The susceptibility of benthic microalgae to periwinkle (Littorina littorea, Gastropoda) grazing in laboratory experiments

Periphyton with different biomass levels were grown in-situ in the Kiel Fjo rd and used for shortterm laboratory experiments to study the grazing of th e periwinkle Littorina littorea. Ingestion rates increased with biomass in a saturating function as described by Holling's type II functional response model, with a maximal ingestion rate of ca. 260 mu g chlorophyll day(-1) f or 2 cm periwinktes. The half saturation constant of the ingestion rate was ca, 2 mu g chlorophyll cm(-2). At subsaturating food levels, L. littorea i ncreased the area grazed per time but not intensity of grazing within that area, L. littorea is a moderately selective periphyton grazer. It fed on al l algal species present, but the tube-dwelling diatoms Amphipleura rutilans and Haslea crucigera suffered significantly smaller grazing losses than th e other species. However, the relative protection of those diatoms against periwinkle grazing decreased with higher biomass and with a higher load of easily ingestible epiphytes. (C) 1999 Elsevier Science B.V. All rights rese rved.