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.     

The influence of flow and season upon leaf-litter breakdown in monsoonal Hong Kong streams

We investigated the effects of water extraction and flow regime on leaf litter breakdown in 10 Hong Kong streams experiencing varying degrees of extraction: five with severe extraction (>85% downstream flow removed), four with moderate extraction (20–85% removed), and one without extraction (~0% removed). Breakdown rates, macroinvertebrate assemblages and nutrients were compared in upstream (reference) reaches and downstream reaches with reduced flow in each stream during high-flow (wet season) and low-flow (dry season) conditions to determine whether responses to flow were influenced by Hong Kong’s monsoonal climate. Litter breakdown was slower in downstream reaches during the wet season, and discharge was the main determinant of breakdown rate in both reaches across the 10 study streams. In contrast, inter-reach differences in breakdown during the dry season were small and unrelated to flow. Instead, breakdown rates among streams were influenced by the abundance of detritivorous shredders and chironomids (Chironominae), as well as phosphate concentrations, indicating the importance of biological processing of litter under low-flow conditions. Although litter breakdown rates appear to offer a promising functional indicator of flow-reduction impacts during the wet season, the pattern seen reflected the physical effects of flow rather than any underlying biological processes, and the lack of response to flow reduction during the dry season was notable.     

Do non-native Platanus hybrida riparian plantations affect leaf litter decomposition in streams?

In forest headwater streams where the riparian canopy limits autochthonous primary production, leaf litter decomposition is a key process controlling nutrient and carbon cycling. Any alteration of the riparian vegetation may influence litter decomposition and detrital food webs. We evaluated the effect of non-native Platanus hybrida riparian plantations on leaf litter decomposition in Mediterranean streams. The experiment was conducted in six headwater streams; three lined by native riparian vegetation and three crossing P. hybrida plantations. We have characterized the processing rates of alder leaves and the assemblages of shredder macroinvertebrates and fungi. Litter decomposition was significantly faster in the P. hybrida than in the reference streams. Although the dissolved inorganic nitrogen concentration was higher in P. hybrida, no significant effect was observed in decomposition rates. Differences in decomposition rates reflected the macroinvertebrate and shredder colonization in alder litter, with higher abundance and richness in the P. hybrida streams. However, aquatic hyphomycete sporulation rate was higher in reference streams, suggesting that the variation in decomposition rates is a direct consequence of shredder abundance. Our findings support part of the substrate quality-matrix quality (SMI) hypothesis, which expects that high-quality litter will show increased decomposition rates in a low-quality litter matrix.     

Effect of small reservoirs on leaf litter decomposition in Mediterranean headwater streams

Leaf litter decomposition is a crucial process providing matter and energy to communities inhabiting headwater streams. This process could be affected by many man-made landscape transformations and its response can vary depending on the climate setting. In this study, we test the hypothesis that the presence of small headwater reservoirs decreases litter decomposition downstream, as reported for temperate Oceanic climatic regions, and that this effect is more accentuated in the Mediterranean. The effect of small dams on the decomposition of alder (Alnus glutinosa) leaves was studied in four headwater streams in Catalonia (NE Spain). The presence of a dam affected litter decomposition rates in three of the four streams studied, and this depended on reservoir typology. In those with seasonal surface release, decomposition rates were slower downstream from the dams, but in the case of a continuous hypolimnetic release, it was faster, with higher DIN and temperature and abundance of shredders. Alder litter decomposition rates were twice those reported for Oceanic climatic conditions. In Mediterranean headwaters, the effect of small dams will even be more evident at an annual scale due to the diminished flow rates in summer and this effect will be more pronounced than in the more Oceanic.     

Leaf-litter processing in headwater streams of northern Iberian Peninsula: moderate levels of eutrophication do not explain breakdown rates

Agricultural and urban runoffs result in increased nitrogen and phosphorus inputs in rivers and are the cause of eutrophication. Headwater streams are less frequently affected by these impairments because of the low-to-moderate human activities there. Eutrophication can affect the structure and function of benthic communities in headwater streams, stimulating the activity of heterotrophic microorganisms and macroinvertebrates on a pivotal process such as leaf-litter decomposition. In this study, we monitored the breakdown of alder leaves in six headwater streams that constitute a moderate nutrient enrichment gradient. Breakdown experiments were conducted in autumn–winter and leaf carbon, nitrogen and phosphorus, and associated aquatic hyphomycetes and macroinvertebrates were determined. The increase in nutrient availability in the stream water enhanced leaf-litter quality and led to an increase in the hyphomycete assemblage evenness and a reduction of shredder densities. However, contrary to our expectations, dissolved nutrient availability did not explain the breakdown rates. Thus, the absence of a clear effect of nutrient enrichment of stream water on the leaf breakdown rate highlights the difficulties of predicting the response of this ecosystem process to slight levels of eutrophication in headwater streams.     

