Diversity of Spirogyra (Chlorophyta) filament types on an altitudinal gradient

A seasonal survey of Spirogyra strains in an intermittent stream in southern Arizona, USA, was conducted from spring 1981 to winter 1982. The diversity and distribution of six morphologically distinct filament types were documented at 47 sites on an altitudinal gradient (915–1463 m) in the Santa Catalina Mountains. Spirogyra filament types were differentiated on the basis of filament width, number of chloroplasts per cell and type of end wall (plane or replicate). Spirogyra filaments of all six types were most abundant in early summer and fall, when temperatures were warm (c. 36°C) and water sufficiently abundant to form temporary pools. Filaments were rare during the month (July) when most pools dried up and during the coldest month (4°C, January). A filament type of intermediate width (∼ 50 μm) and variable chloroplast number was the most abundant and broadly distributed. The two widest filament types were virtually absent from the higher altitudes (1340–1463 m), whereas the narrower filament types were distributed uniformly along the altitudinal gradient. Thus, filament-type diversity at sites was greatest at lower altitudes. This difference in distribution of wide and narrow filament types may reflect a difference in optimal growth temperature for wide and narrow filaments, or it may imply that the wider Spirogyra filaments have a limited ability to disperse upstream.