Introduction

Chrysophyte algae: ecology and biogeographic distribution

The great majority of planktonic chrysophytes (algal class Chrysophyceae sensu Hibberd 1976; incl. Synurophyceae, sensu Andersen 1987) are rather delicate, golden-colored flagellates. Both unicellular and colonial chrysomonads are common in lake plankton and they exhibit three distinct types of cell coverings: ‘naked’ cells (cell membrane only), cells in expanded organic loricas, and cells covered with ornamented siliceous scales and/or bristles. This morphological diversity may affect their palatability for herbivores or may increase the effective diameter of chrysophyte cells as zooplankton ‘food particles’. Chrysophytes range in natural particle size from a few micrometers to several hundred micrometers in diameter; larger colonies are mostly spherical (Synura, JJroglena, Chrysosphaerella), but some are dendroid (Dinobryon) or linear (Chrysidiastrum). Chrysophyte algae demonstrate seasonally restricted population cycles in lakes (Sandgren 1988); they produce siliceous resting cysts and probably recruit annually from sedimentary ‘seed’ populations of these cysts (Sandgren 1991).

Chrysophytes are among the most poorly studied freshwater phytoplankton with regard to their nutrition, physiology and ecology. Those genera of interest here are phototrophs, but many also have a facultative or obligate capacity for supplementary phagotrophic and osmotrophic feeding (Sanders 1991; reviewed in Sandgren 1988; also see Holen & Boraas, this volume). Chrysophyte algae are frequently biomass dominants, together with other algal flagellates, in the myriad of small, softwater, and largely oligotrophic lakes of the north-temperate regions of North America and Scandinavia (as summarized in Sandgren 1988).