Found this answer:
The compensation point is reached when the gross productivity of a producer is equal to the amount it must consume in respiration to stay alive. At the compensation point the net productivity is zero, and no growth can occur.
Each bottle represents the net productivity of the algae at a different depth. The depth at which the net productivity is zero is the compensation point. At this depth there is not enough light to produce growth in the algae. Productivity of the algae increases from this depth upward as more light becomes available.
Light is not the only factor limiting productivity. Decreasing available CO2 can also reach a compensation point. If there is not enough CO2 to make sugar, then the amount of light is not relevant.