte infections (Rhytisma or bullseye) had reduced sporulation of aquatic hyphomycetes, but decomposed significantly faster than lesion‐free or bullseye‐infected litter. Using amplicon‐based sequencing, we found a significant difference in bacterial communities colonizing Rhytisma‐infected and bullseye‐infected leaf litter, a significant difference in fungal communities colonizing Rhytisma‐infected leaf litter compared to the two other treatments, and a change in both community structure and relative abundances of bacterial and fungal taxa throughout the study period. Indicator Species Analysis clarified the drivers of these community shifts at the genus level. Our results show that endophyte‐associated, in‐stream sporulation and microbial community effects are observable within one species of leaf litter.     

Decomposition of Fire Exposed Eucalyptus Leaves in a Portuguese Lowland Stream

We compared fire exposed with normal abscised eucalyptus leaves incubated in a stream running through eucalyptus plantations in central Portugal, in terms of breakdown rates, microbial activity, diversity and macroinvertebrate abundance. Although leaves exposed to fire had lower nutritional value, mass loss was similar for both leaf types (k = 0.0089–0.0095 d^–1 for fire and k = 0.0084–0.00103 d^–1 for normal leaves). Fungal biomass was similar among treatments, whereas sporulation and microbial respiration were lower in fire exposed leaves. Both leaf types had similar aquatic hyphomycetes communities. Physical fragmentation was important in fire exposed leaves breakdown. Invertebrates colonized leaves in low numbers in both treatments. Alteration of leaf litter quality determined by fires in streams does not seem to determine changes in ecosystem functioning in a short term. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Can Metal Nanoparticles Be a Threat to Microbial Decomposers of Plant Litter in Streams?

The extensive use of nanometal-based products increases the chance of their release into aquatic environments, raising the question whether they can pose a risk to aquatic biota and the associated ecological processes. Aquatic microbes, namely fungi and bacteria, play a key role in forested streams by decomposing plant litter from terrestrial vegetation. Here, we investigated the effects of nanocopper oxide and nanosilver on leaf litter decomposition by aquatic microbes, and the results were compared with the impacts of their ionic precursors. Alder leaves were immersed in a stream of Northwest Portugal to allow microbial colonization before being exposed in microcosms to increased nominal concentrations of nanometals (CuO, 100, 200 and 500 ppm; Ag, 100 and 300 ppm) and ionic metals (Cu²⁺ in CuCl₂, 10, 20 and 30 ppm; Ag⁺ in AgNO₃, 5 and 20 ppm) for 21 days. Results showed that rates of leaf decomposition decreased with exposure to nano- and ionic metals. Nano- and ionic metals inhibited bacterial biomass (from 68.6% to 96.5% of control) more than fungal biomass (from 28.5% to 82.9% of control). The exposure to increased concentrations of nano- and ionic metals decreased fungal sporulation rates from 91.0% to 99.4%. These effects were accompanied by shifts in the structure of fungal and bacterial communities based on DNA fingerprints and fungal spore morphology. The impacts of metal nanoparticles on leaf decomposition by aquatic microbes were less pronounced compared to their ionic forms, despite metal ions were applied at one order of magnitude lower concentrations. Overall, results indicate that the increased release of nanometals to the environment may affect aquatic microbial communities with impacts on organic matter decomposition in streams.     

The influence of dissolved nutrients and particulate organic matter quality on microbial respiration and biomass in a forest stream

1. Although dissolved nutrients and the quality of particulate organic matter (POM) influence microbial processes in aquatic systems, these factors have rarely been considered simultaneously. We manipulated dissolved nutrient concentrations and POM type in three contiguous reaches (reference, nitrogen, nitrogen + phosphorus) of a low nutrient, third‐order stream at Hubbard Brook Experimental Forest (U.S.A). In each reach we placed species of leaves (mean C : N of 68 and C : P of 2284) and wood (mean C : N of 721 and C : P of 60 654) that differed in elemental composition. We measured the respiration and biomass of microbes associated with this POM before and after nutrient addition. 2. Before nutrient addition, microbial respiration rates and biomass were higher for leaves than for wood. Respiration rates of microbes associated with wood showed a larger response to increased dissolved nutrient concentrations than respiration rates of microbes associated with leaves, suggesting that the response of microbes to increased dissolved nutrients was influenced by the quality of their substrate. 3. Overall, dissolved nutrients had strong positive effects on microbial respiration and fungal, but not bacterial, biomass, indicating that microbial respiration and fungi were nutrient limited. The concentration of nitrate in the enriched reaches was within the range of natural variation in forest streams, suggesting that natural variation in nitrate among forest streams influences carbon mineralisation and fungal biomass.     

Leaf breakdown and associated macroinvertebrates in alpine glacial streams

1. The relationship between macroinvertebrate assemblages and the breakdown of alder [Alnus viridis (Chaix), Dc.] leaves was examined by exposing leaf packs in four streams in an alpine glacial floodplain over 8 months. Although glacially fed, the four sites (pro-glacial, glacial lake outlet, main channel, and a side-channel with a mix of water sources) differed physically and contained different benthic communities.
2. Leaf breakdown and associated fungal properties differed widely among sites. Leaf decay rate varied by an order of magnitude ( k ranged from 0.0029 to 0.0305 day^–1), and was fastest at the lake outlet (< 20% leaf mass remaining by day 45) and slowest at the pro-glacial site (> 75% remaining on day 45). Rapid processing at the lake outlet was because of the presence of Acrophylax zerberus Brauer, a shredding caddisfly.
3. There were few macroinvertebrate taxa at the pro-glacial site (two to four taxa present in packs) and leaf breakdown was attributed primarily to micro-organisms. Leuctra abundance in leaf packs was strongly correlated with fungal biomass but not with the sporulation activity of any specific aquatic hyphomycete. Other taxa, such as Baetis and chironomids, showed no relationship with any leaf characteristic, suggesting that leaf packs were used mainly as a habitat and not as a food resource.
4. The predatory stonefly Isoperla was significantly associated with the abundance of macroinvertebrate prey ( Baetis , Chironomidae and Leuctra ) in leaf packs at the main and side-channel sites. The results indicate that leaf breakdown can vary widely in alpine lotic environments, reflecting site-specific differences in habitat characteristics, and in macroinvertebrate and fungal composition.