Question

question 6 (1 point)
the main reason for the high productivity of coral reefs is thought to be ________.
the high rate of primary productivity in surrounding waters.
their location in zones of upwelling.
extremely efficient nutrient retention and recycling achieved by the corals and zooxanthellae association.
the high diversity of species found in reef ecosystems.
they are all located in tropical and subtropical zones where light availability is always high.

Explanation:

Coral reefs thrive in nutrient-poor waters, so their high productivity relies on internal nutrient cycling. The symbiosis between corals (animals) and zooxanthellae (photosynthetic algae) allows efficient retention and recycling of limited nutrients—zooxanthellae provide energy via photosynthesis, while corals supply nutrients from waste, creating a closed, efficient system. Other options are incorrect: surrounding waters are nutrient-poor, upwelling zones are not typical for reefs, high diversity is a result (not cause) of productivity, and tropical light alone does not explain productivity without nutrient management.

Answer:

extremely efficient nutrient retention and recycling achieved by the corals and zooxanthellae association.