Emergence and lateral dispersal of adult Plecoptera and Trichoptera from Broadstone Stream, U.K.

1. Emergence and inland dispersal of adult stoneflies (Plecoptera) and caddisflies (Trichoptera) from Broadstone Stream, an acidic and iron-rich stream in southern England, were studied over 10 months in 1996–1997. Fifteen pyramidal emergence traps were placed randomly in a 200-m stretch. Three Malaise traps were placed above the stream and six more on each side (one wooded, one open) along a transect at distances of 1, 15, 30, 45, 60 and 75 m from the channel. 2. More than 16 000 stoneflies, belonging to 11 species, and just under 400 caddisflies (22 species) were caught. Four dominant stoneflies (Leuctra fusca, Leuctra nigra, Leuctra hippopus and Nemurella pictetii) accounted for 96% and 95% of the catches in the emergence and Malaise traps, respectively. Two caddisflies (Plectrocnemia conspersa and Potamophylax cingulatus) accounted for 63% of the catch in the Malaise traps. Few caddisflies were taken in emergence traps. 3. The emergence periods of L. fusca, L. nigra and L. hippopus were well-defined and unimodal, whereas that of N. pictetii was prolonged and erratic. Overall, more females (1285) emerged than males (740). 4. Female stoneflies and caddisflies were in the majority in the Malaise traps above the stream. On land, significantly more females than males of L. fusca, L. nigra and P. cingulatus were caught. The sex ratio of the remaining species did not deviate significantly from 1:1. 5. The three Malaise traps placed above the stream caught most of the stoneflies though there was also dispersal away from the channel, the numbers caught declining with distance. Exponential models explained between 67% and 99% of the variation in numbers of individuals with distance from the channel in the four common stoneflies. Half the individuals went less than 11–16 m from the stream, while 90% travelled less than 51 m. Significantly more L. nigra and N. pictetii were caught in the woodland than on the open side, whereas L. hippopus showed no overall preference for either side.     

The effect of sunlight on leaf litter quality reduces growth of the shredder Klapopteryx kuscheli

• 1 The direct effect of sunlight on the conditioning, breakdown and incorporation of leaf litter in stream food webs has not yet been considered. The aim here was to evaluate the effects of light intensity on the colonization of leaf litter by microorganisms and its resulting quality as food for the stonefly shredder Klapopteryx kuscheli.
• 2 Leaf litter was conditioned for 2 months in an open reach of a second‐order stream in litter bags either exposed to or shaded from direct sunlight. Subsequently, we performed laboratory experiments to test larval consumption, growth, growth efficiency and feeding preference fed on both leaf litter treatments.
• 3 Leaf litter in the unshaded treatment had three times more chlorophyll‐a (Chl‐a) than that in the shaded treatment, 50% lower fungal biomass and similar bacterial abundance. Although larvae did not prefer either food and fed at the same rate on both leaf litter treatments, they grew twice as fast on the shade‐conditioned leaves and attained a two‐fold higher growth efficiency.
• 4 Sunlight can have significant effects on detritus‐based food webs. Riparian modification induced by human activities in forested catchments increases the potential for sunlight to influence detritus dynamics.

     

Fungal alteration of the elemental composition of leaf litter affects shredder feeding activity

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Resource dynamics and detritivore production in an acid stream

1. Life history patterns and production of eight shredder‐detritivore species were studied in relation to the detritus dynamics of a small acidic stream in England. Mean annual detritus inputs (direct and lateral sources combined) were approximately 400 g m^?2 year^?1 and showed significant seasonal and annual variation. 2. Detritus standing stock did not increase significantly during times of high input, reflecting low retention efficiency. However, the mean detritus standing stock was relatively large (108 g m^?2) reflecting a slow decomposition rate typical of acid streams. 3. Four species were univoltine with highly synchronous patterns of emergence and recruitment (Leuctra inermis, Leuctra hippopus, Capnia vidua and Amphinemura sulcicollis). Two species were univoltine with extended patterns of emergence and recruitment (Nemoura cinerea, Potamophylax cingulatus). Leuctra nigra was apparently semivoltine, while Protonemura meyeri showed two successive cohorts in the second year of the study, suggesting either bivoltinism or cohort splitting. 3. Secondary production of the dominant shredders was 1.67 g m^?2 year^?1 in 1997 and 1.99 g m^?2 year^?1 in 1998, which is low compared with other small European streams. This was probably because of an impoverished invertebrate community and poor food quality associated with acid conditions. Food availability probably did not account for the low production as the detritus standing stock far exceeded the estimated shredder ingestion of 42–50 g m^?2 year^?1. 4. Despite low overall shredder production, species‐specific production was high, possibly because of competitive release in this species‐poor acid stream. Periods of high production and growth showed no relationship with detritus availability but were closely related to life history.     

