Forested Headwater Streams Research Articles

Large eddy simulation of turbulence and solute transport in a forested headwater stream

The large eddy simulation (LES) module of the Virtual StreamLab (VSL3D) model is applied to simulate the flow and transport of a conservative tracer in a headwater stream in Minnesota, located in the south Twin Cities metropolitan area. The detailed geometry of the stream reach, which is ∼135 m long, ∼2.5 m wide, and ∼0.15 m deep, was surveyed and used as input to the computational model. The detailed geometry and location of large woody debris and bed roughness elements up to ∼0.1 m in size were also surveyed and incorporated in the numerical simulation using the Curvilinear Immersed Boundary approach employed in VSL3D. The resolution of the simulation, which employs up to a total of 25 million grid nodes to discretize the flow domain, is sufficiently fine to directly account for the effect of large woody debris and small cobbles (on the streambed) on the flow patterns and transport processes of conservative solutes. Two tracer injection conditions, a pulse and a plateau release, and two cross sections of measured velocity were used to validate the LES results. The computed results are shown to be in good agreement with the field measurements and tracer concentration time series. To our knowledge, the present study is the first attempt to simulate via high‐resolution LES solute transport in a natural stream environment taking into account a range of roughness length scales spanning an order of magnitude: from small cobbles on the streambed (∼0.1 m in diameter) to large woody debris up to ∼3 m long.

Increases in humic and bioavailable dissolved organic matter in a forested New England headwater stream with increasing discharge

Understanding the processes controlling the transfer of organic matter from terrestrial to aquatic ecosystems is of fundamental importance for the aquatic sciences. Over the course of a full year, fluorescence, absorbance and bioavailability of dissolved organic matter (DOM) were characterised in Bigelow Brook, a forested headwater stream in Massachusetts, USA. Parallel factor analysis (PARAFAC) identified a four-component model to describe observed DOM fluorescence (C1–C4). Component C2 exhibited the characteristics of a more humic-like fluorophore, with a potentially more reduced redox state and increased with discharge, whereas more fulvic-like (C1) and protein-like (C3, C4) fluorophores decreased. Under both dark and light-exposed conditions, percentage bioavailable dissolved organic carbon (%BDOC) increased with discharge (R2 = 0.37 and R2 = 0.56). C2 and specific absorptivity (SUVA) were reduced following BDOC incubations, whereas C1, C3 and C4 increased. These changes to DOM characteristics with increasing discharge were observed under both baseflow and stormflow conditions, indicating that with rising watertable, loading from a large riparian or hyporheic pool of organic matter is likely occurring. Other headwater streams, where loading is controlled by hillslope processes, are likely to exhibit a similar pattern of increasing export of more humic and bioavailable DOM during hydrologic events.

