Effects of pH and resources on a processing chain interaction in simulated treeholes

1.  Both resources and abiotic factors may affect biotic interactions. One interaction that occurs in treehole habitats involves leaf shredders that facilitate growth of detritivores, and it may be affected by both leaf litter quantity and changes in water quality.
2.  Water chemistry in central Pennsylvania treeholes has been impacted by acid deposition, and the most common insects therein have differential survival under low pH conditions. Experimental microcosms that mimic treehole habitats were used to test the hypothesis that this abiotic factor, pH, also affects facilitative interactions. Leaf litter resources and pH were varied independently of presence of leaf-shredding scirtid beetles ( Helodes pulchella and Prionocyphon discoideus ), and the mosquito Aedes triseriatus , to examine interactions among pH, resources and insects.
3.  pH affected the interaction between the insects, such that effects of scirtids were more evident at pH 4·5 than at 6·5. Female mosquitoes were larger in the presence of scirtids, low resource and low pH conditions than in absence of scirtids, low resource and low pH conditions.
4.  There were also effects of A. triseriatus on scirtids. The size of individual scirtids was smaller in the presence of A. triseriatus , but total scirtid biomass was unaffected as survival was also higher in the presence of A. triseriatus .
5.  The effects observed on a resource-mediated biotic interaction led to the conclusion that this interaction is pH dependent, and gives support to the concept that abiotic factors play a role in determining the outcome of biotic interactions, and that acidification can have complex effects on communities.     

Insect species interactions and resource effects in treeholes: are helodid beetles bottom-up facilitators of midge populations?

The insect community living in central Pennsylvania treeholes in autumn consists primarily of larvae of two species of helodid beetles, Prionocyphon discoideus and Helodes pulchella, and larvae of one species of ceratopogonid midge,Culicoides guttipennis. We manipulated treehole volume and the densities of these insects in laboratory microcosms. We hypothesized that: (1) helodid beetle larvae, which are shredders, would enhance growth and survival of ceratopogonid midge larvae (deposit feeders) in a processing chain commensalism, and (2) the quantity of resources expressed as water volume plus leaf litter would affect helodids and protozoans directly. Intraspecific competition was not found in midges, nor was interspecific competition between the two helodid species. Protozoan population densities decreased or grew slower in the presence of insects and in smaller microcosms. Development time and adult wing length of the midge (C. guttipennis) were affected by both total microcosm volume and insect species combination. Under resource limitation, midges grew larger in the presence of helodids, and in general, midges were larger in treatments with higher ratios of helodids to midges. Water chemistry in the microcosms was affected both temporally and by insect presence. Hydrogen ion levels decreased over time, and microcosms with no insects had lower hydrogen ion levels. Specific conductance increased in all treatments over time, and microcosms with no insects had lower conductivity than most treatments. Helodid larvae have a positive effect on midges, possibly due to a processing chain facilitation. If helodids are keystone decomposers in this system, their presence could affect resource availability and affect other organisms in the community. Similar processing chain commensalisms occur in other phytotelmata. These types of interactions may therefore be important in the structure and function of detritus-based communities. Received: 31 January 1996 /Accepted: 7 August 1996     

Leaf litter quality drives the feeding by invertebrate shredders in tropical streams

