Summary
Fine particulate organic matter (FPOM) provides a key longitudinal link within stream networks, and is the predominant food source for filter‐ and deposit‐feeding invertebrates, collectively ...classified as ‘collectors’.
Organisms involved in producing and using FPOM are sensitive to chemical and other anthropogenic stressors, but information on such impacts, and on FPOM dynamics in general, is limited.
Here, we review information on the ecological role of FPOM in streams, and discuss potential impacts on FPOM dynamics of organic and inorganic chemical stressors, including metals and pesticides. Emphasis is placed on faecal particles produced within the leaf‐litter processing chain.
Key biological factors controlling the resource quality of FPOM for collectors include the identity of the invertebrates producing FPOM, and the nutritional quality of their food resources. FPOM nutrient content is also strongly influenced by microbial colonisation and activity, and FPOM processing rates are thus likely to be sensitive to the impacts of stressors affecting microbes, including nutrients and antimicrobial chemicals.
The potential for FPOM to bind and subsequently transport chemical stressors is high, particularly for hydrophobic compounds, but the extent of such effects and impacts on collectors consuming contaminated particles has attracted only limited attention.
Combining concepts and research approaches from ecotoxicology and basic stream ecology would facilitate development of a common integrated framework for understanding the role of FPOM, and assessing anthropogenic impacts on FPOM dynamics in stream networks.
Summary
Plant leaf litter comprises the major common source of energy and nutrients in forested soil and freshwater ecosystems world‐wide. However, despite the similarity of physical and biochemical ...processes, generalizations across aquatic and terrestrial ecosystems regarding litter decomposition drivers remain elusive.
We re‐analysed data from a published field decomposition experiment conducted in two ecosystems (forest floors and streams) across five biomes (from the tropics to subarctic) with increasing decomposer community complexity (microbes, microbes and mesofauna, microbes and meso‐ and macrofauna).
Using a wide litter quality gradient (15 litter combinations), we aimed to disentangle the roles of decomposer community complexity from that of leaf litter traits (18 traits encompassing four broad trait categories: nutrients, C quality, physical structure and stoichiometry) on litter C and N loss. Comparisons of decomposition drivers between ecosystems were evaluated across and within biomes.
Differences in environmental conditions (e.g. climate, soil/water fertility) and litter nutrients – with a particular focus on Mg and Ca – across biomes were the major drivers of litter C loss in both ecosystems, but decomposer complexity also played a prominent role in streams. Within biomes, we observed consistent effects of litter nutrients and stoichiometry on litter C and N loss between ecosystems, but the effects of decomposer complexity differed between streams and forest floors in the temperate, Mediterranean and tropical biomes.
Our results highlight that, beyond the litter traits commonly identified as controlling decomposition (e.g. C, N and lignin), micronutrients (e.g. Mg and Ca) can also play an important, and globally consistent, role in both aquatic and terrestrial ecosystems. In addition, in forest streams the complexity of decomposer communities had similar importance as litter traits for predicting litter C and N turnover across all five biomes.
The identification of common drivers in our large‐scale ecosystem comparison suggests a basis to develop common models across aquatic and terrestrial ecosystems for C and N dynamics during decomposition. Future modelling efforts should account for the global similarities (litter micronutrient effects) and biome‐level differences (contingent decomposer effects) found between ecosystems.
