Fire impacts on the biology of stream ecosystems: A synthesis of current knowledge to guide future research and integrated fire management.

Freshwater ecosystems host disproportionately high biodiversity and provide unique ecosystem services, yet they are being degraded at an alarming rate. Fires, which are becoming increasingly frequent and intense due to global change, can affect these ecosystems in many ways, but this relationship is not fully understood. We conducted a systematic review to characterize the literature on the effects of fires on stream ecosystems and found that (1) abiotic indicators were more commonly investigated than biotic ones, (2) most previous research was conducted in North America and in the temperate evergreen forest biome, (3) following a control-impact (CI) or before-after (BA) design, (4) predominantly assessing wildfires as opposed to prescribed fires, (5) in small headwater streams, and (6) with a focus on structural and not functional biological indicators. After quantitatively analyzing previous research, we detected great variability in responses, with increases, decreases, and no changes being reported for most indicators (e.g., macroinvertebrate richness, fish density, algal biomass, and leaf decomposition). We shed light on these seemingly contradicting results by showing that the presence of extreme hydrological post-fire events, the time lag between fire and sampling, and whether the riparian forest burned or not influenced the outcome of previous research. Results suggest that although wildfires and the following hydrological events can have dramatic impacts in the short term, most biological endpoints recover within 5-10 years, and that detrimental effects are minimal in the case of prescribed fires. We also detected that no effects were more often reported by BACI studies than by CI or BA studies, raising the question of whether this research field may be biased by the inherent limitations of CI and BA designs. Finally, we make recommendations to help advance this field of research and guide future integrated fire management that includes the protection of freshwater ecosystems.

Invasion of Native Riparian Forests by Acacia Species Affects In-Stream Litter Decomposition and Associated Microbial Decomposers.

The invasion of native riparian forests by exotic tree species can lead to profound changes in the ecological integrity of freshwater ecosystems. We assessed litter decomposition of native (Alnus glutinosa and Quercus robur) and invasive (Acacia melanoxylon and Acacia dealbata) tree species, and associated microbial activity and community structure, after being immersed for conditioning in 3 reference and 3 "invaded" streams in Serra da Lousã (central Portugal) and used in microcosms simulating stream conditions. Litter decomposition differed among species, in the order: Al. glutinosa > Q. robur > (Ac. melanoxylon ~ Ac. dealbata). Alnus glutinosa litter decomposed faster probably because it was soft and had high nitrogen concentration for decomposers. Quercus robur litter decomposed slower most likely because it was tough and had high polyphenol and low nitrogen concentrations. Acacia melanoxylon litter was the toughest and had a thick cuticle that likely acted as a physical barrier for microbial colonization. In Ac. dealbata, the small-sized leaflets and high lignin concentration may have limited microbial litter decomposition. Litter decomposition was faster in "invaded" streams, probably because they were N-limited and increases in nitrogen concentration in water, promoted by Acacia species invasion, stimulated microbial activity on litter. The aquatic hyphomycete community structure differed among litter species and between stream types, further suggesting that microbes were sensitive to litter characteristics and water nutrient concentrations. Overall, the invasion of native riparian forests by Acacia species may affect microbial decomposer activity, thus altering important stream ecosystem processes, such as litter decomposition and nutrient cycles.

Chlorpyrifos interacts with other agricultural stressors to alter stream communities in laboratory microcosms.

Chlorpyrifos is one of the most widely used agricultural insecticides in the world, but to date there is limited empirical information about its potential to interact with other common agricultural stressors. We conducted a 15-d, community-level, microcosm experiment evaluating individual and combined effects of chlorpyrifos, nutrient enrichment, and sedimentation on stream invertebrate communities (abundance, biomass, richness, size structure, composition) and ecosystem processes (primary productivity and leaf decomposition). We found that sedimentation was the most detrimental stressor, with significant negative impacts on most invertebrate community and ecosystem function variables. Even though chlorpyrifos did not cause significant invertebrate mortality in the microcosms, it still altered ecosystem function by lowering leaf decomposition rates, probably through sublethal inhibition of invertebrate shredders. Furthermore, we observed a significant reversal interaction between chlorpyrifos and sediment for small-sized invertebrates collected in gravel (abundance in sediment × insecticide microcosms was 2.4 times lower than predicted by additivity), as well as an antagonistic interaction with nutrients on invertebrate richness in the same microhabitat (richness in nutrient × insecticide microcosms was 1.6 times higher than predicted by additivity). Our results suggest that chlorpyrifos has the potential to alter freshwater ecosystem function and interact non-additively with other common agricultural stressors. These findings are in keeping with a growing body of research highlighting that multiple stressor interactions and ecosystem processes should be considered when evaluating the impacts of organic toxicants on freshwater ecosystems.

