The role of timing, duration, and frequency of inundation in controlling leaf litter decomposition in a river-floodplain ecosystem (Tagliamento, northeastern Italy)

Dam inundation reduces ecosystem multifunctionality following riparian afforestation in the Three Gorges Reservoir Region

Afforestation is an acknowledged method for rehabilitating deteriorated riparian ecosystems, presenting multiple functions to alleviate the repercussions of river damming and climate change. However, how ecosystem multifunctionality (EMF) responds to inundation in riparian afforestation ecosystems remains relatively unexplored. Thus, this article aimed to disclose how EMF alters with varying inundation intensities and to elucidate the key drivers of this variation based on riparian reforestation experiments in the Three Gorges Reservoir Region in China. Our EMF analysis encompassed wood production, carbon storage, nutrient cycling, decomposition, and water regulation under different inundation intensities. We examined their correlation with soil properties and microbial diversity. The results indicated a substantial reduction in EMF with heightened inundation intensity, which was primarily due to the decline in most individual functions. Notably, soil bacterial diversity (23.02%), soil properties such as oxidation-reduction potential (ORP, 11.75%), and temperature (5.85%) emerged as pivotal variables elucidating EMF changes under varying inundation intensities. Soil bacterial diversity and ORP declined as inundation intensified but were positively associated with EMF. In contrast, soil temperature rose with increased inundation intensity and exhibited a negative correlation with EMF. Further insights gleaned from structural equation modeling revealed that inundation reduced EMF directly and indirectly by reducing soil ORP and bacterial diversity and increasing soil temperature. This work underscores the adverse effects of dam inundation on riparian EMF and the crucial role soil characteristics and microbial diversity play in mediating EMF in response to inundation. These insights are pivotal for the conservation of biodiversity and functioning following afforestation in dam-induced riparian habitats.

Flow dynamics in rivers with riffle-pool morphology: a dataset from case studies and field experiments

Bars and pools, common for natural riverbeds, form sequential patterns due to interactions between river flow, alluvium and vegetation. While the morphodynamics of bar-pool units are relatively well understood, far less is known about associated riffle-pool hydrodynamics because of a lack of high-resolution data collected in rivers and problems attaining natural scaling in laboratory studies. Here we present a dataset on turbulent flow structure in riffle-pool sequences of a natural river. Two case studies and two field-based experiments were carried out in a side branch of the braided gravel-bed Tagliamento River in Italy. Our case studies deliver detailed information about the three-dimensional structure of mean and turbulent flows in natural riffle-pool/run and pool-riffle/glide transitions. Field-based experiments completed with the in-stream flume models of a riffle-pool transition and a shallow jet model provide a methodological bridge for linking simplified hydrodynamic theories of shallow jets to complex flow structure documented by our case studies. Therefore, this dataset enables examination of scaling effects and can be widely used for validation of numerical models.

Multiple trophic pathways support fish on floodplains of California's Central Valley

We used compound-specific isotope analysis of carbon isotopes in amino acids (AAs) to determine the biosynthetic source of AAs in fish from major tributaries to California's Sacramento-San Joaquin river delta (i.e., the Sacramento, Cosumnes and Mokelumne rivers). Using samples collected in winter and spring between 2016 and 2019, we confirmed that algae are a critical component of floodplain food webs in California's Central Valley. Results from bulk stable isotope analysis of carbon and nitrogen in producers and consumers were adequate to characterize a general trophic structure and identify potential upstream and downstream migration into our study site by American shad Alosa sapidissima and rainbow trout Oncorhynchus mykiss, respectively. However, owing to overlap and variability in source isotope compositions, our bulk data were unsuitable for conventional bulk isotope mixing models. Our results from compound-specific carbon isotope analysis of AAs clearly indicate that algae are important sources of organic matter to fish of conservation concern, such as Chinook salmon Oncorhynchus tshawytscha in California's Central Valley. However, algae were not the exclusive source of energy to metazoan food webs. We also revealed that other sources of AAs, such as bacteria, fungi and higher plants, contributed to fish as well. While consistent with the well-supported notion that algae are critical to aquatic food webs, our results highlight the possibility that detrital subsidies might intermittently support metazoan food webs.

Litter quality and stream physicochemical properties drive global invertebrate effects on instream litter decomposition

Plant litter is the major source of energy and nutrients in stream ecosystems and its decomposition is vital for ecosystem nutrient cycling and functioning. Invertebrates are key contributors to instream litter decomposition, yet quantification of their effects and drivers at the global scale remains lacking. Here, we systematically synthesized data comprising 2707 observations from 141 studies of stream litter decomposition to assess the contribution and drivers of invertebrates to the decomposition process across the globe. We found that (1) the presence of invertebrates enhanced instream litter decomposition globally by an average of 74%; (2) initial litter quality and stream water physicochemical properties were equal drivers of invertebrate effects on litter decomposition, while invertebrate effects on litter decomposition were not affected by climatic region, mesh size of coarse-mesh bags or mycorrhizal association of plants providing leaf litter; and (3) the contribution of invertebrates to litter decomposition was greatest during the early stages of litter mass loss (0-20%). Our results, besides quantitatively synthesizing the global pattern of invertebrate contribution to instream litter decomposition, highlight the most significant effects of invertebrates on litter decomposition at early rather than middle or late decomposition stages, providing support for the inclusion of invertebrates in global dynamic models of litter decomposition in streams to explore mechanisms and impacts of terrestrial, aquatic, and atmospheric carbon fluxes.

