Healthy Algal-Dominated Reefs

Written By Dan Suthers, based on [1] and conversations with Peter Vroom.

When most people think of reefs and atolls they usually think of coral. Before this expedition, I personally believed that all reefs are made by coral, and the dominant substrate is coral. In some parts of the world where coral is the dominant substrate material, natural or man-made damage to the coral has resulted in the dying coral being replaced (covered) with algae. Because of this, even many coral specialists believed that a prevalence of algae in a reef is a sign of damage to the ecosystem.

The Northwestern Hawaiian Islands (NWHI) show that these assumptions are not necessarily the case. These islands are considered by many specialists to be the best and largest "near pristine" tropical reef ecosystem remaining on the planet. The NWHI therefore provide us with an opportunity to study how healthy reef ecosystems at this latitude function. Let's see what they tell us about algae.

Algae is the Dominant Substrate Life Form

Crustose Coralline Algae (smooth reddish surfaces) with Halimeda macroalgae (green). Photo by Peter Vroom.

Using methods such as towed-diver surveys, video transects, and photoquadrats (explained in the linked articles), Peter Vroom, Kimberly Page, Jean Kenyon & Russell Brainard [1] have found that algae, not coral, are the dominant life forms covering the reef surface in the Northwestern Hawaiian Islands!

In their study benthic (bottom-covering) algae were classified into three major types: macroalgae (large plants that laypeople call "seaweed"), turf algae (small filamentous algae that form dense mats on the seabed overgrowing all hard substrate not colonized by other benthic organisms) and coralline algae (red algae that incorporates calcium carbonate, taking on forms often mistaken for coral). Vroom and colleagues compared percent coverage of these types of algae with coral and other cover (e.g., sand and rubble).

At French Frigate Shoals (FFS), 57.8% of the reef sites are covered with turf algae, 12.7% with coralline algae, and 9.5% with macroalgae. Only 12.4% is actually coral. The figures are similar for Pearl and Hermes Atoll (PHA): 56.3% turf algae, 16.6% macroalgae, 12.0% coralline algae, and 7.1% coral.

Thus, algae is the dominant benthic life form at these two major NWHI reef sites, covering at least 80% of the substrate surfaces.

Algae Contributes Significantly to Reef Habitat

There's a difference between covering a reef and actually building it. Doesn't algae just cover reefs that were previously constructed by coral?

Coralline algae cements the reef together by overgrowing and consolidating loose rubble and other uncolonized materials. It can also form structures that look similar to those built by coral (an example is pictured at right). You can tell the difference by looking closely: if it's coral, you should be able to see polyps (the coral animals) and the calices in which they live. Peter Vroom and colleagues found that the surface of the reef at French Frigate Shoals is 12.7% coralline algae versus 12.4% coral, and at Pearl and Hermes Atoll it is 12.0% coralline algae versus 7.1% coral. Thus, at these locations, algae is actually responsible for at least half of the hard surface of the reef itself!

Some Hawaiian sand consists of coral broken down by weathering or predation. For example, when you are snorkeling or diving near parrotfish, you can hear a crunching noise as they take bites of the reef. After the coral passes through the fish it comes out as sand. However, in some environments, algae is responsible for making most of the sand. In particular, Halimeda algae (pictured at left) incorporate calcium carbonate into their bodies, probably for protection from predation: calcium carbonate chips are not very appetizing! When Halimeda dies, these chips are broken down into small plate-like structures (at right/above). Take a look at the sand at a beach near you and see whether it consists of small plates.

Habitat Variations

The assemblage of life in a given location depends in part on its physical characteristics: the habitat type. Vroom and colleagues classify reef habitats as Fore Reef, Back Reef, Lagoon, Near Island Reef and Patch Reef. They found that in the NWHI corals are dominant primarily in lagoons and protected patch reef; while algae thrive in areas exposed to winter storms. The NWHI include the furthest reefs from the equator in an area where winter and spring storms can result in rough water. The researchers hypothesize that algae is better adapted to these conditions, and therefore may be more dominant in high latitude reefs, while coral may play a greater role in reefs closer to the equator. Further field study is needed to test these hypotheses.

An important lesson to take from this story is that one should not assume that ecosystems to which we assign a given name (such as "reef") are the same everywhere, and therefore we cannot manage an ecosystem without studying it carefully. At different locations we may find different assemblages of organisms that live and die in a dynamic balance unique to the characteristics of the given physical environment and the organisms that happened to find their way to it or evolve within it. The communities of life that exist on earth are diverse and wonderful.

[1] Vroom,Page, Kenyon, & Brainard (submitted)

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See also this related NOWRAMP 2002 "critter feature":

Halimeda Algae - Karla McDermid

Talk About It!

Algae

Asked by Jessica from School of Science and Technology on May 18, 2005.
My name is Jessica Hambleton. I attend the School of Science and Technology in Oregon. I am currently enrolled in a marine biology class, and am studying algae for a project on tropical reefs. I found the information on your page extremely helpful, but I was left with a few questions about algae themselves. First of all, I would like to know what type of organism is algae? Secondly, what do algae eat, or are they photosynthetic? If you could answer either of these questions please email me at the above address. Thank you for your time.
Sincerely,
Jessica Hambleton

Answered by Cindy Hunter, University of Hawaii on Oct 7, 2005.
Hello, Jessica. Algae are plants, and by definition, are photosynthetic. Marine algae are very diverse in that they can live in many different light environments. They are important because, as on land, they form the basis of the food chain for animals that eat them.