Environmental drivers, life strategies and bioindicator capacity of bryophyte communities in high-latitude headwater streams

This study reports an investigation of the bryophyte ecology of minimally impacted high-latitude headwater streams, to assess the hypothesis that underlying geology, water physico-chemistry, current velocity and substrate morphology are significant environmental drivers of the species distribution, community composition and abundance of bryophyte vegetation in these naturally highly stressed and disturbed upland ecosystems. Our secondary aim was to determine whether there is evidence that the assemblage and functional attributes of the species present have potential as indicators of water quality and habitat biointegrity in high-latitude headwater streams. The methodology involved repeated intensive surveys, during 2005–2006, at nine sites located on three streams of contrasting underlying geology, within latitudes 56°–58°N, in upland Scotland. Using a stratified random sampling design, quantitative assessments were made of 5 stream vegetation parameters and 52 environmental variables potentially influencing the target sites. TWINSPAN classification, supported by CCA ordination, of the dataset identified four stream bryophyte community types, plus a fifth sample group (characterized by unstable habitat conditions) devoid of bryophytes. The environmental drivers of the vegetation community were primarily associated with streambed stability (flow, substrate composition) and water chemistry (e.g. pH, conductivity, Ca2+), which were products of the prevailing underlying geology. The abundance of stream bryophytes was generally a function of predominant growth morphology and life strategy of the species present. Two bryophyte assemblages were identified as characterizing either upland oligotrophic stream habitats of acid-sensitive and base-poor waters (indicators: Scapania undulata, Hygrohypnum ochraceum), or calcareous and mineral-rich streamwaters (indicators: Chiloscyphus polyanthus, Hygrohypnum luridum). The results support the incorporation of bryophytes into river biomonitoring schemes, for use in high-latitude upland streams.