Do submerged aquatic plants influence their periphyton to enhance the growth and reproduction of invertebrate mutualists?

It has been suggested that submerged aquatic plants can influence the nutri tional quality of the periphyton which grows on their surfaces, making it m ore nutritious for grazing invertebrates, particularly snails. In return, t hese grazers might preferentially feed on the periphyton and clear the plan ts of a potential competitor, with the plants and grazers both gaining from this mutualistic relationship. A highly replicated experiment was conducte d, in which the nature of the plant (isoetid and elodeid types compared wit h similar-shaped inert substrata), the nutrient loading, and the influence of periphyton grazers (the bladder snail, Physa fontinalis) of similar size and history were controlled. Plant growth and survival significantly incre ased in the presence of the periphyton grazer. Whilst the presence of the g razers had the largest influence on periphyton abundance, nutrient availabi lity and plant type also had effects. Plant type had little influence on th e nutritional quality of the periphyton measured as carbohydrate, protein a nd C:N. Effects of treatment on snail growth, and the timing and extent of snail reproduction disappeared when they were compared with the quantity of periphyton available. There was no evidence of enhanced grazer success in the presence of the live plants compared with inert substrata. Although sub merged plants affect the growth and reproduction of the grazers which feed on their surfaces, through differences in the amount of periphyton which gr ows there, we found no evidence that they manipulate the periphyton to enco urage such grazers.