why productivity is maximum in coral reefs?( related to chapter ecosystem) pls reply fast.🙆😁

The secret to the success of the coral reefs is commonly believed to be the “tight recycling of nutrients” in the system, particularly in the corals, in which tiny plants and animals live together in a symbiosis that conserves key nutrients quite effectively. Algae (zooxanthellae), living inside the tissues of the cnidarian host, harness energy from sunlight and “fix carbon” by photosynthesis. Energy from this source is provided to the polyp host in return for exclusive access to the waste-nutrients produced by the host. These wastes (N and P) function to fertilize the algae. The symbiotic arrangement allows the partners to avoid the loss of fixed N to the water, a process that normally occurs in both free floating algae and solo-living marine animal lifeforms. In larger marine systems, overall fixed N is essentially passed back and forth between the plant and animal compartments of the community - corals have evolved a way to capture and complete this loop inside their own special symbiosis. Plants and animals perpetually passing the precious ball (fixed N) back and forth - it’s a common theme in all living systems on the planet. 
Also, like many organisms, corals are adept at storing food-energy for the lean times. 

Hope it helps.
cHeErS!! :)