If algae populations continue to decline, predict how this might impact the Great Barrier Reef food web.

the food web would crash.

Explanation: the great barrier reef's diversity would decline more and then the food web would crash since algae was the primary source, no ecosystem can live without a primary source to feed off of to gain energy and grow

it's anything that aren't producers and consumers.

explanation:

you've got to be clear on what "this" is