Combined effects of local habitat, anthropogenic stress, and dispersal on stream ecosystems: a mesocosm experiment.

The effects of anthropogenic stressors on community structure and ecosystem functioning can be strongly influenced by local habitat structure and dispersal from source communities. Catchment land uses increase the input of fine sediments into stream channels, clogging the interstitial spaces of benthic habitats. Aquatic macrophytes enhance habitat heterogeneity and mediate important ecosystem functions, being thus a key component of habitat structure in many streams. Therefore, the recovery of macrophytes following in-stream habitat modification may be prerequisite for successful stream restoration. Restoration success is also affected by dispersal of organisms from the source community, with potentially the strongest responses in relatively isolated headwater sites that receive a limited amount of dispersing individuals. We used a factorial design in a set of stream mesocosms to study the independent and combined effects of an anthropogenic stressor (sand sedimentation), local habitat (macrophytes, i.e., moss transplants), and enhanced dispersal (two levels: high vs. low) on organic matter retention, algal accrual rate, leaf decomposition, and macroinvertebrate community structure. Overall, all responses were simple additive effects with no interactions between treatments. Sand reduced algal accumulation, total invertebrate density, and density of a few individual taxa. Mosses reduced algal accrual rate and algae-grazing invertebrates, but enhanced organic matter retention and the number of detritus and filter feeders. Mosses also reduced macroinvertebrate diversity by increasing the dominance by a few taxa. Mosses reduced leaf mass loss, possibly because the organic matter retained by mosses provided an additional food source for leaf-shredding invertebrates and thus reduced shredder aggregation into leaf packs. The effect of mosses on macroinvertebrate communities and ecosystem functioning was distinct irrespective of the level of dispersal, suggesting strong environmental control of community structure. The strong environmental control of macroinvertebrate community composition even under enhanced dispersal suggests that re-establishing key habitat features, such as natural stream vegetation, could aid ecosystem recovery in boreal streams.

Combined effects of insecticide exposure and predation risk on freshwater detritivores.

Insecticides usually present in low concentrations in streams are known to impair behaviour and development of non-target freshwater invertebrates. Moreover, there is growing awareness that the presence of natural stressors, such as predation risk may magnify the negative effects of pesticides. This is because perception of predation risk can by itself lead to changes on behaviour and physiology of prey species. To evaluate the potential combined effects of both stressors on freshwater detritivores we studied the behavioural and developmental responses of Chironomus riparius to chlorantraniliprole (CAP) exposure under predation risk. Also, we tested whether the presence of a shredder species would alter collector responses under stress. Trials were conducted using a simplified trophic chain: Alnus glutinosa leaves as food resource, the shredder Sericostoma vittatum and the collector C. riparius. CAP toxicity was thus tested under two conditions, presence/absence of the dragonfly predator Cordulegaster boltonii. CAP exposure decreased leaf decomposition. Despite the lack of significance for interactive effects, predation risk marginally modified shredder effect on leaf decomposition, decreasing this ecosystem process. Shredders presence increased leaf decomposition, but impaired chironomids performance, suggesting interspecific competition rather than facilitation. C. riparius growth rate was decreased independently by CAP exposure, presence of predator and shredder species. A marginal interaction between CAP and predation risk was observed regarding chironomids development. To better understand the effects of chemical pollution to natural freshwater populations, natural stressors and species interactions must be taken into consideration, since both vertical and horizontal species interactions play their role on response to stress.

