Acid–base status mediates the selection of organic habitats by upland stream invertebrates

Abstract

Freshwater bryophytes support large numbers of invertebrates, but their importance compared to other organic habitats, e.g. submerged terrestrial leaf-litter, is seldom assessed. There is also little information about how water quality might affect bryophyte–invertebrate interactions. Here, we compare macroinvertebrate colonisation of bryophytes and leaf-litter in streams of contrasting acidity. Litterbags containing the liverwort Nardia compressa, a moss mixture dominated by Platyhypnidium riparioides, and oak leaves (Quercus spp.) were exposed in replicate streams at the Llyn Brianne Stream Observatory (Wales, UK), and collected after 42 and 75 days. The family richness and abundance of invertebrates were significantly higher in circumneutral than acid samples. In circumneutral streams, both were significantly higher in oak and N. compressa than in P. riparioides. Under acid conditions, invertebrate colonisation of bryophytes declined markedly relative to oak, due largely to reduced numbers of the otherwise-dominant Leuctridae. Invertebrate assemblage composition differed with both acid–base status and litter type, with acid-tolerant taxa positively associated with bryophytes. We conclude that oak litter and N. compressa are equally valuable habitat under circumneutral conditions, but in acid streams bryophyte quality declines even for acid-tolerant taxa. This effect is consistent with invertebrates using bryophytes as more than physical habitat alone.