Amazon and Cerrado‐forested streams show natural fluctuations in leaf litter quantity along the time and space, suggesting a change on litter quality input. These natural fluctuations of leaf litter have repercussion on the organic matter cycling and consequently effects on leaf decomposition in forested streams. The effects of the quantity of leaf litter with contrasting traits on consumption by larvae of shredder insects from biomes with different organic matter dynamics have still been an understudied question. The Trichoptera Phylloicus spp. is a typical shredder in tropical headwater streams and keep an important role in leaf litter decomposition. Here, we assessed the consumption by shredder Phylloicus spp., from Amazonia and Cerrado biomes, on higher (Maprounea guianensis) and lower quality leaves (Inga laurina) in different proportions and quantities. Experiments were performed concomitantly in microcosms approaches, simulating Cerrado and Amazonian streams. Higher leaf consumption occurred in Cerrado microcosms. Litter quantity influenced negatively leaf consumption by shredders in Cerrado, in opposition to Amazonia, where consumption was not affected by leaf quantity. In both sites, we observed higher consumption by shredders in treatment with only M. guianensis and no difference between other treatments with mixture of leaves. In treatment with litter of I. laurina, we noted the use of substrate for case building (due to the higher leaf toughness), affecting the fragmentation process. Therefore, our results indicate that leaf litter quality drives the preference of consumption by Phylloicus larvae in Cerrado and Amazonia streams.     

Testing for context-dependence in a processing chain interaction among detritus-feeding aquatic insects

Abstract 1. Scirtid beetles may benefit mosquitoes Ochlerotatus triseriatus (Say) by consuming whole leaves and leaving behind fine particles required by mosquito larvae. Such interactions based on the sequential use of a resource that occurs in multiple forms are known as processing chains.
2. Models of processing chains predict that interactions can vary from commensal (0, +) to amensal (0, –), depending on how quickly resource is processed in the absence of consumers.
3. The scirtid– O. triseriatus system was used to test the prediction derived from processing chain models that, as consumer-independent processing increases, scirtids benefit mosquitoes less. Consumer-independent processing rate was manipulated by using different leaf species that vary in decay rate, or by physically crushing a single leaf type to different degrees.
4. Although scirtids increased the production of fine particles, the effects of scirtids on mosquitoes were weak and were not dependent on consumer-independent processing rate.
5. In the leaf manipulation experiment, a correlation between scirtid feeding and consumer-independent processing was detected. Numerical simulations suggest that such a correlation may eliminate shifts from commensal to amensal at equilibrium; because mosquito populations are typically not at equilibrium, however, this correlation may not be important.
6. There was evidence that mosquitoes affected scirtids negatively, which is inconsistent with the structure of processing chain interactions in models. Processing chain models need to incorporate more detail on the biology of scirtids and O. triseriatus , especially alternative mechanisms of interaction, if they are to describe scirtid– O. triseriatus dynamics accurately.     

Priority effects of bamboo-stump mosquito larvae: influences of water exchange and leaf litter input

Abstract 1. A series of laboratory experiments was conducted to examine the influences of leaf litter input and flushing of medium on the priority effects of the two bamboo-stump breeding mosquitoes, Aedes albopictus and Tripteroides bambusa , using 150-ml microcosms. Larvae of either species were introduced to microcosms on day 0 (early cohort), at different densities, and day 14 (late cohort). The effects of the early cohort on survival, pupation, and biomass yield of the late cohort were compared among various combinations of the two species and among different treatments (water exchange, leaf addition, both of them, neither of them) on days 14 and 44.
2. Survivorship and pupation success of the late A. albopictus cohort were affected negatively by the presence of either species of the early cohort to a greater extent than those of the late T. bambusa cohort.
3. Water exchange reduced mortality of the late A. albopictus cohort over a short term in the presence of the early cohort of either species, indicating that a toxic substance was involved in the inhibitory priority effects. The addition of leaf litter enhanced survivorship and pupation of the late A. albopictus cohort, whether or not water was exchanged. The late T. bambusa cohort showed high survival rate with all treatments.
4. The results indicate that leaf-litter input moderates the inhibitory priority effects on A. albopictus larvae, not only by supplying food resources but also by alleviating the toxicity of accumulated dissolved substances.     

