Coral Reefs

Coral Reefs

Coral reefs are unique among marine communities. In addition to being spectacular displays of nature's creativity, coral reefs are reservoirs of biological diversity and are truly the "tropical rain forests of the sea." Coral reefs are also one of the most productive and diverse assemblages on Earth and are important economic resources for many tropical regions of the world, especially islands. For example, in 1991 in Hawai'i, reefs supported over $200 million in revenues from commercial, recreational and subsistence fisheries and $50 million in marine-related tourism. (Hunter, 1995). In addition, reefs buffer shorelines from waves, hurricanes and tsunamis, and generate sediments which create beaches.

However, throughout the world, reefs are being threatened with a variety of human impacts and therefore are at risk of destruction (Richmond, 1993). In order to successfully protect our reefs we must first understand their history and ecology: how they develop and what factors influence their growth and development. Accordingly, this chapter will first survey the geologic history of reefs then move on to their ecology. I will conclude by examining human impacts on reef ecosystems and steps that are being taken to more effectively manage and conserve our reefs.

Reefs are derived from the processes of living animals and plants that colonize rocky islands and shorelines. Therefore, in order to understand the distribution and history of reefs in Hawaii we must first examine the distribution and history of the islands they live on. The Hawaiian Islands are one of the largest and most isolated island chains in the world, stretching from Hawaii Island in the south-east to Kure atoll in the north-west, a distance of over 2300 kilometers. Geological evidence suggests that all of the islands were formed over a volcanic "hot spot" at a location similar to where the island of Hawaii's lies today (Macdonald and Abbott, 1970). Due to continental drift, the ocean floor moves to the north-west, forming a chain of islands over the hot spot. As a result, the islands of Hawaii vary in age from less than a million years to over 26 million years.

As each island formed, living animals and plants colonized the shallow shores and began to develop reefs. Coral reefs are formed over long periods of time by the accumulation of skeletons and sediments from algae, corals, snails, urchins and other calcareous organisms which become accreted together by the actions of encrusting coralline algae. Over hundreds to thousands of years these accretions form a solid framework, or reef, close to shore called a fringing reef. Most of the reefs in the main Hawaiian islands are fringing reefs. However, as time progresses and an island moves off the volcanic hot-spot, they begin to sink and erode and reefs grow outward, away from shore, to form a barrier reef. The only true barrier reefs in the main Hawaiian Islands are in Bay on Oahu and on the north coast of Kauai. Eventually, as islands sink below the surface of the ocean all that remains above the surface is a ring of living reef and its accumulated sediment, an atoll. All of the Hawaiian Islands northwest of Gardener Pinnacles, the last rocky island in the chain, are either atolls or submerged shoals. Thus, due to their geologic history, there is considerable variation in reef structure in Hawaii.

In comparison to the islands themselves, current reefs are geologically young due to changes in sea level which expose or drown living reefs. About 17,000 years ago sea level was as much as 121 m less than today and has been rising ever since. Consequently, most reefs in the world are 7,000-9,000 years old, a time when sea level change slowed to less than 2 m per century, which is generally considered to be the maximum rate of reef growth. However, in many cases coral re-colonizes older drowned reefs and continued pattern of reef development began earlier, thus preserving long-term patterns of fringing reef, barrier reef, and atoll development.

Because people have a propensity to live near the ocean it is not surprising that we have a major impact on the coastal zone. Unfortunately, near shore coral reefs often receive the brunt of these impacts. Because corals contain internal microscopic plants called zooxanthellae, they receive much of their energy from sunlight via photosynthesis. In addition, their calcareous skeletons are fragile and grow slowly. As a result, corals are easily broken are sensitive to changes in the quality of coastal waters.

For example, pollution from sewage and a variety of non-point source contaminants changes the nutrient content of local waters and can alter the community structure of our reefs. Large sewage discharges off of O'ahu have had major negative impacts on coral reefs which have taken many years to recover. In Bay, changes in nutrient concentration associated with sewage discharges are responsible for the proliferation of bubble algae, which overgrew reefs and killed coral. Perhaps a similar mechanism is responsible for the west Maui "algal problem" where species of the green alga Cladophora and the introduced red alga Hypnea are covering corals and killing the reef.

Reefs are also damaged by the runoff of terrestrial sediments, which smother and kill reefs. Sugar mills can produce large amounts of sediments and can create a "sludge bank" devoid of coral in an area 0.5 km from the mill's discharge. On Kaho'olawe, bombing by the military and grazing by feral animals has stripped the land of terrestrial vegetation resulting in massive amounts of sediments washing into the ocean and destroying reefs.

Other problems include damage by boat anchors and swimmers, which smash and crush reefs; and the massive removal of herbivorous fishes through over fishing. Perhaps of greater long-term concern is that we are slowly increasing the Earth's temperature through global warming, which promotes reef destruction through coral bleaching. Thus, our highly-valued reefs are currently in a great global experiment involving many simultaneous manipulations, the outcomes of which are obvious to anybody. Why then, if we value reefs, is this happening? I believe it is a symptom that our current management of coral reefs has run astray. We have separated the people that manage the resources from the people that use them. We need to return to a more traditional form of management: community-based management.

There are many examples of effective management in traditional Pacific Island cultures but most of these have been destroyed by western culture. Most of