Watersheds: Kamalō, Kawela, Kamiloloa, Kaunakakai, Kalama‘ula, Manawainui, Kāluape‘elua, Waiahewahewa
Figure 1: South Moloka‘i watersheds.
Geographic coordinates of watershed boundary:
(21° 08‘N, 157° 00‘ W) Located between the islands of O‘ahu and Maui.
Kamalō Stream & Kawela Stream
Figure 2: Perennial streams within south Moloka‘i watersheds.
7,400 (2000 census)
The Island of Moloka‘i is the fifth largest of the eight main Hawaiian Islands. The joining of two ancient volcanoes formed Moloka‘i: Mauna Loa to the west and Kamakou to the east. Erosion, deposition, slumping and secondary volcanic events modified the topography. The resulting shape of the island is elongated in the east-west direction, much in the shape of a peanut. The crest of the eastern volcano is named Kamakou, the highest peak on Moloka‘i (1515 m or 4970 ft). Steep mountain slopes that are deeply eroded and cut by numerous gorges characterize East Moloka‘i. In contrast, West Moloka‘i is a barren tableland that reaches only 400 m (1200 ft) in elevation. West Moloka‘i supports plantations, ranches and small farms.
Two-thirds of the island is privately owned, 31% is State land and only 0.2% is federal lands (Atlas of Hawai‘i, 1973). Private lands are mainly in the hands of large landowners (45% of the island), the largest being Moloka‘i Ranch, Ltd. Small private landowners hold only 6% of the island land area.
Coral zonation from shore to deep ocean (Figure 3).
Figure 3. This generalized profile shows the distribution of the 5 most abundant species of corals along a typical section of reef between Kamalō and Kawela. Relative abundance of each species at each point along the cross section is represented by the width of the distribution line.
The deep fore-reef extends from a depth of approximately 10 m (30 ft.) to the sand terrace at 30-40 m (90-120 ft.). This zone consists of coral platforms bisected by sand channels. In areas of high coral coverage, a Porites compressa community dominates the platforms, while a Montipora spp. community dominates the vertical sides. Shallow fore-reef extends from a depth of approximately 2 m (6 ft.) to a depth of approximately 10 m (30 ft.). This zone is characterized by "spur and groove" morphology consisting of sand channels, ridges and buttresses. Often there is a shallow terrace extending seaward that receives the brunt of the wave action.
Diverse wave-resistant coral species and crustose coralline algae characterize this area. The reef crest receives extremely heavy wave impact resulting in low coral cover. Diversity of coral species, however, can be relatively high. Dominant species such as Porites compressa, Montipora flabellata, small Pocillopora meandrina, Pocillopora ligulata and Montipora studeri are often found in this zone as well as in the upper fore-reef and the back reef. Leptastrea purpurea is an encrusting species with a hard skeleton and is found on the reef crest, shallow fore-reef and back reef with a somewhat scattered distribution. Pavona duerdeni occurs here and extends deeper into the shallow fore-reef. This species has a hard dense skeleton and is resistant to waves and sediment abrasion.
Shoreward of the reef crest there is a strong environmental gradient on the shallow reef flats of south Moloka‘i. The seaward edge of the reef flat is subjected to breaking waves over the reef crest and strong currents. Small carbonate rock outcrops surrounded by coral rubble characterize the seaward reef flat. Depth diminishes shoreward of the reef crest. Wave energy is dissipated as the water decreases in depth, moving toward the shoreline (Denny, 1988). Moving shoreward, the substrate becomes sandy with scattered rock. Near the shoreline the sand grades into mud.
Mounds and ridges. The outer reef flat coral community is characterized by high coral diversity and low coral cover. Wave disturbance prevents the dominance of substratum by the more rapidly growing species, so many species rarely found in deeper water occur here. All of the common species are found here mixed with several species not common in deeper water. The "mushroom coral" Fungia scutaria is a solitary "free living" coral that is not attached to the reef. This coral can live on the unstable rubble and fore-reef. Porites lichen is a small encrusting species that is characteristically apple green in color with calices arranged in rows. Colonies are generally less than 2 cm (1 inch) in diameter and found hidden in crevices and under overhangs on the carbonate outcrops. Cyphastrea ocellina is another common encrusting species that occurs here in similar habitats.