岷江上游高山森林溪流木质残体碳、氮和磷贮量特征

PDF HTML阅读 XML下载 导出引用 引用提醒 岷江上游高山森林溪流木质残体碳、氮和磷贮量特征 DOI: 10.5846/stxb201409081772 作者: 作者单位: 四川农业大学生态林业研究所,四川农业大学生态林业研究所,理县林业局,四川农业大学生态林业研究所,四川农业大学生态林业研究所,四川农业大学生态林业研究所 作者简介: 通讯作者: 中图分类号: S718.5 基金项目: 国家自然科学基金重点项目(31270498, 31170423); 国家"十二五"科技支撑计划(2011BAC09B05);四川省杰出青年学术与技术带头人培育项目(2012JQ0008, 2012JQ0059); 中国博士后科学基金特别资助项目(2012T50782) Carbon, nitrogen, and phosphorus storage of woody debris in headwater streams in an alpine forest in the upper reaches of the Mingjiang River Author: Affiliation: Key Laboratory of Ecological Forestry Engineering,Institute of Ecology Forestry,Long-term Research Station of Alpine Forest Ecosystem,Sichuan Agricultural University,Key Laboratory of Ecological Forestry Engineering,Institute of Ecology Forestry,Long-term Research Station of Alpine Forest Ecosystem,Sichuan Agricultural University,Forestry Bureau of Lixian County,Lixian,Key Laboratory of Ecological Forestry Engineering,Institute of Ecology Forestry,Long-term Research Station of Alpine Forest Ecosystem,Sichuan Agricultural University,Key Laboratory of Ecological Forestry Engineering,Institute of Ecology Forestry,Long-term Research Station of Alpine Forest Ecosystem,Sichuan Agricultural University,Key Laboratory of Ecological Forestry Engineering,Institute of Ecology Forestry,Long-term Research Station of Alpine Forest Ecosystem,Sichuan Agricultural University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:森林溪流木质残体是森林生态系统与水域之间物质循环和能量流动的主要联结之一,其碳、氮和磷贮量不仅可影响森林与溪流生态系统的结构和功能,而且与下游水体环境密切相关。因此,于2013年8月雨季以岷江上游典型高山森林为研究对象,调查了12条森林溪流木质残体的碳、氮和磷贮量分配特征,并汇算了研究区域内碳、氮和磷在溪流中单位面积的总贮量。结果表明,高山森林溪流木质残体碳、氮和磷的溪流单位面积总贮量分别为312.1 g/m2、809.5 mg/m2和110.9 mg/m2;在溪流中,木质残体碳、氮和磷贮量以径级为1-2.5 cm和2.5-5 cm的木质残体分布居多,分别共占碳、氮和磷总贮量的86.71%、87.20%和84.55%;木质残体碳、氮和磷贮量以Ⅴ腐烂级分配最多,分别共占碳、氮和磷总贮量的65.86%、67.86%和60.31%;尽管溪流各项特征与碳、氮和磷元素贮量的相关性不显著,但基本达到中度相关关系。这些结果为认识森林生态系统中以木质残体为载体的碳、氮和磷输出潜力提供了基础数据。 Abstract:Woody debris, an essential component in headwater streams in the forest, plays an important linkage role in material cycling and the flow of energy between the ecosystems of the land and water. As the decomposition proceeds, carbon, nitrogen, phosphorus, and other elements in the woody debris are gradually released into the streams. These released elements, especially nitrogen and phosphorus, are not only an important contribution into the forest ecosystems but also affect the water environment of the forest streams and the downstream ecosystems. However, little information has been available on the storage of carbon, nitrogen, and phosphorus in woody debris in the forest streams. Therefore, for this study in August, 2013, we selected 12 headwater streams, in the upper reaches of the Mingjiang River, located in an alpine primary forest in western Sichuan. In each stream, we investigated the carbon, nitrogen, and phosphorus storage of the woody debris, as well as their distribution patterns, pertaining to the diameters and the decay classes, from the head to the end, in the woody debris. The Akaike information criterion, corrected for sample size (AICC), was used for the selection of the optimum linear fit model to evaluate the total carbon, nitrogen, and phosphorus storage of woody debris in the headwater streams of the area of investigation. The results showed that the carbon, nitrogen, and phosphorus storages of woody debris per unit area of streams were 312.1 g/m2, 809.5 mg/m2, and 110.9 mg/m2, respectively, in these alpine forest headwater streams. Large variations were observed among the headwater streams pertaining to the carbon, nitrogen, and phosphorus storage of the woody debris. Moreover, both the woody debris diameter and the decay class could affect the storage of the carbon, nitrogen, and phosphorus. Woody debris at diameters of 1-2.5 cm and 2.5-5 cm contributed significantly more carbon (86.71%), nitrogen (87.20%), and phosphorus (84.55%) storage to the total storage, compared with those at other diameters. Woody debris at a diameter of >10 cm contributed the least and accounted for only 1.17%, 0.94%, and 1.56% of the total carbon, nitrogen, and phosphorus storage, respectively. In contrast, the woody debris of decay class Ⅴ showed the highest contribution to carbon, nitrogen, and phosphorus storage; which were 65.86%, 67.86%, and 60.31%, respectively, of the total carbon, nitrogen and phosphorus storage. However, the woody debris of decay classes Ⅱ and Ⅲ accounted for only 8.14%, 8.70%, and 8.94% of the total carbon, nitrogen, and phosphorus storage, respectively. There were insignificant correlations between the storages of carbon, nitrogen, and phosphorus (storage per unit stream area) of the woody debris and the characteristics (length, width, depth, area, velocity, and discharge) of streams. The results obtained here could provide the basic data and sufficient scientific evidence to understand the carbon, nitrogen, and phosphorus contribution potential of woody debris in headwater streams in alpine forest ecosystems. 参考文献 相似文献 引证文献