Predation, resource availability, and community structure in Neotropical water-filled tree holes

Predation and resource availability influence community structure in many aquatic ecosystems. Predators (odonates) and resources (leaf litter) were manipulated to determine their independent effects on macroorganism species richness, abundance, and composition in water-filled tree holes of Barro Colorado Island, Panama. Interactive effects of these factors were also investigated in artificial tree holes. Large odonates reduced species richness in natural tree holes, but did not significantly reduce macroorganism abundance. The presence of larvae of the mosquito Culex urichii and the ceratopogonid midge Bezzia snowi were negatively associated with the presence of large odonate larvae. In natural tree holes, leaf litter addition and removal respectively increased and decreased richness by c. 1 species relative to controls, and macroorganism abundance was greater in litter addition holes than in litter removal holes. Independent effects of predation showed similar patterns in artificial holes, but there was no predator2resource interaction, partly due to the short duration of the experiment. Predators grew faster when litter was abundant, and indirectly reduced litter degradation rates when resources were scarce in artificial holes. Both resource availability and predation influence species richness in water-filled tree holes, but act at different time scales; richness follows productivity (litter quantity) over a period of weeks, whereas effects of predation may span several months.     

Effects of aqueous extracts from leaves and leaf litter on the abundance and diversity of soil gymnamoebae in laboratory microcosm cultures

The distribution and abundance of microbiota in soil and litter may be significantly affected by the quality and quantity of localized patches of leaf organic matter. This study examined the relative effects of aqueous extracts of shed autumn leaves from American beech (Fagus grandifolia), sugar maple (Acer saccharum), red oak (Quercus rubra), and white oak (Quercus alba) on the density and diversity of gymnamoebae in laboratory cultures. Overall, the beech leaf extract produced the most growth of gymnamoebae followed by white oak with leaf extracts from maple and red oak producing least growth. Cultures using natural leaf litter from beneath beech trees had higher densities and diversity of gymnamoebae than leaf-litter cultures from a maple-oak stand. Soil microcosms confirmed that beech leaf extracts produced a higher density of gymnamoeba growth when added to soil cultures compared with maple and oak leaf extracts. Protein content, CHN (carbon and nitrogen content), and pH of the leaf extracts were assayed, but these alone were not sufficiently different to account for the effects. A dilution experiment indicated that some other concentration-dependent factor in the extract may produce the effects.     

Decomposition of leaf litter exposed to simulated acid rain in a buffered lotic system

SUMMARY. 1. Exposure to simulated acid rain resulted in changes in the chemical content of riparian vegetation and terrestrial leaf litter and had significant effects on leaf litter decay rates in a well-buffered lotic ecosystem.
2. Foliar nitrogen and phosphorus decreased with decreasing pH of the simulated acid rain and microbial activity was greater on leaf litter exposed to rain of pi I 5.4 than on leaf litter exposed to pH 3.0 or pH 4.0.
3. Detritivore numbers and biomass were significantly higher on leaf litter exposed to pH 5.4, probably due to the greater palatability of the leaf substrate.
4. Decay rates of leaf litter processed in the summer months were significantly lower than decay rates of leaf litter processed during the autumn/winter due to reduction in nitrogen content and microbial respiration.     

Effects of leaf litter species on macroinvertebrate community properties and mosquito yield in Neotropical tree hole microcosms

Detritus quality and quantity affect macroinvertebrate productivity and distribution in many freshwater ecosystems. This study experimentally investigated the effects of leaf litter from Ceiba pentandra, Dipteryx panamensis, Ficus yoponensis, and Platypodium elegans on macroinvertebrate species composition, richness, and abundance in artificial water-filled tree holes in a lowland moist forest of Panama. Species composition was similar among treatments, but species richness and longevity differed among litter types and were consistently highest with Platypodium litter. Similar patterns were observed in natural tree holes of the focal tree species. The mosquito Culex mollis was the most abundant species in the field experiment. Average conductivity and dissolved oxygen concentration differed among leaf species, but pH did not. Leaf toughness was positively correlated with mean macroinvertebrate abundance and cumulative species richness. A laboratory experiment measured C. mollis yield and pupation time in tree hole microcosms containing the four litter species. Cumulative mosquito mass and time to pupation differed among leaf litter species, with Platypodium litter supporting the greatest yield. Pupation was slowest on Ceiba litter. Grazing by mosquito larvae facilitated leaf decomposition in all treatments. Results suggest that differences in macroinvertebrate species richness and mosquito yield can be attributed to differences in nutritional quality among litter species. Received: 14 October 1998 / Accepted: 21 February 1999     

