Meromictic Lakes as Habitats for Protists.

The meromictic lake is, in effect, two different habitats separated by a distinct vertical gradient ("the chemocline", Hutchinson, 1937). By passing through the chemocline, the conditions with regard to dissolved salts and organic matter, particulates, gases and pH/eH may be altered significantly. Contrary to holomictic lakes where gradients may be established seasonally by temperature differences, the chemical gradient in truly meromictic lakes is sufficiently robust to withstand seasonal mixis. While the meromictic condition in temperate lakes may last for several to thousands or more years, some basins of marine origin may have conserved a gradient towards anoxia for millions of years, if unperturbed by glaciations. The origin, terminology, properties and significance of land-locked waters (fjords being isolated from the sea by postglacial isostatic equilibration) and various aspects of meromixis have repeatedly been discussed in papers and reviews (e.g., Strøm, 1936; Findenegg, 1937; Hutchinson, 1937; Kjensmo, 1967; Walker and Likens, 1975; Degens and Stoffers, 1976; Hakala, 2004) and textbooks (e.g., Ruttner, 1940; Hutchinson, 1957; Wetzel, 2001; Kalff, 2002). A review of biological implications at the chemocline level has been written by Tyler and Vyverman (1995). More information is hidden in the literature, and emerging from further studies of meromictic lakes of different origin (cf. Hakala, 2004). Here, information about protists inhabiting the chemocline and possibly the monimolimnion (or anoxic water of isolated fjords) are presented from unpublished observations and from literature. Since these authors' experience is mainly from Norway and Sweden, examples will be chosen from the geo-diversity of fjords and lakes here. This presentation is a sequel and extension to the review by Tyler and Vyverman (1995) and the treatise on meromictic lakes in Finland by Hakala (2004). [ABSTRACT FROM AUTHOR]