Lay Summary
We compared conventional microscope-based methods for quantifying biomass and community composition of stream benthic algae with output obtained for these parameters from a new instrument (the ...BenthoTorch), which measures fluorescence of algal pigments in situ. Benthic algae were studied in 24 subarctic oligotrophic (1.7-26.9, median 7.2 μg total phosphorus L(-1)) streams in Northern Sweden. Readings for biomass of the total algal mat, quantified as chlorophyll a, did not differ significantly between the BenthoTorch (median 0.52 μg chlorophyll a cm(-2)) and the conventional method (median 0.53 μg chlorophyll a cm(-2)). However, quantification of community composition of the benthic algal mat obtained using the BenthoTorch did not match those obtained from conventional methods. The BenthoTorch indicated a dominance of diatoms, whereas microscope observations showed a fairly even distribution between diatoms, blue-green algae (mostly nitrogen-fixing) and green algae (mostly large filamentous), and also detected substantial biovolumes of red algae in some streams. These results most likely reflect differences in the exact parameters quantified by the two methods, as the BenthoTorch does not account for variability in cell size and the presence of non-chlorophyll bearing biomass in estimating the proportion of different algal groups, and does not distinguish red algal chlorophyll from that of other algal groups. Our findings suggest that the BenthoTorch has utility in quantifying biomass expressed as μg chlorophyll a cm(-2), but its output for the relative contribution of different algal groups to benthic algal biomass should be used with caution.
Species interactions can influence ecosystem functioning by enhancing or suppressing the activities of species that drive ecosystem processes, or by causing changes in biodiversity. However, one ...important class of species interactions – parasitism – has been little considered in biodiversity and ecosystem functioning (BD-EF) research. Parasites might increase or decrease ecosystem processes by reducing host abundance. Parasites could also increase trait diversity by suppressing dominant species or by increasing within-host trait diversity. These different mechanisms by which parasites might affect ecosystem function pose challenges in predicting their net effects. Nonetheless, given the ubiquity of parasites, we propose that parasite–host interactions should be incorporated into the BD-EF framework.
Biodiversity affects ecosystem functioning.
Biodiversity may decrease or increase parasitism.
Parasites impair individual hosts and affect their role in the ecosystem.
Parasitism, in common with competition, facilitation, and predation, could regulate BD-EF relationships.
Parasitism affects host phenotypes, including changes to host morphology, behavior, and physiology, which might increase intra- and interspecific functional diversity.
The effects of parasitism on host abundance and phenotypes, and on interactions between hosts and the remaining community, all have potential to alter community structure and BD-EF relationships.
Global change could facilitate the spread of invasive parasites, and alter the existing dynamics between parasites, communities, and ecosystems.
How are resource consumption and growth rates of litter-consuming detritivores affected by imbalances between consumer and litter C:N:P ratios? To address this question, we offered leaf litter as ...food to three aquatic detritivore species, which represent a gradient of increasing body N:P ratios: a crustacean, a caddisfly and a stonefly. The detritivores were placed in microcosms and submerged in a natural stream. Four contrasting leaf species were offered, both singly and in two-species mixtures, to obtain different levels of stoichiometric imbalance between the resources and their consumers. The results suggest that detritivore growth was constrained by N rather than C or P, even though 1) the N:P ratios of the consumers' body tissue was relatively low and 2) microbial leaf conditioning during the experiment reduced the N:P imbalance between detritivores and leaf litter. This surprisingly consistent N limitation may be a consequence of cumulative N-demand arising from the production of N-rich chitin in the exoskeletons of all three consumer species, which is lost during regular moults, in addition to N-demand for silk production by the caddisfly. These N requirements are not commonly quantified in stoichiometric analyses of arthropod consumers. There was no evidence for compensatory feeding, but when offered mixed-species litter varying in C:N:P ratios, detritivores consumed more of the litter species showing the highest N:P and lowest C:N ratio, accelerating the mass loss of the preferred leaf species in the litter mixture. These results show that imbalances in consumer–resource stoichiometry can have contrasting effects on coupled processes, highlighting a challenge in developing a mechanistic understanding of the role of stoichiometry in regulating ecosystem processes such as leaf litter decomposition.
Forest clearance is a pervasive disturbance worldwide, but many of its impacts are regarded as transient, diminishing in intensity as forest recovers. However, forests can take decades to centuries ...to recover after severe disturbances, and temporal lags in recovery of ecosystem properties for different forest habitats are mostly unknown. This includes forest streams, where most studies of the impacts of forest clearance are restricted to the first years of recovery, typically finding that temporary increases in light and nutrient run‐off diminish as forest recovers. Implications of longer term changes remain little investigated.
