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Wide variation in a tiny space: The microdistribution of meiobenthos in an artificial pond

Gansfort, Birgit, Traunspurger, Walter, Threis, Ilka, Majdi, Nabil
Freshwater biology 2018 v.63 no.5 pp. 420-431
Nematoda, Rotifera, autocorrelation, biocenosis, community structure, fauna, invertebrates, lentic systems, locomotion, organic matter, oxygen, sediments, straw
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.