Seasonal food webs were constructed for the whole invertebrate assemblage (meio‐ and macrofauna) inhabiting Broadstone Stream (southeast England). High and uniform taxonomic resolution was applied in a dietary analysis, by resolving the complete benthic community to species, including the meiofauna, protozoa, and algae. Meiofauna accounted for 70% of all species in the summary web and for 73% and 63% of those in the summer/autumn and spring webs, respectively. The web structure changed between summer/autumn and winter/spring, due to differences in species composition. Many stream invertebrates fed on meiofauna and organic matter. Addition of meiofauna to the Broadstone web increased the percentage of intermediate species. Seasonal webs contained between 54 (spring 1997) and 86 (autumn 1996) interactive taxa and 229–378 trophic links. Marked differences in web complexity were found between the summer/autumn and winter/spring periods. Meiofauna accounted for most of the links in the web with a high proportion of intermediate–intermediate links in summer and autumn (0.421–0.440) and also of intermediate–basal links during winter and spring (0.509–0.628). In general, the summary web showed that intermediate species and basal resources were numerically dominant components in this stream. Web connectance rose slightly between summer (0.052) and winter (0.061) and increased further in spring (0.079), coinciding with a reduction in species number. A high fraction of detritivores was combined with omnivorous predators, many of which supplemented their diets with organic matter and, depending on season, with algae and invertebrate eggs. In addition, a wide range of feeding modes was found among meiofaunal species. The diversity of the Broadstone community suggests that the impact of top predators tends to dissipate. A low proportion of top predators in the web was combined with a low mean number of prey items, other than detritus, in their guts (large predators, 1.08–1.26 prey/individual gut; small‐sized tanypods, 2.15–2.32 prey/individual gut). Dietary similarity was highest in autumn and winter 1996, and observed feeding links of the most common predatory species showed low overlap in their diets. The web architecture of this stream is reticulate and complex, and the patterns observed in these seasonal webs differed from previous stream webs, resulting in low connectance, high linkage density, long food chains, and a high proportion of intermediate species and of intermediate–intermediate links. The food web derived from Broadstone Stream clearly demonstrates that the meiofauna increases web complexity and thus, taking into account their functional diversity, may be crucial to the understanding of food web properties and ecosystems processes in streams.

Summary This paper summarises the most important contributions on trophic relationships of lotic meiofauna. In contrast to marine research, the few quantitative studies of the freshwater meiobenthos have shown that these invertebrates not only take up particulate/fine organic matter, but also dissolved organic substances attached to organic particles. In lotic ecosystems, further estimates of grazing rate and bacterial/algal ingestion rate are needed, particularly in situ measurements. The effects of macroinvertebrate predators upon meiofauna are still under debate. Depending on the type of experiments (laboratory vs. field) it seems that macrofauna may or may not affect meiofauna. Field samples and analyses of gut contents of larval tanypod chironomids have shown that the impact upon meiofauna was low and larvae were nonselective predators. Predation amounted to 2.2% of the combined prey density and prey consumption averaged 1.3 individuals per predator individual per year. Adding taxonomic resolution by including the meiofaunal component within lotic food webs distinctly increases the number of total species and, as a consequence, changes food web statistics. Webs that included meiofauna revealed that these metazoans contributed substantially to the percentage of intermediate species (species with predators and prey). The resolution of dietary analyses of major consumers of macro‐ and meiobenthos showed that many stream invertebrates feed on meiofauna.

The existence of a general relation between population density and body size in animal assemblages has been debated because of known biases and ambiguities in the published data and data handling. Using new comprehensive data sets from two geographically separated stream communities that encompass 448 and 260 invertebrate taxa with a wide range of body sizes, we show that an inverse proportionality between density and body size is a consistent feature in these communities. The scaling across taxa is not statistically different between the two systems, indicating a convergent pattern of communities. Variation in the regression slope among different taxonomic groups indicates that these communities are not governed universally by a single ecological or energetic rule.

Summary1. The properties of food webs are important both in theoretical ecology and environmental management, yet remain elusive. Here, we examined 12 new stream food webs of higher taxonomic resolution and completeness than any previously published data set and combined them with other 10 published stream webs. 2. Compared with most previously published food webs, these stream communities (containing between 22 and 212 species) had more feeding links per species, a higher fraction of intermediate species (0·78-0·88) and lower fractions of top (0·04-0·09) and 'basal' species (0·07-0·15). 3. The exponent of the log link-log species relationship ( S 1·3 ) differed significantly (  : P < 0·001) from the link-species 'law' ( S 1 ) and the constant connectance hypothesis ( S 2 ). Based on these results, the link scaling law and the constant connectance hypothesis must be rejected for food webs in stream systems. 4. Connectance was markedly lower than previously reported and decreased with web size. We attribute this to the body-size disparity of organisms at the top and the bottom of the web, flow disturbance and to the high physical heterogeneity and complexity of the sedimentary habitat. All these may reduce the fraction of possible feeding links that are realized.

