The periphyton of stony hard substrates in the littoral zone of lakes is inhabited by an abundant and diverse algae and meiofauna assemblage that is regulated by grazing. The objective of our study was to examine the effect of macroinvertebrate grazers on periphytic algae and meiofaunal organisms at different depths in the littoral zone of Lake Erken (Sweden). To this end, a 2-factorial field experiment was carried out in which macrofaunal access to the periphyton was manipulated at 3 different water depths (30, 70, and 200 cm). Grazing effects on algal biomass varied among depths and were highest in both deeper zones. Compared to algal biomass, the abundance of meiofaunal organisms was less affected by grazing because macrograzers significantly reduced meiofauna only at the 200-cm depth. The organism mainly responsible for the depth-dependent differences in grazer effects was the snail Theodoxus fluviatilis, which dominated macrofaunal assemblage at depths where grazing effects were the strongest (70 and 200 cm). The abundance of T. fluviatilis was positively correlated with the proportions of reduced algal biomass and reduced meiofaunal abundance. Meiofaunal groups and individual nematode species responded differently to grazing, so our results further revealed that grazing macroinvertebrates affected the composition of the meiofaunal assemblage. However, meiofaunal and nematode assemblages differed more across depths than across grazing treatments, suggesting that other depth-related factors (e.g., hydrodynamics or light) also were important for structuring periphytic meiofaunal assemblages.
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