Direct and indirect effects of soil structure on the density of an antlion larva in a tropical dry forest

Abstract.  1. We used structural equation models to discriminate direct and indirect effects of soil structure on the abundance of the antlion Myrmeleon crudelis , a neuropteran larva that digs conical pits in soil to capture small arthropods. We proposed that soil structure may modify antlion density indirectly through its influence on tree cover, which in turn directly alters the amount of sun and rain that can reach the forest floor and the amount of litter fall.
2. The proportion of finer soils positively affected antlion density directly, but negatively tree cover. Tree cover positively affected both the amount of leaf litter and antlion density. Leaf litter negatively affected antlion density. The indirect effects of soils varied in strength and sign depending on whether trees are considered shelters against sun and rain, or leaf litter sources. The relative importance of these effects might also vary between years and seasons.
3. Antlions may select patches of finer soils not only because they are easy substrates in which to build pits, but also for their indirect benefit as sites with low leaf litter, illustrating how indirect interactions may affect the local abundance of semi-sedentary insects.     

The significance of differences in the mechanisms of photosynthetic acclimation to light, nitrogen and CO2 for return on investment in leaves

1. We report changes in photosynthetic capacity of leaves developed in varying photon flux density (PFD), nitrogen supply and CO₂ concentration. We determined the relative effect of these environmental factors on photosynthetic capacity per unit leaf volume as well as the volume of tissue per unit leaf area. We calculated resource-use efficiencies from the photosynthetic capacities and measurements of leaf dry mass, carbohydrates and nitrogen content.
2. There were clear differences between the mechanisms of photosynthetic acclimation to PFD, nitrogen supply and CO₂. PFD primarily affected volume of tissue per unit area whereas nitrogen supply primarily affected photosynthetic capacity per unit volume. CO₂ concentration affected both of these parameters and interacted strongly with the PFD and nitrogen treatments.
3. Photosynthetic capacity per unit carbon invested in leaves increased in the low PFD, high nitrogen and low CO₂ treatments. Photosynthetic capacity per unit nitrogen was significantly affected only by nitrogen supply.
4. The responses to low PFD and low nitrogen appear to function to increase the efficiency of utilization of the limiting resource. However, the responses to elevated CO₂ in the high PFD and high nitrogen treatments suggest that high CO₂ can result in a situation where growth is not limited by either carbon or nitrogen supply. Limitation of growth at elevated CO₂ appears to result from internal plant factors that limit utilization of carbohydrates at sinks and/or transport of carbohydrates to sinks.     

Stoichiometric imbalances between detritus and detritivores are related to shifts in ecosystem functioning

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.     

Rainfall, nitrogen and host plant condition: consequences for the processionary caterpillar, Ochrogaster lunifer

1. At the end of November in subtropical areas of Australia, second-instar larvae of the processionary caterpillar Ochrogaster lunifer (Lepidoptera: Thaumetopoeidae) initiate feeding on the peripheral shoots of acacias (first-instar larvae do not feed). Field surveys at ten localities in south-east Queensland showed that larval survival was highly variable both among and within localities. Within-locality variation in larval growth was low compared with variation among localities. Larval growth and survival rates were higher at coastal and island localities, where November rainfall was high, than at drier inland localities.
2. Potted Acacia concurrens were grown in the greenhouse under high and low watering regimes, with and without nitrogen-rich fertilizer. Plant vigour (height, foliar water content and quantity of flush growth) was significantly greater in high-water treatments than in low-water treatments. Watering also affected foliar nitrogen, with plants in the high-water/no fertilizer treatment having similar nitrogen levels to those in fertilized treatments. Fertilizer increased foliar nitrogen levels of plants in low-water treatments and increased the number of shoots in high-water treatments. Different treatments had no effect on leaf toughness. After the first 3 weeks of feeding, size and survival of larvae were significantly reduced on the small, less vigorous plants in low water treatments. These results do not support the plant stress hypothesis.
3. Early-instar larvae (instar II–IV) developed more quickly and grew larger when reared on flush leaves than when reared on senescent leaves of A. concurrens . As water uptake affects the quantity of flush growth available to early stage larvae as well as foliar quality, rainfall and water availability may have important consequences for the distribution and population dynamics of the moth at local and regional scales.     