Air temperature more than drought duration affects litter decomposition under flow intermittency

Stream intermittency - periodic sequences of water flow cessation and resumption - occurs throughout the year, across seasons. Even though temperature is a known regulator of litter decomposition in both terrestrial and aquatic environments, comparative experiments on drought durations at distinct temperatures on microbial-mediated decomposition in streams experiencing intermittency are still lacking. Here, three drought temperatures (5, 15 and 25 °C) and two durations (short: 2.5 weeks; long: 5 weeks) were applied in a microcosm study to oak leaf discs colonized in a reference stream; mass loss and associated microbial parameters (fungal biomass, microbial activity, and sporulation rates) were evaluated following re-submersion for 2 weeks. Higher mass loss was found at 15 °C than 25 °C. A prolongation of the drought exposure period had no effect on mass loss, suggesting an early (≤ 2.5 weeks) inhibitor effect of drought on microbial-mediated leaf degradation. Fungal biomass was highest at 25 °C following a short drought, and decreased with a longer drought period at both 15 °C and 25 °C. Microbial activity was not affected by either drought duration or temperature. Sporulation rates and fungal diversity were significantly reduced by the longer drought period; in the short treatment, maximum values were found at 15 °C. In contrast to longer droughts, aquatic fungal communities during short dry periods seem to invest in energetically-expensive physiological responses to desiccation (e.g., ergosterol production) promoting biomass accrual at the expense of mass loss and reproductive output. Under more severe desiccation (higher duration and temperature), the lower diversity of fungal communities seem to result in negative legacy effects for fungal growth and reproductive capacity after flow resumption. These results suggest that native riparian vegetation, through its ability to regulate temperature in streams, may be critical in protecting freshwaters from intensified severity of drought periods in streams experiencing intermittency.

Transient Flooding and Soil Covering Interfere with Decomposition Dynamics of Populus euphratica Leaf Litter: Changes of Mass Loss and Stoichiometry of C, N, P, and K

Litter decomposition plays a critical role in carbon and nutrient cycling in terrestrial and aquatic ecosystems. However, the effects transient flooding and soil covering have on leaf litter decomposition remain unclear. The changes of litter mass loss and stoichiometric ratio of C:N (the ratio of carbon to nitrogen), C:K (the ratio of carbon to potassium), C:P (the ratio of carbon to phosphorus), N:P (the ratio of nitrogen to phosphorus), and N:K (the ratio of nitrogen to potassium) of fresh Populus euphratica (P. euphratica) leaves in surface, transient flooding, and soil covering treatments were studied using litterbags in a desert riparian forest in a field decomposition experiment for a period of 640 d. The results showed that there was a significant influence of disturbance type and incubation time on litter mass loss rate and stoichiometric ratios of C:N, C:K, C:P, N:P, and N:K of fresh P. euphratica leaves, but no significant influence of the interaction between disturbance type and incubation time on leaf litter mass loss. In three treatments, five sequential phases of leaf litter mass loss rate and different temporal change patterns of stoichiometric ratio were identified within 640 d. Transient flooding was shown to affect P. euphratica leaf litter mass loss phases compared to that in no-disturbance conditions, and especially promote leaf litter mass loss within 0–173 d of incubation time. It was also demonstrated that transient flooding and soil covering can influence leaf litter decomposition, which led to the leaf litter mass loss rate and the stoichiometric ratios of C:N, C:K, C:P, N:P, and N:K exhibiting varied patterns and phases in different treatments during decay.

Contrary effects of flow intermittence and land uses on organic matter decomposition in a Mediterranean river basin

