EPIPHYTE-GRAZER INTERACTIONS ON ZOSTERA-MARINA (ANTHOPHYTA, MONOCOTYLEDONES) - EFFECTS OF DENSITY ON COMMUNITY FUNCTION

Interactions between algal epiphytes and their grazers can have a sign ificant impact on the structure and function of eelgrass (Zostera mari na L.) meadows. In Puget Sound, the herbivorous gastropod Lacuna varie gata Carpenter and its congeners appear to remove large quantities of the epiphytic community from eelgrass blades. When snails at typical f ield densities were used in microcosms, Lacuna significantly reduced e piphytic biomass and areal productivity. Biomass-specific productivity of the epiphytic community showed an increasing trend with increasing snail density. Epiphytic productivity increased nonlinearly with incr easing epiphytic biomass. The commonly used logistic population growth formula adequately described this relationship. Grazing rate also inc reased nonlinearly with increasing epiphytic biomass. The Holling equa tion adequately described the relationship between grazing rate and ep iphytic biomass. The proportion of the epiphytic biomass found on the oldest blade of an eelgrass shoot was related linearly to epiphytic bi omass, suggesting that a constant fraction of the epiphytic community is lost regardless of epiphytic density. Lacuna clearly removed large quantities of epiphytic material from eelgrass blades, significantly a ltering community function. Modified Lotka-Volterra equations, incorpo rating the logistic growth form and holling grazing equation, should p rove useful in modeling the epiphyte-grazer interaction.