The diversity of beetle assemblages in different habitat types in Sabah, Malaysia

The diversity of beetle assemblages in different habitat types (primary forest, logged forest, acacia plantation and oil palm plantation) in Sabah, Malaysia was investigated using three different methods based on habitat levels (Winkler sampling, flight-interception-trapping and mist-blowing). The overall diversity was extremely high, with 1711 species recorded from only 8028 individuals and 81 families (115 family and subfamily groups). Different degrees of environmental changes had varying effects on the beetle species richness and abundance, with oil palm plantation assemblage being most severely affected, followed by acacia plantation and then logged forest. A few species became numerically dominant in the oil palm plantation. In terms of beetle species composition, the acacia fauna showed much similarity with the logged forest fauna, and the oil palm fauna was very different from the rest. The effects of environmental variables (number of plant species, sapling and tree densities, amount of leaf litter, ground cover, canopy cover, soil pH and compaction) on the beetle assemblage were also investigated. Leaf litter correlated with species richness, abundance and composition of subterranean beetles. Plant species richness, tree and sapling densities correlated with species richness, abundance and composition of understorey beetles while ground cover correlated only with the species richness and abundance of these beetles. Canopy cover correlated only with arboreal beetles. In trophic structure, predators represented more than 40% of the species and individuals. Environmental changes affected the trophic structure with proportionally more herbivores (abundance) but fewer predators (species richness and abundance) in the oil palm plantation. Biodiversity, conservation and practical aspects of pest management were also highlighted in this study.     

Effect of Inorganic Nutrients on Relative Contributions of Fungi and Bacteria to Carbon Flow from Submerged Decomposing Leaf Litter

The relative contributions of fungi and bacteria to carbon flow from submerged decaying plant litter at different levels of inorganic nutrients (N and P) were studied. We estimated leaf mass loss, fungal and bacterial biomass and production, and microbial respiration and constructed partial carbon budgets for red maple leaf disks precolonized in a stream and then incubated in laboratory microcosms at two levels of nutrients. Patterns of carbon flow for leaf disks colonized with the full microbial assemblage were compared with those colonized by bacteria but in which fungi were greatly reduced by placing leaf disks in colonization chambers sealed with membrane filters to exclude aquatic hyphomycete conidia but not bacterial cells. On leaves colonized by the full microbial assemblage, elevated nutrient concentrations stimulated fungi and bacteria to a similar degree. Peak fungal and bacterial biomass increased by factors of 3.9 and 4.0; cumulative production was 3.9 and 5.1 times higher in the high nutrient in comparison with the low nutrient treatment, respectively. Fungi dominated the total microbial biomass (98.4 to 99.8%) and cumulative production (97.3 and 96.5%), and the fungal yield coefficient exceeded that of bacteria by a factor of 36 and 27 in low- and high-nutrient treatments, respectively. Consequently, the dominant role of fungi in leaf decomposition did not change as a result of nutrient manipulation. Carbon budgets indicated that 8% of leaf carbon loss in the low-nutrient treatment and 17% in the high-nutrient treatment were channeled to microbial (essentially fungal) production. Nutrient enrichment had a positive effect on rate of leaf decomposition only in microcosms with full microbial assemblages. In treatments where fungal colonization was reduced, cumulative bacterial production did not change significantly at either nutrient level and leaf decomposition rate was negatively affected (high nutrients), suggesting that bacterial participation in carbon flow from decaying leaf litter is low regardless of the presence of fungi and nutrient availability. Moreover, 1.5 and 2.3 times higher yield coefficients of bacteria in the reduced fungal treatments at low and high nutrients, respectively (percentage of leaf carbon loss channeled to bacterial production), suggest that bacteria are subjected to strong competition with fungi for resources available in leaf litter.     