Flow interruption in intermittent rivers (IRs) generates a mosaic of terrestrial and aquatic habitats across the river network affecting ecosystem processes, as organic matter (OM) decomposition. Water use for farming in arid and semi-arid climates intensifies the dry conditions and affects local river characteristics. In that way, flow intermittence and the distribution of land uses may affect the OM processing along the river. To understand the role of IRs in global OM dynamics and how global change affecting water flow regimes determines these dynamics, it is important to estimate OM-processing rates at a basin scale. The aim of this study was to evaluate the effect of the intensity of flow intermittence on OM processing, and how this effect was modulated by local environmental factors related to land uses across a Mediterranean river basin. To do this, wood decomposition (mass loss and fungal biomass) was selected as a functional indicator. Drying duration and frequency were measured to characterize flow intermittence in different reaches along the river, as well as local environmental factors. Linear models stablished the role of factors on decomposition. The results showed that differences in decomposition rates across the river network were negatively related to the duration of flow interruption. Dissolved inorganic nitrogen associated with agriculture counteracted the negative effect of intermittence on mass loss (increasing by up to three times); but with a higher duration of dry conditions, its effect was insignificant. An increase of 20% of canopy (higher in natural areas) resulted in increases of up to 5% of mass loss. Overall, our study is relevant to understanding the interaction between flow intermittence and land uses on OM processing, especially considering the intensification of flow intermittence and its increased distribution to other regions, which is expected to be a consequence of climate warming and human activities.

Original Leaf Colonisers Shape Fungal Decomposer Communities of Phragmites australis in Intermittent Habitats

Common reed (Phragmites australis) has high biomass production and is primarily subjected to decomposition processes affected by multiple factors. To predict litter decomposition dynamics in intermittent lakes, it is critical to understand how communities of fungi, as the primary decomposers, form under different habitat conditions. This study reports the shotgun metagenomic sequencing of the initial fungal communities on common reed leaves decomposing under different environmental conditions. We demonstrate that a complex network of fungi forms already on the plant persists into the decomposition phase. Phragmites australis leaves contained at least five fungal phyla, with abundant Ascomycota (95.7%) and Basidiomycota (4.1%), identified as saprotrophs (48.6%), pathotrophs (22.5%), and symbiotrophs (12.6%). Most of the correlations between fungi in fresh and decomposing leaves were identified as co-occurrences (positive correlations). The geographic source of litter and leaf age did not affect the structure and diversity of fungal communities. Keystone taxa were mostly moisture-sensitive. Our results suggest that habitat has a strong effect on the formation of the fungal communities through keystone taxa. Nevertheless, it can also alter the proportions of individual fungal groups in the community through indirect effects on competition between the fungal taxa and their exploitation of favourable conditions.

Hydrologic flushing rates drive nitrogen cycling and plant invasion in a freshwater coastal wetland model

Coastal wetlands intercept significant amounts of nitrogen (N) from watersheds, especially when surrounding land cover is dominated by agriculture and urban development. Through plant uptake, soil immobilization, and denitrification, wetlands can remove excess N from flow-through water sources and mitigate eutrophication of connected aquatic ecosystems. Excess N can also change plant community composition in wetlands, including communities threatened by invasive species. Understanding how variable hydrology and N loading impact wetland N removal and community composition can help attain desired management outcomes, including optimizing N removal and/or preventing invasion by nonnatives. By using a dynamic, process-based ecosystem simulation model, we are able to simulate various levels of hydrology and N loading that would otherwise be difficult to manipulate. We investigate in silico the effects of hydroperiod, hydrologic residence time, N loading, and the NH4+ : NO3- ratio on both N removal and the invasion success of two nonnative species (Typha × glauca or Phragmites australis) in temperate freshwater coastal wetlands. We found that, when residence time increased, annual N removal increased up to 10-fold while longer hydroperiods also increased N removal, but only when residence time was >10 d and N loading was >30 g N·m^-2 ·yr^-1 . N removal efficiency also increased with increasing residence time and hydroperiod, but was less affected by N loading. However, longer hydrologic residence time increased vulnerability of wetlands to invasion by both invasive plants at low to medium N loading rates where native communities are typically more resistant to invasion. This suggests a potential trade-off between ecosystem services related to nitrogen removal and wetland invasibility. These results help elucidate complex interactions of community composition, N loading and hydrology on N removal, helping managers to prioritize N removal when N loading is high or controlling plant invasion in more vulnerable wetlands.

Effects of dry-wet cycles on nitrous oxide emissions in freshwater sediments: a synthesis

Background

Sediments frequently exposed to dry-wet cycles are potential biogeochemical hotspots for greenhouse gas (GHG) emissions during dry, wet and transitional phases. While the effects of drying and rewetting on carbon fluxes have been studied extensively in terrestrial and aquatic systems, less is known about the effects of dry-wet cycles on N₂O emissions from aquatic systems. As a notable part of lotic systems are temporary, and small lentic systems can substantially contribute to GHG emissions, dry-wet cycles in these ecosystems can play a major role on N₂O emissions.

Methodology

This study compiles literature focusing on the effects of drying, rewetting, flooding, and water level fluctuations on N₂O emissions and related biogeochemical processes in sediments of lentic and lotic ecosystems.

Results

N₂O pulses were observed following sediment drying and rewetting events. Moreover, exposed sediments during dry phases can be active spots for N₂O emissions. The general mechanisms behind N₂O emissions during dry-wet cycles are comparable to those of soils and are mainly related to physical mechanisms and enhanced microbial processing in lotic and lentic systems. Physical processes driving N₂O emissions are mainly regulated by water fluctuations in the sediment. The period of enhanced microbial activity is driven by increased nutrient availability. Higher processing rates and N₂O fluxes have been mainly observed when nitrification and denitrification are coupled, under conditions largely determined by O₂ availability.