Scientific briefing: quantifying streambed heat advection associated with groundwater-surface water interactions

Stream thermal regimes are controlled by the interactions of external and internal energy fluxes with the water in the channel. Solar radiation is typically the dominant driver of stream water temperature, but streambed heat fluxes can be important in forested headwater streams. Past studies have presented seemingly disparate formulae for quantifying streambed heat advection from upwelling groundwater. This note details the sources of the differences in these alternative formulations. The equations illustrate the difficulties of attempting to isolate the thermal influence of groundwater–surface water interactions and highlight future research opportunities. Copyright © 2015 John Wiley & Sons, Ltd.

Lentic and lotic habitats as templets for fungal communities: traits, adaptations, and their significance to litter decomposition within freshwater ecosystems

Decomposition of plant matter is a key ecosystem process and considerable research has examined plant litter decay processes in freshwater habitats. Fungi are common inhabitants of the decomposer microbial community and representatives of all major fungal phyla have been identified within freshwater systems. Development and application of quantitative methods over the last several decades have firmly established that fungi are central players in the decomposition of plant litter in freshwaters and are important mediators of energy and nutrient transfer to higher trophic levels. Despite the critical roles that fungi play in carbon and nutrient cycling in freshwater ecosystems, there are notable differences in the types and adaptations of fungal communities between lotic and lentic habitats. These differences can be explained by the wide range of hydrologic, physical, chemical and biological conditions within freshwater systems, all of which can influence the presence, type, and activity of fungal decomposers and their impact on litter decomposition. This paper seeks to provide a brief overview of the types, adaptations, and role of fungi within lotic and lentic freshwater ecosystems, with a particular emphasis on their importance to litter decomposition and the key environmental conditions that impact their growth and decay activities. This discussion will specifically focus on fungal dynamics occurring on plant litter in forested headwater streams and emergent freshwater marshes, since published data concerning their role in these systems is considerably more abundant in comparison to other freshwater habitats.

Community patterns and ecosystem processes in forested headwater streams along a gradient of riparian canopy openness

Community patterns and ecosystem processes in forested headwater streams along a gradient of riparian canopy openness

Large wood in central Appalachian headwater streams: controls on and potential changes to wood loads from infestation of hemlock woolly adelgid

AbstractLarge wood (LW) is an important component of forested headwater streams. The character of LW loads reflects a balance between adjacent valley processes that deliver LW to the channel (herein recruitment processes) and stream channel processes that either retain or transport LW through the reach (herein retention processes). In the central Appalachian Mountains, USA, LW characteristics in headwater streams located in eastern hemlocks (Tsuga candensis) forests are expected to change because of infestation of hemlock woolly adelgid (Adelges tsugae, HWA), an exotic, invasive insect. We examined LW characteristics in 24 headwater streams ranging from un‐infested to severe infestation, as determined by hemlock canopy health. The objectives of this work were to: (i) quantify wood loads; (ii) assess the relative importance of valley recruitment and in‐stream retention mechanisms in controlling reach‐scale wood loads; and (iii) assess if there was a detectable influence of HWA on LW loads. We hypothesized that LW loads would be similar to other forested streams in eastern USA and dominated by recruitment processes. In addition, higher LW loads would correspond with advanced HWA infestation. Mean wood frequency was 38 pieces/100 m ± 17 (standard deviation); mean wood volume was 3.69 m3/100 m ± 2.76. In general, LW load characteristics were influenced by both recruitment and retention parameters; jam (accumulations ≥ 3 pieces) characteristics were dominated by retention parameters. Results suggest that adjacent stand basal area influences LW loads and once LW is recruited to the channel, streams lack sufficient hydraulic driving forces, despite having lower resistance structures, to transport LW out of the reach. Sites in moderate decline had higher proportions of short (1–2 m and 1–4 m) and very long (>10 m) LW with higher frequency of jams that were low in volume. We present a hypothesized conceptual model of expected changes to LW loads associated with HWA infestation and hemlock mortality. Copyright © 2015 John Wiley & Sons, Ltd.

