Are aquatic plants producers?

Aquatic plants are producers in the aquatic food chain, converting the basic chemical nutrients in the water and soil into plant matter, which becomes food for all other life.

What about corals? That made me curious.

The corals get energy from the algae so in this case they are primary consumers. Corals are also secondary consumers at the third trophic level, because they also eat zooplankton and other small organisms they catch with their tentacles.