Climate extremes make reef fish less diverse and productive  

Coral reef fish communities that are exposed to heat are much less diverse and abundant compared to fish in nearby reefs with less extreme temperatures. An international team of researchers led by NYU Abu Dhabi compared bottom-dwelling reef fish in the southern Arabian Gulf and the Sea of Oman.

“The Arabian Gulf is the world’s hottest sea each summer with temperatures well beyond those experienced by fishes elsewhere in the tropics, while the adjacent Sea of Oman is considerably more benign,” said study senior author Professor John Burt. “This makes this region a useful natural laboratory for understanding how climate extremes affect fish function and diversity.”

The researchers found that the more thermally intense habitat in the Arabian Gulf lowers the diversity and productivity of cryptobenthic reef fish, which are important species that form the base of coral reef food webs.

The team used a variety of techniques to investigate how the small, bottom-dwelling reef fish varied as a result of different environmental conditions.

Cryptobenthic fish communities in the Arabian Gulf were found to be half as diverse and less than 25 percent as abundant as those in the Gulf of Oman, even though the amount of live corals were largely similar.

The researchers were surprised to find that variations in cryptobenthic fish diversity and abundance were not the result of differing temperature tolerances.

“We expected to see much lower temperature tolerances in species that occur in the Sea of Oman, but not in the southern Arabian Gulf,” said study co-lead author Professor Jacob Johansen. “Yet, the critical thermal tolerances of all species found in the Sea of Oman were, in theory, sufficient to survive even the maximum summer temperatures of 36°C in the southern Arabian Gulf.”

The team determined that the thermal extremes of the Arabian Gulf are more energetically challenging for the small fish. Beyond the fact that warmer water temperatures require more energy, fewer prey are present in the Arabian Gulf to satisfy the energetic needs of cryptobenthic fish.

According to the researchers, the reduced diversity and abundance of cryptobenthic fish greatly impairs their ability to fulfill their unique functional role.

“These tiny fishes normally feed a lot of the larger, predatory animals on coral reefs by growing and dying rapidly, while constantly replenishing their populations,” said study co-lead author Simon Brandl. “But on reefs in the southern Arabian Gulf, cryptobenthic fish communities provide only a fraction of the fish flesh that they can produce in more benign environments.”

“Our findings highlight an imminent threat to cryptobenthic reef fishes and their essential role for coral reef functioning. These smallest marine ectotherms may struggle to compensate for increasing costs of growth and maintenance as they adapt to more extreme temperatures,” said Professor Johansen. 

“Extreme environmental conditions, as predicted for the end of the 21st century, could, therefore, disrupt the community structure and productivity of coral reefs in the Arabian Gulf and beyond.”

The study is published in the journal Nature Communications.

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By Chrissy Sexton, Earth.com Staff Writer