Leaf-litter breakdown in tropical streams: is variability the norm?

Many forested headwater streams are heterotrophic ecosystems in which allochthonous inputs of plant litter are a major source of energy. Leaves of riparian vegetation entering the stream are broken down by a combination of biotic and abiotic processes and, in most temperate and boreal streams, provide food and habitat for dense populations of detritivorous invertebrates. However, tropical streams in different parts of the world show substantial variability in the number and diversity of leaf-shredding detritivores (hereafter detritivores). We used data obtained with standardized methods from multiple streams in Africa, the Americas, Asia, and Australia to test the hypothesis that this variability would lead to differences in the relative role of detritivores and microorganisms in the breakdown process. We also tested the hypotheses that variability in litter breakdown rates changes with litter type (native litter mixtures vs nonnative alder [Alnus glutinosa]) and is higher across regions within than outside the tropics. We found that litter breakdown rates were highly variable across sites, with no consistent pattern within geographic areas, although litter consumption by detritivores was negligible at several sites, all in America. Geographic patterns of litter breakdown also varied between litter types, with higher breakdown rates for alder than for native litter in most but not all regions. When litter breakdown rates at the tropical sites were compared to previously reported values from temperate and boreal regions, we found that differences in variability between tropical and temperate sites were inconsistent, with great differences among studies. Further global-scale studies will be needed to assess the extent to which latitudinal changes in the diversity and composition of microbial and detritivore assemblages contribute to variability in litter breakdown rates.

Stream invertebrate productivity linked to forest subsidies: 37 stream-years of reference and experimental data.

Riparian habitats provide detrital subsidies of varying quantities and qualities to recipient ecosystems. We used long-term data from three reference streams (covering 24 stream-years) and 13-year whole-stream organic matter manipulations to investigate the influence of terrestrial detrital quantity and quality on benthic invertebrate community structure, abundance, biomass, and secondary production in rockface (RF) and mixed substrates (MS) of forested headwater streams. Using a mesh canopy covering the entire treatment stream, we examined effects of litter ex'clusion, small- and large-wood removal, and addition of artificial wood (PVC) and leaves of varying quality on organic matter standing crops and invertebrate community structure and function. We assessed differences in functional feeding group distribution between substrate types as influenced by organic matter manipulations and long-term patterns of predator and prey production in manipulated vs. reference years. Particulate organic matter standing crops in MS of the treatment stream declined drastically with each successive year of litter exclusion, approaching zero after three years. Monthly invertebrate biomass and annual secondary production was positively related to benthic organic matter in the MS habitats. Rockface habitats exhibited fewer changes than MS habitats across all organic matter manipulations. With leaf addition, the patterns of functional group distribution among MS and RF habitats returned to patterns seen in reference streams. Secondary production per unit organic matter standing crop was greatest for the leaf addition period, followed by the reference streams, and significantly less for the litter exclusion and wood removal periods. These data indicate that the limited organic matter remaining in the stream following litter exclusion and wood removal was more refractory than that in the reference streams, whereas the added leaf material was more labile and readily converted into invertebrate production. Predator production and total production were tightly coupled in reference and treatment streams, indicating strong relationships between predators and their prey. Results from the artificial wood addition demonstrate that physical structure alone will not restore invertebrate productivity without detrital resources from the riparian forest. Our long-term studies conducted over three decades at the ecosystem scale unequivocally show the necessity of maintaining and restoring aquatic-terrestrial linkages in forested headwater streams.

