EPIPHYTE‐GRAZER INTERACTIONS ON ZOSTERA MARINA (ANTHOPHYTA: MONOCOTYLEDONES): EFFECTS OF DENSITY ON COMMUNITY FUNCTION1

Abstract

ABSTRACTInteractions between algal epiphytes and their grazers can have a significant impact on the structure and function of eelgrass (Zostera marina L.) meadows. In Puget Sound, the herbivorous gastropod Lacuna variegata Carpenter and its congeners appear to remove large quantities of the epiphytic community from eelgrass blades. When snails at typical field densities were used in microcosms, Lacuna significantly reduced epiphytic biomass and areal productivity. Biomass‐specific productivity of the epiphytic community showed an increasing trend with increasing snail density. Epiphytic productivity increased nonlinearly with increasing epiphytic biomass. The commonly used logistic population growth formula adequately described this relationship. Grazing rate also increased nonlinearly with increasing epiphytic biomass. The Hailing equation adequately described the relationship between grazing rate and epiphytic biomass. The proportion of the epiphytic biomass found on the oldest blade of an eelgrass shoot was related linearly to epiphytic biomass, 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 altering community function. Modified Lotka‐Volterra equations, incorporating the logistic growth form and Hailing grazing equation, should prove useful in modeling the epiphyte–grazer interaction.