Primary production is very high in coral reefs but low in surrounding waters.One reason for this is that:

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Quizplus · Accepted Answer

Coral reefs have an efficient system for recycling nutrients within the community, which contributes to the high primary productivity observed in these ecosystems. The nutrient-rich waters surrounding the reefs are effectively used by the organisms in the reef, and this helps to concentrate the nutrients and produce a relatively nutrient-poor surrounding environment.

Primary Production Is Very High in Coral Reefs but Low