Aulacoseira subarctica: Taxonomy, physiology, ecology and palaeoecology

Abstract

Knowledge of the morphology, taxonomy, physiology, ecology and palaeoecology of the filamentous freshwater diatom Aulacoseira subarctica (O. Müller) Haworth is reviewed from the literature. The species is widely distributed across Northern Europe, Scandinavia and North America, Japan, China, Australia and New Zealand, in the last usually as fo. subborealis. The species is rare in the Tropics and positive identifications are lacking for Africa. The importance of correct identification is stressed because literature reports of its occurrence often need revision. A comparison is made between two sites with detailed records of its seasonal cycle. Blelham Tarn, England, is a clear-water seasonally stratified lake; Lough Neagh, Northern Ireland, is a very large turbid and unstratified lake. In spite of their contrasting characters, Aulacoseira subarctica succeeds in both lakes. In Blelham Tarn, there is a strong seasonal cycle of growth, dormancy on the sediment and further growth on re-suspension. In Lough Neagh, there is a large deposition of cells at the end of the spring bloom but few of these cells survive grazing by benthic animals. Fossil occurrences are summarized from the species' initial appearance in the Pliocene and its later occurrence during the last inter-glacial (Eemian) deposits in NW Europe, where it is an important ecological indicator. Palaeolimnological studies are reviewed to describe the contemporary distribution and ecological preference of this widespread diatom. Aulacoseira subarctica usually appears in response to moderate increases in nutrients but is disadvantaged by further enrichment. In Blelham Tarn the species only became abundant during the twentieth century, reaching a peak between 1930 and 1960. With nutrient enrichment, abundance has declined. In Lough Neagh, it has been present for much of the post-glacial period and still thrives today in spite of the highly enriched state of the lake. The species has disappeared from many eutrophic lakes and its sustained existence in Lough Neagh is anomalous. Lough Neagh may continue to offer a favourable habitat in spite of the poor light climate because the water depth, lack of stratification and long water retention time reduce losses to a sustainable level.