Nutrient pollution can lead to excessive growth of autotrophic biofilms (“benthic eutrophication”) with severe consequences for riverine ecosystems. However, there is large variation in the relationship between nutrients availability and algae biomass showing that some ecosystems can cope with elevated nutrients better than others. Besides nutrients, top-down pressure on algae plays an important role for the control of eutrophication. While the general phenomenon has been investigated extensively, it is still unresolved, which variables explain the strength of biofilm top-down control and ultimately the resilience of ecosystems towards nutrient stress. We here present a series of laboratory and mesocosm (stream bypass) experiments on different scales and different organizational levels. We test the control of grazing pressure on natural autotrophic biofilms communities by the snail Ancylus fluviatilis. In a first short-term experiment, we demonstrate that the nutrient stoichiometry of the resources and correspondingly of the algal community alters the grazing pressure due to compensatory feeding behaviour. In subsequent longer-term experiments, we addressed the roles of biofilm community composition, grazer behaviour and feedback between resources and grazer growth for the overall strength of the top-down control. Together, the results show how both nutrient quantity and nutrient quality directly and indirectly alter the ability of grazers to control eutrophication and reveal underlying mechanisms how the biofilm-grazer system can buffer adverse effects of nutrients.

Primary Presenter: Markus Weitere, Helmholtz Centre for Environmental Research - UFZ (markus.weitere@ufz.de)

Authors:

Markus Weitere, Helmholtz Centre for Environmental Research - UFZ  (markus.weitere@ufz.de)

Alessandra Iannino, Helmholtz Centre for Environmental Research - UFZ  (alex.iannino@gmail.com)

Alexander Vosshage, Helmnoltz Centre for Environmental Research - UFZ  (a.t.l.vosshage@t-online.de)

Alexandra Schlenker, Helmholtz Centre for Environmental Research - UFZ  (alexandra.schlenker@ufz.de)

Mario Brauns, Helmholtz Centre for Enviromental Research - UFZ  (mario.brauns@ufz.de)

Patrick Fink, Helmholtz Centre for Enviromental Research - UFZ  (patrick.fink@ufz.de)