Hawai'i Coral Reef Network

 

The Hawaiian Reef Ecosystem

Brian N. Tissot
Marine Education Center
University of Hawai段 at Hilo

"Hanau ka 'Uku-ko'ako'a, Hanau kana, he Ako'ako'a, puka"
[Born the coral polyp, Born of him a coral colony emerged]
-- Kumulipo, The Hawaiian Hymn of Creation, compiled by Rubellite Kawena Johnson, 1981

Introduction

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段, 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.

Geologic history

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 Hawai段 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 Hawai段 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 Hawai段 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 Hawai段 vary in age from less than a million years (Hawai段 island) to over 26 million years (Midway).

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 (Hawai段 to Kaua段) 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 inBay on O誕hu and on the north coast of Kaua段. 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 (Grigg, 1983). Thus, due to their geologic history, there is considerable variation in reef structure in Hawai段.

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 (Jackson, 1992). 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 (Jackson, 1992). 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.

Reef growth and development

Because corals grow slowly it would take many decades to directly observe how they develop into mature coral communities. However, on the island of Hawai段, reef development can be inferred by comparing reefs that colonized lava flow of varying age. In a hallmark paper, Grigg and Maragos (1974) describe variation in coral reef communities among lava flows in Puna and Kona that vary in age from 1.6 to 102 years (Figure 1). They found that reefs generally take 20-50 years to develop, but that the process is influenced by the degree of exposure to ocean waves. Reefs on the wave-exposed Puna coast were fully developed after 20 years, while it took over 50 years for reefs to fully develop in the more wave-protected Kona coast (Grigg and Maragos, 1974). Young reefs were characterized by a low abundance (or bottom cover) of coral, which consisted primarily of the highly branched cauliflower coral or ko誕 (Pocillopora meandrina). On older reefs, coral cover was higher, and other species had colonized and increased in abundance such as the massive lobe coral or puna (Porites lobata) and flat, encrusting rice corals (Montipora verrucosa and M. patula). Thus, reef communities may also vary due to differences in age which interacts ecologically with their exposure to waves.

Mature reef structure also varies in response to wave-exposure as well as other factors such as the extent of terrestrial runoff and latitude. The best developed reefs occur on wave-protected leeward coasts, such as the Kona coast, the southeast coast of Maui, the north coast of Lana段, and the southeast coast of Moloka段, and in sheltered bays such as Kealakekua Bay, Hanauma Bay, and Molokini Inlet (UNEP/IUCN, 1988). Corals have a very low tolerance for fresh-water and terrestrial sediments and hence do not form well-developed reefs near large steams and rivers. In addition, coral growth rates are highest in the main Hawaiian islands and decrease in the northwest Hawaiian islands with decreasing temperatures and available sunlight (Grigg, 1983). Towards the northwest end of the island chain rates of reef growth and accretion are insufficient to keep up with reef erosion and island subsidence and reefs drown. This threshold is known as the "Darwin Point" (Grigg, 1982).

Reef ecology and zonation

Ecological interactions among reef organisms and their relationship with the physical environment also have a strong effect on the abundance and distribution of organisms on coral reefs. On the Kona coast of the Big Island, Dollar (1982) described zonation patterns typical of Hawaiian reefs (Figures 2 & 3). Close to shore in shallow water, reefs are characterized by a low cover of cauliflower coral intermixed with algal-covered boulders. This area is called the boulder zone. In this habitat high wave energy and natural terrestrial runoff (fresh water and sediments) prevents most reef development but is it a good habitat for many species of limu, small invertebrates and shore-fishes (see Chapter 6)

Immediately makai (seaward), at 2-4 m depths is the reef bench zone; a wave-swept area dominated by a higher cover coral, mostly lobe coral, which builds massive wave-resistant colonies. In this habitat the highest diversity of corals is found including lobe coral, cauliflower coral, rice corals, and at least 5 other common coral species. In addition, this area is a good habitat for a wide variety of seaweeds, other invertebrates, and shore fishes.

Christmas tree worms (Spirobranchus gigantea) are commonly found burrowed into lobe coral while feather duster worms (Sabellastarte sanctjosephii) burrow into rocks and rubble. Both of these species filter suspended particles in seawater to obtain food. Another burrower are the boring urchins or ina (Echinometra mathaei and E. oblonga). These burrowing animals help create sediments which ultimately contribute to reef growth. In contrast, collector urchins or (Tripneustes gratilla) are common out in the open grazing on encrusting and filamentous seaweeds which cover the rocks. Laying around on sand patches you can find loli or black sea cucumbers (Holothuria atra and H. nobilis) and brown speckled sea cucumbers (Actinopyga mauritiana). These species obtain food by digesting organic material in sediments. Also found here are the highly prized but increasingly harder to find slipper lobsters or ula papa (Paribaccus antarcticus) and spiny lobsters or ula (Panulirus marginatus), which occur under rocks and in caves.

