Do submerged aquatic plants influence periphyton community composition forthe benefit of invertebrate mutualists?

1. It has been suggested that submerged aquatic plants can influence the pe riphyton which grows on their surfaces, making it nutritionally beneficial to snails. In return, preferential feeding by snails clears the plants from a potential competitor, with both plants and grazers gaining from this mut ualistic relationship. 2. A highly replicated experiment was conducted, in which the nature of the plant (isoetid and elodeid types compared with similar shaped inert substr ata), the nutrient availability (10-200 mu g L-1 P, 0.2-4 mg L-1 N) and the influence of periphyton grazers, Physa fontinalis, were controlled. The pl ants were cleaned of periphyton before use and an algal inoculum added to a ll treatments. At the end of the growth period, quantitative measures of th e periphyton community composition were made and related to the treatments using both ordination and analysis of variance. 3. Grazing had the largest influence on community composition and algal num bers. A community of unicellular and adpressed filamentous forms developed in the presence of snails, and of erect filamentous forms in their absence. Three algal species, Cocconeis placentula, Chamaesiphon incrustans and Aph anochaete repens, increased in real numbers in the presence of snails, prob ably as a result of reduced competition whilst being able to withstand graz ing. 4. The second largest effect was the influence of host plant. However, diff erences between the two artificial plants were as great as between the real plants and their artificial counterparts, indicating that physical structu re was as important as any active contribution by the plants. Nutrients had a small but significant effect on community composition, but not all speci es responded in the same way to nutrient enrichment. 5. Although submerged aquatic plants exert an influence over the community composition of the periphyton which develops on their surfaces, it is unlik ely that they manipulate it to make it more attractive to grazers such as s nails.