Conclusions

The studies evidence the driving role of dry-wet cycles leading to temporarily high N₂O emissions in sediments from a wide array of aquatic habitats. Peak fluxes appear to be of short duration, however, their relevance for global emission estimates as well as N₂O emissions from dry inland waters has not been quantified. Future research should address the temporal development during drying-rewetting phases in more detail, capturing rapid flux changes at early stages, and further explore the functional impacts of the frequency and intensity of dry-wet cycles.

Habitat patchiness, ecological connectivity and the uneven recovery of boreal stream ecosystems from an experimental drought

Ongoing climate change is increasing the occurrence and intensity of drought episodes worldwide, including in boreal regions not previously regarded as drought prone, and where the impacts of drought remain poorly understood. Ecological connectivity is one factor that might influence community structure and ecosystem functioning post-drought, by facilitating the recovery of sensitive species via dispersal at both local (e.g. a nearby habitat patch) and regional (from other systems within the same region) scales. In an outdoor mesocosm experiment, we investigated how impacts of drought on boreal stream ecosystems are altered by the spatial arrangement of local habitat patches within stream channels, and variation in ecological connectivity with a regional species pool. We measured basal ecosystem processes underlying carbon and nutrient cycling: (a) algal biomass accrual; (b) microbial respiration; and (c) decomposition of organic matter, and sampled communities of aquatic fungi and benthic invertebrates. An 8-day drought event had strong impacts on both community structure and ecosystem functioning, including algal accrual, leaf decomposition and microbial respiration, with many of these impacts persisting even after water levels had been restored for 3.5 weeks. Enhanced connectivity with the regional species pool and increased aggregation of habitat patches also affected multiple response variables, especially those associated with microbes, and in some cases reduced the effects of drought to a small extent. This indicates that spatial processes might play a role in the resilience of communities and ecosystem functioning, given enough time. These effects were however insufficient to facilitate significant recovery in algal growth before seasonal dieback began in autumn. The limited resilience of ecosystem functioning in our experiment suggests that even short-term droughts can have extended consequences for stream ecosystems in the world's vast boreal region, and especially on the ecosystem processes and services mediated by algal biofilms.

Diversity or Redundancy in Leaf Physiological and Anatomical Parameters in a Species Diverse, Bottomland Hardwood Forest?

Research Highlights: Bottomland hardwood forests exhibit seasonal flooding, are species diverse, and provide numerous ecosystem services including floodwater storage, wildlife habitat and nutrient mitigation. However, data are needed to adequately predict the potential of individual species to achieve these services. Background and Objectives: In bottomland hardwood forests, increasing tree species richness may increase functional diversity unless species exhibit an overlap in physiological functioning. Therefore, the objectives of this study were to (1) compare physiological and anatomical leaf parameters across species, (2) determine if leaf anatomical and nutrient properties were correlated with physiological functioning, (3) determine intra-species variability in leaf stomatal properties and determine how whole crown metrics compare with leaves measured for gas exchange and (4) measure soil nitrogen for evidence of denitrification during inundation periods. Materials and Methods: We measured gas exchange, leaf nutrients and anatomical properties in eight bottomland hardwood species including Carya ovata, Fraxinus pennsylvanica, Quercus michauxii, Quercus nigra, Quercus pagoda, Quercus phellos, Ulmus alata and Ulmus americana. Additionally, we quantified soil ammonium and nitrate content during winter inundated conditions to compare with non-inundation periods. Results: We found that leaf-level water use parameters displayed greater variability and diversity across species than photosynthesis and leaf nitrogen parameters, but green ash and shagbark hickory exhibited generally high leaf N concentrations and similar physiological functioning. Elms and oaks displayed larger variability in leaf physiological functioning. Stomatal density was significantly correlated with photosynthetic capacity and tree-level water use and exhibited high intra-species variability. Conclusions: This bottomland hardwood forest contains more diversity in terms of water use strategies compared with nitrogen uptake, suggesting that differences in species composition will affect the hydrology of the system. Green ash and shagbark hickory exhibit higher leaf nitrogen concentrations and potential for nutrient mitigation. Finally, leaf anatomical parameters show some promise in terms of correlating with leaf physiological parameters across species.