Nutrients, algae and grazers in some British streams of contrasting pH

1. The relationship between algal biomass accumulation, invertebrate colonization, and stream-water pH was investigated in seven streams in three regions of England and Wales. Possible nutrient limitation of algal production at all sites was examined with diffusion substrata. 2. Periphyton assemblages on experimental substrata after 30 days were dominated by diatoms, notably Eunotia spp., at all sites. Algal pigment concentration (chlorophyll a and phaeopigments) was not correlated with stream-water pH, and mean concentrations on control (unenriched) substrata ranged from 0.08 to 1.94 μg cm^?2. 3. The growth response of periphyton to nutrient additions was site specific. Algal production was stimulated by nutrient additions at sites in the English Lake District and Llyn Brianne (south-west Wales), but not in the Ashdown Forest (southern England). 4. Larval Chironomidae were the main invertebrates retrieved from substrata at all sites. Within all three regions, larval abundance was positively related to algal pigment concentration (biomass). Abundance of the stonefly Nemurella pictetii was also positively correlated with algal biomass at the one site where it occurred. 5. Our results indicate that epilithic algal production in small, oligotrophic streams is unlikely to be determined primarily by pH. Neither do they support the view that an absence of grazers from acid streams is necessarily due to an inadequate food supply.     

Leaf diversity influences in-stream litter decomposition through effects on shredders

1. The functioning of many aquatic ecosystems is controlled by surrounding terrestrial ecosystems. In a view of growing interest in linking biodiversity to ecosystem‐level processes, we examined whether and how leaf diversity influences litter decomposition and consumers in streams. 2. We tested experimentally the hypothesis that the effects of leaf diversity on decomposition are determined by the responses of leaf consumers to resource–habitat heterogeneity. Leaves from three common riparian trees, beech (Fagus sylvatica), hazel (Corylus avellana) and ash (Fraxinus excelsior), were exposed alone and in all possible mixtures of two and three species in a stream. We analysed individual leaf species for decomposition rate, microbial respiration and mycelial biomass, and we determined the species composition, abundance and biomass of shredders in leaf bags. 3. We found that the decomposition of the fastest decomposing leaves (hazel and ash) was substantially stimulated (up to twofold higher than single species leaf packs) in mixtures containing beech leaves, which are refractory. In contrast, the decomposition of beech leaves was not affected by leaf mixing. Such species‐specific behaviour of leaves in species mixtures has been overlooked in previous studies that examined the overall decomposition of litter mixtures. 4. The effects of leaf diversity on decomposition varied with the abundance and biomass of shredders but not with microbial parameters. Beech leaves alone were less attractive to shredders than leaf packs made of hazel, ash or any mixture of species. Moreover, the presence of beech leaves in mixtures led to higher shredder abundance and biomass than we had expected from data from single species exposed alone. Lastly, we found that early instars of the caddisfly Potamophylax (the dominant shredder in terms of biomass) almost exclusively used the toughest material (i.e. beech leaves) to construct their cases. 5. Leaf pack heterogeneity may have altered shredder‐mediated decomposition. Shredders colonising diverse leaf packs benefited from the stable substratum provided by beech leaves, whereas ash and hazel leaves were primarily used as food. Thus, our findings provide strong evidence for an intimate linkage between the diversity of riparian vegetation and aquatic communities.     

