Stephanocostis chantaicus (Bacillariophyceae): morphology and population dynamics of a rare centric diatom growing in winter under ice in the oligotrophic Lake Stechlin, Germany

Abstract

Phytoplankton succession was studied weekly based on integrated samples from the euphotic zone (0-25 m) of the deep oligotrophic Lake Stechlin (Germany, Brandenburg) between 1994-1999. The rare centric diatom Stephanocostis chantaicus appeared regularly int he winter plankton building up population maximum int he winter of 1995/1996 when long-lasting ice, partly with snow, covered the lake. The paper describes morphological features of the Lake Stechlin population of St. chantaicus, its population dynamics and growth conditions. The morphological analysis indicates that by giving less taxonomic importance to rib formation and the development of a ring along the margin of the valves, and instead, giving more weight to similarities with the generic features of Cyclostephanos, it would worth to consider to include stephanocostis chantaicus into the genus Cyclostephanos. Concerning ecophysiology of the species it is supposed that underice convection currents driven by solar heating are essential pattern of population to remain in suspension and this explains the highly variable interannual pattern of population size and sexual reproduction (with maximum frequency of auxospores and initial cells of 0,94%) which were observed only int he winter of 1995/1996. An analysis of the known localities for stephanocostis chantaicus indicates that the species has a north circumpolar distribution and its occurence is predictable in large, deep, oligo-mesotrophic waters with high transparency, neutral or slightly alkaline pH and Ca(2+) concentrations of 6-46 mg 1(-1). In such lakest he species is probably more widely distributed than indicated by floristic records.