Fishpond dams affect leaf‐litter processing and associated detritivore communities along intermittent low‐order streams

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Future increase in temperature more than decrease in litter quality can affect microbial litter decomposition in streams

The predicted increase in atmospheric CO(2) concentration for this century is expected to lead to increases in temperature and changes in litter quality that can affect small woodland streams, where water temperature is usually low and allochthonous organic matter constitutes the basis of the food web. We have assessed the individual and interactive effect of water temperature (5 and 10°C) and alder litter quality produced under ambient CO(2) levels (ambient litter) or under CO(2) concentrations predicted for 2050 (elevated litter) on litter decomposition and on fungal activity and assemblage structure. Litter decomposition rates and fungal respiration rates were significantly faster at 10 than at 5°C, but they were not affected by litter quality. Litter quality affected mycelial biomass accrual at 5 but not at 10°C, while increases in temperature stimulated biomass accrual on ambient but not on elevated litter. A similar pattern was observed for conidial production. All variables were stimulated on elevated litter at 10°C (future scenario) compared with ambient litter at 5°C (present scenario), but interactions between temperature and litter quality were additive. Temperature was the factor that most strongly affected the structure of aquatic hyphomycete assemblages. Our results indicate that if future increases in atmospheric CO(2) lead to only slight modifications in litter quality, the litter decomposition and fungal activities and community structure will be strongly controlled by increased water temperature. This may have serious consequences for aquatic systems as faster litter decomposition may lead to food depletion for higher trophic levels.     

Effects of nutrient enrichment on boreal streams: invertebrates, fungi and leaf-litter breakdown

1. The effect of nutrient enrichment on structural (invertebrate indices) and functional (leaf‐litter breakdown rates) characteristics of stream integrity was studied in nine boreal streams. 2. The results showed predicted changes in biotic indices and leaf‐litter breakdown along a complex (principal component) nutrient gradient. Biotic indices were better correlated with nutrient effects than leaf‐litter breakdown. 3. Fungal biomass and invertebrate densities in the litter bags were positively correlated with leaf‐litter breakdown, and both were also positively related to the nutrient gradient. 4. Invertebrate community composition influenced breakdown rate. High breakdown rates at one site were associated with the high abundance of the detritivore Asellus aquaticus. 5. This study lends support to the importance of invertebrate and fungi as mediators of leaf‐litter decomposition. However, our study also shows that study design (length of incubation) can confound the interpretation of nutrient‐induced effects on decomposition.     

Influences of low-head dams on the fish assemblages in the headwater streams of the Qingyi watershed, China

The influences of low-head dams on the fish assemblages were examined in this study, using fish data collected in six treatment and five reference sites at three low-head dams in the headwater streams of the Qingyi watershed, China. Comparing with those in the reference sites, local habitat variables were significantly altered by low-head dams in the treatment sites, involving wider channel (only in the impoundment area), deeper water and slower flow. Fish species richness varied significantly across seasons, not across site categories, suggesting that these low-head dams did not alter species richness. However, significant decreases in fish abundance and density were observed in the impoundment areas immediately upstream of dams, but not in the plunge areas downstream. Fish assemblage structures kept relative stability across seasons, and their significant difference between-site was only observed between the impoundment areas and the sites far from dams upstream. This variation in assemblage structures was due to the differing relative abundance of some co-occurring species; more lentic but less lotic fish was observed in the impoundment areas. The spatial and temporal patterns of fish assemblages were correlated with local habitat in this study area. Wetted width had negative correlation with fish species richness, abundance and density, respectively. Water temperature also positively affected species richness. In addition, wetted width, water depth, current velocity and substrate were the important habitat variables influencing assemblage structures. Our results suggested that, by modifying local habitat characteristics, low-head dams altered fish abundance and density in the impoundment areas immediately upstream of dam, not in the plunge areas immediately downstream, and thereby influenced fish assemblage structures in these stream segments.     

