What drives small-scale spatial patterns in lotic meiofauna communities?

1. Lotic meiofaunal communities demonstrate extremely variable dynamics, especially when viewed at small spatial scales (less than or equal to metres). Given the limited amount of research on lotic meiofauna, we chose to organise our discussion of their small-scale spatial patterns around the dominant factors we believe drive their spatial distributions in streams. We separate scale-dependent effects that structure lotic meiofauna into biotic factors (e.g. predation, food quantity/quality, dispersal) and abiotic factors (e.g. local flow dynamics and substratum characteristics). 2. The impact of predation on the distribution of meiofauna varies with the scale over which predators forage (e.g, fish predation influences meiofauna in different ways and at broader spatial scales than do invertebrate predators), the type of streambed substrata in which the predator-prey interactions occur, and the dispersal ability of different meiofauna. The latter is greatly influenced by predator and prey (meiofauna) interactions with the flow envrironment. 3. Organic matter influences the small-scale distribution of meiofauna in streams. Both its quality as food (as indicated by C:N content, ATP content, or microbial biomass) and its spatial distribution on the streambed, influence meiofauna patchiness, community structure and life history characteristics. As a habitat, the structure that organic matter provides (e.g. wood or leaves) can influence predator-prey interactions, offer materials for case-building and offer refugia during disturbance events - all of which influence the small-scale spatial distribution of meiofauna. 4. Stream flow influences the distribution of meiofauna at broad scales (10s-100s of metres), primarily because of the high susceptibility of meiofauna to passive drift; small-scale interactions between flow and substrata are also important, however, particularly at more localised (less than or equal to metre) scales. At both scales, substratum particle size is important to interstitial-dwelling fauna, influencing the probability of passive drift by meiofauna as well as local microhabitat conditions (e.g. dissolved oxygen; upwelling/ downwelling in the hyporheic zone) and, thus, the small-scale distribution among microhabitats. 5. In general, the processes governing the distribution of meiofauna at small scales cannot be separated entirely from those processes working at larger scales. A conceptual diagram is presented illustrating the relative importance of various factors in influencing the spatial patterns of meiofauna and over what scales these factors act.