Distribution of stream macroalgae in south-central Alaska

Forty stream segments were sampled throughout the Cook Inlet drainage basin in June 1984. There was a great range in stream size, current velocity, temperature, shading, conductance and water color. This range in variables presumably has resulted in a diverse flora of 40 species of lotic macroalgae. The major divisions in terms of species numbers were the Chlorophyta (43%), Bacillariophyta (25%), Rhodophyta (13%) and Xanthophyta (13%). Filaments were the predominant form (60% of species), though mats were common (28% of species). Species numbers and abundance were significantly correlated to water temperature. Distribution was patchy in the basin, with total cover varying from less than 1% to 90% of the stream bottom. Lowland brown-water streams flowing through emergent wetlands tended to have the highest species diversity and abundance. In the drainage basin, the chrysophyte Hydrurus foetidus and the blue-green alga Phormidium retzii exhibited strongly developed dominance. This pattern of niche pre-emption appears to be typical of stream macroalgal communities in general.