Microbial and photochemical cycling of dissolved organic matter in boreal headwater streams

Franke, Doreen (2013) Microbial and photochemical cycling of dissolved organic matter in boreal headwater streams. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Dissolved organic matter (DOM) is a key global energy source and carbon reservoir that links terrestrial and aquatic biogeochemical cycling. Allochthonous organic matter is abundant in boreal headwater streams, and environmental changes such as variation in nutrient availability and changes to watershed landscape composition have great potential for altering the DOM source, its composition and cycling. This dissertation focuses on two of the main drivers of aquatic carbon and nutrient cycling: the photochemical and the microbial processing of DOM in boreal headwater streams; specifically (i) how the photochemical lability of DOM varies between reaches within headwater streams, among headwater streams and an associated large river reach, (ii) how stream biofilm mineralization may be regulated by watershed organic matter source and composition, increased labile carbon, nitrogen, and phosphorus availability, and (iii) whether algal carbon sources are important to supporting stream biofilms and if such sources influence the use of allochthonous DOM in nutrient-enriched streams. The results suggest changes in landscape and nutrient availability have the potential to alter the photochemical and biogeochemical cycling of DOM. DOM photolability was increased upstream relative to downstream and the river DOM. This may be due to differences in DOM source and composition, and suggests losses in photolabile DOM downstream and in the lower reaches of the watershed. The phototransformation of DOM into low molecular weight compounds and nutrients such as ammonium is likely relevant to the carbon and nutrient cycling in boreal watersheds. Results here further suggest that boreal stream biofilm mineralization of DOM is regulated by watershed DOM source and composition. Labile carbon sources, such as algal inputs, may also play an important role in regulating DOM mineralization and the processing of nutrients by these biofilms. In nutrient-impacted streams, where primary production is high relative to nutrientpoor streams, biofilms may be stimulated to incorporate algal carbon sources. Yet in the boreal streams studied here, added labile carbon rarely enhanced the mineralization of extant stream DOM suggesting autotrophic-heterotrophic interactions represent a more important priming effect relative to changing DOM source in boreal streams.

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