Here's my imaginary version. In the the raised center a field is a sealed algae production unit. This will need to have a fairly significant surface area to produce a significant quantity of algae, but the production there will only be a small fraction of the total.
Algae from that plant will be continuously harvested live, mixed with water, and piped to the end of some spiraling troughs. Similar to what you suggested, the troughs would have CO2 bubbled through them.
There would be a slow but continuous flow of water through the troughs. The diluted algae would grow in the sun and thus become more concentrated when the reached the outer edges of the troughs were they could be harvested. The water flow would have to be fast enough to keep the algae flowing, and the troughs would probably need periodic cleaning even so, but if the algae were only in the trough for a day, I wouldn't think foreign algae would be too much of a problem; they would just get swept up and processed along with all the other trough algae.
>>I wouldn’t think foreign algae would be too much of a
>>problem; they would just get swept up and processed
>>along with all the other trough algae.
Especially if the system were a large recirculating closed loop. Fresh spores of the “good” algae could be introduced just after the extraction point.