Composition of speciose leaf litter alters stream detritivore growth, feeding activity and leaf breakdown

Leaf litter derived from riparian trees can control secondary production of detritivores in forested streams. Species-rich assemblages of leaf litter reflect riparian plant species richness and represent a heterogeneous resource for stream consumers. Such variation in resource quality may alter consumer growth and thus the feedback on leaf breakdown rate via changes in feeding activity. To assess the consequences of this type of resource heterogeneity on both consumer growth and subsequent litter breakdown, we performed a laboratory experiment where we offered a leaf-shredding stream detritivore (the stonefly Tallaperla maria, Peltoperlidae) ten treatments of either single- or mixed-species leaf litter. We measured consumer growth rate, breakdown rate and feeding activity both with and without consumers for each treatment and showed that all three variables responded to speciose leaf litter. However, the number of leaf species was not responsible for these results, but leaf species composition explained the apparent non-additive effects. T. maria growth responded both positively and negatively to litter composition, and growth on mixed-litter could not always be predicted by averaging estimates of growth in single-species treatments. Furthermore, breakdown and feeding rates in mixed litter treatments could not always be predicted from estimates of single-species rates. Given that species richness and composition of senesced leaves in streams reflects riparian plant species richness, in-stream secondary production of detritivores and organic matter dynamics may be related to species loss of trees in the riparian zone. Loss of key species may be more critical to maintaining such processes than species richness per se.     

Effect of Leaf Type and Pesticide Exposure on Abundance of Bacterial Taxa in Mosquito Larval Habitats

Lentic freshwater systems including those inhabited by aquatic stages of mosquitoes derive most of their carbon inputs from terrestrial organic matter mainly leaf litter. The leaf litter is colonized by microbial communities that provide the resource base for mosquito larvae. While the microbial biomass associated with different leaf species in container aquatic habitats is well documented, the taxonomic composition of these microbes and their response to common environmental stressors is poorly understood. We used indoor aquatic microcosms to determine the abundances of major taxonomic groups of bacteria in leaf litters from seven plant species and their responses to low concentrations of four pesticides with different modes of action on the target organisms; permethrin, malathion, atrazine and glyphosate. We tested the hypotheses that leaf species support different quantities of major taxonomic groups of bacteria and that exposure to pesticides at environmentally relevant concentrations alters bacterial abundance and community structure in mosquito larval habitats. We found support for both hypotheses suggesting that leaf litter identity and chemical contamination may alter the quality and quantity of mosquito food base (microbial communities) in larval habitats. The effect of pesticides on microbial communities varied significantly among leaf types, suggesting that the impact of pesticides on natural microbial communities may be highly complex and difficult to predict. Collectively, these findings demonstrate the potential for detritus composition within mosquito larval habitats and exposure to pesticides to influence the quality of mosquito larval habitats.     

Higher temperature reduces the effects of litter quality on decomposition by aquatic fungi

