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

Capacity of Vitiver grass in treatment of a mixture of labaratory and domestic wastewaters

 In this study, laboratory wastewater containing organic matters, heavy metals and aromatic compounds, was treated by vetiver grass (Vetiveria zizanioides) as a phytoremediation method to remove the above three groups of pollutants. Sewage effluent, as a source of nutrient supply for plant growth, was firstly fed to two wetland systems: mini horizontal subsurface flow (HSSF) and floating raft (FR) wetlands. Next, laboratory wastewater was added gradually to mix with sewage. Nominal hydraulic retention time in both wetlands are 12 hours. Pollutants removal efficiencies were monitored. Microbial community change corresponding with each stages of sewage only and mixture with laboratory wastewater was also examined. The examined microbial community includes Nitrogen-fixing (N-fixing) bacteria, Phosphate-solubilizing (P-solubilizing) microorganism, Pseudomonas sp., and Zoogloea sp.  In HSSF wetland, base materials (gravel and sand), algae, and vetiver root were in turn investigated for pollutant removal efficiencies. The results reveal that even with the presences of heavy metals and aromatic compounds, vetiver presented reasonable removal efficiencies of about 62%, 68.6%, and 58.3% for BOD, TN, and TP removal, respectively. Base materials showed almost no effect on pollutant removal. Algae was slightly responsible for approximate 6.3%, 16.6%, and 19.7% of BOD, TN, and TP removals, respectively. On the other hand vetiver roots, in term of heavy metals, had an impressive removal efficiencies of 99.2, 95.8, 96.2, and 96.7% of Cr+6 (in K2Cr2O7), Mn (MnSO4), Fe (FeSO4), and Cu (CuSO4), respectively. For aromatic compounds, the wetland is responsible for 96.8 and almost 100% of correspondingly phenol and benzene removal efficiencies. For microbial aspect, N-fixing microorganisms (e.g. Azospirillum sp., Azotobacter sp.) and Phosphate-solubilizing bacteria (Bacillus sp.) increased gradually in population during domestic wastewater feeding stage. When laboratory wastewater was added, N-fixing and P-solubilizing bacteria were quantitatively decreased slightly while population of Pseudomonas sp. increased. Besides, Zoogloea sp. was also found increasing through out the experiment and keeping a stable growth even during laboratory wastewater adding.  In FR wetland, both algae and vetiver root were also investigated for BOD and aromatic compounds and heavy metals. The outcomes show similar tendencies in treatment and microbial behaviours as in HSSF wetland. Vetiver grass, mainly responsible for organic matters and nutrients removal, presented slightly lower removal efficiencies than those in HSSF wetland. The average values of removal efficiencies are 59%, 63.5%, and 53.0% for BOD, TN, and TP removal, respectively. Algae, also, took minor responsibility for approximate 3.3%, 9.1%, and 8.9% of BOD, TN, and TP removals, respectively. Heavy metals of Cr+6 (in K2Cr2O7), Mn (MnSO4), Fe (FeSO4), and Cu (CuSO4) were found removing less than in HSSF wetland with average removal efficiencies values of 92.4, 85.1, 91.8, and 91.5%, respectively, by  vetiver root. Algae show almost no effect on heavy metals and aromatic removals. The vetiver root likewise plays important role in phenol and benzene removals with values of 91.5 and 96% in efficiency, respectively. N-fixing and P-solubilizing microorganisms, Pseudomonas sp., and Zoogloea sp. presented similar responses tendencies to different living condition when domestic and laboratory wastewaters, in turn, were fed

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 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