Background
"Because terrestrial leaf litter supports the food webs of many freshwater aquatic ecosystems, providing upwards of 99% of their energetic requirement (Minshall, 1967; Petersen and Cummins, 1974)," the authors say that "the implications of an elevated CO2 atmosphere could be great for aquatic ecosystems."

What was done
After characterizing the initial chemical characteristics of groups of senesced leaves collected from trembling aspen (Populus tremuloides Michx.) seedlings growing in open-top chambers in ambient air and air enriched with an extra 350 ppm of CO2, the leaves were enclosed in porous bags and suspended slightly above the rocky substrate of a small third-order headwater stream in Cheboygan County, Michigan, USA, where their rates of decomposition and concurrent patterns of invertebrate colonization were determined at monthly intervals for a period of 120 days.

What was learned
Both green leaves and senesced leaves from the seedlings growing in CO2-enriched air had higher carbon-to-nitrogen ratios, percent total phenolic compounds and lignin concentrations, as well as lower levels of foliar nitrogen, than their ambient-treatment counterparts, which negatively affected both microbial respiration and leaf litter decomposition rates during the first month of stream incubation.  However, subsequent enhanced leaching and microbial decomposition of leaves appeared to compensate for these initial differences, such that "longer term leaf processing rates and invertebrate colonization patterns did not significantly differ between CO2 treatments of leaf litter."

What it means
Even though there may be significant CO2-induced differences in the quality of leaf litter produced by certain trees in certain circumstances (which might be thought to produce negative consequences for freshwater food webs), this particular study found only a brief one-month perturbation of the initial stage of leaf litter decomposition that had no effect on subsequent invertebrate colonization patterns (density and biomass).  Hence, based on this study, one would not expect atmospheric CO2 enrichment to have a negative effect on the aquatic food webs of freshwater streams.

References
Minshall, G.W.  1967.  Role of allochthonous detritus in the trophic structure of a woodland springbrook community.  Ecology 48: 139-149.

Petersen, R.C. and Cummins, K.W.  1974.  Leaf processing in a woodland stream.  Freshwater Biology 4: 343-368.