What was done
The authors "used free air CO2 enrichment to compare effects of eCO2 (i.e., double ambient ~ 720 ppm) relative to ambient CO2 (aCO2 ~ 360 ppm) on several ecosystem properties and functions in large, outdoor, experimental mesocosms that mimicked shallow sand-bottom prairie streams."

What was learned
Hargrave et al. report that "eCO2 decreased water-column pH," as climate alarmists say it will also do in the world's oceans, thereby leading to devastating ocean acidification; but the three U.S. scientists discovered that the all-important primary productivity of benthic algae "was about 1.6, 1.9, 2.5 and 1.3 times greater in the eCO2 treatment on days 30, 45, 60 and 75, respectively." On the other hand, they determined that the carbon/phosphorus (C/P) ratio of the algae was on average 2 and 1.5 times greater in the eCO2 treatment than in the aCO2 treatment on days 45 and 90, respectively; and this result implies a reduced availability of phosphorus, which would supposedly make the algae less nutritious and, therefore, less beneficial for the ecosystem's consumers. However, and quite to the contrary of this hypothesis, they observed that eCO2 "had positive effects on benthic invertebrates, significantly increasing chironomid density, biomass, and average size." In fact, they state that "chironomid density was about 3, 5 and 2.5 times greater in the eCO2 treatment than in the aCO2 treatment on days 30, 60 and 90, respectively," that "biomass was about 4, 3 and 3 times greater in the eCO2 treatment than in the aCO2 treatment on days 30, 60 and 90, respectively," and that "individual mass was about two times greater on days 30 and 60."

What it means
"Contrary to the dominating hypotheses in the literature," in the words of Hargrave et al., "eCO2 might have positive, bottom-up effects on secondary production in some stream food webs." As a result, they conclude that their experimental findings, as well as "the large literature from terrestrial and marine ecosystems suggests that future [i.e., higher] atmospheric CO2 concentrations are likely to have broad reaching effects on autotrophs and consumers across terrestrial and aquatic biomes," which effects could well be hugely positive, as were those observed in their important study.