Benthic and hyporheic biofilms play a key role in stream nutrient cycling, ecosystem metabolism and greenhouse gas emissions. However, the relative roles of dissolved organic carbon (DOC), inorganic carbon (via light availability for photosynthesis), nitrogen (N) and phosphorus (P) availability for biofilm properties and the associated control of C, N and P assimilation by biofilms remain poorly understood. Here we present results from a full factorial stream mesocosm experiment to investigate how reactive DOC:N:P ratios (as DOC, nitrate-N (NO₃-N) and phosphate-P (PO₄-P)) and light availability (10 and 46 µmol/m²/s) affect the heterotrophic and autotrophic structure and function of benthic and hyporheic biofilms. Due to high NO₃-N concentrations, the background reactive DOC:N:P ratio was 317:1572:1, indicating low relative availability of DOC and P. The addition of reactive C and P sources resulted in a reactive DOC:N:P ratio of 97:47:1, more similar to typical ratios of bacteria and algae. Altered reactive nutrient ratios increased bacterial density and heterotrophic functional diversity in both benthic and hyporheic biofilms, whereas light increased autotrophic functional diversity only when combined with altered DOC:N:P ratios. Our data confirm that the ratio of reactive macronutrients is an important driver of benthic and hyporheic biofilm composition and function, and interacts differently with autotrophic and heterotrophic components. These results highlight the need for coupled C:N:P studies to better assess stream ecosystem responses to human impacts.

Primary Presenter: Anika Grosse, Helmholtz Centre for Environmental Research - UFZ (anika.grosse@ufz.de)

Authors:

Anika Grosse, Helmholtz Centre for Environmental Research - UFZ  (anika.grosse@ufz.de)

Nuria Perujo, Helmholtz Centre for Environmental Research - UFZ  ()

Patrick Fink, Helmholtz Centre for Environmental Research - UFZ  ()

Alexander Reisinger, University of Florida  ()

Dietrich Borchardt, Helmholtz Centre for Environmental Research - UFZ  ()

Daniel Graeber, Helmholtz Centre for Environmental Research - UFZ  ()