Elevated light and nutrients alter the nutritional quality of stream periphyton

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Growth and emergence of the stonefly Leuctra nigra in coniferous forest streams with contrasting pH

1. Growth and emergence of the acidification tolerant stonefly Leuctra nigra were studied in four coniferous forest streams with contrasting pH. Invertebrate samples were taken bimonthly during May 1997 to April 1998. Emerging L. nigra were captured in two of the streams during April to July 1998 using pyramidal emergence traps. In addition, a laboratory experiment was conducted to evaluate growth of L. nigra in response to variations in food quality (neutral or acid conditioned leaves) and water pH. 2. The invertebrate community in low pH streams had lower diversity and density compared with neutral streams. Abundance of L. nigra was also lower, despite the dominance of this stonefly in the invertebrate community in the acidic streams. 3. Secondary production of L. nigra was 2.04 and 3.07 g DW m^–2 year^–1 in the neutral streams compared with 1.02 and 1.27 g DW m^–2 year^–1 in the acidic streams. The P/B ratio differed only slightly among streams. 4. More adult L. nigra emerged in the neutral stream compared with the acidic stream, despite the density of the pre‐emerging fifth instar nymphs of L. nigra being similar in the two streams. Adult size (expressed as wing length) and female fitness were also significantly greater in the neutral stream. 5. Laboratory growth of L. nigra nymphs was significantly lower when fed acid conditioned alder leaves rather than neutral conditioned leaves. There was no difference in growth when nymphs were fed neutral conditioned leaves in acid and neutral water. 6. Our study shows multiple, sub‐lethal effects on life history of the acidification tolerant stonefly L. nigra in coniferous streams with low pH. The results suggest that these effects could be mediated primarily by low food quality. These findings add to the existing evidence that coniferous plantation forestry has a negative impact on stream ecosystems.     

Top-down and bottom-up influences on populations of a stream detritivore

1. Knowledge of the influence of predatory fish in detritus‐based stream food webs is poor. We tested whether larval abundance of the New Zealand leaf‐shredding caddisfly, Zelandopsyche ingens (family Oeconesidae), was affected by the presence of predatory brown trout, Salmo trutta and the abundance of their primary detrital resource (Nothofagus leaves). 2. The density of Z. ingens and the biomass of leaves were determined in seven fishless streams and four trout streams in the Cass region, central South Island, on four occasions spanning 5 years. 3. Physicochemical conditions were similar in trout and fishless streams, but ancova indicated that Z. ingens numbers were positively related to leaf biomass and that caddisfly numbers were significantly greater in fishless streams than trout streams for any given biomass of leaf. The cases of trout stream larvae were also heavier per unit length than those in fishless streams. 4. Our results provide evidence for both top‐down and bottom‐up influences on a detritus‐based stream food web. Although stream detritivores may benefit from a habitat that provides both food and a degree of protection from predators, top‐down effects of predators on detritivore population abundance were still important. Thus, detrital resource availability may determine maximum attainable population size, whereas predation is likely to reduce the population to a level below that.     

Density‐ and trait‐mediated top–down effects modify bottom–up control of a highly endemic tropical aquatic food web

Benthic invertebrates mediate bottom–up and top–down influences in aquatic food webs, and changes in the abundance or traits of invertebrates can alter the strength of top–down effects. Studies assessing the role of invertebrate abundance and behavior as controls on food web structure are rare at the whole ecosystem scale. Here we use a comparative approach to investigate bottom–up and top–down influences on whole anchialine pond ecosystems in coastal Hawai‘i. In these ponds, a single species of endemic atyid shrimp (Halocaridina rubra) is believed to structure epilithon communities. Many Hawaiian anchialine ponds and their endemic fauna, however, have been greatly altered by bottom–up (increased nutrient enrichment) and top–down (introduced fish predators) disturbances from human development. We present the results of a survey of dissolved nutrient concentrations, epilithon biomass and composition, and H. rubra abundance and behavior in anchialine ponds with and without invasive predatory fish along a nutrient concentration gradient on the North Kona coast of Hawai‘i. We use linear models to assess 1) the effects of nutrient loading and fish introductions on pond food web structure and 2) the role of shrimp density and behavior in effecting that change. We find evidence for bottom–up food web control, in that nutrients were associated with increased epilithon biomass, autotrophy and nutrient content as well as increased abundance and size of H. rubra. We also find evidence for top–down control, as ponds with invasive predatory fish had higher epilithon biomass, productivity, and nutrient content. Top–down effects were transmitted by both altered H. rubra abundance, which changed the biomass of epilithon, and H. rubra behavior, which changed the composition of the epilithon. Our study extends experimental findings on bottom–up and top–down control to the whole ecosystem scale and finds evidence for qualitatively different effects of trait‐ and density‐mediated change in top–down influences.     