The combined effects of chlorine and ammonia on litter breakdown in outdoor experimental streams

The response of Potamogeton crispus L. breakdown to controlled doses of different levels of chlorine and chlorine + ammonia was investigated over two years in outdoor experimental streams. In 1985, downstream riffles of 2 streams were dosed (observed in-stream concentrations) at ca. 10 μg/L Total Residual Chlorine (TRC), one stream at 64 μg/L TRC and one stream at 230 μg/L TRC. Two control streams were not dosed and the upstream riffles of each stream served as within stream controls. In 1986, the downstream riffle of one stream was dosed at 70 μg/L TRC and a second stream was dosed at 200 μg/L TRC. Four streams were also dosed with 2.5 mg/L NH₃-N: one stream with no chlorine, one stream with ca. 10 μg/L TRC, one with 56 μg/L TRC, and one with 150 μg/L TRC. A seventh stream was dosed for 2 h at 2000 μg/L TRC and 2.5 mg/L ammonia and then allowed to recover (recovery stream). Each year, litter decomposition (degree day k values) was measured during two 35 day trials (Jun–Jul and Aug–Sep). In 1985, when streams were dosed with chlorine alone, decomposition was significantly reduced with the high (230 μg/L TRC) chlorine dose. Downstream decomposition was 27% (Jun–Jul) and 59% (Aug–Sep) of the upstream (control) rate. No other chlorine effects were found during this period. In Jun–Jul 1986, there was significantly lower decomposition in the downstream dosed sites of the 200 μg/L TRC alone stream, the 146 μg/L TRC + ammonia stream and the recovery stream; downstream decay rates were (respectively) 56%, 42% and 64% of the upstream control sites. No other up-down pairs were different in July 1986. In Aug–Sep, all three streams with chlorine + ammonia (6, 56 and 146 μg/L TRC + 2,5 mg/L ammonia) and the 70 μg/L TRC alone stream had significantly lower decomposition rates in the downstream dosed sites. For these streams, downstream decay rates ranged from 46% (high chlorine + ammonia) to 73% (low chlorine + ammonia) of the upstream control rates. No other up-down pairs were different during this trial. Up and downstream sites of the stream dosed with 2.5 mg/L ammonia alone were nearly identical for both trials (< 3% difference). These results indicate that TRC at less than 250 μg/L can significantly reduce litter decomposition and strongly suggest that addition of ammonia to chlorinated water can increase the toxic effect of chlorine. currently at the Department of Fisheries and Wildlife currently at the Department of Fisheries and Wildlife     

Limited evidence that mixing leaf litter accelerates decomposition or increases diversity of decomposers in streams of eastern Canada

Most studies of terrestrial litter decomposition in streams and rivers have used leaves from a single tree species, but leaf packs in streams in eastern North America are usually mixtures of two or more species. Litter mixtures may decay more quickly than either of the component species. If so, estimates of stream energy and nutrient budgets may be inaccurate. In northern Nova Scotia, Canada, we measured mass loss from binary mixtures (1:1 mass ratio) of leaf litter in mesh bags, using freshly fallen or air-dried litter from five species of canopy trees. We repeated the experiment eight times, in summer and fall, in two streams and a small river, over 3 years. In some trials we enumerated benthic invertebrate and fungal colonization of decaying litter. Although there were marked differences in mass loss rates among litter types, decomposition was accelerated in mixtures relative to the mean of the component species in only three of eight trials, and only in mixtures containing N-rich speckled alder leaves. Mixing yellow birch and red maple leaves inhibited decomposition. Diversity (Shannon–Weaver Index), species richness, and abundance of aquatic hyphomycete fungi, as indexed by conidial production, were never greater (and sometimes less) on litter mixtures than on the component species. Total numbers, taxonomic richness and diversity of benthic invertebrates generally, and litter-feeding species in particular, were not augmented by mixing litter types. Litter mixtures appear to dilute a preferred substrate with patches of a less preferred substrate. Our results provide only weak support for the contention that combining two litter types leads to acceleration of decomposition rates. Handling editor: K. Martens     

Shifts in leaf litter breakdown along a forest–pasture–urban gradient in Andean streams