One of the unique features of the south Moloka‘i reef is the system of deep "blue holes" that lie to the east of Kamalō (Figure 4). This portion of the reef is bisected by areas of deeper water. The holes are somewhat in alignment and may be remnants of submerged valleys that have been partially cut off by reef growth. Perhaps the holes are areas where reef development was retarded by fresh water and sediment discharged from Kamalō gulch. Another hypothesis is that the blue holes are "Karst" features caused by ground water dissolution with consequent undermining of the carbonate reef structure. Eastern (upwind) vertical faces of the "blue holes" show high coral cover with low cover on the western (wave impacted) edge. The coral community growing along the edges of the blue holes is similar in composition to the coral communities found in Kāne‘ohe Bay, O‘ahu. The dominant coral is Porites compressa, with a small amount of Pocillopora damicornis, Cyphastrea ocellina and Montipora spp.
Figure 4. One of the most striking features of the reef flats off Kamalo is the presence of deep “blue holes” as well as an aborted attempt to dredge a harbor (square hole near upper right).
Estimated coral cover by dominant species along the 10 m (30 ft.) depth contour is shown in Figure 5. It is apparent that the coral distribution along the 10 m (30 ft) depth contour is bi-modal, with two areas having high coral cover and two areas with very low coral cover. On the western end of the south Moloka‘i reef the extremely high wave energy from the North Pacific swell wraps around La‘au Point and suppresses coral development (Storlazzi, et. al., 2003). Coral coverage increases rapidly to the east of Hale O Lono and remains high along the coastline to Kaunakakai, where a sharp decrease is observed. Coral cover from Kaunakakai to Kawela is quite low. Coverage increases sharply between Kawela to Kamalo. The existence of low coral cover between Kaunakakai and Kawela is not easily explained.
Figure 5. Distribution of reef corals along the fore-reef at a depth of 10 m (30 ft).
A detailed visual survey was conducted by teams of SCUBA divers from Kauankakai to Kawela along a series of transects run perpendicular to shore from 30 m (100 feet) to 10 m (30 feet) (Fig 6). Two important features of the coral distribution were noted. First, the damage increases with depth as we move west from Kawela to Kaunakakai. West of Kaunakakai and east of Kawela the cover is very high at all depths. Second, the distribution is essentially bi-modal with either low coverage (0-20%) or high coral coverage (80-100%), and with a very sharp intervening boundary.
Figure 6. This figure is essentially a view from offshore looking toward the south Moloka‘i reef showing coral cover from Kaunakakai (left) eastward to Kawela (right) at depths of from 10 m (30 feet) to 25 m (80 feet).
Two hypotheses can be advanced to explain the low coral cover in the Kaunakakai to Kawela sector of the reef:
Hypothesis 1 is that waves selectively destroyed this section of reef corals. This hypothesis is difficult to support. The reefs on both sides of the damaged area along this straight coastline have the same wave exposure, yet these reefs show very high coral cover at all depths. Also, it is hard to account for the extremely sharp boundaries between areas of low coral cover and high cover with this hypothesis.
Hypothesis 2 states that construction of the solid-fill Kaunakakai wharf changed the pattern of long-shore sediment transport and thereby caused the reef to decline. Sediment transport includes movement of sand bed-load in addition to fine muds and silt. Movement of muddy sand down the reef face can bury and abrade corals. The flow of this material moving down-slope could create the sharp boundaries observed at the edge of the damaged zone.
The second hypothesis affords an explanation based on what we already know about transport of sediments along such fringing reefs. Reefs grow and develop in a predictable manner. Rapid vertical growth cannot occur above sea level, but high coral cover and rapid growth can continue to extend the reef seaward on the reef face. There is continual production of carbonate material on the reef flat by calcareous algae, foraminifera, echinoderms, mollusks, and reef corals. Production of carbonate sediment is accelerated by grazing and bio-erosion of the framework. A great deal of sediment in the form of rubble, sand and mud will continue to build-up behind the reef crest on the reef flat. Terrigenous mud from the land is also carried onto the reef flat. Rejuvenation of the fringing reef system is accomplished by long-shore transport of sediments from the reef flat into deep channels where they are carried into deep water without impacting the reef face. Such reef flat – channel transport systems are seen throughout Hawai‘i and indeed throughout the world. Figure 7 shows the pattern of circulation that probably existed prior to human activity. Waves breaking over the reef crest transport clear oceanic water onto the reef flat (blue arrows). The dynamic head and prevailing wind direction from the east creates a strong long-shore current into the deep Kaunakakai Gulch channel. This channel most likely represents a drowned river valley created during the last low sea level stand. The turbid water and bed-load of sediments are carried into deeper water during periods of high surf or flooding. However, this pattern has been disrupted by human activity.