In a space‐for‐time substitution experiment, we assessed changes in organic matter processing and in the functional and taxonomic composition of litter‐consuming detritivores along a riparian forest age gradient ranging from 1 to 120 years since last timber harvesting.
Variation in organic matter processing and detritivore functional diversity along the forest succession gradient were both expressed as second‐order polynomial relationships (peaking at ~50 years along the forest age gradient). Decomposition rates were lowest in both the more recently clear‐cut and older riparian forest streams.
Variation of litter decomposition rates among litter bags within streams, measured by the coefficient of variation, was lowest in recent clear‐cuts and increased linearly along the succession gradient. This result indicates higher within‐stream heterogeneity in decomposition rates in older forest streams.
Synthesis and applications. We found that the decomposition of leaf litter, a component of carbon cycling in forests, was higher in streams flowing through intermediately aged forest, and that several key attributes of the organisms regulating litter decomposition also varied systematically with forest age. These findings highlight the longer term consequences of forest succession following forest clear‐cutting for stream habitats. Our findings further illustrate complications arising from the use of forested sites as references for newly cleared sites without properly accounting for forest age, given conclusions regarding biotic responses will depend on the age of the reference forests. Finally, our results emphasise the potential of intensive forest management centred on vast, one‐time clear‐cutting events to drive long‐term homogenisation not only in forest age structure but also in the functioning of associated forest stream habitats.
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We found that the decomposition of leaf litter, a component of carbon cycling in forests, was higher in streams flowing through intermediately aged forest, and that several key attributes of the organisms regulating litter decomposition also varied systematically with forest age. These findings highlight the longer term consequences of forest succession following forest clear‐cutting for stream habitats. Our findings further illustrate complications arising from the use of forested sites as references for newly cleared sites without properly accounting for forest age, given conclusions regarding biotic responses will depend on the age of the reference forests. Finally, our results emphasise the potential of intensive forest management centred on vast, one‐time clear‐cutting events to drive long‐term homogenisation not only in forest age structure but also in the functioning of associated forest stream habitats.
Despite ample experimental evidence indicating that biodiversity might be an important driver of ecosystem processes, its role in the functioning of real ecosystems remains unclear. In particular, ...the understanding of which aspects of biodiversity are most important for ecosystem functioning, their importance relative to other biotic and abiotic drivers, and the circumstances under which biodiversity is most likely to influence functioning in nature, is limited. We conducted a field study that focussed on a guild of insect detritivores in streams, in which we quantified variation in the process of leaf decomposition across two habitats (riffles and pools) and two seasons (autumn and spring). The study was conducted in six streams, and the same locations were sampled in the two seasons. With the aid of structural equations modelling, we assessed spatiotemporal variation in the roles of three key biotic drivers in this process: functional diversity, quantified based on a species trait matrix, consumer density and biomass. Our models also accounted for variability related to different litter resources, and other sources of biotic and abiotic variability among streams. All three of our focal biotic drivers influenced leaf decomposition, but none was important in all habitats and seasons. Functional diversity had contrasting effects on decomposition between habitats and seasons. A positive relationship was observed in pool habitats in spring, associated with high trait dispersion, whereas a negative relationship was observed in riffle habitats during autumn. Our results demonstrate that functional biodiversity can be as significant for functioning in natural ecosystems as other important biotic drivers. In particular, variation in the role of functional diversity between seasons highlights the importance of fluctuations in the relative abundances of traits for ecosystem process rates in real ecosystems.
