The ecological role of bryophytes in alpine streams of New Zealand.

The ecological roles of aquatic bryophytes in 2 small New Zealand alpine streams were investigated. The streams differed with respect to algal biomass and detrital inputs, reflecting their location either above the tree-line (Mouse Stream) or flowing through mountain beech forest (Tim's Creek). Streambed instability was also higher in the forested site. Quantitative sampling of bryophytes and riffles over 18 months revealed the existence of discrete macroinvertebrate (>250 µm) and meiofaunal (i.e., <250µm) communities within bryophytes. Here, macroinvertebrate and meiofaunal densities in bryophytes were up to 10, and 24 times greater than in riffles at Mouse Stream, and up to 7, and 15 times greater at Tim's Creek. Faunas colonizing non-edible bryophyte analogues were similar to those colonising real plants, suggesting that invertebrates colonise these plants for their nontrophic properties. Manipulations of artificial bryophytes altered quantities of potential food (i.e., periphyton and detritus) and degree of shelter (i.e., "stem" density) to ascertain the importance of these in regulating invertebrate colonization. Functional responses to these variables differed between species and between sites. Positive relationships existed between invertebrate density and algal and detrital biomass, and shelter at Mouse Stream; the trends at Tim's Creek were not as evident and were masked by increased streambed instability and flooding. Assessment of bryophyte consumption by gut content analysis revealed that only three of 23 taxa examined contained >5% of bryophyte material in their guts. The cranefly Limonia hudsoni was the only taxa that appeared to graze bryophytes extensively. Lack of consumption may be related to that fact that bryophytes contained more refractory, and less "digestible" material than selected riparian vegetation, or to the presence of antiherbivore compounds within some species. The importance of bryophytes in affecting energy inputs into streams was finally examined. Algal biomass was higher on structures mimicking bryophytes than stones, and biomass was higher above the tree-line than below. Natural bryophytes trapped more FPOM than riffles, and mimics at Tim's Creek trapped more FPOM than at Mouse Stream. Bryophyte biomass at both sites however was similar, reflecting the ability of these plants to tolerate a wide range of light regimes. Retention of introduced organic matter into streams was influenced by bryophytes, whereby streams with these plants retained material better than streams without