1. We investigated the effects of riparian plant diversity (species number and identity) and temperature on microbially mediated leaf decomposition by assessing fungal biodiversity, fungal reproduction and leaf mass loss. 2. Leaves of five riparian plant species were first immersed in a stream to allow microbial colonisation and were then exposed, alone or in all possible combinations, at 16 or 24 °C in laboratory microcosms. 3. Fungal biodiversity was reduced by temperature but was not affected by litter diversity. Temperature altered fungal community composition with species of warmer climate, such as Lunulospora curvula, becoming dominant. 4. Fungal reproduction was affected by litter diversity, but not by temperature. Fungal reproduction in leaf mixtures did not differ or was lower than that expected from the weighted sum of fungal sporulation on individual leaf species. At the higher temperature, the negative effect of litter diversity on fungal reproduction decreased with the number of leaf species. 5. Leaf mass loss was affected by the identity of leaf mixtures (i.e. litter quality), but not by leaf species number. This was mainly explained by the negative correlation between leaf decomposition and initial lignin concentration of leaves. 6. At 24 °C, the negative effects of lignin on microbially mediated leaf decomposition diminished, suggesting that higher temperatures may weaken the effects of litter quality on plant litter decomposition in streams. 7. The reduction in the negative effects of lignin at the higher temperature resulted in an increased microbially mediated litter decomposition, which may favour invertebrate‐mediated litter decomposition leading to a depletion of litter stocks in streams.     

Effects of augmentation of coarse particulate organic matter on metabolism and nutrient retention in hyporheic sediments

SUMMARY 1. Metabolic and biogeochemical processes in hyporheic zones may depend on inputs of coarse particulate organic matter. Our research focused on how differing quantity and quality of organic matter affects metabolism and nutrient retention in the hyporheic zone of a first-order Appalachian stream.
2. Sixteen plots were established on a tributary of Hugh White Creek, NC, U.S.A. Sediment was extracted and treated with leaves, wood, plastic strips or remained unamended. Following treatment, sediment was returned to the stream and, approximately 3 months later, samples were removed from each plot.
3. Aerobic and anaerobic metabolism were measured as the change in O₂ and CO₂ in recirculating microcosms. At the same time, we monitored other possible terminal electron accepting processes and changes in nutrient concentrations. Aerobic metabolism was low in all treatments and respiratory quotients calculated for all treatments indicated that metabolism was dominated by anaerobic processes.
4. Rates of anaerobic respiration and total (combined aerobic and anaerobic) respiration were significantly greater ( P  < 0.05) in plots treated with leaf organic matter compared to controls.
5. Addition of leaves, which had a low C:N ratio, stimulated respiration in hyporheic sediments. Anaerobic processes dominated metabolism in both control and amended sediments. Enhanced metabolic rates increased retention of many solutes, indicating that energy flow and nutrient dynamics in the subsurface of streams may depend upon the quantity and quality of imported carbon.     

Detritivore stoichiometric diversity alters litter processing efficiency in a freshwater ecosystem

Many studies have estimated relationships between biodiversity and ecosystem functioning, and observed generally positive effects. Because detritus is a major food resource in stream ecosystems, decomposition of leaf litter is an important ecosystem process and many studies report the full range of positive, negative and no effects of diversity on decomposition. However, the mechanisms underlying decomposition processes in fresh water remain poorly understood. Organism body stoichiometry relates to consumption rates and tendencies, and decomposition processes of litter may therefore be affected by diversity in detritivore body stoichiometry. We predicted that the stoichiometric diversity of detritivores (differences in C: nutrient ratios among species) would increase the litter processing efficiency (litter mass loss per total capita metabolic capacity) in fresh water through complementation regarding different nutrient requirements. To test this prediction, we conducted a microcosm experiment wherein we manipulated the stoichiometric diversity of detritivores and quantified mass loss of leaf litter mixtures. We compared litter processing efficiency among litter species in each microcosm with single species detritivores, and observed detritivores with nutrient‐rich bodies tended to prefer litter with lower C: nutrient ratios over litter with higher C: nutrient ratios. Furthermore, litter processing efficiencies were significantly higher in the microcosms containing species of detritivores with both nutrient‐rich and ‐poor bodies than microcosms containing species of detritivores including only nutrient‐rich or ‐poor bodies. This might mean a higher stoichiometric diversity of detritivores increased litter processing efficiency. Our results suggest that ecological stoichiometry may improve understanding of links between biodiversity and ecosystem function in freshwater ecosystems.