Note: This is NOT about global warming/climate change! PB forum isn't the place for a debate on that subject, and that is not my intent.
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One thing everyone can agree on is that atmospheric CO2 is rising. Still just a trace gas, only 0.041 percent of the atmosphere, but the number is higher than it used to be.

And CO2 is plant food. Terrestrial plants, especially C4, have benefited from rising CO2 levels. Indeed, many greenhouses pump in lots of extra CO2 to encourage plant growth.

So here are my questions: How does rising CO2 affect pond plants? Does it have the same effects as on land plants? If so, what are the likely impacts on ponds?

I am not sure about the rising atmospheric co2 and pond plants, but co2 is great for submerged plants. i have a freshwater planted aquarium with pressurized co2 injection. it moves you to an entire different level of aquascaping in an aquarium. fast growing, colorful plants in an aquarium require higher light and co2 injection. the goal in an aquarium is to keep the co2 bubbles in the water as long as possible. eliminating/reducing surface action helps keep in the co2. but I wonder if the rising atmospheric gas could be captured in ponds with fountains enough to make a difference?

That's interesting, Scott. Never knew about CO2 injection in aquariums, just in greenhouses.

Nobody that I know does CO2 injection in ponds, though of course aeration is becoming more common. As CO2 levels rise in the ambient air, seems to make sense that aeration not only circulates water, but also increases CO2 content. Result should be more plant growth, and maybe fertility.

Downside could be excess fertility, I guess, in the wrong situation. Cyanobacteria might get a leg up. Naturally infertile ponds, like mine, might actually benefit.

First...In an aquarium, you normally do frequent water changes, and sizes are minuscule in comparison to even a .05 acre pond.

Second, in a pond, excess (even normal) amounts of CO2 production from fish and plant respiration creates Carbonic acid.

If you have water with a naturally high pH, the excess acids created may be buffered naturally, but if your soil/water pH is in the 7.4 to low 8 pH range, you will be creating an acid bath for your plants and fish.

If one quarter of your pond water is exchanged weekly, naturally, pump in all the CO2 you'd like without much concern, unless you like fishing and grow so much plant life you can't make a cast....