Effects of Zn and Ti Nanoparticles on the Survival and Growth of Sclerophrys arabica Tadpoles in a Two Level Trophic System.

Under natural conditions pollutants rarely affect only a single trophic level. This study investigated the effects of titanium dioxide (Ti-NPs) and zinc oxide (Zn-NPs) nanoparticles on survival and growth of Sclerophrys arabica tadpoles exposed directly in water, indirectly through their food source (decomposing leaves), or a combination of both. Zn-NPs did not cause significant mortality. The LC50 for tadpoles exposed to Ti-NPs directly was 74.9 µg/L (95% CI 16.6-338.7 µg/L) and 18.3 µg/L (95% CI 6.5-51.9 µg/L) for tadpoles exposed directly and with pre-treated leaves. Tadpoles fed only pre-exposed food did not show significant mortality. Exposure route also affected growth. On average the tadpoles with the lowest body mass were found in the water-only exposures, followed by the water and leaf treatments. However, the tadpoles with the greatest body mass were in the treatments fed with leaves pretreated with Ti-NPs. This suggests that the NPs made their contents more available to the tadpoles.

Invertebrates, Fungal Biomass, and Leaf Breakdown in Pools and Riffles of Neotropical Streams.

We evaluated fungal biomass (as ergosterol concentration) and invertebrate colonization during leaf breakdown of Picramnia sellowii Planch. (Picramniaceae) in pools and riffles of three low-order forested streams in southeastern Brazil. We hypothesized that leaf breakdown will be higher in riffles due to the high physical fragmentation and fungal activity. The experiment was carried out during the dry season of 2012, using 108 litter bags, each containing 3 ± 0.05 g of air-dried leaves. After 7, 15, 30, 60, 90, and 120 d of incubation, six litter bags (riffle = 3 and pool = 3) were removed from each stream. Leaf breakdown rate (k) was classified as intermediate in pools and fast in riffles. We recorded similar values of remaining leaf mass in two habitats until 60 d. However, at 90 and 120 d, this process was faster in riffles. The mean fungal biomass was similar between habitats and showed an increase during the experiment at 90 d. Fauna composition differed between habitats and across sampling dates, with Chironomidae most contributing to these differences and being particularly abundant in riffles and in the initial period of leaf breakdown (until 30 d). Shredder abundance and biomass were not different between habitats and among incubation durations. Leaf breakdown (remaining leaf mass) was positively associated with fungal and shredder biomasses. However, water velocity was not related to leaf breakdown. These findings emphasize the importance of fungal and shredder organisms, as well as the low importance of water velocity, on mass loss in low-order tropical streams.

Adaptation of microbial communities to multiple stressors associated with litter decomposition of Pterocarya stenoptera.

To understand the further impacts of multiple stressors in freshwater, we investigated the effects of heavy metal (HM, Cu and Zn) and nutrient enrichments (nitrogen and phosphorus, NP) on microbial decomposition of Pterocarya stenoptera litter and the associated extracellular enzyme activities and microbial biomass with microcosms. Results showed that the decomposition rates were slower in the polluted stream waters than those in the unpolluted ones, which corresponded to lower microbial biomass and integrated enzyme activities of cellulose and ß-glucosidase. The decomposition rates were accelerated at low HM level, which was associated with the stimulated enzyme activities of hydrolytic enzymes or was stimulated by both NP levels in polluted stream waters. In particular, the hydrolase enzyme activities of microbial communities in polluted stream waters were stimulated by low HM level, suggesting that low HM level-stimulated litter decomposition may be due to the increased enzymatic activities. When microbial communities were exposed to HM and NP simultaneously, the inhibitory effect (in unpolluted stream waters) or the stimulated effect (in polluted stream waters) of low HM concentration was enhanced and attenuated, respectively, which suggests that the NP antagonistic effect against HM toxicity on litter decomposition may contribute to the litter-associated extracellular enzyme activities. These results suggest that the co-occurrence of HM and NP may have antagonistic effects on stream ecosystem functioning.

Microplastics alter the leaf litter breakdown rates and the decomposer community in subtropical lentic microhabitats.

