6. Brown Algae (Phaeophyceae) in Rivers

Freshwater brown algae can be abundant in streams, but represent just seven species in the Phaeophyceae, a class of ~2000 species, most from marine environments. Freshwater species do not form parenchyma, but are based on one of the three filamentous growth forms: (1) uniseriate, creeping filaments infrequently or frequently branched; (2) complex branched forms producing basal and vertical series of filaments forming a crust; and (3) multiseriate, frequently branched forms. Their evolutionary history and relation to marine taxa is unclear, with molecular data suggesting some closely related to existing marine taxa, while others forming a separate clade of freshwater forms. Most species are known from few locations worldwide, with all but a few reports from sites in the northern hemisphere. All species are benthic and most colonize epilithic substrata, but a few are epiphytes on macrophytes or other algae. A key to all known species is provided.