Litter Decomposition as an Indicator of Stream Ecosystem Functioning at Local-to-Continental Scales

RivFunction is a pan-European initiative that started in 2002 and was aimed at esta- blishing a novel functional-based approach to assessing the ecological status of rivers. Litter decomposition was chosen as the focal process because it plays a central role in stream ecosystems and is easy to study in the field. Impacts of two stressors that occur across the continent, nutrient pollution and modified riparian vegetation, were exam- ined at >200 paired sites in nine European ecoregions. In response to the former, decomposition was dramatically slowed at both extremes of a 1000-fold nutrient gra- dient, indicating nutrient limitation in unpolluted sites, highly variable responses across Europe in moderately impacted streams, and inhibition via associated toxic and addi- tional stressors in highly polluted streams. Riparian forest modification by clear cutting or replacement of natural vegetation by plantations (e.g. conifers, eucalyptus) or pasture produced similarly complex responses. Clear effects caused by specific riparian distur- bances were observed in regionally focused studies, but general trends across different types of riparian modifications were not apparent, in part possibly because of important indirect effects. Complementary field and laboratory experiments were undertaken to tease apart the mechanistic drivers of the continental scale field bioassays by addressing the influence of litter, fungal and detritivore diversity. These revealed generally weak and context-dependent effects on decomposition, suggesting high levels of redundancy (and hence potential insurance mechanisms that can mitigate a degree of species loss) within the food web. Reduced species richness consistently increased decomposition variability, if not the absolute rate. Further field studies were aimed at identifying impor- tant sources of this variability (e.g. litter quality, temporal variability) to help constrain ranges of predicted decomposition rates in different field situations. Thus, although many details still need to be resolved, litter decomposition holds considerable potential in some circumstances to capture impairment of stream ecosystem functioning. For instance, species traits associated with the body size and metabolic capacity of the con- sumers were often the main driver at local scales, and these were often translated into important determinants of otherwise apparently contingent effects at larger scales. Key insights gained from conducting continental scale studies included resolving the appar- ent paradox of inconsistent relationships between nutrients and decomposition rates, as the full complex multidimensional picture emerged from the large-scale dataset, of which only seemingly contradictory fragments had been seen previously

Continental-Scale Effects of Nutrient Pollution on Stream Ecosystem Functioning

Excessive nutrient loading is a major threat to aquatic ecosystems worldwide that leads to profound changes in aquatic biodiversity and biogeochemical processes. Systematic quantitative assessment of functional ecosystem measures for river networks is, however, lacking, especially at continental scales. Here, we narrow this gap by means of a pan-European field experiment on a fundamental ecosystem process—leaf-litter breakdown—in 100 streams across a greater than 1000-fold nutrient gradient. Dramatically slowed breakdown at both extremes of the gradient indicated strong nutrient limitation in unaffected systems, potential for strong stimulation in moderately altered systems, and inhibition in highly polluted streams. This large-scale response pattern emphasizes the need to complement established structural approaches (such as water chemistry, hydrogeomorphology, and biological diversity metrics) with functional measures (such as litter-breakdown rate, whole-system metabolism, and nutrient spiraling) for assessing ecosystem health

Global Patterns and Controls of Nutrient Immobilization On Decomposing Cellulose In Riverine Ecosystems

Microbes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low-nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization are poorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low-nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, and were strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature-dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature

Small Patches of Riparian Woody Vegetation Enhance Biodiversity of Invertebrates

Patches of riparian woody vegetation potentially help mitigate environmental impacts of agriculture and safeguard biodiversity. We investigated the effects of riparian forest on invertebrate diversity in coupled stream-riparian networks using a case study in the Zwalm river basin (Flanders, Belgium). Agriculture is one of the main pressures in the basin and riparian forest is limited to a number of isolated patches. Our 32 study sites comprised nine unshaded “unbuffered” sites which were paired with nine shaded “buffered” sites on the same stream reach, along with five ‘least-disturbed’ sites and nine downstream sites. We sampled water chemistry, habitat characteristics and stream and riparian invertebrates (carabid beetles and spiders) at each site. Three methods were used to quantify riparian attributes at different spatial scales: a visually-assessed qualitative index, quantitative estimates of habitat categories in six rectangular plots (10 × 5 m) and geographic information system (GIS)-derived land cover data. We investigated relationships between invertebrates and riparian attributes at different scales with linear regression and redundancy analyses. Spiders and carabids were most associated with local riparian attributes. In contrast, aquatic macroinvertebrates were strongly influenced by the extent of riparian vegetation in a riparian band upstream (100–300 m). These findings demonstrate the value of quantifying GIS-based metrics of riparian cover over larger spatial scales into assessments of the efficacy of riparian management as a complement to more detailed local scale riparian assessments in situ. Our findings highlight the value of even small patches of riparian vegetation in an otherwise extensively disturbed landscape in supporting biodiversity of both terrestrial and freshwater invertebrates and emphasize the need to consider multiple spatial scales in riparian management strategies which aim to mitigate human impacts on biodiversity in stream-riparian networks

Diversity and distribution patterns of benthic invertebrates along alpine gradients. A study of remote European freshwater lakes

