Enhancement of invertebrate food resources by bryophytes in New Zealand alpine headwater streams

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Abstract
The variability and abundance of potential invertebrate foods in two contrasting alpine streams were investigated. Potential foods examined included periphyton and detritus on artificial substrates, and naturally occurring organic matter in gravel and bryophyte habitats. Artificial substrates simulating rocks (plain tiles), and bryophytes (tiles covered with grass carpet) were used to sample periphyton. Chlorophyll a levels were higher at the unshaded site (3.42 μg cm−2 of tile) than the shaded (0.56 μ.g cm−2 of tile) and were also higher on artificial bryophytes (3.61 μg cm−2 of tile) than rocks (0.38 μg cm−2 of tile). Periphyton colonising artificial bryophytes at the unshaded site also had a much lower temporal variability than that colonising plain tiles (coefficient of variation for total periphyton pigments = 5.7% on artificial bryophytes; 10.8% on plain tiles). Variation in periphyton biomass was also higher at the unshaded site (CV = 8.3%) than the shaded site (CV = 4.4%). This reflects the presence of a high biomass of the flocculent, filamentous Diatoma hiemale var. mesodon at the unshaded site which was easily washed away, whereas the crustose Epithemia sorex dominating periphyton community at the shaded site was more flood‐resistant. Artificial bryophytes trapped more detritus at the shaded site (0.32 g cm−2 ) than the unshaded site (0.13 g cm−2 ), reflecting the enhanced allochthonous inputs below the tree‐line. Bryophyte biomass was similar in both sites indicating that low light intensities did not adversely affect these plants. Bryophytes at both sites trapped more detritus than unstable gravels, and detrital biomass within these plants varied less. Bryophytes thus increase quantities of periphyton and detritus by increasing habitat stability in these normally unstable streams, a role similar to debris jams in forested North American streams. Consequently, they provide an abundant, persistent food source for the many invertebrates that dwell among them in streams where these foods are often transient and of low biomass.