Relationships among nutrient enrichment, detritus quality and quantity, and large-bodied shredding insect community structure

Anthropogenic nutrient enrichment of forested headwater streams can enhance detrital quality, decrease standing stocks, and alter the community structure of detrivorous insects, reducing nutrient retention and decreasing ecosystem functioning. Our objective was to determine if stoichiometric principles could be used to predict genus-specific shifts in shredding insect abundance and biomass across a dissolved nutrient and detritus food quality/quantity gradient. Detritus, insect, and water samples were collected from 12 Ozark Highland headwater streams. Significant correlations were found between stream nutrients and detrital quality but not quantity. Abundance and biomass responses of four out of five tested genera were accurately predicted by consumer-resource stoichiometric theory. Low carbon:phosphorus (C:P) shredders responded positively to increased total phosphorus and/or food quality, and high C:P shredders exhibited neutral or negative responses to these variables. Genus-specific declines were correlated with decreased overall biomass in shredder assemblages, potentially causing disruptions in nutrient flows to higher level consumers with nutrient enrichment. This work provides further evidence that elevated nutrients may negatively impact shredding insect communities by altering the stoichiometry of detritus–detritivore interactions. A better understanding of stoichiometric mechanisms altering macroinvertebrate populations is needed to help inform water quality criteria for the management of headwater streams.

Dynamics of dissolved organic matter during four storm events in two forest streams: source, export, and implications for harmful disinfection byproduct formation.

Dynamics of river dissolved organic matter (DOM) during storm events have profound influences on the downstream aquatic ecosystem and drinking water safety. This study investigated temporal variations in DOM during four storm events in two forest headwater streams (the EH and JH brooks, South Korea) and the impacts on the disinfection byproducts (DBPs) formation potential. The within-event variations of most DOM quantity parameters were similar to the flow rate in the EH but not in the larger JH brook. The dissolved organic carbon (DOC) showed clockwise and counterclockwise hysteresis with the flow rate in the EH and JH brooks, respectively, indicating the importance of both flow path and DOM source pool size in determining the effects of storm events. The stream DOM became less aromatic/humified from the first to the last event in both brooks, probably due to the increasing fresh plant pool and the decreasing leaf litter pool during the course of rainy season. The DOC export during each event increased 1.3-2.7- and 1.1-7.0-fold by stormflows in the EH and JH brooks, respectively. The leaf litter and soil together was the major DOM source, particularly during early events. The enhanced DOM export probably increases the risks of DBPs formation in disinfection, as indicated by a strong correlation observed between DOC and trihalomethanes formation potential (THMFP). High correlations between two humic-like fluorescent components and THMFP further suggested the potential of assessing THMFP with in situ fluorescence sensors during storms.

Export, biodegradation, and disinfection byproduct formation of dissolved and particulate organic carbon in a forested headwater stream during extreme rainfall events

Abstract. Despite an increasing recognition of the importance of extreme rainfall events for organic carbon export to inland waters, little attention has been paid to the export and reactivity of particulate organic carbon (POC) and dissolved organic C (DOC) in mountainous headwater watersheds under monsoon climates. To investigate environmental implications of storm-enhanced export of POC and DOC in mountainous headwater streams, we examined the relationships between storm magnitude and C export from a forested watershed in the Haean Basin, South Korea, during 13 storm events over 4 years and compared potentials of DOC and POC for biodegradation and disinfection byproduct (DBP) formation during an extreme rainfall event with a total rainfall of 209 mm. Event mean concentrations and export of POC increased nonlinearly above thresholds of precipitation and discharge, far exceeding the relatively small increases of DOC. The export of POC during a few storm events with a total rainfall above 200 mm per event exceeded the annual organic C export during dry years. During the 209 mm storm event, concentrations of total trihalomethanes formed by POC-derived dissolved components changed synchronously with POC concentrations, exhibiting lower levels than those formed by DOC. During a 30-day incubation at 25 °C, DOC exported during peak flow exhibited rapid biodegradation of labile components within 7 days. In contrast, the concentrations of DOC leached from POC gradually increased following the initial decline. Gradual transformation of POC-derived dissolved materials resulted in greater increases in the intensity of fulvic- and humic-like fluorescent components compared to the DOC treatment. The results highlight the significance of extreme rainfall events as "hot moments" for POC export from mountainous watersheds and also suggest that storm pulses of POC can provide potential sources of reactive organic components that can rapidly biodegrade and form DBPs after being released into headwater streams.