Effects of Drying and Re-Wetting on Litter Decomposition and Nutrient Recycling: A Manipulative Experiment

Climate change and water abstraction may change stream flow from perennial into intermittent lotic systems, modifying their abiotic and biotic benthic environment and impacting ecosystem processes such as nutrient turnover. We conducted a microcosm experiment to investigate the interactive effect of water intermittency, macrofauna and leaf size (Populus nigra leaves) on nutrient mineralization and recycling. Leaf disks (1 or 5 cm diameter) were incubated for 40 days with or without the leaf-consumer, Potamophylax cingulatus larvae (Trichoptera, Limnephilidae) and with or without an intervening, 10-days simulation of stream drying and subsequent rewetting. Nutrient fluxes, residual leaf biomass and leaf elemental composition were measured to evaluate how intermittency, macrofauna and leaf size affect organic matter mineralization rates and stoichiometry. Results suggest that drying slows decomposition rates, impacting both the microbial and setting to zero macrofauna activities. The presence of macrofauna increases mineralization and nutrient (C, N and P) regeneration rates. Our findings also suggest that leaf disks with higher diameter display higher microbial activity and NH4+ regeneration. During the experiment, the C:N:P ratios of residual litter changed, as the leaf material became enriched with N and P. Our study suggests that increasingly frequent dry events might slow mineralization rates and downstream nutrient transport.

Microbial decomposition is highly sensitive to leaf litter emersion in a permanent temperate stream

Drought frequency and intensity in some temperate regions are forecasted to increase under the ongoing global change, which might expose permanent streams to intermittence and have severe repercussions on stream communities and ecosystem processes. In this study, we investigated the effect of drought duration on microbial decomposition of Populus nigra leaf litter in a temperate permanent stream (Oliveira, NW Portugal). Specifically, we measured the response of the structural (assemblage composition, bacterial and fungal biomass) and functional (leaf litter decomposition, extracellular enzyme activities (EEA), and fungal sporulation) parameters of fungal and bacterial communities on leaf litter exposed to emersion during different time periods (7, 14 and 21d). Emersion time affected microbial assemblages and litter decomposition, but the response differed among variables. Leaf decomposition rates and the activity of β-glucosidase, cellobiohydrolase and phosphatase were gradually reduced with increasing emersion time, while β-xylosidase reduction was similar when emersion last for 7 or more days, and the phenol oxidase reduction was similar at 14 and 21days of leaf emersion. Microbial biomass and fungal sporulation were reduced after 21days of emersion. The structure of microbial assemblages was affected by the duration of the emersion period. The shifts in fungal assemblages were correlated with a decreased microbial capacity to degrade lignin and hemicellulose in leaf litter exposed to emersion. Additionally, some resilience was observed in leaf litter mass loss, bacterial biomass, some enzyme activities and structure of fungal assemblages. Our study shows that drought can strongly alter structural and functional aspects of microbial decomposers. Therefore, the exposure of leaf litter to increasing emersion periods in temperate streams is expected to affect decomposer communities and overall decomposition of plant material by decelerating carbon cycling in streams.

A framework for evaluating food-web responses to hydrological manipulations in riverine systems

Environmental flows are used to restore elements of the hydrological regime altered by human use of water. One of the primary justifications and purposes for environmental flows is the maintenance of target species populations but, paradoxically, there has been little emphasis on incorporating the food-web and trophic dynamics that determine population-level responses into the monitoring and evaluation of environmental flow programs. We develop a generic framework for incorporating trophic dynamics into monitoring programs to identify the food-web linkages between hydrological regimes and population-level objectives of environmental flows. These linkages form the basis for objective setting, ecological targets and indicator selection that are necessary for planning monitoring programs with a rigorous scientific basis. Because there are multiple facets of trophic dynamics that influence energy production and transfer through food webs, the specific objectives of environmental flows need to be defined during the development of monitoring programs. A multitude of analytical methods exist that each quantify distinct aspects of food webs (e.g. energy production, prey selection, energy assimilation), but no single method can provide a basis for holistic understanding of food webs. Our paper critiques a range of analytical methods for quantifying attributes of food webs to inform the setting, monitoring and evaluation of trophic outcomes of environmental flows and advance the conceptual understanding of trophic dynamics in river-floodplain systems.

Structural and functional recovery of macroinvertebrate communities and leaf litter decomposition after a marked drought: Does vegetation type matter?

Climate change and anthropogenic disturbances are expected to lead to more intense and frequent droughts, with potentially severe effects on structure and function of perennial temperate streams. However, more information is required on whether streams flowing through basins already affected by exotic plantations will respond to droughts in the same way as streams under native forests. The recolonisation dynamics of benthic macroinvertebrate communities and leaf litter decomposition rates were examined in nine streams of oceanic-temperate climate that differed in catchment vegetation (three streams draining native deciduous forest, three in pine plantations and three in eucalypt plantations) after a marked drought. In each stream, five benthic samples were collected three times (ca. 1.5months between sampling dates) after flow recovery, and the taxonomic and functional trait compositions of the macroinvertebrate communities were analysed. The decomposition rate of Alnus glutinosa was measured in fine- and coarse-mesh litter bags. Benthic macroinvertebrate density, richness and diversity increased with time after flow recovery but only richness and diversity differed among stream types, with eucalypt streams showing the lowest values. Both the taxonomic and functional compositions of the macroinvertebrate community were dependent on vegetation type and time, with the differences among stream types diminishing over time. While leaf-litter decomposition rate did not depend on catchment vegetation after drought, detritivore activity was the lowest under eucalypt streams and it was positively correlated to benthic shredder density. Our results indicated that in these perennial temperate streams the catchment vegetation influenced the recovery of benthic macroinvertebrate communities after a period of drought, although the decomposition rate of leaf litter was not strongly affected. Greater understanding of the structural and functional responses of stream ecosystems to different stressors is required before the effects of expected more intense and frequent hydrological changes caused by climate change can be adequately forecast.