Grazer diversity affects resistance to multiple stressors in an experimental seagrass ecosystem

When multiple stressors act simultaneously, their effects on ecosystems become more difficult to predict. In the face of multiple stressors, diverse ecosystems may be more stable if species respond differently to stressors or if functionally similar species can compensate for stressor effects on focal species. Many habitats around the globe are threatened by multiple stressors, including highly productive seagrass habitats. For example, in Chesapeake Bay, USA, regional climate change predictions suggest that elevated temperature and freshwater inputs are likely to be increasingly important stressors. Using seagrass mesocosms as a model system, we tested whether species richness of crustacean grazers buffers ecosystem properties against the impacts of elevated temperature and freshwater pulse stressors in a fully factorial experiment. Grazer species responded to pulsed salinity changes differently; abundance of Elasmopus levis responded negatively to freshwater pulses, whereas abundance of Gammarus mucronatus and Erichsonella attenuata responded positively or neutrally. Consistent with the hypothesis that biodiversity provides resistance stability, biomass of epiphytic algae that form the base of the food web was less affected by stressors in species‐rich grazer treatments than in single‐species grazer treatments. Stochastic (among‐replicate) variation of sessile invertebrate biomass within treatments was also reduced in more diverse grazer treatments. Therefore, grazer species richness tended to increase the resistance stability of both major components of the seagrass fouling community, algae and invertebrates, in the face of environmental stressors. Finally, in our model system, multi‐stressor impacts suggested a pattern of antagonism contrary to previous assumptions of synergistic stressor effects. Overall, our results confirm that invertebrate grazer species are functionally diverse in their response to environmental stressors, but are largely functionally redundant in their grazing effects leading to greater resistance stability of certain ecosystem properties in diverse grazer assemblages even when influenced by multiple environmental stressors.     

Fish determine macroinvertebrate food webs and assemblage structure in Greenland subarctic streams

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Shifts in elemental composition,methylmercury content and δ15N ratio during growth of a High Arctic copepod

1. We examined how ontogenetic development in a calanoid copepod from the High Arctic, Limnocalanus macrurus, influenced its elemental composition (carbon, nitrogen, phosphorus), methylmercury (MeHg) content and stable nitrogen and carbon isotope ratios in populations from nine lakes. 2. Population structure explained 33–83% of among‐lake variation in the C, N and P composition of the biomass. Biomass dominated by early‐stage copepodites had a greater P content, which declined in more mature populations, as indicated by significant changes in % P and the molar N/P ratio. Carbon and lipid contents increased with the proportion of adult biomass. Copepod populations sampled in warmer waters had a greater proportion of adult biomass, and water temperature was the most significant environmental variable explaining elemental composition. 3. A δ¹⁵N enrichment of 3.3 ± 1.0‰ was associated with copepodite development. Gut contents of L. macrurus showed no evidence of animal (invertebrate) prey, indicating no change in trophic position. 4. Unexpectedly, MeHg concentration was negatively correlated with the proportion of adult biomass. However, this trend was not significant after correcting MeHg concentration to non‐lipid dry mass, suggesting a lipid dilution effect in more mature copepods. Lake surface area, rather than ontogeny, best explained MeHg concentrations in L. macrurus. 5. Ontogenetic influences on chemical constituents of this common Arctic copepod, particularly δ¹⁵N ratios and uncorrected MeHg concentrations, highlight the relevance of developmental processes for studies of food webs and mercury in species‐poor High Arctic lakes.     

Non-additive, identity-dependent effects of detrital species mixing on soft-sediment communities