Tropical montane ecosystems of the Andes are critically threatened by a rapid land‐use change which can potentially affect stream variables, aquatic communities, and ecosystem processes such as leaf litter breakdown. However, these effects have not been sufficiently investigated in the Andean region and at high altitude locations in general. Here, we studied the influence of land use (forest–pasture–urban) on stream physico‐chemical variables (e.g., water temperature, nutrient concentration, and pH), aquatic communities (macroinvertebrates and aquatic fungi) and leaf litter breakdown rates in Andean streams (southern Ecuador), and how variation in those stream physico‐chemical variables affect macroinvertebrates and fungi related to leaf litter breakdown. We found that pH, water temperature, and nutrient concentration increased along the land‐use gradient. Macroinvertebrate communities were significantly different between land uses. Shredder richness and abundance were lower in pasture than forest sites and totally absent in urban sites, and fungal richness and biomass were higher in forest sites than in pasture and urban sites. Leaf litter breakdown rates became slower as riparian land use changed from natural to anthropogenically disturbed conditions and were largely determined by pH, water temperature, phosphate concentration, fungal activity, and single species of leaf‐shredding invertebrates. Our findings provide evidence that leaf litter breakdown in Andean streams is sensitive to riparian land‐use change, with urban streams being the most affected. In addition, this study highlights the role of fungal biomass and shredder species (Phylloicus; Trichoptera and Anchytarsus; Coleoptera) on leaf litter breakdown in Andean streams and the contribution of aquatic fungi in supporting this ecosystem process when shredders are absent or present low abundance in streams affected by urbanization. Finally, we summarize important implications in terms of managing of native vegetation and riparian buffers to promote ecological integrity and functioning of tropical Andean stream ecosystems.     

A meta‐analysis of the effects of nutrient enrichment on litter decomposition in streams

The trophic state of many streams is likely to deteriorate in the future due to the continuing increase in human‐induced nutrient availability. Therefore, it is of fundamental importance to understand how nutrient enrichment affects plant litter decomposition, a key ecosystem‐level process in forest streams. Here, we present a meta‐analysis of 99 studies published between 1970 and 2012 that reported the effects of nutrient enrichment on litter decomposition in running waters. When considering the entire database, which consisted of 840 case studies, nutrient enrichment stimulated litter decomposition rate by approximately 50%. The stimulation was higher when the background nutrient concentrations were low and the magnitude of the nutrient enrichment was high, suggesting that oligotrophic streams are most vulnerable to nutrient enrichment. The magnitude of the nutrient‐enrichment effect on litter decomposition was higher in the laboratory than in the field experiments, suggesting that laboratory experiments overestimate the effect and their results should be interpreted with caution. Among field experiments, effects of nutrient enrichment were smaller in the correlative than in the manipulative experiments since in the former the effects of nutrient enrichment on litter decomposition were likely confounded by other environmental factors, e.g. pollutants other than nutrients commonly found in streams impacted by human activity. However, primary studies addressing the effect of multiple stressors on litter decomposition are still few and thus it was not possible to consider the interaction between factors in this review. In field manipulative experiments, the effect of nutrient enrichment on litter decomposition depended on the scale at which the nutrients were added: stream reach > streamside channel > litter bag. This may have resulted from a more uniform and continuous exposure of microbes and detritivores to nutrient enrichment at the stream‐reach scale. By contrast, nutrient enrichment at the litter‐bag scale, often by using diffusing substrates, does not provide uniform controllable nutrient release at either temporal or spatial scales, suggesting that this approach should be abandoned. In field manipulative experiments, the addition of both nitrogen (N) and phosphorus (P) resulted in stronger stimulation of litter decomposition than the addition of N or P alone, suggesting that there might be nutrient co‐limitation of decomposition in streams. The magnitude of the nutrient‐enrichment effect on litter decomposition was higher for wood than for leaves, and for low‐quality than for high‐quality leaves. The effect of nutrient enrichment on litter decomposition may also depend on climate. The tendency for larger effect size in colder regions suggests that patterns of biogeography of invertebrate decomposers may be modulating the effect of nutrient enrichment on litter decomposition. Although studies in temperate environments were overrepresented in our database, our meta‐analysis suggests that the effect of nutrient enrichment might be strongest in cold oligotrophic streams that depend on low‐quality plant litter inputs.     

