Periphyton architecture and susceptibility to grazing by periwinkles (Littorina littorea, Gastropoda)

Periphyton was grown on transparent plastic substrata in the Kiel Fjord and used for short-term laboratory experiments to study the feeding selectivit y of the periwinkle Littorina littorea in response to the vertical structur e of the periphyton. The susceptibility of algae to periwinkle grazing was assessed by comparing the species-specific biomass within the grazing track s of the snails to the biomass outside the tracks. After 3 weeks of incubat ion, the periphyton consisted of a scattered monolayer of algal cells witho ut vertical structure. No apparent grazing could be found. After 6 weeks of incubation, periphyton consisted of a tightly attached undergrowth (mainly Cocconeis scutellum, Bacillariophyceae, and Myrionema sp., Phaeophyceae) a nd canopy of filamentous (Melosira moniliformis, Bacillariophyceae) and sta lked forms (Achnanthes longipes, Bacillariophyceae). The unicellular diatom s Fragilaria tabulata and Stauroneis constricta grew partly on the primary substratum and partly as epiphytes on the canopy species. The canopy specie s and the epiphytes were decimated inside the grazing tracks, while the tig htly attached undergrowth species appeared ungrazed.