The distribution of biomass production and its allocation across populations under environmental constraints draw a picture of community dynamics and energy flows in ecosystems. However, microscopic benthic invertebrates (meiofauna) are often overlooked in stream production budgets. We monitored the meiofauna dwelling in the sediment at two headwater stream sites (Ems and Furlbach, NW Germany) during 1 year. The two streams were similar in their granulometries and temperature regimes, but they differed in their flow velocities and nutrient balances, which allowed investigations of the effects of these factors on the density and production of different taxonomic groups of meiofauna. Meiofaunal production in the top 10‐cm sediment at Ems and Furlbach was 2.58 and 5.46 gC m−2 yr−1, respectively; these values are among the highest reported so far for a streambed. Allocation of density and production across taxonomic groups differed between the two streams. Tardigrades, rotifers, oligochaetes, and gastrotrichs thrived in the phosphate‐rich, slow‐flowing waters of the Ems, whereas nematodes, micro‐turbellarians, and harpacticoid copepods were better adapted to the nitrate‐rich, fast‐flowing waters of the Furlbach. Body‐size distribution varied across site and depth and was mostly multi‐modal, with important contributions of minute individuals weighing between 0.01 and 0.1 μgC. Our study shows that, despite their small size, meiofauna can produce substantial amounts of biomass and should thus be better considered in budgets and models of stream ecosystems.

We monitored for 1 year the sediment nematofauna of two headwater streams located near water resurgence from typical helocrene springs. Nematode community composition, benthic organic matter (BOM) and chlorophylla(BChl a) were assessed in two sediment layers (0-5 and 5-10 cm depth), providing insights into nematode vertical distribution. Globally, the density, diversity and functional richness of nematodes were lower in the upper sediments, although important amounts of BChl awere available there. The assemblages differed markedly between springs, with only 41 shared species from a total of 120 species, suggesting stochastic horizontal distribution even at small geographical scales (<5 km). Bacterial-feeding nematodes dominated in the upper sediments, whereas omnivorous, fungal- and plant-feeding nematodes thrived in the deeper sediments. Reproduction by the dominant algal-feeding species,Prodesmodora arctica, was detected in August. The substantial presence of juveniles throughout the year highlighted the importance of parthenogenetic reproduction.

The potential of copepods as predators of free-living nematodes was tested by presenting Diacyclops bicuspidatus, a common holarctic cyclopoid copepod, with Panagrolaimus sp. as prey in the laboratory. Diacyclops bicuspidatus readily fed on nematodes of all size classes, including prey longer than itself. No size preference was observed. Handling times varied between a few seconds and several minutes, depending on the size of the prey. At different prey densities, the feeding rates of D. bicuspidatus followed a type II functional response. Starved D. bicuspidatus consumed up to 45.1 nematodes in 2 h, equivalent to 43.5% of the copepod's body mass. Indications that nematophagous nutrition is common among freshwater copepods are discussed.

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Meiofauna play an important ecological role in benthic ecosystems. However, little is known about their small‐scale (<1 m2) distribution. Yet, this knowledge has both theoretical implications, by improving our understanding of how processes generate patterns; and practical outcomes by improving sampling designs. The goal of this study was to obtain insights into the small‐scale distribution of meiofaunal invertebrates dwelling in the sediment of a lentic ecosystem (in this study, an artificial pond). We addressed this aim at coarse and fine taxonomical levels using meiofaunal groups and nematode species assemblages, respectively. For this purpose, 225 sediment samples were collected over a surface of 729 cm2 using a straw corer. Spatial patterns (dispersion index) and the typical patch sizes of the dominant meiofaunal groups were estimated using spatial autocorrelation analysis (Moran's I). We hypothesized that (1) we would find a patchy distribution of organisms with patch radii of a few centimetres, (2) the distribution of microbivorous meiofauna would follow that of OM, and (3) distribution would be patchier at the species level. The densities of the dominant meiobenthic groups (nematodes, gastrotrichs and rotifers) decreased with sediment depth. In all sediment layers, nematodes contributed 61%–82% to total invertebrate densities. Eleven nematode species were identified, with Eumonhystera filiformis (Bastian 1865) and Monhystera cf. paludicola de Man 1881 dominating the assemblage. All meiofaunal groups showed an aggregated distribution pattern (dispersion index >1). Spatial autocorrelation analysis revealed aggregations of nematodes in a patch radius of 3–8 cm according to the different sediment layers. Rotifers were also patchy distributed but with smaller patch radius (3 cm), whereas the respective distributions of gastrotrichs were less patchy. Therefore, our results mainly confirmed hypothesis (1). Our results provide the first insights into rotifer patch sizes. Also, these findings highlight the need for sampling designs better able to capture small‐scale heterogeneities during larger‐scale investigations. We ruled out the organic matter (OM) supply as an important driver of meiofaunal community structure as OM content did not correlate with the density of either meiofaunal groups or nematode species, and therefore arguing against hypothesis (2). Instead, the different distributions suggested responses to qualitative differences in resource availability, oxygen availability, biotic interactions and specific population‐level processes, such as behaviour, dispersal and locomotion. Patchiness did not increase from coarse to fine taxonomic levels, thus contradicting hypothesis (3). Our results instead suggest that the patchiness of the spatial distribution was more closely linked to functional traits.