Who feeds on whom in semi-aquatic moss ecosystems?

Moss patches growing at the interface between terrestrial and limnic ecosystems (in the splash-water zone) are colonized by terrestrial, semi-aquatic, and aquatic animals. Besides being a refuge, mosses provide food resources, such as micro-algae, particulate organic matter, and moss tissues themselves. In this study, the standing stocks and isotopic signatures of both resources and consumers were measured to analyze the trophic channels utilized by moss-dwelling consumers in two semi-aquatic moss ecosystems of a stream and a lake. The mean total invertebrate abundance (+/- SD) in the lake mosses was 1109.08 +/- 230.33 ind.10(gmoss)DW(- 1), which was ~4-fold higher than in the stream (265.37 +/- 184.62 ind.10(gmoss)DW(-1)). We found that moss tissues were a fairly minor contributor to the diets of the invertebrates; instead the most important resource for invertebrates was the epiphytic biofilms growing on them. The moss-dwelling community in the lake occupied a larger isotopic space than in the stream, suggesting a lower trophic redundancy of the community in the lake. This was especially the case for the delta C-13 range, which was 39% larger in the lake than in the stream, where delta C-13 signatures appeared more focused. In contrast, the delta N-15 range of the invertebrate community associated with mosses was rather large and did not differ substantially between stream and lake. Dorylaimid nematodes, predatory dipters and water mites showed enriched delta N-15 signatures suggesting they could occupy apex positions in moss food webs despite their small body sizes. Our results suggested that mosses were not an important food resource, but instead their textural complexity enabled a coherent and relatively large invertebrate food web composed of detritivores, algivores, omnivores, predators, and probably parasites.