Differential use of DOM sources to sustain respiratory activity in epiphytic and epipelic biofilms in an open-canopy stream

Dissolved organic matter in open-canopy streams is mostly derived from autochthonous sources, but the role of autochthonous DOM to sustain heterotrophic communities has been poorly studied. We compared respiration rates of epiphytic and epipelic biofilms (measured by the Resazurin—Resorufin tracer system) when processing autochthonous and allocthonous DOM sources with different chemical and optical properties. We performed laboratory experiments to determine aerobic respiration rates of epipelic and epiphytic communities in different leachates from autochthonous (macroalgae and emergent and submerged macrophytes) and allochthonous (riparian grasses) DOM sources from a Pampean stream. We also determined dissolved organic carbon and nutrient contents and DOM optical properties of each leachate. We found that epipelon was more heterotrophic and showed a lower respiratory activity than epiphyton, regardless of the type of leachate. Epiphytic respiratory activity was positively associated with DOM of microbial origin with a lower degree of humification (i.e., autochthonous microbial-algal derived materials), while epipelic respiratory activity was related to autochthonous and allochthonous humic-like materials, with higher aromaticity, derived from grasses and macrophytes. This indicates that both autochthonous and allochthonous sources can sustain microbial respiratory activity in the stream, and highlights the role of primary producers in the carbon cycle of open-canopy streams.