Drought and detritivores determine leaf litter decomposition in calcareous streams of the Ebro catchment (Spain)

Drought, an important environmental factor affecting the functioning of stream ecosystems, is likely to become more prevalent in the Mediterranean region as a consequence of climate change and enhanced water demand. Drought can have profound impacts on leaf litter decomposition, a key ecosystem process in headwater streams, but there is still limited information on its effects at the regional scale. We measured leaf litter decomposition across a gradient of aridity in the Ebro River basin. We deployed coarse- and fine-mesh bags with alder and oak leaves in 11 Mediterranean calcareous streams spanning a range of over 400km, and determined changes in discharge, water quality, leaf-associated macroinvertebrates, leaf quality and decomposition rates. The study streams were subject to different degrees of drought, specific discharge (Ls^-1km^-2) ranging from 0.62 to 9.99. One of the streams dried out during the experiment, another one reached residual flow, whereas the rest registered uninterrupted flow but with different degrees of flow variability. Decomposition rates differed among sites, being lowest in the 2 most water-stressed sites, but showed no general correlation with specific discharge. Microbial decomposition rates were not correlated with final nutrient content of litter nor to fungal biomass. Total decomposition rate of alder was positively correlated to the density and biomass of shredders; that of oak was not. Shredder density in alder bags showed a positive relationship with specific discharge during the decomposition experiment. Overall, the results point to a complex pattern of litter decomposition at the regional scale, as drought affects decomposition directly by emersion of bags and indirectly by affecting the functional composition and density of detritivores.

Combined effects of drought and the fungicide tebuconazole on aquatic leaf litter decomposition

Loss of biodiversity and altered ecosystem functioning are driven by the cumulative effects of multiple natural and anthropogenic stressors affecting both quantity and quality of water resources. Here we performed a 40-day laboratory microcosm experiment to assess the individual and combined effects of drought and the model fungicide tebuconazole (TBZ) on leaf litter decomposition (LLD), a fundamental biogeochemical process in freshwater ecosystems. Starting out from a worst-case scenario perspective, leaf-associated microbial communities were exposed to severe drought conditions (four 5-day drought periods alternated with 4-day immersion periods) and/or a chronic exposure to TBZ (nominal concentration of 20μgL(-1)). We assessed the direct effects of drought and fungicide on the structure (biomass, diversity) and activity (extracellular enzymatic potential) of fungal and bacterial assemblages colonizing leaves. We also investigated indirect effects on the feeding rates of the amphipod Gammarus fossarum on leaves previously exposed to drought and/or TBZ contamination. Results indicate a stronger effect of drought stress than fungicide contamination under the experimental conditions applied. Indeed, the drought stress strongly impacted microbial community structure and activities, inhibiting the LLD process and leading to cascading effects on macroinvertebrate feeding. However, despite the lack of significant effect of TBZ applied alone, the effects of drought on microbial functions (i.e., decrease in LLD and in enzymatic activities) and on Gammarus feeding rates were more pronounced when drought and TBZ stresses were applied together. In a perspective of ecological risk assessment and ecosystem management for sustainability, these findings stress the need for deeper insight into how multiple stressors can affect the functioning of aquatic ecosystems and associated services.

Effects of flow scarcity on leaf-litter processing under oceanic climate conditions in calcareous streams

Although temporary streams represent a high proportion of the total number and length of running waters, historically the study of intermittent streams has received less attention than that of perennial ones. The goal of the present study was to assess the effects of flow cessation on litter decomposition in calcareous streams under oceanic climate conditions. For this, leaf litter of alder was incubated in four streams (S1, S2, S3 and S4) with different flow regimes (S3 and S4 with zero-flow periods) from northern Spain. To distinguish the relative importance and contribution of decomposers and detritivores, fine- and coarse-mesh litter bags were used. We determined processing rates, leaf-C, -N and -P concentrations, invertebrate colonization in coarse bags and benthic invertebrates. Decomposition rates in fine bags were similar among streams. In coarse bags, only one of the intermittent streams, S4, showed a lower rate than that in the other ones as a consequence of lower invertebrate colonization. The material incubated in fine bags presented higher leaf-N and -P concentrations than those in the coarse ones, except in S4, pointing out that the decomposition in this stream was driven mainly by microorganisms. Benthic macroinvertebrate and shredder density and biomass were lower in intermittent streams than those in perennial ones. However, the bags in S3 presented a greater amount of total macroinvertebrates and shredders comparing with the benthos. The most suitable explanation is that the fauna find a food substrate in bags less affected by calcite precipitation, which is common in the streambed at this site. Decomposition rate in coarse bags was positively related to associated shredder biomass. Thus, droughts in streams under oceanic climate conditions affect mainly the macroinvertebrate detritivore activity, although macroinvertebrates may show distinct behavior imposed by the physicochemical properties of water, mainly travertine precipitation, which can override the flow intermittence effects.