Microplastics, pervasive pollutants in aquatic environments, have been primarily studied for their impact on marine ecosystems. However, their effects on freshwater systems, particularly in forested phytotelmata habitats, remain understudied in Subtropical systems. This research examines the influence of varying microplastic concentrations (0.0, 200, 2,000, 20,000, and 200,000 ppm) on leaf litter breakdown of Inga vera (in bags of 10 and 0.05 mm mesh) and the naturally associated invertebrate community occurring in forested phytotelmata. The study employs an experimental design with microplastic concentration treatments in artificial microcosms (buckets with 800 mL of rainwater) arranged in an area of Atlantic Rain Forest native vegetation of Subtropical systems. The results indicate that elevated concentrations of microplastics may enhance leaf litter breakdown (6-8%), irrespective of the bag mesh, attributed to heightened decomposer activity and biofilm formation. Consequently, this contributes to increased invertebrate richness (33-37%) and greater shredder abundance (21-37%). Indicator analysis revealed that Culicidae, Stratiomyidae, Chironomidae, Empididae, Planorbidae, and Ceratopogonidae were indicative of some microplastic concentrations. These findings underscore the significance of accounting for microplastics when evaluating the taxonomic and trophic characteristics of invertebrate communities, as well as the leaf breakdown process in Subtropical systems.

Elevated Fungicide and Nutrient Concentrations Change Structure but not Function of Aquatic Microbial Communities.

Leaf decomposition is a key process in stream ecosystems within forested catchments; it is driven by microbial communities, particularly fungi and bacteria. These microorganisms make nutrients and energy bound in leaves available for wider parts of the food web. Leaf-associated microorganisms are subjected to anthropogenic pressures, such as the increased exposure to nutrients and fungicides associated with land-use change. We assessed the sensitivity of leaf-associated microbial communities with differing exposure histories, namely, from pristine (P) streams, and streams impacted by wastewater (W) and agricultural run-off (vineyards; V). In the laboratory, microbial communities were exposed to elevated nutrient (NO3-N: 0.2-18.0 mg/L, PO4-P: 0.02-1.8 mg/L) and fungicide concentrations (sum concentration 0-300 µg/L) in a fully crossed 3 × 4 × 4-factorial design over 21 days. Leaf decomposition and exoenzyme activity were measured as functional endpoints, and fungal community composition and microbial abundance served as structural variables. Overall, leaf decomposition did not differ between fungicide treatments or exposure histories. Nonetheless, substantial changes in the fungal community composition were observed after exposure to environmentally relevant fungicide concentrations. Elevated nutrient concentrations assisted leaf decomposition, and the effect size depended on the exposure history. The observed changes in the fungal community composition support the principle of functional redundancy, with highly efficient decomposers maintaining leaf decomposition. Environ Toxicol Chem 2024;43:1300-1311. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Intraspecific leaf trait variation mediates edge effects on litter decomposition rate in fragmented forests.

There is strong trait dependence in species-level responses to environmental change and their cascading effects on ecosystem functioning. However, there is little understanding of whether intraspecific trait variation (ITV) can also be an important mechanism mediating environmental effects on ecosystem functioning. This is surprising, given that global change processes such as habitat fragmentation and the creation of forest edges drive strong trait shifts within species. On 20 islands in the Thousand Island Lake, China, we quantified intraspecific leaf trait shifts of a widely distributed shrub species, Vaccinium carlesii, in response to habitat fragmentation. Using a reciprocal transplant decomposition experiment between forest edge and interior on 11 islands with varying areas, we disentangled the relative effects of intraspecific leaf trait variation versus altered environmental conditions on leaf decomposition rates in forest fragments. We found strong intraspecific variation in leaf traits in response to edge effects, with a shift toward recalcitrant leaves with low specific leaf area and high leaf dry matter content from forest interior to the edge. Using structural equation modeling, we showed that such intraspecific leaf trait response to habitat fragmentation had translated into significant plant afterlife effects on leaf decomposition, leading to decreased leaf decomposition rates from the forest interior to the edge. Importantly, the effects of intraspecific leaf trait variation were additive to and stronger than the effects from local environmental changes due to edge effects and habitat loss. Our experiment provides the first quantitative study showing that intraspecific leaf trait response to edge effects is an important driver of the decrease in leaf decomposition rate in fragmented forests. By extending the trait-based response-effect framework toward the individual level, intraspecific variation in leaf economics traits can provide the missing functional link between environmental change and ecological processes. These findings suggest an important area for future research on incorporating ITV to understand and predict changes in ecosystem functioning in the context of global change.

