GROWTH ENHANCEMENT OF THE MACROPHYTE CERATOPHYLLUM-DEMERSUM IN THE PRESENCE OF THE SNAIL PLANORBIS-PLANORBIS - THE EFFECT OF GRAZING AND CHEMICAL CONDITIONING

1. The effects of epiphyton grazing and nutrient recycling by the freshwater snail Planorbis planorbis on the growth of the macrophyte Ceratophyllum demersum were examined in laboratory experiments. 2. Ceratophyllum plants grown in the presence of snails, or in water chemically conditioned by snails, were significantly longer, had more healthy nodes of leaves, and more growing tips than Ceratophyllum plants grown in isolation. There were no significant differences between the growth of plants that were grazed by snails, and those plants which received water chemically conditioned by snails. 3. Adult P. planorbis released 3.74 x 10(-4)mu-g phosphate, 23.9 x 10(-4)mu-g nitrate, 4.38 x 10(-4)mu-g ammonia, and 0.79 x 10(-4)mu-g of urea mg-1 wet weight of snail h-1. Measurements of these chemicals in the experimental systems suggested that ammonia was an important nutrient in stimulating the growth of Ceratophyllum. 4. Removal of epiphyton by grazing snails significantly prolonged the life of individual Ceratophyllum leaves compared to ungrazed leaves.