Fishes are abundant and diverse in this area and are dominated primarily by parrotfish or uhu and surgeonfishes, such as convict tangs or manini (Acanthurus triostagus), brown surgeonfish or (A. nigrofuscus), orange-band surgeonfish or na弾na弾 (A. olivaceus), goldring surgeonfish or kole (Ctenochaetus stigosus), and yellow tangs or lau段pala (Zebrasoma flavescens). These fishes are primarily plant-eaters (herbivores) and graze the rocks and dead coral clean of seaweeds. Some parrotfish also eat live coral and can generate a large amount of sediment which contributes to beach development and reef growth (Randall, 1996). Other common nearshore fishes are moray eels or puhi (mostly Gymnothorax meleagris and G. flavomarginatus), several types of wrasses, triggerfish, puffers and butterflyfishes. Green sea turtles or honu (Chelonia mydas) are also commonly seen in this area sleeping under ledges and in caves.

Further makai the reef drops off steeply to the reef slope zone, which is dominated by a very high cover of finger coral or (Porites compressa) to a depth of 20-30 m. Here wave forces are minimal and conditions for reef growth are optimal. Hawaiian reefs protected from waves are almost always dominated by finger coral, whose thin vertical branches quickly overgrow other coral species. These patterns suggest that finger coral is the dominant coral competitor on Hawaiian reefs (Maragos, 1972). Common in holes on the reef are herbivorous slate-pencil urchins (Heterocentrotus mammillatus) and black sea urchins or wana (Echinothrix calamaris and E. diadema). Wana have small poisonous spines nestled between the larger ones which can cause a mild burn if accidentally touched. Occasionally you may find a crown-of-thorns seastar (Acanthaster planci) or cushion seastar (Culcita novaeguineae), both of which live by eating live coral.

Butterflyfishes or are very common in this area especially threadfin butterflyfish (Chaetodon auriga), four-spot butterflyfish (C. quadrimaculatus), raccoon butterflyfish (C. lunula), ornate butterflyfish (C. ornatissimus) and multi-band butterflyfish (C. multicinctus). Several of these species are obligate coral eaters and the "health" of the coral reef can be assessed by examining their abundance and behavior (Reese, 1993). Other fishes common in this area include hawkfish (Cirrhitus and Paracirrhites), snappers (Lutjanus), and damselfish such as Hawaiian sergeants or mamo (Abudefduf abdominalis), the Pacific gregory (Stegastes fasciolatus), the Hawaiian dascyllus or alo-'ilo'i (Dascyllus albisella), which is commonly found in cauliflower coral heads, and a diverse mix of wrasses. Goatfish (Parupeneus and Mulloidichthys) are also frequently seen digging in sand patches on the reef for worms, crustaceans and small mollusks. It is not uncommon in some areas to see white-tipped reef sharks or (Traenodon obesus) resting in caves and ledges or swimming slowly along the reef. In some areas, such as Kealakekua Bay on the Big Island, spinner dolphins (Stenella longirostris) may also be seen near the reef.

Below 20-30 m is the rubble zone which is characterized by accumulations of broken coral fragments intermixed with a small amount of live lobe coral and sand (Dollar, 1982). Common invertebrates in this area include black sea cucumbers and many types of burrowing worms. Hawaiian dascyllus may occur here on isolated coral heads along with an occasional triggerfish. On sand flats in areas with moderate currents you may see garden eels (Gorgasia hawaiiensis) extending out of their sand burrows. Humpback whales or (Megaptera novaeangliae) may sometimes approach the outer edges of reefs and hearing their vibrant underwater songs is truly an exceptional experience.

Human impacts and coral conservation

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, nearshore 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誕hu have had major negative impacts on coral reefs which have taken many years to recover (Grigg, 1995). In Bay, changes in nutrient concentration associated with sewage discharges are responsible for the proliferation of bubble algae (Dictosphaeria caveronasa), which overgrew reefs and killed coral (Maragos, et al., 1985). 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 (Rogers, 1990). 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痴 discharge (Grigg, 1972). On Kaho弛lawe, 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 (KIR, 1997).

Other problems include damage by boat anchors (Davis, 1977) and swimmers (Talge, 1990), which smash and crush reefs; and the massive removal of herbivorous fishes through overfishing. 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 (Glynn, 1991, 1993). 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 (see Johannes, 1978). Most of these cultures lived sustainably with their resources for millennia, despite large population sizes. For example, Hawai'i may have had as many as 800,000 people prior to European contact (Stannard, 1989). Thus, there were more people on every island in the state then the present (except O'ahu), and they lived entirely on the marine and terrestrial resources on each island. In traditional island cultures marine resource management began by dividing up islands into small areas, each controlled by a local group of people. People in these areas were strongly involved with their resources and thus had a lot of experience with their reefs and knowledge of their history and capacity. Laws were created to close areas deemed overused or to create seasons where fishing wasn't allowed. These laws were backed up with penalties, many of them quite severe. Perhaps more importantly the local community received both the benefits and the costs from their reef management. Well-cared for reefs were prosperous and so were its people but overused reefs did not support their community.

