EPIPHYTIC AND EPILITHIC DIATOM COMMUNITIES ALONG ENVIRONMENTAL GRADIENTS IN THE NEPALESE HIMALAYA - IMPLICATIONS FOR THE ASSESSMENT OF BIODIVERSITY AND WATER-QUALITY

Variations in diatom communities between river habitats might have ram ifications for their use as bioindicators and for the assessment of th eir contribution to aquatic biodiversity. Diatoms therefore were sampl ed from similar surface areas of the epilithon and bryophytes in 24 hi ll-streams in the Nepalese Himalaya to assess whether communities vari ed more strongly between habitats or between streams. Some diatom spec ies (e.g. Eunotia minor and Cocconeis placentula) were significantly m ore abundant on bryophytes than in the epilithon. In addition, bryophy tes held, on average, a significantly larger proportion of the total s pecies pool of diatoms at each site. However, neither diversity H' nor evenness E varied between habitats, suggesting no gross differences i n community structure. Also, DECORANA ordination and TWINSPAN classifi cation showed that communities were more similar be tween habitats wit hin sites, than between sites. DECORANA scores from both epilithon and bryophytes showed the same linear relationships with water chemistry (acid-base status, sulphate concentration and conductivity), so that d iatom samples from either habitat reflected the same chemical gradient . These results support previous conclusions that comprehensive sampli ng will be required in the assessment of diatom biodiversity, while sa mples from just one habitat may be sufficient for water quality monito ring.