Accelerating rates of species extinction and invasion have sparked recent interest in how changes in plant community composition can be propagated through food webs. Research in this area has, however, been largely restricted to considerations of how detrital species mixing affects litter decay processes. The consequences of changing detrital resources for whole assemblages of sediment‐dwelling invertebrates remain largely unknown. We manipulated the availability of three detrital sources, Avicennia marina leaves, Posidonia australis blades and Sargassum sp. thalli, on an Australian mudflat to test hypotheses about how changes in the type and number of macrophytes contributing to detrital resources might impact benthic invertebrate assemblages of estuarine soft‐sediments. By controlling for changes in total detrital biomass and ensuring that each detrital source was present in two‐ and three‐species mixes as well as monocultures, our experimental design was able to distinguish among effects of mixing, identity and biomass. Three months after detrital manipulation, macroinvertebrate abundance and species richness differed among treatments according to the biomass of detritus added and non‐additive effects of detrital species mixing. Whereas the mixing of two detrital species generally had an antagonistic effect on macroinvertebrate abundance and richness, faunal assemblages did not appreciably differ between three‐species mixes and monocultures. Generally negative effects of two‐species mixes on macroinvertebrates were opposed by positive effects on microphytobenthos, an important food‐source for many of the animals. Non‐additive effects on sediment communities were particularly apparent when Sargassum sp., the most labile of the three detrital sources considered, was included in two‐species mixes. This demonstration of non‐additive and identity‐dependent effects of detrital species mixing on soft‐sediment communities suggests that predicted compositional changes to aquatic macrophyte communities, resulting from coastal development and climate change, will flow on to effect other components of the estuarine food‐web.     

Trophic coupling of the microbial and the classical food web in Lake Baikal,Siberia

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Food preference, growth and maturation of Nemurella pictetii (Plecoptera: Nemouridae)

1. The stonefly Nemurella pictetii has a highly variable life cycle and is semi‐ and uni‐, but also plurivoltine in different geographical areas. 2. The influence of the type of food on growth and maturation of larvae was studied in laboratory experiments. Food preference was also assessed. 3. Larvae of N. pictetii are strongly opportunistic feeders utilising a broad range of food sources. However, biofilm was preferred over detritus (conditioned leaves), green algae and animal food. The highly mobile larvae quickly found the preferred food. Larvae fed on biofilm grew and developed significantly faster than larvae fed on conditioned leaves. 4. Larval growth and maturation in experiments as well as in the field differed with available food qualities. Phases of faster growth and maturation corresponded well with maximum availability of biofilm in the field. This contributes to the interpretations of differences in life cycles in the field.     

Diet quality influences isotopic discrimination among amino acids in an aquatic vertebrate

Stable nitrogen isotopic composition of amino acids (δ¹⁵N_AA) has recently been employed as a powerful tool in ecological food web studies, particularly for estimating the trophic position (TP) of animal species in food webs. However, the validity of these estimates depends on the consistency of the trophic discrimination factor (TDF; = Δδ¹⁵N_AA at each shift of trophic level) among a suite of amino acids within the tissues of consumer species. In this study, we determined the TDF values of amino acids in tadpoles (the Japanese toad, Bufo japonicus) reared exclusively on one of three diets that differed in nutritional quality. The diets were commercial fish‐food pellets (plant and animal biomass), bloodworms (animal biomass), and boiled white rice (plant carbohydrate), representing a balanced, protein‐rich, and protein‐poor diet, respectively. The TDF values of two “source amino acids” (Src‐AAs), methionine and phenylalanine, were close to zero (0.3–0.5‰) among the three diets, typifying the values reported in the literature (~0.5‰ and ~0.4‰, respectively). However, TDF values of “trophic amino acids” (Tr‐AAs) including alanine, valine, leucine, isoleucine, and glutamic acid varied by diet: for example, the glutamic acid TDF was similar to the standard value (~8.0‰) when tadpoles were fed either the commercial pellets (8.0‰) or bloodworms (7.9‰), but when they were fed boiled rice, the TDF was significantly reduced (0.6‰). These results suggest that a profound lack of dietary protein may alter the TDF values of glutamic acid (and other Tr‐AAs and glycine) within consumer species, but not the two Src‐AAs (i.e., methionine and phenylalanine). Knowledge of how a nutritionally poor diet can influence the TDF of Tr‐ and Src‐AAs will allow amino acid isotopic analyses to better estimate TP among free‐roaming animals.     

Riparian plant species loss alters trophic dynamics in detritus-based stream ecosystems