In Hawai'i the land on each island was historically divided into areas know as Ahupua'a, which ran from the mountains to the sea. Thus, these areas contained both the reefs and the watershed that sustained them, an early example of integrated watershed management. Within each Ahupua'a there was a resource manager (Konohiki), people knowledgeable about resources (Kahuna and Kapuna), and strict laws which restricted resource uses (the Kapu system). The penalty for many violations was severe (death in same cases). More importantly, the inhabitants of each Ahupua'a benefited from healthy reefs with abundant resources, which could be traded to adjacent areas for additional resources not locally available. In contrast, overused or damaged reefs would have provided less resources and less community prosperity. In modern Hawai'i we have replaced this management scheme with a new one. Although many Ahupua'a in Hawai'i remain, the islands are managed at the county- (generally island) level and/or at the state-level. Moreover, the land is managed separately from the ocean and the ocean water is managed separately from the organisms. Furthermore, we have placed much of the local knowledge and teaching of our communities in the educational system, and law enforcement is notoriously weak due to limited personnel, inconsistent enforcement, and generally low fines. Thus, we have transformed an integrated system into an inefficient splintered one and subsequently spend a great deal of our time and energy trying to communicate among agencies rather than managing resources. Moreover, the resource users are largely left out of the picture.

The major principle of community-based management is that all of the resource stakeholders are involved in the process (White et al., 1994). This situation promotes involvement and participation of the community in activities that impact reefs. It also incorporates a wide range of knowledge and experiences into management processes and decisions. Thus, management decisions are more likely to be made on social and cultural issues, not just short-time economic ones as is typically the case. Perhaps most importantly, community-issues will drive management, which promotes interactions among the various agencies to help solve real problems. The end result is to promote reef stewardship in the community: people will care and nurture their reefs and communicate to others this respect.

The biggest issues to tackle are education and communication. The first step is to bring people together: to educate the community about basic reef ecology and local threats to reefs; to share their knowledge and experiences; and to hear their concerns about reef resources. The next step is to communicate these activities to other communities in order to build momentum and provide examples of progress. Ultimately, these activities need to be integrated at a level that is consistent with the agencies involved in the actual decision-making and implementation. Based on national and international efforts to conserve coral reefs through the Coral Reef Initiative (Crosby et al., 1995), the Hawai'i Coral Reef Network was established in 1995. The network is an attempt to communicate coral reef activities at the state-level and provide educational and research information to a wide variety of people. If we value our coral reef ecosystems then we must strive to protect them through education, research and conservation. Reefs are clearly a very important part of our island lifestyle. We all need to work together to keep it this way.

References

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Davis, G. E. 1977. Anchor damage to a coral reef on the coast of Florida. Biol. Conserv. 11: 29-34.

Dollar, S. J. 1982. Wave stress and coral community structure in Hawaii. Coral Reefs 1:71-81.

Dollar, S. J. and G. W. Tribble. 1993. Recurrent storm disturbance and recovery: a long-term study of coral communities in Hawaii. Coral Reefs 12: 223-233.

Glynn, P. W. 1991. Coral bleaching in the 1980's and possible connections with global warming. Trends in Ecology and Evolution. 6(6): 175-179.

Glynn, P. W. 1993. Coral bleaching: ecological perspectives. Coral Reefs 12: 1-17.

Grigg, E. W. 1972. Some ecological effects of discharged sugar mill wastes on marine life along the Hamakua Coast, Hawaii. Unpublished Technical Report. 45 pp.

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Grigg, R. W. 1983. Community structure, succession and development of corals reefs in Hawaii. Marine Ecology Progress Series 11: 1-14.

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Grigg, R. W. and J. E. Maragos. 1974. Recolonization of hermatypic corals on submerged lava flows in Hawaii. Ecology 55: 387-395.

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Johnson, R. K. 1981. Kumulipo, The Hawaiian Hymn of Creation. Vol. 1. TopGallant Publishing Co., LTD. Honolulu, HI. 188 pp.

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Maragos, J. E. C. Evans and P. Holthus. 1985. Reef corals in Kaneohe Bay six years before and after termination of sewage discharges. Proc. 5th Int. Coral Reef Symp., 4: 189-194.

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Stannard, David E. 1989. Before the horror: the population of Hawai'i on the eve of Western contact. Social Science Research Institute, University of Hawaii, Honolulu, HI. 149 pp.

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White, A. T., L. Z. Hale, Y. Renard and L. Cortesi (Editors). 1994. Collaborative and community-based management of coral reefs: lessons from experience. Kumarian Press, West Hartford, CN.130 pp.

 


Last update: 1/25/2005