Review: Vroom PS, Page KN, Kenyon JC, Brainard RE (2006). Algae-dominated reefs. American Scientist, 94:430-437.

Feature Paper:  Vroom PS, Page KN, Kenyon JC, Brainard RE (2006). Algae-dominated reefs. American Scientist, 94:430-437.

Author Abstract: Numerous reports suggest that reefs must be dominated by coral to be healthy, but many thriving reefs depend more on algae. Most people now recognize that coral reefs worldwide are in decline, and fingers are commonly pointed at algae as the culprit. The authors have had the good fortune to study pristine reefs at more than 50 uninhabited islands in the Pacific Ocean. Although unaffected by human activity, these reefs show considerable variety in their percentage of coral versus algal cover, with some being dominated by various species of algae. It has thus become clear that, although algae may pose a problem in some situations, these plants are an essential part of nearly every healthy reef system.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: This paper is important because many scientists mistakenly believe that any conspicuous presence of algae on a reef means that nutrients are overloaded and that corals are suffering. What is more important to look at when determining the health of a reef is not the percent cover of living coral, algae, or other organisms, but rather to understand what the "baseline" is for the reef in question. As this article points out, some healthy and unfished reefs surrounding uninhabited islands can have luxurious algal (or seaweed) communities.

As the authors point out "over the past few years seemingly every publication -- from newsstand magazines and newspapers to peer-reviewed journals and textbooks -- has reported on the degradation of 'coral' reefs." This is a good time to point out that a reef is merely a rocky structure and that it does not have to have (and more often does not have) corals attached to its substrate. However, many scientists use the term "coral reef" to specifically note reefs in tropical waters with coral growth without realizing that there are many situations when corals for communities but are not dominant as reef structures. Most of the history of corals showed them as coral communities, with the large coral reef structures (e.g., the Great Barrier Reef) only developing geologically recently.

Nevertheless, it is true that in a healthy coral reef environment dominated by hard coral cover (the "typical" reef in many scientists and the public's minds), when under stress (e.g., from overfishing of herbivorous fishes to bleaching and death of the coral communities) can have algae (which grow quicker than corals) overgrow and smother living or recently dead corals. This phenomenon is called a "phase shift" (defined by TM Work and colleagues as when "coral reefs undergo relatively rapid changes in the dominant biota, a phenomenon referred to as phase shift") and can hamper the recovery of the reef in question to its former state.

The authors do not dispute that such phase shifts occur and that algae are often a culprit. Rather, the authors try to detail their work with the National Oceanic and Atmospheric Administration (NOAA) on more than 50 uninhabited US-territorial islands (and associated reefs) in the Pacific Ocean, where they discovered reefs with no signs of human impact that nevertheless had healthy, thriving, and visibly conspicuous algal communities.

A special note that I should make is that the authors are mainly discussing macroalgae (the fleshy seaweeds that include kelps as their largest forms) rather than crustose coralline algae (CCA), which cement many reefs together. CCA contribute as much to the fixing of atmospheric carbon dioxide as do living hermatypic (reef-building) corals (which do so through symbiotic single-celled algae called zooxanthellae and photosynthesis). The authors do note that CCA and other algae have been shown "decades ago that a reef can depend more on algae than coral."

The authors note that to distinguish disturbed reefs from healthy reefs (and thus determine whether the community composition is representative of a healthy community or one of a phase shift), scientists "must create a set of diagnostic indicators." As the authors further state, "researchers are beginning to understand that reefs from different latitudes and in different successional stages differ dramatically from one another [and] consequently, indices appropriate for measuring reef health at one island might not be appropriate for another." For the more than 50 reefs the authors surveyed (in the NW Hawaiian Islands, main Hawaiian Islands, Mariana Islands, Wake Island, Howland & Baker Islands, American Samoa, and the US-territorial Line Islands), the majority of surveyed reefs "contained only 7.1 to 32.7 percent live-coral cover" and often surrounding uninhabited islands. Algae, for the most part, occupied the remaining space. Sometimes those algae were "turf algae" that were kept cropped low by healthy herbivorous fish communities (e.g., surgeonfishes and parrotfishes) and the authors rightfully state that "turf algae are some of the most overlooked macroscopic organisms on tropical reefs" and that "casual divers swimming over boulder fields or hard pavements often don't realize that the fuzzy layer covering every inch of space between corals or other sessile organisms consists of hundreds of species of little plants that help form the base of the food chain."

The main "take-home message" of this paper is that when a coral reef biologist (whether a graduate student, participant in a Reef Check program, or an established field scientist) surveys a given reef, they should understand the geographical context of the reef community in question and not jump to conclusions of stress should algae be found to be a significant makeup of the community structure. This makes the seemingly simple question from managers of whether their reefs are healthy one that can only be answered when their reefs are placed in the context of "globally accepted indices that define a healthy reef." Until those are developed, such questions are, in the author's words, often "lacking definitive answers."

The work of NOAA, AIMS, GCRA, Reef Base, and others helps to create such standardized indices and surveying techniques and every four years at the International Coral Reef Symposium, top scientists and managers around the world inch closer to a globally accepted standard. In the meantime, while some coral reefs certainly hold true to the romantic images of the general public, perhaps it is time to revise our collective imagery of reefs as coral-dominated communities that are healthiest when they are sterilized from the presence of algae.

In carrying through with the theme of "non-traditional" reef communities, next we'll review mesophotic deep reefs (not coldwater coral communities below 300m, but rather, deeper reef communities that still have impact from the sun but with significantly reduced photosynthesis, often between 30 - 200m depth).