Grazing Effects of Four Marine Intertidal Herbivores on the Microflora

Abstract

The impact of 4 species of intertidal gastropods (3 limpets and 1 periwinkle) on microalgal species composition and biomass has been evaluated and the degree of overlap of grazer diets has been examined. A series of clay pipes set in a mud flat became artificial substrates for diatom assemblages; grazers were added to the outside of these pipes after an initial period during which the flora developed. The mud proved to be an effective barrier to both immigration and emigration. All 4 gastropods selectively removed 3 diatom species from the pipes: Melosira moniliformis, Melosira nummuloides, and Fragilaria striatula var. californica. These species consist of long chains of moderate—sized cells that form an overstory within the diatom mat; a total of 130 additional species may be found within the diatom community. Selective removal may be facilitated by the chain morphology, by the location of these diatoms in the outermost portion of the mat, and by loose adhesion of these species to the rock surface. Diatoms with potentially tighter adhesion such as the stalked Achnanthes spp. are less affected by grazers. The abundance of 3 species of blue—green algae was little changed by grazing. All 4 grazers had remarkably similar diets, apparently using the same resource in about the same way. Data on the amount of chlorophyll a per m2 on natural rock surface indicate that grazers greatly reduced microalgal standing crops. Differences in grazer zonation and use of macroalgae, coupled with increased food availability during reproductive seasons (winter and spring), allow continued coexistence of these very similar herbivores.