Patterns of bedload entrainment and transport in forested headwater streams of the Columbia Mountains, Canada

AbstractWe monitor bedload transport and water discharge at six stations in two forested headwater streams of the Columbia Mountains, Canada. The nested monitoring network is designed to examine the effects of channel bed texture, and the influence of alluvial (i.e. step pools and riffle pools) and semialluvial morphologies (i.e. boulder cascades and forced step pools) on bedload entrainment and transport. Results indicate that dynamics of bedload entrainment are influenced by differences in flow resistance attributable to morphology. Scaled fractional analysis shows that in reaches with high form resistance most bedload transport occurs in partial mobility fashion relative to the available bed material, while calibers finer than 16 mm attain full mobility during bankfull flows. Equal mobility transport for a wider range of grain sizes is achieved in reaches exhibiting reduced form resistance. Our findings confirm that the Shields value for mobilization of the median surface grain size depends on channel gradient and relative submergence; however, we also find that these relations vary considerably for cobble and gravel bed channels due to proportionality between dimensionless shear stress and grain size. Exponents of bedload rating curves across sites correlate most with the D90s of the mobile bed, however, where grain effects are controlled (i.e. along individual streams), differences in form resistance across morphologies exert a primary control on bedload transport dynamics. Application of empirical formulae developed for use in steep alpine channels present variable success in predicting transport rates in forested snowmelt streams. Formulae that explicitly account for reductions in mobile bed area and high morphological resistance associated with woody debris provide the best approximation to observed empirical data. Copyright © 2014 John Wiley & Sons, Ltd.

The effects of riparian logging on terrestrial invertebrate inputs into forested headwater streams

Terrestrial invertebrates (TIs) are an important food source for stream salmonids, especially in low-productivity headwater streams. Studies have shown that TIs can account for >50% of a salmonid’s summer diet, and the consumption of larger TIs is often the difference between feeding above or below maintenance ration. Riparian disturbance is assumed to affect the biomass and composition of TIs entering a stream, but consistent patterns have not emerged. In this study, pan traps were used to quantify the effects of riparian tree harvesting at 0, 50, and 90% basal area harvest (BAH) on TI inputs into 8 Appalachian headwater streams in West Virginia during the summers of 2007 and 2008. The biomass and composition of TI inputs were affected by both treatment and year. The 90% BAH treatments had significantly higher TI input biomass due to a shift in composition toward larger taxa. At greater harvest intensities, increases of grasshoppers (Acrididae) and katydids (Tettigoniidae), wood-associated Coleoptera, ants (Formicidae), and pollinating hymenopterans were observed. No significant changes were attributed to the 50% BAH treatments. These results indicate forestry practices can significantly affect TI inputs into Appalachian headwater streams and have the potential to alter fish production.

Downstream alteration of the composition and biodegradability of particulate organic carbon in a mountainous, mixed land-use watershed

Despite increasing recognition of storm-induced organic carbon (C) export as a significant loss from the terrestrial C balance, little is known about the biodegradation and chemical transformation of particulate organic carbon (POC) in mountainous river systems. We combined analyses of C isotopes, solution-state 1H NMR, and lipid biomarkers with biodegradable dissolved organic C (BDOC) measurements to investigate downstream changes of POC composition and biodegradability at a mountainous, mixed land-use watershed in South Korea. Water and suspended sediment (SS) samples were collected in a forested headwater stream, a downstream agricultural stream, and two downstream rivers during peak flow periods of four storm events, using either sequential grab sampling from the headwater stream to the most downstream river within a few hours around the peak flow or sediment samplers deployed during the whole storm event. DOC concentrations exhibited relatively small changes across sites, whereas POC concentrations were highest in the agricultural stream, and tapered along downstream reaches. The δ13C and δ15N of SS in the agricultural stream were distinct from up- and downstream signatures and similar to those for erosion source soils and lake bottom sediment, although increases in radiocarbon age indicated continuous compositional changes toward the lake. 1H NMR spectra of SS and deposited sediment exhibited downstream decreases in carbohydrates and lignin but enrichment of organic structures related to microbial proteins and plant wax. The downstream sediments had more microbial n-alkanes and lipid markers indicating anthropogenic origin such as coprostanol compared to the forest soil n-alkanes dominated by plant wax. While the BDOC concentrations of filtered waters differed little between sites, the BDOC concentrations and protein- to humic-like fluorescence ratios of DOC leached from SS during a 13-day incubation were higher in downstream rivers, pointing to contribution of labile POC components to the enhanced biodegradation. Overall, inputs of microbial and anthropogenic origin, in interplay with deposition and mineralization, appear to substantially alter POC composition and biodegradability during downstream transport, raising a question on the conventional view of mountainous river systems as a passive conduit of storm pulses of POC.

