Introduction

At the seventh workshop of the International Association for Phytoplankton Taxonomy and Ecology (IAP) held on 5–15 May 1989 at Konnevesi Research Station, University of Jyväskylä, Finland, phytoplankton was collected in nearby Lake Konnevesi. At that time of year the lake was extremely rich in scaled chrysophytes (Table 14.1) and dominated by different species of Mallomonas and Synura. In the samples investigated, a Mallomonas species similar to M. alpina Pasch. & Ruttn. and M. tonsurata, Teil, was observed. Between 6 and 11 May this Mallomonas species developed stomatocysts (statospores, cysts). Preserved plankton samples were later examined with electron microscopy (EM). The alpina-tonsurata-like Mallomonas was found to be a new species and has been given the name Mallomonas variabilis sp. nov.

M. variabilis had, in fact, been recorded earlier from several Finnish lakes, even from Lake Konnevesi. However, no complete cells, only isolated scales were found; thus not enough information was available to make a new description of the species (Hällfors & Hällfors 1988). This species has also been recorded under the name M. tonsurata fa. in large lakes in northwestern Ontario, central Northwest Territories, and in the Alaskan Toolik Lake area (H. Kling, unpublished data).