1. Three predatory chironomid species constituted numerically 8.8% (± 95% CL 2.2) of the macro‐ and meiobenthic community at the sediment surface and in the hyporheic zone of Oberer Seebach, a gravel stream in Lower Austria. Larvae of Thienemannimyia geijskesi (Goetghebuer) and Nilotanypus dubius (Meigen) occurred in higher densities in sediment depths between 10 and 40 cm, whereas Conchapelopia pallidula (Meigen) achieved higher densities at the sediment surface. The three species completed one generation in a year. 2. A total of ninety‐seven prey species and instars were identified by gut analyses, of which forty‐one benthic rotifer species constituted 69.5% of individuals and twenty‐three chironomid species and their instars, 22.9%. The three tanypod species showed shifts from mainly rotifer species in early instars to chironomids and diverse other meio‐ and macrofaunal taxa in later instars. Rather than shifting towards larger prey sizes, growing predators expanded their upper size thresholds and continued to include smaller prey species in their diet. The extent to which tanypod instars fed on similar prey size classes declined with increasing larval size. Predation by tanypods amounted to 2.2% (± 95% CL 0.1) of the combined prey densities and prey consumption averaged 1.32 (bootstrap 95% CL 1.26–1.39) individuals per predator individual. 3. Preferences for microhabitat flow differed between predator species and in the prey assemblage. Prey densities and densities of T. geijskesi and C. pallidula were highest in pool areas, whereas N. dubius achieved high densities in riffle sites. 4. Tanypod larvae fed non‐selectively among prey types. To test the significance of observed size(instar)‐specific spatial and dietary overlap values amongst tanypod species, simulations were generated from random models for pairs of intra‐ and interspecific associations of individuals and groups of prey and predator species. Groups and individuals of tanypod instars fed near randomly on groups of prey types and a high proportion (P > 0.60) of prey individuals are quasi‐randomly chosen by tanypods in those patches. Tanypod instar‐pairs did not show a sustained trophic resource partitioning in time, thus reducing the degree of competitive interactions for food in this predator guild. Spatially segregated and non‐segregated tanypod instars formed random aggregations independent of each other at different flow microhabitats. 5. Species‐rich prey assemblages such as benthic rotifers and larval chironomids increased the probability of non‐selective feeding upon a wide spectrum of prey species by tanypods. Prey choice was governed by prey availability and tanypod individuals fed on many species at rather even proportions independent of each other.

Summary 1.We investigated abundance-size spectra and body size-density allometry using an exceptionally detailed data set from a stony stream. The body size distribution of the whole metazoan community was expressed in terms of both density and biomass over a 14-month survey, and the relationships of body size with population density and taxon richness were also evaluated. 2. The distribution of density among size classes varied considerably between sampling occasions, although that of biomass was rather more consistent. This suggests that the body size distribution is dynamic and does not always fall into any single pattern or category, as has been assumed hitherto. 3. Analysis of the modes in the density-body size distribution in individual sample-units revealed small-scale spatial variation, with little consistency in the number or position of modes in any one month. 4. This stream community did not conform to the − 0·75 (or indeed to any other significant regression) relationship between body size and population density, although such patterns are very sensitive to methodological details. 5. In data aggregated over the whole study, most metazoan taxa fell into the size range < 10-100 µ g. The annual average density-and biomass-body size spectra appeared triand bimodal, respectively (on log-log scales), with minor troughs in the density spectrum at about 100 and 0·1 µ g body mass. This could indicate, at this scale of analysis, discontinuities in the way in which benthic species exploit their environment. 6. Spatial and temporal heterogeneity in size spectra, and the lack of clear density-body size allometry, could be due to biotic or physicochemical fluctuations and environmental change (including seasonal and longer-term changes in acidity) that cause rapid species turnover and changes in density and distribution, particularly among the meiofauna. Small and rare species may thus be in the process of colonization or decline and the pattern would indicate a lack of equilibrium, particularly among organisms in the smallest size classes, at this temporal scale of the study.

In recent years, biodiversity has become an issue of broad academic interest, and its assessment and maintenance are now recognized as an important area of ecological research. While the concept of biodiversity encompasses, first and foremost, the total number of species co-occurring in a locality, it has increasingly been realized that information on the relative abundances of co-occurring species is also required for a better understanding of the patterns and dynamics of biodiversity. In many areas of ecological research, "abundance" constitutes a key variable that characterizes populations and communities. The relative abundances of species in natural communities reflect evolutionary and contemporary processes occurring on different spatiotemporal scales. The idea of niche apportionment has been developed to provide an integrated conceptual framework for the study of species abundance patterns in communities. This article reviews a number of important issues surrounding the concept of niche apportionment, including some aspects that have received very little or no consideration in previous ecological literature. The main emphasis here is on possible evolutionary implications and backgrounds. Further, as a universal factor which affects species abundance in one way or another, body size is highlighted and its relationship with abundance ("density-body-size relation") is considered, referring in particular to a recent comprehensive analysis of freshwater benthic data. Consideration of this and other studies has led to the formulation of the biomass equivalence rule, that suggests the independence of the biomass measure of abundance from body size, which strengthens the logical basis of niche apportionment models. It is suggested that, compared with Hubbell's neutral theory of biodiversity, niche apportionment with the biomass equivalence rule represents a conceptually more sound and widely applicable approach to elucidating species abundance patterns.

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