Artificial light as a disturbance to light‐naïve streams

Summary Artificial light at night is prevalent in human‐dominated landscapes, and streams in these landscapes can be expected to be affected by artificial lights. We hypothesised that artificial light at night would reduce the activity of aquatic insects, resulting in reduced drift rates, lower fish growth rates and lower leaf litter decomposition rates. We tested these hypotheses by installing street lights to reaches in four forested, natural streams of coastal British Columbia each paired with a control reach. Cutthroat trout (Oncorhynchus clarkii) are the top predators in these streams and feed mostly on terrestrial and drifting aquatic invertebrates. We found that the night‐time drift of aquatic invertebrates in lit reaches was ˜50% of the drift in dark reaches. However, the density of emerging aquatic insects, the density of insects falling into reaches, leaf litter decomposition rate and the number and growth rate of trout caught were not significantly different between the dark and experimentally lit reaches. We conclude that, while short‐term exposure to artificial light during the summer changes invertebrate behaviour, it does not significantly alter other trophic levels in forested headwater streams. Our results suggest that low levels of artificial light do not strongly influence stream ecosystems, but future research should determine whether this is true for all seasons and longer‐term exposure to light.

Litter processing and shredder distribution as indicators of riparian and catchment influences on ecological health of tropical streams

Terrestrial plant litter is the main source of energy for food webs in forest headwater streams. Leaf litter quality often changes when native tree species are replaced by exotic ones and land use change in the watershed can alter physico-chemistry and functional composition of invertebrate communities, ultimately impairing associated ecosystem processes. We used the composition of invertebrate functional feeding groups (FFGs) and the ecosystem process of leaf breakdown as structural and functional indicators, respectively, of ecosystem health in upland Kenyan streams. During dry and wet conditions, invertebrates were sampled in 24 streams within forest (10), mixed (7) and agriculture (7) catchments. Five forest and five agriculture streams were subsequently used for leaf litter breakdown experiments using two native (Croton macrostachyus and Syzygium cordatum) and one exotic (Eucalyptus globulus) species differing in quality. Coarse- and fine-mesh litterbags were used to compare microbial (fine-mesh) with shredder+microbial (coarse-mesh) breakdown rates, and by extension, determine the role of shredders in litter processing in these streams. Seasonal influences on water quality were observed across catchment land uses. Total suspended sediments, turbidity and total dissolved nitrogen were consistently higher during the wet than dry season. However, seasonal influences on FFGs were inconsistent. Catchment land use influenced invertebrate functional composition: 21 taxa, including eight shredders, were restricted to forest streams, but abundance was a poor discriminator of disturbance. Breakdown rates were generally higher in coarse- compared with fine-mesh litterbags for the native leaf species and the relative differences in breakdown rates among leaf species remained unaltered in both agriculture and forest streams. Shredder and microbial breakdown of leaf litter displayed contrasting responses with shredders relatively more important at forest compared with agriculture streams. However, these patterns were inconsistent across leaf species over the dry and wet seasons. Overall, shredder mediated leaf litter breakdown was dependent on leaf species, and was highest for C. macrostachyus and lowest for E. globulus. This suggests that replacement of indigenous riparian vegetation with poorer quality Eucalyptus species has the potential to reduce nutrient cycling in streams, with foodwebs becoming more reliant on microbial processing of leaf litter, which cannot support diverse consumers and complex food webs.