The microbial control agent
Bacillus thuringiensis
var.
israelensis
(Bti) has been successfully used worldwide to reduce abundances of biting Nematocera (Diptera), often with little direct impact on ...non-target organisms observed. However, the potential for additional indirect effects on other ecosystem properties, including on trophic linkages within food webs, is poorly known. We investigated the effects of multiple-year mosquito control treatments using the Bti product VectoBac
®
-G on the stable isotope composition of epigeal and soil-based consumers inhabiting replicate floodplains along the River Dalälven, Sweden. We observed significant changes in the isotopic composition of detritivores feeding at the base of floodplain food webs. Enchytraeid worms were characterised by 3.5% higher δ
13
C values in treated floodplains, suggesting increased consumption of δ
13
C-enriched food. The overall range of community-wide δ
15
N values was 56% greater in the treated floodplains, whilst δ
15
N values of oribatid mites were elevated by 97%. These results suggest extra fractionation in the transfer of nitrogen through floodplain food chains. We conjecture that the ecological mechanisms driving these food web shifts are (1) the mass mortality of high δ
13
C
A. sticticus
larvae, which leaves high concentrations of dead mosquito biomass deposited on soils at local scales, after the floodwaters have receded and (2) incorporation of the very high δ
13
C-enriched corn particles comprising the bulk of the VectoBac
®
-G product into floodplain food webs. Our results suggest that repeated applications of Bti might have wider, still largely unknown implications for nutrient and energy cycles within floodplain ecosystems.
The decomposition of dead organic matter is a major determinant of carbon and nutrient cycling in ecosystems, and of carbon fluxes between the biosphere and the atmosphere. Decomposition is driven by ...a vast diversity of organisms that are structured in complex food webs. Identifying the mechanisms underlying the effects of biodiversity on decomposition is critical given the rapid loss of species worldwide and the effects of this loss on human well-being. Yet despite comprehensive syntheses of studies on how biodiversity affects litter decomposition, key questions remain, including when, where and how biodiversity has a role and whether general patterns and mechanisms occur across ecosystems and different functional types of organism. Here, in field experiments across five terrestrial and aquatic locations, ranging from the subarctic to the tropics, we show that reducing the functional diversity of decomposer organisms and plant litter types slowed the cycling of litter carbon and nitrogen. Moreover, we found evidence of nitrogen transfer from the litter of nitrogen-fixing plants to that of rapidly decomposing plants, but not between other plant functional types, highlighting that specific interactions in litter mixtures control carbon and nitrogen cycling during decomposition. The emergence of this general mechanism and the coherence of patterns across contrasting terrestrial and aquatic ecosystems suggest that biodiversity loss has consistent consequences for litter decomposition and the cycling of major elements on broad spatial scales.
The importance of aquatic plant diversity in regulating nutrient cycling in wetlands remains poorly understood. We investigated how variation in macrophyte growth form (emerging, submerged and ...bryophyte) combinations and species mixtures affect nitrogen (N) removal from the water and N accumulation in plant biomass. We conducted a wetland mesocosm experiment for 100 days during July–September 2015. Twelve species were grown in mono- and in two-species mixed cultures for a total of 32 single and two-growth form combinations. Nitrogen removal from the water was quantified on three occasions during the experiment, while N accumulation in plant biomass was determined following termination of the experiment. The number of species and growth forms present increased N removal and accumulation. The growth form combinations of emerging and bryophyte species showed the highest N accumulation and N removal from water, followed by combinations of emerging species. By contrast, submerged species growing in the presence of emerging or other submerged species showed the lowest levels of N accumulation and N removal. Temporal variation in N removal also differed among growth form combinations: N removal was highest for emerging-bryophyte combinations in July, but peaked for the emerging-submerged and emerging-bryophyte combinations in August. Indeed, the occurrence of complementarity among macrophyte species, particularly in combinations of bryophyte and emerging species, enhanced N removal and uptake during the entire growing season. Our study highlights the importance of bryophytes, which have been neglected in research on nutrient cycling in wetlands, for aquatic N cycling, especially given their worldwide distribution across biomes. Overall, our findings point towards the potential important role of the diversity of macrophyte growth forms in regulating key ecosystem processes related to N cycling in wetlands.
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•Number of species and growth forms enhanced nitrogen removal and accumulation.•Nitrogen removal varied temporally and among growth-form combinations.•Co-occurrence of bryophytes and emerging species showed highest removal of nitrogen.
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