Decomposition of Three Common Moist-Soil Managed Wetland Plant Species

Moist-soil wetland management is used to precisely control delivery, duration, and timing of water addition to, and removal from, managed wetlands with targeted responses including germination and growth of desirable moist-soil plant species. Similarly, water delivery and removal drives decomposition of moist-soil plants as well as nutrient cycling within these systems, which is a key driver of productivity in such managed wetlands. Through deployment of litter bags, we examined rate of mass loss and decay coefficients of three locally abundant moist-soil annual species that are potentially valuable wintering-waterfowl food sources (nodding smartweed Persicaria lapathifolia, red-rooted flatnut sedge Cyperus erythrorhizos, and toothcup Ammannia coccinea) within man-made moist-soil managed wetlands on the Richland Creek Wildlife Management Area in East-central Texas. All three species lost nearly 100% of their mass during an 11-mo deployment period, where rate of mass lost and decay coefficient rates were driven by time, because all moist-soil managed wetlands used were inundated for the duration of this study. Plant materials exposed to persistent inundation in shallow wetlands exhibited rates of mass loss typical of the first two stages of decomposition, during which a majority of plant material mass was lost. However, during this study, typical inundation and drawdown regimes were not implemented, which may have delayed or prolonged decomposition processes, because litter bags of focal species were inundated for the duration of this study. Both locally and regionally specific moist-soil management hydroperiod manipulation should include both drawdown and inundation, to incorporate temporal transitions between these conditions. Such practices will allow wetland managers to more expeditiously meet plant management and waterfowl food production goals within moist-soil managed wetlands.

Long-term impact of hydrological regime on structure and functions of microbial communities in riverine wetland sediments

In a context of global change, alterations in the water cycle may impact the structure and function of terrestrial and aquatic ecosystems. Wetlands are particularly at risk because hydrological regime has a major influence on microbially mediated biogeochemical processes in sediments. While the influence of water availability on wetland biogeochemical processes has been comprehensively studied, the influence of hydrological regime on microbial community structure has been overlooked. We tested for the effect of hydrological regime on the structure and functions of microbial communities by comparing sediments collected at multiple sites in the Ain département (Eastern France). Each site consisted of two plots, one permanently and one seasonally inundated. At the time of sampling, all plots were continuously inundated for more than 6 months but still harboured distinct bacterial communities. This change in community structure was not associated with marked modifications in the rates of microbial activities involved in the C and N cycles. These results suggest that the observed structural change could be related to bacterial taxa responding to the environmental variations associated with different hydrological regimes, but not strongly associated with the biogeochemical processes monitored here.

Leaf-litter decomposition across three flooding regimes in a seasonally flooded Amazonian watershed

Decomposition of leaf litter is an important process that releases energy and nutrients in both terrestrial and aquatic environments (Moore et al. 2004, Wallace et al. 1997); therefore, the physical, chemical and biological processes controlling leaf-litter decomposition rates can affect nutrient cycling and productivity in these systems (Cross et al. 2007, Wood et al. 2009). Several studies have shown that leaf decomposition is faster in aquatic than in terrestrial habitats due to relatively constant temperatures, continuous leaching and the physical breakdown of leaves by flowing water (Hutchens & Wallace 2002, Langhans & Tockner 2006, Langhans et al. 2008). Yet, comparatively few studies have examined these relationships in tropical systems with flooded forests. Flooding is a predominant feature of the upper Amazon Basin, but its occurrence and magnitude is complex and not strictly seasonal (Junk et al. 1989). To identify the dominant energy pathways and understand the nutrient dynamics of upper Amazon rain forests, it is imperative to investigate organic matter processing in the aquatic/terrestrial transition zones of these ecosystems.

