BENEFICIAL-EFFECTS OF HERBIVORES ON STREAM MACROALGAE VIA EPIPHYTE REMOVAL

The densities of two epiphyton-feeding stream insects (Baetis sp. and Agapetus celata) were experimentally manipulated in stream-side channels containing macroalgae (Cladophora glomerata) to determine grazer influences on the host alga. After 16 days Cladophora biomass increased significantly more in the presence of Baetis (248.0%) and high densities of Agapetus (153.6%) than in grazerless controls (68.6%); biomass in low density Agapetus treatments (57.8%) did not differ from controls. Without grazers macroalgal thalli were heavily encrusted with a complex microalgal assemblage, while grazers tended to eliminate all epiphyton except for small, adnate diatoms (esp. Cocconeis pediculus). The proportion of macroalgal biomass exported from Baetis treatments (1.7%) was one third that from controls (6.2%), while greater loss occurred in high Agapetus channels (10.0%). The scraping mode of Agapetus apparently damaged filaments and caused breakage, while collector-gatherers like Baetis removed epiphyton without damaging tissue. Although both insects inhibit early establishment of Cladophora, these results indicate that grazers can be beneficial to macroalgal growth once the alga is established. Thus, effects of the same consumer species can be diametrically opposed, depending upon characteristics of the resource, suggesting that effects may differ among assemblage patches and during succession.