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Study finds two-thirds of Hawaii’s reefs covered with algae

Coral reefs are considered the marine equivalent of tropical rainforests. The very mention of them tends to bring to mind the corals themselves, which are tiny animals belonging to the group Cnidaria and living in colonies consisting of many individuals, each called a polyp. Or, images are evoked of fish and other marine inhabitants that rely on this diverse and beautiful ecosystem for habitat, shelter and food.

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Coral reefs are considered the marine equivalent of tropical rainforests. The very mention of them tends to bring to mind the corals themselves, which are tiny animals belonging to the group Cnidaria and living in colonies consisting of many individuals, each called a polyp. Or, images are evoked of fish and other marine inhabitants that rely on this diverse and beautiful ecosystem for habitat, shelter and food.

Often overlooked and underappreciated is the algae, which is important to reef ecology and productivity.

There are many types of algae and not all are bad, said Jack Kittinger, director of Conservation International-Hawaii. Results of surveys in healthy, fairly unimpacted islands, such as those in the Northwestern Hawaiian Islands, have shown reef habitats containing an abundance of algae groups in conjunction with corals.

Also, some areas not only have naturally occurring algae, but appear to be dominated by it rather than the coral species. Still, algae-dominated reefs are the degraded remains of once-diverse coral communities and knowing how they got that way is critical information, he added.

A new study found just one-third of Hawaii’s coral reef ecosystems are dominated by healthy corals and calcareous algae. “Identifying multiple coral reef regimes and their drivers across the Hawaiian archipelago” was published Monday in the Philosophical Transaction of Royal Society — Biological Sciences. It was a collaboration between the Stockholm Resilience Centre, Stanford University’s Center for Ocean Solutions, University of Hawaii, National Oceanic and Atmospheric Administration, Scripps Institute of Oceanography and Conservation International-Hawaii.

“While some algal cover is natural on Hawaiian reefs, it is unexpected to find that turf algae dominate more than half of all reefs we examined,” said Jean-Baptiste Jouffray of the Stockholm Resilience Center at Stockholm University. “This raises the question whether turf-dominated reefs are stable configurations or transitional states moving toward macroalgae or conversely towards coral recovery.”

Kittinger explained turf algae is an assemblage of small, more delicate filamentous species that are a food source for many herbivores and reef grazers. These herbivores are key to keeping algal colonization and growth in check.

Macroalgae, on the other hand, is a large algal species that spans a wider range of growth forms and can be big enough to essentially smother corals. Macroalgae can get out of hand if reef grazers are fished out, as well as lead to competition and a reef collapse.

Researchers for this new study used a comprehensive data set to identify distinct and stable regimes of Hawaii reef ecosystems, such as hard corals, turf algae and macroalgae. They also considered the role that tipping points play in changes from coral- to algae-dominated reefs. So besides figuring out the current makeup of Hawaii’s reefs, they identified the key stressors of these reef systems, including declines in herbivorous fish abundance, ocean temperature and pollution runoff from land.

Kittinger said researchers have long known ecosystems can tip into different states and there are thresholds, or tipping points, that can be crossed. Knowing where those thresholds are is important because the ecosystem can take a series of disturbances or stresses before tipping into a state. Understanding where that threshold is, is very relevant, especially in regard to decision-making by managers with limited funds and resources. It’s also important because once in a changed regime state, it can be very hard to get out of it.

Kittinger stressed more research is still needed on understanding where those tipping points and thresholds are, which is why continuing to track the drivers and changes seen are important. He spoke about resilience science and how the best examples have been in the Caribbean where scientists have documented these state changes and the pressures that drove them.

“This study tells us that there are tipping points for coral reefs, where even small increases in human stressors can impart major changes in the status of reefs,” Kittinger said. “From a management perspective, understanding the safe operating space is critical, and these tipping points give us concrete milestones to shoot for to protect healthy reefs, recover reefs that are in decline, and increase the benefits that reefs provide to communities across our archipelago.”

Coral reefs are among the most biologically diverse ecosystems on earth, and harbor approximately 25 percent of all marine species. They also provide a wide range of ecosystem goods and services that are crucial for economic and societal development, such as food, coastal protection and income from tourism.

Coral reefs are of critical importance in Hawaii, supporting nearshore fisheries. The average resident of Hawaii consumes three times as much seafood — up to 60 percent of which is imported — as other Americans. Healthy reef ecosystems can help restore the islands’ seafood security.

“The condition of coral reefs around Hawaii is determined largely by the abundance of herbivorous fish,” said Magnus Nyström, associate professor at the Stockholm Resilience Centre at Stockholm University. “Hence, keeping herbivore populations healthy should be of key concern for safe-guarding Hawaiian coral reefs. However, other human stressors such as land runoff also seem to be important, especially for the occurrence of turf algae.”

Along with identifying the balances between the coral and algae, the disruptions, and the “safe operating space” for Hawaii reefs, Kettinger stressed the importance of identifying early warning indicators, or “guideposts that can tell us how much a reef can take before tipping,” focusing and prioritizing management efforts, as well as taking a more holistic management approach.

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“While place-based management is an effective way of bringing people in communities together and improving conditions in our marine environment, we cannot lose sight of the big picture,” he added.

Email Carolyn Lucas-Zenk clucas-zenk@westhawaiitoday.com.