A New Culprit Hindering Coral Recovery

The traditional way scientists understand the resilience and recovery of corals on reefs may no longer apply in explaining their decline in Florida. A recently published study has identified a new culprit hindering the recovery of corals in Florida: long sediment-laden algal turfs.

Decades of research has shown that for reefs to maintain coral populations that provide structural complexity, they need healthy herbivores (parrotfish, sea urchins, etc.). These herbivores act as lawnmowers, eating seaweeds and other macroalgae. The logic is straightforward: If fish eat the seaweed growing on reefs, baby corals will have a clean place to land and grow. If there are no fish to eat the seaweed, then seaweed will take over and the corals cannot recover. The issue is that Florida's reefs may not fit into this school of thought anymore. Florida has very low coral cover, but lots of grazing fish and low seaweed cover. By the old rulebook, Florida's coral populations should be showing some signs of recovery, not declining.

The Missing Puzzle Piece

Enter long sediment-laden algal turfs, the missing piece of the puzzle. Instead of traditional big seaweeds, Florida's reefs are being taken over by these thick, hairy carpets of turf algae that trap sediments and sand on the surface of reefs. The study suggests that under particular ecological contexts, herbivores might actually be contributing to the establishment and dominance of LSATs. When parrotfish graze, they can also scrape the surface of the reef, removing structural complexity (the 3D shape of coral) and promoting flat surfaces. This makes it easier for LSATs to establish themselves on the reef.

The authors of this study propose that once LSATs establish, herbivores are less likely to try to eat the turf algae full of sediments . Instead, they're likely to target algae growing on structurally complex areas on the reef where sediments have not accumulated. This in turn promotes further "reef flattening," making it even easier for LSATs to establish themselves on the surface of a reef. It creates a self-reinforcing feedback loop that may help explain why corals fail to recover on many reefs in Florida.

"LSATs are now the most common group covering the surface of many Florida reefs, yet receive very little attention. Here, we draw on ecological theory, research, and decades of observation to propose numerous pathways by which LSATs can establish and inhibit the recovery of corals on Florida's reefs. Understanding the mechanisms behind LSAT establishment and persistence is urgently needed to develop realistic strategies to address this ubiquitous but underappreciated bottleneck to the recovery of Florida's coral populations." - Dr. Mark Ladd, Coral Ecology Unit Lead, NOAA Fisheries Southeast Fisheries Science Center

When reefs are covered in large seaweed, the usual solution is to incorporate more herbivores to help "reset" the reef and let it heal. With LSATs, scientists have a new problem with no known "reset" solution. With this new information, scientists are now pivoting their focus from merely helping coral grow to breaking the cycle that keeps reefs in this newly degraded, sandy state.

Looking Forward

Coral reef scientists are looking at new ways to help Florida's reefs on their road to recovery. Next steps include designing studies to test the hypotheses laid out in the paper, which will allow us to better understand how LSATs are formed and retained. This information is critical to developing strategies to slow or halt the proliferation of LSATs on our reefs and to work towards developing realistic solutions to reverse this impaired ecological state.