Invertebrates inhabiting alpine water bodies are sensitive to environmental variability and lake faunal communities can therefore be important indicators of long-range airborne pollution, climate change and other human impacts. Information about the trophic structure and species composition of alpine lake ecosystems over space and time should therefore give important insights into environmental change effects. To explore the sensitivity of the faunal communities to environmental variability at local and regional scales we sampled biological and environmental variables from alpine lakes in seven different alpine lake districts in six European mountain regions. The fauna of the lakes was mainly composed of oligotrophic/ultraoligotrophic species, predominantly chironomids which made up more than 60% of all individuals. Species accumulation curves were used to explore patterns of species turnover and distribution, and the results indicated strong patterns in species diversity and also environmentally driven patterns in species distributions among lake districts. We used ordination analyses to explore these patterns in more detail. The results showed that the over-riding patterns in faunal assemblages were found between the lake districts. Lake-water chemistry appeared to be the major driver of the faunal assemblages at this scale, but altitude and geography could also account for significant fractions of the variability. In addition to these broad-scale patterns, repeatable trends in the faunal assemblages could be found in the faunal assemblages within lake districts. At this scale, the strongest compositional trends were found along the altitudinal gradient, but the faunal communities also responded to within-district variability in lake-water chemistry. Lake size or maximum depth did not appear to have any statistically significant effect on the littoral faunal assemblages at either scale.Gli invertebrati che colonizzano gli ambienti acquatici d\u27alta quota sono sensibili alle variazioni ambientali e le loro comunit? possono essere utili indicatori di trasporto di inquinanti, cambiamenti climatici e altri impatti antropici. Informazioni relative alla struttura trofica e alla composizione in specie degli ambienti lacustri possono fornire utili informazioni sugli effetti dei cambiamenti climatici. Per definire la sensibilit? delle comunit? lacustri alle variabilit? ambientali su scala regionale e locale abbiamo campionato laghi in 7 diversi distretti alpini appartenenti a 6 regioni montane europee. Le curve di accumulo utilizzate hanno fornito informazioni relative al turn-over di specie ed alla loro distribuzione. L\u27idrochimica lacustre sembra essere uno dei principali fattori determinanti la struttura di comunit? fra i diversi distretti, l\u27altitudine e la distribuzione geografica hanno anche un ruolo importante

Stream ecosystem functioning in an agricultural landscape : the importance of terrestrial-aquatic linkages

The loss of native riparian vegetation and its replacement with non-native species or grazing land for agriculture is a worldwide phenomenon, but one that is prevalent in Europe, reflecting the heavily-modified nature of the continent's landscape. The consequences of these riparian alterations for freshwater ecosystems remain largely unknown, largely because bioassessment has traditionally focused on the impacts of organic pollution on community structure. We addressed the need for a broader perspective, which encompasses changes at the catchment scale, by comparing ecosystem processes in woodland reference sites with those with altered riparian zones. We assessed a range of riparian modifications, including clearance for pasture and replacement of woodland with a range of low diversity plantations, in 100 streams to obtain a continental-scale perspective of the major types of alterations across Europe. Subsequently, we focused on pasture streams, as an especially prevalent widespread riparian alteration, by characterising their structural (e.g. invertebrate and fish communities) and functional (e.g. litter decomposition, algal production, herbivory) attributes in a country (Ireland) dominated by this type of landscape modification, via field and laboratory experiments. We found that microbes became increasingly important as agents of decomposition relative to macrofauna (invertebrates) in impacted sites in general and in pasture streams in particular. Resource quality of grass litter (e.g., carbon : nutrient ratios, lignin and cellulose content) was a key driver of decomposition rates in pasture streams. These systems also relied more heavily on autochthonous algal production than was the case in woodland streams, which were more detrital based. These findings suggest that these pasture streams might be fundamentally different from their native, ancestral woodland state, with a shift towards greater reliance on autochthonous-based processes. This could have a destabilizing effect on the dynamics of the food web relative to the slower, detrital-based pathways that dominate in woodland streams

Global Patterns and Controls of Nutrient Immobilization on Decomposing Cellulose in Riverine Ecosystems

International audienceMicrobes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low‐nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization arepoorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low‐nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, andwere strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature‐dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature. Collectively, we demonstrated that exogenous nutrient supply and immobilization are critical control points for decomposition of organic matter

Global patterns and controls of nutrient immobilization on decomposing cellulose in riverine ecosystems

Abstract Microbes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low-nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization are poorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low-nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, and were strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature-dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature. Collectively, we demonstrated that exogenous nutrient supply and immobilization are critical control points for decomposition of organic matter

Global patterns and drivers of ecosystem functioning in rivers and riparian zones

Abstract River ecosystems receive and process vast quantities of terrestrial organic carbon, the fate of which depends strongly on microbial activity. Variation in and controls of processing rates, however, are poorly characterized at the global scale. In response, we used a peer-sourced research network and a highly standardized carbon processing assay to conduct a global-scale field experiment in greater than 1000 river and riparian sites. We found that Earth’s biomes have distinct carbon processing signatures. Slow processing is evident across latitudes, whereas rapid rates are restricted to lower latitudes. Both the mean rate and variability decline with latitude, suggesting temperature constraints toward the poles and greater roles for other environmental drivers (e.g., nutrient loading) toward the equator. These results and data set the stage for unprecedented “next-generation biomonitoring” by establishing baselines to help quantify environmental impacts to the functioning of ecosystems at a global scale