Global atmospheric CO2 levels are predicted to double by the year 2100. Under elevated atmospheric CO2 conditions, C:N ratios typically increase in leaf tissues of C3 plants, whose leaf litter constitutes a major source of allochthonous carbon in headwater streams. Upon entering the stream, soluble constituents leach from leaf litter as dissolved organic matter (DOM). A 35-d experiment analyzed the effects of ambient (AMB = 360 ppm) versus elevated (ELEV = 720 ppm) DOM (UV and non UV-exposed) enrichment on growth, productivity, and community structure of a periphyton community from a northern Michigan stream. Dissolved organic matter (DOM) from ELEV CO2-grown Populus tremuloides leaf litter exhibited significantly higher concentrations of dissolved organic carbon (DOC) and higher C:N than DOM from AMB CO2-grown leaf litter. Periphyton communities exposed to ELEV and ELEV-UV DOM underwent a shift in community composition of common algal taxa relative to AMB and AMB-UV communities. These results indicate that ELEV and ELEV-UV DOC enrichment in an artificial stream ecosystem may limit nutrient availability to periphyton and therefore alter the community composition of periphytic algae. Chapter III abstract: Global atmospheric levels of CO2 are expected to double from the current level (360 ppm) by the year 2100. Under elevated atmospheric CO2 conditions leaf tissue C:N increases in C3 plants, leaf litter of which a major source of allochthonous carbon to headwater streams. Upon entering streams, much of the soluble carbon content of leaf litter is leached as dissolved organic carbon (DOC). DOC from elevated (ELEV = 720 ppm) CO2-grown Populus tremuloides leaf litter exhibited significantly higher C:N than DOC from ambient (AMB = 360 ppm) CO2-grown leaf litter. DOC is an abundant carbon source in streams and is a nutrient source for benthic periphyton. ELEV and AMB DOC was administered to periphyton for 35 d and resulted in different algal community composition. A crayfish choice experiment was conducted in a Y-maze to determine crayfish olfactory preference for periphyton grown on AMB or ELEV DOC. Pairwise combinations of AMB, ELEV, and control (CON) were offered to crayfish, and their preferences were compared. Crayfish chose the arm receiving AMB periphyton chemicals significantly more often when given a choice of either AMB and CON or AMB and ELEV peiphyton stimuli. However, there were no significant differences in preference for ELEV versus CON. These results demonstrate that crayfish can discriminate between AMB and ELEV DOM-enriched periphyton and show preference for AMB. However, when given the choice of periphyton enriched with DOM containing higher concentrations of DOC (ELEV) or no periphyton source (CON), crayfish exhibit no preference.