Decomposition of leaf litter from a native tree and an actinorhizal invasive across riparian habitats

Dynamics of nutrient exchange between floodplains and rivers have been altered by changes in flow management and proliferation of nonnative plants. We tested the hypothesis that the nonnative, actinorhizal tree, Russian olive (Elaeagnus angustifolia), alters dynamics of leaf litter decomposition compared to native cottonwood (Populus deltoides ssp. wislizeni) along the Rio Grande, a river with a modified flow regime, in central New Mexico (U.S.A.). Leaf litter was placed in the river channel and the surface and subsurface horizons of forest soil at seven riparian sites that differed in their hydrologic connection to the river. All sites had a cottonwood canopy with a Russian olive-dominated understory. Mass loss rates, nutrient content, fungal biomass, extracellular enzyme activities (EEA), and macroinvertebrate colonization were followed for three months in the river and one year in forests. Initial nitrogen (N) content of Russian olive litter (2.2%) was more than four times that of cottonwood (0.5%). Mass loss rates (k; in units of d(-1)) were greatest in the river (Russian olive, k = 0.0249; cottonwood, k = 0.0226), intermediate in subsurface soil (Russian olive, k = 0.0072; cottonwood, k = 0.0031), and slowest on the soil surface (Russian olive, k = 0.0034; cottonwood, k = 0.0012) in a ratio of about 10:2:1. Rates of mass loss in the river were indistinguishable between species and proportional to macroinvertebrate colonization. In the riparian forest, Russian olive decayed significantly faster than cottonwood in both soil horizons. Terrestrial decomposition rates were related positively to EEA, fungal biomass, and litter N, whereas differences among floodplain sites were related to hydrologic connectivity with the river. Because nutrient exchanges between riparian forests and the river have been constrained by flow management, Russian olive litter represents a significant annual input of N to riparian forests, which now retain a large portion of slowly decomposing cottonwood litter with a high potential for N immobilization. As a result, retention and mineralization of litter N within these forests is controlled by hydrologic connectivity to the river, which affects litter export and in situ decomposition.

Understanding the impact of flooding on trait-displacements and shifts in assemblage structure of predatory arthropods on river banks

1. Species assemblages of naturally disturbed habitats are governed by the prevailing disturbance regime. Consequently, stochastic flood events affect river banks and the inhabiting biota. Predatory arthropods occupy predominantly river banks in relation to specific habitat conditions. Therefore, species sorting and stochastic processes as induced by flooding are supposed to play important roles in structuring riparian arthropod assemblages in relation to their habitat preference and dispersal ability. 2. To ascertain whether assemblages of spiders and carabid beetles from disturbed river banks are structured by stochastic or sorting mechanisms, diversity patterns and assemblage-wide trait-displacements were assessed based on pitfall sampling data. We tested if flooding disturbance within a lowland river reach affects diversity patterns and trait distribution in both groups. 3. Whereas the number of riparian spider species decreased considerably with increased flooding, carabid beetle diversity benefited from intermediate degrees of flooding. Moreover, regression analyses revealed trait-displacements, reflecting sorting mechanisms particularly for spiders. Increased flooding disturbance was associated with assemblage-wide increases of niche breadth, shading and hygrophilic preference and ballooning propensity for spider (sub)families. Trait patterns were comparable for Bembidiini carabids, but were less univocal for Pterostichini species. Body size decreased for lycosid spiders and Bembidiini carabids with increased flooding, but increased in linyphiid spiders and Pterostichini carabids. 4. Our results indicate that mainly riparian species are disfavoured by either too high or too low degrees of disturbance, whereas eurytopic species benefit from increased flooding. Anthropogenic alterations of flooding disturbance constrain the distribution of common hygrophilous species and/or species with high dispersal ability, inducing shifts towards less specialized arthropod assemblages. River banks with divergent degrees of flooding impact should be maintained throughout dynamic lowland river reaches in order to preserve typical riparian arthropod assemblages.

Uptake of Cd, Zn and Mn by willow increases during terrestrialisation of initially ponded polluted sediments

Metal concentration of plants growing on contaminated soils among other factors may depend on changes in the hydrological regime of the soil. Foliar and stem metal concentrations in Salix cinerea (grey sallow) were measured in 2 consecutive growing seasons on a submerged sediment-derived soil that underwent gradual terrestrialisation. Foliar and stem cutting concentrations for Cd, Zn and Mn increased on plots that were submerged during the first year, but emerged in the second year of monitoring. The litter layer was sampled under the shrubs of a plot with a recent abrupt change in hydrological regime and on the reference plot. It was separated in three size fractions through sieving. Analysis of the litter fractions suggested that Cd and Zn concentrations remained constant during fragmentation. However, Cr, Cu, Ni and Pb concentrations increased, which was attributed to adhesion of mineral soil particles on the fine fraction. After correction for the metal content in the mineral fraction, an increase in Cd, Mn and Cu concentration during fragmentation of the organic part of the litter layer was observed for the polluted plot. Net litter layer decomposition rate was low, which may indicate low colonisation by the decomposing community. Terrestrialisation resulted in higher Cd, Mn and Zn uptake by willows. The deviant litter layer metal concentrations for Cd, Zn and Mn and low decomposition rate must be further monitored. Feasibility of measures aiming at re-establishing wetland conditions for the dredged sediment landfill must be considered.