Standard Versus Natural: Assessing the Impact of Environmental Variables on Organic Matter Decomposition in Streams Using Three Substrates.

The decomposition of allochthonous organic matter, such as leaves, is a crucial ecosystem process in low-order streams. Microbial communities, including fungi and bacteria, colonize allochthonous organic material, break up large molecules, and increase the nutritional value for macroinvertebrates. Environmental variables are known to affect microbial as well as macroinvertebrate communities and alter their ability to decompose organic matter. Studying the relationship between environmental variables and decomposition has mainly been realized using leaves, with the drawbacks of differing substrate composition and consequently between-study variability. To overcome these drawbacks, artificial substrates have been developed, serving as standardizable surrogates. In the present study, we compared microbial and total decomposition of leaves with the standardized substrates of decotabs and, only for microbial decomposition, of cotton strips, across 70 stream sites in a Germany-wide study. Furthermore, we identified the most influential environmental variables for the decomposition of each substrate from a range of 26 variables, including pesticide toxicity, concentrations of nutrients, and trace elements, using stability selection. The microbial as well as total decomposition of the standardized substrates (i.e., cotton strips and decotabs) were weak or not associated with that of the natural substrate (i.e., leaves, r² < 0.01 to r² = 0.04). The decomposition of the two standardized substrates, however, showed a moderate association (r² = 0.21), which is probably driven by their similar composition, with both being made of cellulose. Different environmental variables were identified as the most influential for each of the substrates and the directions of these relationships contrasted between the substrates. Our results imply that these standardized substrates are unsuitable surrogates when investigating the decomposition of allochthonous organic matter in streams. Environ Toxicol Chem 2023;42:2007-2018. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Leaf-associated macroinvertebrate assemblage and leaf litter breakdown in headwater streams depend on local riparian vegetation.

Headwater streams harbor diverse macroinvertebrate communities and are hotspots for leaf litter breakdown. The process of leaf litter breakdown mediated by macroinvertebrates forms an important link between terrestrial and aquatic ecosystems. Yet, how the vegetation type in the local riparian zone influences leaf-associated macroinvertebrate assemblages and leaf litter breakdown rates is still not resolved. We investigated how leaf-associated macroinvertebrate assemblages and leaf litter fragmentation rates differ between forested and non-forested sites using experimental leaf litter bags in sixteen sites paired across eight headwater streams in Switzerland. Our results show that sensitive taxa of the invertebrate orders Ephemeroptera, Plecoptera and Trichoptera (EPT) and the functional group of shredders were strongly associated with forested sites with overall higher values of abundance, diversity, and biomass of EPTs in forested compared to non-forested sites. However, the importance of riparian vegetation differed between study regions, especially for shredders. Fragmentation rates, which are primarily the result of macroinvertebrate shredding, were on average three times higher in forested compared to non-forested sites. Our results demonstrate that not only the composition of the aquatic fauna but also the functioning of an essential ecosystem process depend on the vegetation type in the local riparian zone. Supplementary Information: The online version contains supplementary material available at 10.1007/s10750-022-05049-7.

A Preconditioning Paradox: Contrasting Effects of Initial Phyllosphere and Early Leaf Decomposer Microfungi on Subsequent Colonization by Leaf Decomposing Non-Unit-Restricted Basidiomycetes.

