If an ecosystem's balance is disturbed by a volcanic eruption, what will most likely occur?

Many plants will be destroyed and animals will be killed. Famine and many other things will surely occur because of the eruption and there would even be more related to it such as earthquakes or tsunamis. The aquatic life would be greatly affected due to changes in the temperature and its acidity. Thus, the fishermen would surely have a hard time in their living.

I would say that the consumers in the ecosystem would begin to consume carbon dioxide.

Consumers always consume carbon dioxide, whether it's a cause of volcanism or not.