Elevated Aluminium Concentration in Acidified Headwater Streams Lowers Aquatic Hyphomycete Diversity and Impairs Leaf-Litter Breakdown

Aquatic hyphomycetes play an essential role in the decomposition of allochthonous organic matter which is a fundamental process driving the functioning of forested headwater streams. We studied the effect of anthropogenic acidification on aquatic hyphomycetes associated with decaying leaves of Fagus sylvatica in six forested headwater streams (pH range, 4.3-7.1). Non-metric multidimensional scaling revealed marked differences in aquatic hyphomycete assemblages between acidified and reference streams. We found strong relationships between aquatic hyphomycete richness and mean Al concentration (r = -0.998, p < 0.0001) and mean pH (r = 0.962, p < 0.002), meaning that fungal diversity was severely depleted in acidified streams. By contrast, mean fungal biomass was not related to acidity. Leaf breakdown rate was drastically reduced under acidic conditions raising the issue of whether the functioning of headwater ecosystems could be impaired by a loss of aquatic hyphomycete species.     

The effect of temperature on leaf decomposition and diversity of associated aquatic hyphomycetes depends on the substrate

We examine the relative importance of substrate quality and temperature in the establishment of aquatic hyphomycete assemblages and in their ability to decompose leaves. We used leaves of alder (Alnus glutinosa) and oak (Quercus robur) and we tested four temperatures (5°, 10°, 15° and 20 °C). Differences in decomposition rates and fungal assemblages were higher substrata than across temperatures. In both species, decomposition efficiency measured as the ratio of decay rate to fungal biomass, was greater at higher temperatures. Oak leaves were colonized by fewer aquatic hyphomycete species than was alder. Decomposition rates of oak increased with temperature but that of alder was not affected. We conclude that the substratum is a key driver of aquatic hyphomycete assemblages and can attenuate the effects of temperature differences on litter decomposition.     

Effects of whole-stream nutrient enrichment on the concentration and abundance of aquatic hyphomycete conidia in transport

The concentrations and relative abundances of aquatic hyphomycete conidia in water were followed during a three-year study in two headwater streams at Coweeta Hydrologic Laboratory, North Carolina, using the membrane-filtration technique. After a one-year pretreatment period, one of the streams was enriched continuously with inorganic nutrients (N+P) for two years while the other stream served as the reference. This ecosystem-level nutrient manipulation resulted in concentrations of aquatic hyphomycete conidia in the water of the treated stream that were 4.5-6.9 times higher than the concentrations observed during the pretreatment period and in the reference stream. Nutrient enrichment led to an increase in the number of fungal species detected on each sampling date. Changes in dominance patterns and relative abundances of individual species also were detected after treatment. Nutrient addition stimulates the reproductive activity of aquatic hyphomycetes, their colonization success and fungal-mediated leaf-litter decomposition. Such changes in the activity of the fungal community might affect higher trophic levels in lotic ecosystems.     

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

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

Leaf litter identity alters the timing of lotic nutrient dynamics

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Response of aquatic hyphomycete communities to enhanced stream retention in areas impacted by commercial forestry

1. Aquatic hyphomycetes are an important component of detritus processing in streams. Their response to enhanced stream retentiveness was tested by manipulating three streams located in Kielder Forest (northern England), a large plantation of exotic conifers, and two streams in Montagne Noire (south-west France) dominated by native broadleaf woodland. Treatment was by placement of logs or plastic litter traps into a 10–20 m stream section. Fungal spores were collected from stream water upstream and downstream of the treated sections over 1–2 years.
2. The average concentration of fungal spores in reference sections was nearly 10× greater in the French streams than in the English streams. The number of hyphomycete species was also higher in the French streams. These differences between regions were probably a consequence of the much lower standing stock and diversity of leaf litter in the English streams.
3. Despite these large regional differences, the treatment had a clear effect in all streams. Detrital standing stocks were enhanced in treated sections by up to 90% in French streams and 70% in English streams.
4. Mean spore density below treated sections increased by 1.8–14.8% in French streams and 10.2–28.9% in the naturally less retentive English streams. The number of fungal species increased significantly below the treated sections of the English streams, although not the French ones.
5. In biologically impoverished plantation streams, input of woody debris can increase detritus retention and enhance hyphomycete diversity and productivity. This may have consequent benefits for detritus processing and macroinvertebrate production.     

Leaf litter decomposition in remote oceanic island streams is driven by microbes and depends on litter quality and environmental conditions

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