EPILITHIC DIATOMS FROM RHEOCRENE SPRINGS IN THE EASTERN ALPS (VORARLBERG, AUSTRIA)

Abstract

Benthic epilithic diatom assemblages were studied along with physico-chemical variables from 27 spring habitats situated in the western margin mountain area of the Eastern Alps (Vorarlberg, Austria) and were collected between the years 2003–2005. The sampling sites are situated in five of the seven aquatic ecoregions, as defined by the Austrian ecological quality assessments program of rivers. The sites occur over a wide altitudinal range (580 to 1800 m above sea level). The present study attempts to test the bioregional approach for the differentiation of rheocrene spring habitats, a spring type closely related to small running waters. Spring diatom assemblages primarily reflected the influence of the nutrient content, altitude, flow regime and mineral content but also are differentiated on a smaller scale by the main habitat peculiarities. The most frequent and abundant taxa were common elements of alpine lotic sites, the differentiation of assemblages appeared in the subdominant or even rare taxa (e.g. Gomphonema occultum, Encyonopsis microcephala, Delicata delicatula). Altogether the diatom composition comprised 197 taxa with >50% typical from lotic environments and 9% crenophilous taxa (having a strong affinity to springs, e.g. Delicata delicatula, Diatoma mesodori). Nitrate concentration and pH showed the greatest effect on the species composition but site-to-site characteristics also depended on the availability of substrates (e.g. wet rocks) and/or irregular discharge. Moisture indicator values, taking into consideration the distribution of taxa linked to water level fluctuations, were given for 57% of the species found (112 taxa). Fifteen percent of these taxa were generally occurring on wet and moist or temporarily dry places (springs—Navicula stromii, Rhopalodia gibba, moss habitats—Encyonopsis falaisensis, Pinnularia borealis) and another 4% nearly exclusively outside water bodies (e.g. Diadesmis gallica var. perpusilld). The latter group indicated either non-permanent sites or springs emerging from unstable ground. The number of threatened taxa (25%) according to the German Red List for diatoms was less than what was expected when compared to former studies based on Southern Alpine springs (50%) or Central Germany (31%). This reduction in red list species went along with higher nitrate concentrations, irregular discharge and minor human impact, since only 50% of the sites were still undisturbed.