Detritus decomposition and nutrient dynamics in a forested headwater stream

Field experiment and modeling studies have shown that microbial processes during leaf decomposition can modify detritus nutrients and inorganic nutrients in the system. In this study, we developed three models (Models I, II, and III) for leaf decomposition in a shaded headwater stream with little nutrient input, and we predicted the nutrient patterns in the stream. We used data from Hugh White Creek, NC, and synthesized several important ecological concepts from previous studies, i.e., ecological stoichiometry, microbial nutrient mining, and microbial–substrate interaction. In Model I, we calculated decomposition based on microbial metabolism rather than using first-order decay; in Model II, we further added an alternative microbial nutrient acquisition mechanism in which microorganisms (miners) mine detritus nutrients rather than using nutrients from stream water; and in Model III, we additionally broke detritus into three substrate groups (labile, intermediate, and recalcitrant material). Results showed that Model I fit the nutrient data the poorest; for the other two models, Model III performed better in fitting nitrate and Model II did slightly better in fitting ammonium; Models II and III predicted similar nutrient patterns, which were different from the patterns by Model I, suggesting that microbial mining may play an important role in decomposition.

Phosphorus availability modulates the toxic effect of silver on aquatic fungi and leaf litter decomposition

The functioning of forested headwater streams is intimately linked to the decomposition of leaf litter by decomposers, mainly aquatic hyphomycetes, which enables the transfer of allochthonous carbon to higher trophic levels. Evaluation of this process is being increasingly used as an indicator of ecosystem health and ecological integrity. Yet, even though the individual impacts of contaminants and nutrient availability on decomposition have been well studied, the understanding of their combined effects remains limited. In the current study, we investigated whether the toxic effects of a reemerging contaminant, silver (Ag), on leaf litter decomposition could be partly overcome in situations where microorganisms were benefitting from high phosphorus (P) availability, the latter being a key chemical element that often limits detritus decomposition. We also investigated whether these interactive effects were mediated by changes in the structure of the aquatic hyphomycete community. To verify these hypotheses, leaf litter decomposition by a consortium of ten aquatic hyphomycete species was followed in a microcosm experiment combining five Ag contamination levels and three P concentrations. Indirect effects of Ag and P on the consumption of leaf litter by the detritivorous crustacean, Gammarus fossarum, were also evaluated. Ag significantly reduced decomposition but only at the highest concentration tested, independently of P level. By contrast, P and Ag interactively affected fungal biomass. Both P level and Ag concentrations shaped microbial communities without significantly affecting the overall species richness. Finally, the levels of P and Ag interacted significantly on G. fossarum feeding rates, high [Ag] reducing litter consumption and low P availability tending to intensify the feeding rate. Given the high level of contaminant needed to impair the decomposition process, it is unlikely that a direct effect of Ag on leaf litter decomposition could be observed in situ. However, subtle Ag effects in relation to nutrient levels in ecosystems could be expected. In particular, owing to higher consumption of low P leaf litter, shredding invertebrates could increase the ingestion of contaminated resources, which could, in turn, represent an important threat to headwater stream ecosystems.

A cross-continental comparison of stream invertebrate community assembly to assess convergence in forested headwater streams

We studied two forested, headwater streams to compare patterns of invertebrate community structure and consequences for ecosystem functioning in two temperate locations, Galicia (NW Spain) and Vancouver (SW Canada). The two sites were selected due to the similar dominance of congeneric invertebrate species, as well as similarity in their hydromorphological and physico-chemical characteristics. Field experiments tested for similarities and dissimilarities in the invertebrate community assembly in leaf packs in streams. Our results indicated that alder leaves always decomposed faster than eucalyptus leaves, from threefold higher in Galicia to tenfold in Vancouver. At the species level, the biogeographic factor was the main source of variation on invertebrate assemblages (84.9 %), but this percentage quickly decreased at higher levels of taxonomic resolution, i.e. family. Moreover, there was a strong leaf species influence in both sites. There were more invertebrates colonizing leaves (per unit mass) in Vancouver than in Galicia (fourfold on average), though alder leaves seemed to be always the preferred resource (5.5-fold higher density on average). Regardless, a similar trophic structure was found between sites and leaf species. Brillia spp. and Corynoneura spp., a shredder and a collector-gatherer, respectively, seemed to be the most important species and showed similar colonization patterns in both sites with potential to strongly influence the leaf processing and nutrient cycling in these ecosystems. Even though our results are limited to the similarity found between only two sites, results from other studies, where the same species have been found coexisting during leaf pack processing, reinforces our results that common rules and mechanisms determine patterns of key ecological processes on a biogeographical scale.