Fungal interactions during leaf decomposition can facilitate or inhibit other fungi. This experiment focused on whether preconditioning of leaf litter by microfungi that were confined to one leaf (Unit-Restricted) made leaf litter less likely to be colonized and decomposed by basidiomycetes that bind litter into mats (Non-Unit-Restricted) than non-preconditioned litter. Leaves of Manilkara bidentata in litterbags were preconditioned by incubating them for 0, 1, 2 or 3 months in flat litter/seed rain baskets 10 cm above the forest floor to avoid colonization by basidiomycete fungi. Preconditioned and non-preconditioned leaves were transferred to 5 replicate basidiomycete fungal mats of Gymnopus johnstonii for 6 weeks. Both attachment by basidiomycete fungi and percent mass loss after 6 weeks decreased significantly with increasing preconditioning time. In non-preconditioned leaves, gamma irradiation did not affect mass loss or percent white-rot despite having significantly increased numbers of basidiomycete fungal connections as compared to non-irradiated leaves. In non-preconditioned leaves, more basidiomycetes attachmented to non-irradiated than irradiated leaves suggest facilitation by phyllosphere microfungi. While basidiomycete colonization was initially facilitated by phyllosphere fungi, we inferred that degradation of resource quality led to fewer fungal attachments and less mass loss after 1-3 months of preconditioning by microfungi. The date suggest there is a 1-month time window for basidiomycete fungi to incorporate fallen leaves into their litter mats.

Effects of norfloxacin on decomposition and nutrient release in leaves of the submerged macrophyte Vallisneria natans (Lour.) Hara.

It is well known that antibiotic residuals affect the composition and structure of microbial communities. However, the consequences of these biological changes in terms of ecosystem function remain poorly understood, particularly in aquatic ecosystems. Here, we investigated the impacts of norfloxacin (NOR, 0, 0.5, and 8 mg L-1), a widely used antibiotic, on the microbial community structure on leaf surfaces of the submerged macrophyte Vallisneria natans, and the corresponding variations in litter decomposition, litter nutrient release, and water properties. Results showed that after 40 days of exposure, bacterial richness consistently decreased with increasing NOR concentration, and that richness of fungi was significantly lower in treatments adding NOR than in the control treatment. Moreover, NOR shifted the community toward NOR resistant phyla and genera, especially in the bacteria community. These community shifts resulted in the inhibition of litter decomposition and nutrient release from leaf litter to system water, accompanied by increases in dissolved oxygen concentration and pH of system water. Our results indicate that, by affecting microbial communities, NOR had significant effects on litter decomposition, litter nutrient release, and water properties, highlighting the potential harmful effects of NOR on aquatic ecosystem function.

Stimulation or inhibition: Leaf microbial decomposition in streams subjected to complex chemical contamination.

Leaf litter decomposition is a key mechanism in headwater streams, allowing the transfer of nutrients and energy into the entire food web. However, chemical contamination resulting from human activity may exert a high pressure on the process, possibly threatening the structure of heterotrophic microbial communities and their decomposition abilities. In this study, the rates of microbial Alnus glutinosa (Alnus) leaf decay were assessed in six French watersheds displaying different land use (agricultural, urbanized, forested) and over four seasons (spring, summer, autumn, winter). In addition, for each watershed at each sampling time, both upstream (less-contaminated) and downstream (more-contaminated) sections were monitored. Toxicities (estimated as toxic units) predicted separately for pesticides and pharmaceuticals as well as environmental parameters (including nutrient levels) were related to microbial decay rates corrected for temperature and a range of fungal and bacterial community endpoints, including biomass, structure, and activity (extracellular ligninolytic and cellulolytic enzymatic activities). Results showed that agricultural and urbanized watersheds were more contaminated for nutrients and xenobiotics (higher pesticides and pharmaceuticals predicted toxicity) than forested watersheds. However, Alnus decay rates were higher in agricultural and urbanized watersheds, suggesting compensatory effects of nutrients over xenobiotics. Conversely, fungal biomass in leaves was 2-fold and 1.4-fold smaller in urbanized and agricultural watersheds than in the forested watersheds, respectively, which was mostly related to pesticide toxicity. However, no clear pattern was observed for extracellular enzymatic activities except that ß-glucosidase activity positively correlated with Alnus decay rates. Together, these results highlight microbial communities being more efficient for leaf decomposition in polluted watersheds than in less contaminated ones, which is probably explained by changes in microbial community structure. Overall, our study showed that realistic chemical contamination in stream ecosystems may affect the biomass of Alnus-associated microbial communities but that these communities can adapt themselves to xenobiotics and maintain ecosystem functions.

