GRAZER CONTROL OF ALGAE IN AN OZARK MOUNTAIN STREAM - EFFECTS OF SHORT-TERM EXCLUSION

Stony substrata in streams of the southwestern Ozark Mountains are covered with cyanobacterial felts dominated by Calothrix sp., an active nitrogen fixer. To study the effect of grazers on these producer assemblages, we exposed or protected periphyton from grazers for periods ranging from 4 to 33 d. When protected from grazing fishes and invertebrates, cyanobacterial felts are overgrown by turfs of benthic diatoms within 4-10 d. Diatom turfs also develop on bare substrata that are incubated in sites in the stream inaccessible to grazers. Diatom turfs exposed to grazing minnows are stripped off in 3-5 min, and when left exposed to grazers, are replaced by cyanobacterial felts within 11 d. Basal regeneration of Calothrix trichomes may contribute to their persistence under intense grazing.