Coral Reef Food Web

Coral reef food webs and food chains are exceedingly complex.

It has taken scientists nearly two centuries to begin to unravel or understand the energy sources and processes that enable the development and growth of such large and biologically rich biomes as coral reefs in waters highly depleted of dissolved nutrients. Yet these are exactly the kind of marine environments in which coral reefs thrive.

While considerable progress has been made in answering some of the more fundamental questions in this regard, there still remains much to be learned in terms of more detailed understanding of the roles and interactions of the many forms of life that comprise coral reef communities.

Plant Life

An obvious anomaly of coral reefs is the marked paucity of visible plant life in those parts of these ecosystems where the most vigorous growth of hard corals occurs.

Coral reefs often appear devoid of plant life. Photo: Courtesy NOAA

The main groups of plants associated with coral reef ecosystems are discussed elsewhere on this web site, but in the context of understanding coral reef food webs it is particularly noteworthy that microscopic algae living within and upon hard corals, though barely visible to the naked eye, are characterized by very high production rates.

As has often been noted by plant ecologists, a smaller biomass of rapidly-growing plants (like coral reef microalgae) may, in the same amount of time, produce as much new food as a larger biomass of far slow-growing plants (like pine trees).

Coral reef microalgae are classic illustrations of this principle, achieving high production rates with low standing crops (biomass).

But are microscopic algal cells alone sufficient to support the entire coral reef food web? Not completely; rapidly growing, inconspicuous plant life growing within and among the hard coral colonies is only part of the answer.

Contributions From The Lagoon

The abundance of animal life concentrated in the outer reef zones (i.e., the reef crest and fore reef, which lie seaward of the lagoon, receive considerable assistance from plant and animal assemblages occupying nearby adjacent habitats within the back reef zone.

This exchange of energy and nutrients is due to both abiological factors (e.g., water movement) as well as biological factors (active transport by living animals), and is an essential aspect of feeding the coral reef community.

For example, animal assemblages associated with the extensive hard coral formations of the fore reef zone often receive substantial nutritional support from back reef (lagoon) seagrass meadows.

These highly productive back reef habitats support enormous numbers of invertebrate animals, vital links in the coral reef food web. Many daytime reef-sheltering fishes such as grunts (Pomadasyidae) and snappers (Lutjanidae) make a living by feeding on such animal prey at night.

After returning to their day shelters on lagoonal patch reefs and the upper reef face, the nutrient-rich remains of this food are eliminated. In this way, part of the prodigous biological production of seagrass meadows and mangroves is used to promote the growth of hard corals throughout the ecosystem.

Contributions From the Open Sea

Coral reef food webs also gain a good deal of energy input from a less obvious source: the open sea.

Although poor in dissolved nutrients that might support the growth of reef plant life, tropical waters contain zooplankton, tiny animals of the open sea that move passively at the mercy of winds and currents.

Through the capture and ingestion of these tiny creatures by myriad invertebrate animals and plankton-feeding reef fishes (see photo, right), some of the organic production of the open ocean assists in feeding the reef community.

The overall energetic contribution of zooplankton to coral reef communities is variable in both time and space, and is difficult to measure.

Nonetheless, reef planktivores have been shown to be highly efficient at what they do, removing as many as 60-70% of the drifting animals from the passing water. Thus, the contribution of zooplankton to feeding the reef community is generally believed to be substantial.

The Reef Recycling Program

Finally, coral reef communities are extremely efficient at retaining and recycling the nutrients received from all sources. The key element in this recycling process is the plant-animal symbiotic relationship between hard corals and their zooxanthellae.

It is the sum of organic contributions from myriad sources, along with an efficient "recycling program", that allows for the sustenance and growth of coral reef ecosystems in the midst of waters so stripped of dissolved nutrients that they are often referred to as biological deserts.