Riparian vegetation is closely connected to stream food webs through input of leaf detritus as a primary energy supply, and therefore, any alteration of plant diversity may influence aquatic ecosystem functioning. We measured leaf litter breakdown rate and associated biological parameters in mesh bags in eight headwater streams bordered either with mixed deciduous forest or with beech forest. The variety of leaf litter types in mixed forest results in higher food quality for large-particle invertebrate detritivores (‘shredders’) than in beech forest, which is dominated by a single leaf species of low quality. Breakdown rate of low quality (oak) leaf litter in coarse mesh bags was lower in beech forest streams than in mixed forest streams, a consequence of lower shredder biomass. In contrast, high quality (alder) leaf litter broke down at similar rates in both stream categories as a result of similar shredder biomass in coarse mesh bags. Microbial breakdown rate of oak and alder leaves, determined in fine mesh bags, did not differ between the stream categories. We found however aquatic hyphomycete species richness on leaf litter to positively co-vary with riparian plant species richness. Fungal species richness may enhance leaf litter breakdown rate through positive effects on resource quality for shredders. A feeding experiment established a positive relationship between fungal species richness per se and leaf litter consumption rate by an amphipod shredder (Gammarus fossarum). Our results show therefore that plant species richness may indirectly govern ecosystem functioning through complex trophic interactions. Integrating microbial diversity and trophic dynamics would considerably improve the prediction of the consequences of species loss.     

Comparison of glucose/xylose cofermentation of poplar hydrolysates processed by different pretreatment technologies

The inhibitory effects of furfural and acetic acid on the fermentation of xylose and glucose to ethanol in YEPDX medium by a recombinant Saccharomyces cerevisiae strain (LNH‐ST 424A) were investigated. Initial furfural concentrations below 5 g/L caused negligible inhibition to glucose and xylose consumption rates in batch fermentations with high inoculum (4.5–6.0 g/L). At higher initial furfural concentrations (10–15 g/L) the inhibition became significant with xylose consumption rates especially affected. Interactive inhibition between acetic acid and pH were observed and quantified, and the results suggested the importance of conditioning the pH of hydrolysates for optimal fermentation performance. Poplar biomass pretreated by various CAFI processes (dilute acid, AFEX, ARP, SO₂‐catalyzed steam explosion, and controlled‐pH) under respective optimal conditions was enzymatically hydrolyzed, and the mixed sugar streams in the hydrolysates were fermented. The 5‐hydroxymethyl furfural (HMF) and furfural concentrations were low in all hydrolysates and did not pose negative effects on fermentation. Maximum ethanol productivity showed that 0–6.2 g/L initial acetic acid does not substantially affect the ethanol fermentation with proper pH adjustment, confirming the results from rich media fermentations with reagent grade sugars. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Effects of microbivore species composition and basal resource enrichment on trophic-level biomasses in an experimental microbial-based soil food web

Previous theoretical and empirical evidence suggests that species composition within trophic levels may profoundly affect the response of trophic-level biomasses to enhanced basal resources. To test whether species composition of microbivorous nematodes has such an effect in microbial-based soil food webs, I created three microcosm food webs, consisting of bacteria, fungi, bacterial-feeding nematodes (Acrobeloides tricornus, Caenorhabditis elegans), fungal-feeding nematodes (Aphelenchus avenae, Aphelenchoides sp.) and a predatory nematode (Prionchulus punctatus). The food webs differed in species composition at the second trophic level: food web A included A. tricornus and Aph. avenae, food web B included C. elegans and Aphelenchoides sp., and food web AB included all four species. I increased basal resources by adding glucose to half of the replicates of each food web, and sampled microcosms destructively four times during a 22-week experiment to estimate the biomass of organisms at each trophic level. Microbivore species composition significantly affected bacterivore and fungivore biomass but not bacterial, fungal or predator biomass. Greatest bacterivore and fungivore biomass was found in food web A, intermediate biomass in food web AB, and smallest biomass in food web B. Basal resource addition increased the biomass of microbes and microbivores but did not affect predator biomass. Importantly, microbivore species composition did not significantly modify the effect of additional resources on trophic-level biomasses. The presence of a competitor reduced the biomass of A. tricornus and Aph. avenae, in that the biomass of these species was less in food web AB than in food web A, whereas the biomass of C. elegans and Aphelenchoides sp. was not affected by their potential competitors. The biomass of Aph. avenae increased with additional resources in the absence of the competitor only, while the biomass of A. tricornus and Aphelenchoides sp. increased also in the presence of their competitors. The results imply that microbivore species composition may determine the second-level biomass in simple microbe-nematode food webs, but may not significantly affect biomass at other levels or modify the response of trophic-level biomasses to enhanced basal resources. The study also shows that even if the role of predation in a food web is diminished, the positive response of organisms to increased resource availability may still be hindered by competition. Received: 22 June 1998 / Accepted: 28 August 1998