Landscape patterns in top-down control of decomposition: omnivory disrupts a tropical detrital-based trophic cascade.

Detrital-based trophic cascades are often considered weak or absent in tropical stream ecosystems because of the prevalence of omnivorous macroconsumers and the dearth of leaf-shredding insects. In this study, we isolate top-down effects of three macroconsumer species on detrital processing in headwater streams draining Trinidad's northern mountains. We separated effects of different macroconsumers by experimentally manipulating their temporal access to isolated benthic habitat over the diel cycle. We found no evidence that omnivorous macroconsumers, including a freshwater crab (Eudaniela garmani) and guppy (Poecilia reticulata), increased leaf decomposition via consumption. By contrast, above a waterfall excluding guppies, the insectivorous killifish, Anablepsoides hartii, reduced the biomass of the leaf-shredding insect Phylloicus hansoni 4-fold, which consequently reduced leaf decomposition rates 1.6-fold. This detrital cascade did not occur below the barrier waterfall, where omnivorous guppies join the assemblage and reduce killifish densities; here killifish had no significant effects on Phylloicus or decomposition rates. These patterns of detrital processing were also observed in upstream-downstream comparisons in a landscape study across paired reaches of six streams. Above waterfalls, where killifish were present, but guppies absent, leaf decomposition rates and Phylloicus biomass were 2.5- and ~35-fold lower, respectively, compared to measurements below waterfalls. Moreover, the strength of top-down control by killifish is reflected by the 20- and 5-fold reductions in variability (±SE) surrounding mean Phylloicus biomass and leaf decomposition rates in upstream relative to downstream reaches where no top-down control was detected. Findings show a clear, detrital-based trophic cascade among killifish, a leaf-shredding insect, and leaf decomposition rates. Results also show how omnivorous guppies disrupt this cascade by depressing killifish densities, thereby releasing invertebrate shredders from predation, and significantly increasing decomposition rates. Moreover, this combination of direct and indirect trophic interactions drives patterns in decomposition rates in stream networks at a landscape scale, resulting in significantly lower rates of decomposition above vs. below barrier waterfalls. Our findings reveal that omnivory can result in significant indirect effects on a key ecosystem process, illustrating the importance of these hidden trophic pathways in detrital-based systems and suggesting that resource control in tropical systems may be even more complex than previously envisioned.

Leaf decomposition and nutrient release of three tree species in the hydro-fluctuation zone of the Three Gorges Dam Reservoir, China.

In order to understand the effect of submergence on nutrient release of the reforested tree leaves and assess the environmental risk of leaf decomposition under submergence, the mass loss and nutrient release rates of three reforestation tree species, Taxodium ascendens Brongn, Taxodium distichum (L.) Rich., and Salix matsudana Koidz., at different elevation in the hydro-fluctuation zone of the Three Gorges Dam Reservoir (TGDR) region were tested in situ. Results showed that the initial macroelement contents of the leaves of the three tree species varied among different elevations due to different submergence stresses. All foliar mass loss rates of the three tree species at 165 m a.s.l. were significantly higher than that at 175 m a.s.l. (except that of S. matsudana at 165 m a.s.l.), after 179 days of incubation commenced September 20. After 138 days of incubation commenced October 5, the foliar mass loss rates of the three tree species at 170 m a.s.l. were significantly higher than that at 175 m a.s.l. Moreover, the leaf mass loss rates of S. matsudana were higher than the other two species when compared at the same elevation of the same incubation period. In addition, foliar release rates of N and Ca in T. ascendens, C, N, and Ca in T. distichum as well as Ca in S. matsudana at 165 m a.s.l. after 179 days of incubation and at 170 m a.s.l. after 138 days of incubation were significantly higher than that at 175 m a.s.l., respectively. Leaf mass loss rates of T. ascendens were significantly correlated with its initial leaf K, Ca, and Mg contents. In contrast, leaf mass loss rates of T. distichum had significant correlations with the initial leaf P and K contents, as well as C/P and N/P ratios. However, the mass loss rates of S. matsudana significantly correlated with initial leaf N, P, and Ca contents and C/N, C/P, and N/P ratios. Foliar nutrient release rates, especially the foliar release of C, N, and P of the three tree species, had significant correlations with initial leaf C/P and N/P ratios. Our results suggested that submergence facilitated the decomposition and nutrient release rates of the leaves of the three woody species, especially the broad leaves of S. matsudana, which may potentially cause secondary pollution to the water body of the TGDR. Thus, we suggested that a sustainable harvest of leaves of the reforested forest stands prior to submergence should be considered in the hydro-fluctuation zone so as to protect the water quality of the TGDR.

