Work package 4

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Influence of organic carbon on the nutrient uptake

Agriculture has changed the pristine stoichiometric ratio of organic carbon, nitrogen, and phosphorus in streams via excess nutrient inputs over decades, thus diminishing the natural self-purification and nutrient retention capacity of the aquatic systems. Riparian forest buffers have the potential to shift the stoichiometry of the water column back to pristine conditions through the supply with particulate organic carbon of high complexity (e.g. leaves, wood).

Riparian forests also influence the metabolism of streams via shading, which, in turn, may affect the nutrient uptake by algae and bacteria. For an efficient implementation of forest buffers, it is inevitable to study the interactive effects of organic carbon supply, light availability, and water temperature on the self-purification and nutrient retention capacity of streams.

WP3 focuses on the following research questions and hypotheses:

  • Can particulate organic carbon significantly increase the microbial uptake of excess dissolved inorganic nitrogen and phosphorus?
  • Do autotrophic (algal-dominated) and heterotrophic (bacteria-dominated) biofilms differ in their response to stoichiometric shifting?
  • How does water temperature affect the potential of stoichiometric shifting to improve microbial nutrient uptake?
  • How does the addition of leaves influence the metabolism of the benthic biofilms under nutrient-enriched conditions?

We will perform flume experiments under controlled lab conditions to investigate the effects of particulate organic matter on the microbial nutrient uptake. At different light conditions and water temperature, the P- and N-uptake with and without addition of leaves to the flumes should be determined. The use of stable isotopes enables the identification of organic and inorganic P- and N sinks.