The second flank of Reef 21-187 is exposed to the prevailing wind and sea conditions found within the lagoon of the Great Barrier Reef. Manta tow surveys were conducted on the reef slope. The reef slope is generally steep dropping off into deep water with no discernable bottom. The reef structure has few features including pavement.

The benthic community was dominated by hard coral, but coralline/turf algae was also common. Hard coral cover on the reef second flank was moderate with no one coral genus dominant. The dominant coral lifeform was branching, but other forms such as foliose, encrusting and tabulate corals were also present in decreasing order of abundance. Coral bleaching was present on a few individual colonies.

The fish abundance on the second flank of Reef 21-187 was moderate. Conspicuous groups of reef fishes included parrotfish, surgeonfish, damselfish and fusiliers. No crown-of-thorns starfish were observed on the second flank of Reef 21-187 . White syndromes (such as white band disease or shutdown reaction) were present on hard corals.