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.

SUMMARY 1. The whole metazoan community (i.e. including the meiofauna) of an acidic, fishless stream in south‐east England was surveyed over 14 months between March 1999 and April 2000. Invertebrate density, biomass and taxonomic richness were assessed on each sampling occasion in relation to physico‐chemical variables. 2. The meiofauna were more numerous and diverse than the macrofauna, while their total biomass occasionally equalled that of the macrofauna. 3. The meiofaunal and macrofaunal assemblages appeared to respond to different environmental factors. The meiofauna showed genuine species turnover through the year, while the macrofauna varied less in taxonomic composition though there were substantial variations in density. 4. These data suggest that the meiofauna and macrofauna exist at different temporal and spatial scales and perceive their environment with a different ‘grain’.

The benthic communities of streams contain invertebrates of a wide range of body size and from many taxa. Owing mainly to methodological problems, however, the contribution of smaller and more obscure metazoans to community structure and dynamics, including production, is poorly known compared with that of larger size fractions and, particularly, insects and macrocrustaceans. Based on a monthly survey of a first‐order, acidic English stream, we used the size‐ frequency method to estimate annual production of the whole metazoan benthos (down to organisms retained on a 42‐ µm mesh and being as taxonomically inclusive as possible). Mean total secondary production (5.22 g dry weight m−2 yr−1) was low, presumably mainly due to the stream's acidity. About 15% (0.76 g m−2 yr−1) of this total production, however, was contributed by the permanent meiofauna (species always small enough to pass through a 500‐ µm mesh, and mainly made up of ostracods and copepods but also including rotifers, microturbellarians, and others). By estimating separately production from the macrofaunal and meiofaunal net fractions (500 µm and 42‐500 µm, respectively), we found that about 51% of total production could be accounted for by the permanent and temporary meiofauna together, the latter being defined as organisms that potentially grow into the macrofaunal size class and here consisting primarily of oligochaetes, chironomids, and plecopterans. This study points to the potentially substantial underestimation of production arising from the problems of assessing the meiofauna, including the former use of coarse‐meshed (e.g., 500 µm) sampling devices, the requirement for live sorting of many soft‐bodied taxa, and other difficulties of counting and identifying less well known groups.