Groundwater contamination and land drainage induce divergent responses in boreal spring ecosystems.

Degradation of freshwater ecosystems has engendered legislative mandates for the protection and management of surface waters while groundwater-dependent ecosystems (GDEs) have received much less attention. This is so despite biodiversity and functioning of GDEs are currently threatened by several anthropogenic stressors, particularly intensified land use and groundwater contamination. We assessed the impacts of land drainage (increased input of dissolved organic carbon, DOC, from peatland drainage) and impaired groundwater chemical quality (NO3--N enrichment from agricultural or urban land use) on biodiversity and ecosystem functioning in 20 southern Finnish cold-water springs using several taxonomic and functional measures. Groundwater contamination decreased macroinvertebrate and bacterial diversity and altered their community composition. Changes in macroinvertebrate and bacterial communities along the gradient of water-quality impairment were caused by the replacement of native with new taxa rather than by mere disappearance of some of the original taxa. Also species richness of habitat specialist (but not headwater generalist) bryophytes decreased due to impaired groundwater quality. Periphyton accrual rate showed a subsidy-stress response to elevated nitrate concentrations, with peak values at around 2500 µg L-1, while drainage-induced spring water brownification (increased DOC) reduced both periphyton accrual and leaf decomposition rates already at very low concentrations. Our results highlight the underutilized potential of ecosystem-level functional measures in GDE bioassessment as they seem to respond to the first signs of spring ecosystem impairment, at least for the anthropogenic stressors studied by us.

Shelters of leaf-tying herbivores decompose faster than leaves damaged by free-living insects: Implications for nutrient turnover in polluted habitats.

Leaf-eating insects can influence decomposition processes by modifying quality of leaf litter, and this impact can be especially pronounced in habitats where leaf-eating insects reach high densities, for example in heavily polluted areas. We hypothesized that the decomposition rate is faster for shelters of leaf-tying larvae than for leaves damaged by free-living insects, in particular due to the accumulation of larval frass within shelters. We exposed litter bags containing samples of three different compositions (shelters built by moth larvae, leaves damaged by free-living insects and intact leaves of mountain birch, Betula pubescens ssp. czerepanovii) for one year at two heavily polluted sites near the nickel-copper smelter at Monchegorsk in north-western Russia and at two unpolluted sites. The decomposition rate of leaves damaged by free-living insects was 91% of that of undamaged leaves, whereas the mass loss of leaves composing shelters did not differ of that of undamaged leaves. These differences between leaves damaged by different guilds of herbivorous insects were uniform across the study sites, although the decomposition rate in polluted sites was reduced to 77% of that in unpolluted sites. Addition of larval frass to undamaged leaves had no effect on the subsequent decomposition rate. Therefore we suggest that damaged leaves tied by shelter-building larvae decompose faster than untied damaged leaves due to a looser physical structure of the litter, which creates favourable conditions for detritivores and soil decomposers. Thus, while leaf damage by insects per se reduces litter quality and its decomposition rate, structuring of litter by leaf-tying insects counterbalances these negative effects. We conclude that leaf-tying larvae, in contrast to free-living defoliators, do not impose negative effects on nutrient turnover rate even at their high densities, which are frequently observed in heavily polluted sites.