Mechanisms and consequences of vertical light-nutrient competition between benthic and pelagic primary producers in running-water systems

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In aquatic systems primary production critically depends on the vertical distribution of light and nutrients which particularly influences the competition of pelagic and benthic algae. Pelagic algae can shade benthic algae which, in turn, can intercept the nutrient flux from the sediment or hyporheic zone to the pelagic habitat. In running water systems the vertical distribution of light and nutrients changes in a longitudinal gradient. With changing light-to-nutrient availability also algal carbon-to-nutrient stoichiometry changes which usually is correlated with food quality for herbivores. The aim of this study is to experimentally analyse the influence of pathways of nutrient supply and characteristic longitudinal shifts - which are: magnitude of light and nutrient supply, and recruitment and advection of pelagic populations - on the interaction of pelagic and benthic primary producers and their carbon-to-nutrient stoichiometry. The experiments will be first carried out without grazers and in a second year with site-typical dominant herbivores to examine the effect of the algal communities on the grazers, but also the effect of the grazers on these communities. The results will provide an understanding of fundamental ecological mechanisms and processes and will also help to evaluate the impact of anthropogenic pressures like surface influx from sewage treatment plants in relation to diffuse sources in agricultural environments via hyporheic pathways and would be of particular relevance for the fields of theoretical and applied ecology and environmental management.