Coral Reef Algae

Coral reef algae (along with all other algae) are the most primitive of plants, without the organization or structure of their terrestrial counterparts.

They have neither roots, stems, or true leaves, although some of the larger, multicellular forms have developed analogous structures that serve some of the same functions.

Various forms of marine algae are common in all portions of the marine environment sufficiently shallow to permit photosynthesis. This includes the surface layer of the open sea, as well as inshore waters harboring coral reefs.


algal ridge

Types of Coral Reef Algae

Algae that are permanent members of the coral reef community are benthic (living attached to or within the substrate). Some forms are macroscopic (visible to the naked eye), while others are microscopic (discernable to humans only with magnification).

Macroscopic Algae

The macroscopic forms of algae found on coral reefs are comparatively large, multicellular algal species popularly known as “seaweeds”.

However, in terms of biomass and production, these forms make up only a tiny fraction of the plant life of healthy coral ref ecosysstems.

Benthic macroscopic algae (macroalgae) are classified into three main types based on color: green, brown, and red.

Each of these types contains pigments specialized to best utilize sunlight at different depths. The characteristic color of each type is attributable to the particular nature of their photosynthetic pigments; colorful compounds used by plants to capture the energy of sunlight.

Macroscopic Algae

Green algae (red absorbing) are common on reef flats and on the upper portions of shallow patch reefs in the back reef zone, while brown algae most commonly occupy intermediate depths of the upper reef face (reef face).

In contrast, red coral reef algae (blue absorbing) may be found over one hundred feet deep on the lower fore reef. Red coral reef algae is also found on shallow reefs. Here, the red algae is mainly the coralline type that helps to build and stabilize the reef structure.

Microscopic Algae

The best known and most abundant microscopic form of microscopic algae found on healthy coral reefs are the single-celled, symbiotic zooxanthellae that dwell within the tissues of reef building coral animals.

The biology and ecological role of these plants are discussed elsewhere on this web site, as part of our discussion on how coral reefs are formed.

NUISANCE ALGAE AND THE REEF AQUARIUM: Excessive unwanted algal growth is often the most problematic maintenance issue for reef aquarists. A highly useful resource for those seeking solutions to such issues is The Reef Tank, a website loaded with informative discussions and forums devoted to establishing and maintaining coral reef aquaria.
reef types

Abundance Patterns in Coral Reef Algae

Clear tropical waters are naturally low in the dissolved nutrients needed to support the rapid growth of seaweeds. Thus, except for a few specialized types of coralline red algae that actually help build the reef, macroalgae are naturally rare on healthy coral reefs.

However, in many parts of the world, rapid increases in coastal human populations have led to increases in dissolved nutrients in waters bathing nearby coral reefs. The result is often “blooms” of excessive algal growth that smothers and kills hard coral colonies (see photo, left).

Even on “healthy” reefs there are some obvious seaweeds growing amidst the coral colonies, but the vast bulk of marine algae on healthy coral reefs is not so easily observed.

Microscopic algae growing on interstitial reef surfaces and other types of reef ecosystem substrates is simply too small to be seen. Likewise, the presence and abundance of tiny zooxanthellae within coral polyp tissues is only betrayed by the distinctive colors they impart to the colonies of their animal hosts.

Inconspicuous as these tiny algal forms may be, they nonetheless are ubiquitous in coral reef ecosystems, and provide most of the primary production on the reef crest and reef face.

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