Facultes Universitaires Notre

Grazing impact on periphyton
Much research has been devoted to the ecological interactions in food webs in the littoral. Different
kinds of grazers such as snails play a very important role in this ecological system. I investigated the
snail-periphyton interactions in 3 laboratory experiments. In the first experiment, I tested whether
different snails select their food based its quaIity. The results showed that Valvata viviparum chose the
high P-content algae, whereas Theodoxus fluviatilis had no preference and Bythinia tentaculata, a more
sedentary species, did not discriminate between food iterns.
The second experiment (6 d) dealt with the influence of grazing by these same three snails on periphytic
biomass and algal growth forms. A shift in algae cornposition, removing unicellular with raphe and
chains diatoms to allow green colonies to grow up, was caused by T. fluviatilis and less intensely by V
viviparum, but not by B. tentaculata. The biomass of the periphyton was not significantlyaffected,
probably because of the short duration of this experiment.
Experiment three studied the impact of grazing in a long-term laboratory experiment (23 d) on epilithic
periphyton with two different grazers (T. fluviatilis and B. tentaculata). The biomass of the
periphython decreased in the "grazed" treatments. Moreover, grazing enhanced the spatial heterogeneity
of periphton. This decrease of difference of Chl.a was not explained by the distances between each tile.
The snails grazed periphyton in a hornogeneous way: they were not concentrated to feed on the algae,
which were close to each other. The grazing pressure of these two snails and its effetcs on spatial
heterogeneity were similar .
Memoire de licence en Sciences Biologiques
AoGt 2002
Promoteur: H.Hillebrand (Erken Laboratory. University of Uppsala, Sweden)
Co-promoteur: J .-P. Descy