Separating the grazing effects of periwinkles and amphipods on a seaweed community dominated by Fucus Distichus

In situ caging experiments showed that both littorinid snails and gammarid amphipods exerted minimal effects on the suite of algae comprising the canopy of littoral fringe tide pools dominated by Fucus distichus. In the absence of grazers, the Fucus canopy was dramatically reduced by fatal endophyte infections. Among the 10dependent variables making up the algal species and forms at the substratum level below the canopy, 3 were highly affected by grazing. The germlings of F. distiehus were greatly reduced by littorinids. Gamrnarids also reduced germling cover, but were less effective. In addition, the two grazer guilds interactively reduced the cover of algae belonging to the Uniseriate functional form at the substratum level. Sheet/Tube form algae were reduced in cover only by littorinids. Other forms, including calcareous crusts, thalli of Chondrus crispus and 3 Cladophora species were unaffected by grazing. There was no indication of food resource partitioning between littorinids and gammarids