The Role of Epiphytic Algae and Grazing Snails in Stable States of Submerged and of Free-Floating Plants

Dominant floating and submerged rootless vegetation can be regarded as alternative stable states world-wide. The competition between these two vegetation types can be strongly influenced by epiphytic algae. These algae, on the other hand, are partially controlled by grazers like snails. However, how this interaction between snails and epiphyton affects the competition between floating and submerged rootless vegetation remains rather unclear. Here, we investigate this interaction. Floating (Lemna gibba) and submerged rootless (Ceratophyllum demersum) plants were co-cultured with the presence and absence of the grazing snail Radix labiata. Biomass and nitrogen uptake of algae were strongly reduced in the presence of grazing snails. Ceratophyllum-epiphyton complex without snails reduced N and P concentration of the medium faster and had higher pH values than with the presence of grazing snails. These changes resulted in more unfavourable conditions for free-floating plants. The presence of snails indirectly increased the growth, tissue N concentration and N uptake for both Lemna and Ceratophyllum. Submerged plants together with epiphyton caused 20% more growth limitation on Lemna than Ceratophyllum alone. Structural equations modelling together with experimental results revealed that grazing snails seem to weaken the negative impact of macrophyte-epiphyton complex on Lemna. Large-scale field observations showed that the abundance of L. gibba negatively correlated with Ceratophyllum cover. Abundance of C. demersum and L. gibba negatively correlated with algal biomass; however, correlated positively with the group of larger sized grazing snails. Our findings strengthen the hypothesis that under a certain nutrient range, epiphytic algae stabilize the submerged vegetated state preventing colonization of lentic ponds by free-floating plants.