Up to 99% of the carbon fuelling the food webs of temperate woodland streams is derived from inputs of terrestrial leaf litter. Aquatic bacteria, fungi, and detritivore invertebrates directly utilize these inputs, transferring this energy to other components of the food web. Increases in atmospheric CO2 could indirectly impact woodland stream food webs by chemically altering leaf litter. This study evaluated CO2‐induced chemical changes in aspen (Populus tremuloides) leaf litter, and the corresponding effects on stream bacteria, fungi and leaf‐shredding cranefly larvae (Tipula abdominalis: Diptera). Leaf litter from plants grown under elevated CO2 had decreased nutritional value to aquatic decomposers and detritivores because of higher levels of structural compounds and lower nitrogen content. Consequently, elevated CO2‐grown leaf litter supported 59% lower bacterial production in a stream than litter grown at ambient CO2 levels, while not affecting fungal biomass. Larval craneflies fed elevated CO2‐grown microbially colonized leaves consumed less, assimilated less, and grew 12 times slower than their ambient fed counterparts.

The heterotrophic utilization of organic substrates by diatoms is likely an important survival strategy when light levels are too low for photosynthesis. The objectives of this study were: (1) to determine if heterotrophic utilization of a large array of organic compounds by eight common freshwater benthic diatom taxa was light-dependent, and (2) to determine if organic substrate utilization patterns differed between darkgrown diatoms and bacteria as a possible means of reducing competition by niche separation. Eight lightand dark-grown diatom taxa and five bacterial species were incubated in 96-well Biolog Ò Microtiter plates with each well containing 1 of 95 different organic substrates. Oxidation rates of each organic substrate were measured through time. There was a substantial increase in the number of organic substrates oxidized by diatoms grown in the dark compared to their light-grown counterparts, indicating that the transport systems for these molecules may be light activated. Therefore, diatoms likely only utilize these metabolically expensive uptake mechanisms when they are necessary for survival, or when substrates are plentiful. A principal components analysis indicated discernible differences in the types of organic-C substrates utilized by dark-grown diatoms and bacteria. Although bacteria were able to oxidize a more diverse array of organic substrates including carboxylic acids and large polymers, diatoms appeared to more readily utilize the complex carbohydrates. By oxidizing different organic substrates than bacteria, heterotrophically metabolizing diatoms may be reducing direct competition and enhancing coexistence with bacteria.

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Global atmospheric CO(2) levels are expected to double within the next 50 years. To assess the effects of increased atmospheric CO(2) on soil ecosystems, cloned trembling aspen (Populus tremuloides) seedlings were grown individually in 1 m(3) open bottom root boxes under either elevated (720 ppm, ELEV) or ambient CO(2) (360 ppm, AMB). After 5 years, soil cores (40 cm depth) were collected from the root boxes and divided into 0-20 cm and 20-40 cm fractions. ELEV treatment resulted in significant decreases in both soil nitrate and total soil nitrogen in both the 0-20 cm and 20-40 cm soil fractions, with a 47% decrease in soil nitrate and a 50% decrease in total soil nitrogen occurring in the 0-20 cm fraction. ELEV treatment did not result in a significant change in the amount of soil microbial biomass. However, analysis of indicator phospholipid fatty acids (PLFA) indicated that ELEV treatment did result in significant increases in PLFA indicators for fungi and Gram-negative bacteria in the 0-20 cm fraction. Terminal restriction fragment length polymorphism (T-RFLP) analysis was used to analyze the composition of the soil bacterial communities (using primers targeting the 16SrRNA gene) and the soil fungal communities (using primers targeting the intergenic transcribed spacer region). T-RFLP analysis revealed shifts in both bacterial and fungal community structure, as well as increases in both bacterial and fungal species richness with ELEV treatment. These results indicated that increased atmospheric CO(2) had significant effects on both soil nutrient availability and the community composition of soil microbes associated with aspen roots.