Figure 7. Wave action over the reef crest produces flow of clear oceanic water (blue arrows) onto the reef flat. The resulting current drives suspended sediment and bed-load of sand and mud along the shoreline (red arrows) and eventually into the Kaunakakai Gulch deep channel where it is transported into deeper water.
Shoreline structures (fishponds) were constructed during the period before western contact. The ponds retain some of the terrigenous mud that normally would have been quickly removed by the long-shore current system. The storage capacity of the ponds is limited and cannot possibly hold all the soil that has come down the streams over the past millennium. These structures created barriers to long-shore flow that resulted in pockets of mud on the upstream and downstream sides of the fishponds as shown in Figure 8. The ponds serve as groins and impede long shore transport. Extensive agriculture on the watershed at that time increased the rate of erosion and sediment delivery to the reefs compared to conditions that existed prior to the arrival of humans (Roberts 2000).
Figure 8. Shoreline structures (fish ponds) and increased terrigenous sediment input result in disruption of the long-shore current system along the beach with consequent buildup of inshore sediment.
The construction of a solid-fill causeway and dock area in recent times has totally blocked the original long-shore transport of sediments into the Kaunakakai Gulch channel (Figure 9). Sediments have accumulated and formed a beach along the east side of the causeway in an area that previously was deep enough to float small vessels (Joe Reich, personal communication). Turbid water and the associated bed-load of mud and sand (red arrows) now must exit the reef system over the reef face. Movement of sand and other sediments offshore as small “dunes” (especially during storm wave events) provide a mechanism for intermittent burial and uncovering of corals that could readily explain the pattern of reef damage (Figure 5).
Figure 9. Under present conditions wave action continues to bring clear oceanic water onto the reef (blue arrows), but long-shore flow is diminished. Blockage of transport of sand and sediments into the deep channel causes extensive sediment buildup on the reef flat. Movement of turbid water and bed-load sediment transport must occur over the reef face (red arrows), with consequent destruction of coral cover.
The south shore of Moloka‘i is protected from large swell from the north. The NE trade winds bend around the eastern end of the island and tend to run parallel to the south shore. These winds increase during the day and generally die out at night. Therefore the reef flat experiences more wave action and more re-suspension of the mud during midday. Wind driven currents generally run from east to west. A more detailed description is as follows:
The dominant wave regimes and their wave direction for the Hawaiian Islands are the North Pacific Swell (320o), Kona Storm waves (170o), Southern Ocean Swell (180o) and Tradewind Swells (50o). The North Pacific Swell has a large impact on the north, west, and east sides of the islands. In contrast, the south shores of the Hawaiian Islands are protected from this swell which produces high bed sheer stress. The reef structures on southern shores have developed in a significant wave shadow.
An extensive fringing reef has developed off the south shore of Moloka‘i. The crest of this reef extends up to 1500 m offshore and approximately 50 km across the south shore. The dominant wave regime that appears to be the forcing function controlling coral reef development occurs in the winter months. The North Pacific Swell generates high velocity waves that inhibit substantial coral development in waters less than 10 m. The central section of the reef is protected from this swell while a wrap-around effect occurs at both the east and west ends of the island. In these sections the reef gradually narrows and eventually disappears. Thus, the ends of the island experience substantially greater wave energy that is reflected in the species, morphology and abundance of corals occurring there (Storlazzi et al. 2001).
Wave energy is an important factor controlling development of coral reef communities (Grigg 1983). Anomalous and extreme wave events such as hurricanes can fragment and remove corals, but such catastrophic events are very infrequent. The normal wave regime is of greater importance over the long run. Many fast-growing species do not recruit, grow or survive in high wave energy regimes. Corals adapted to high water motion cannot compete effectively in low water-motion environments (Maragos 1972). Each species will modify its morphology to environmental conditions, so the prevailing wave regime shapes the structure of the coral community. For example, the growth form of the coral Montipora capitata is delicate and branching in calm water, but becomes encrusting in high water motion areas.
Figure 10. The force of large storm waves from the North Pacific scours the bottom and prevents reef development off the north coast of Moloka‘i (left) at Kalaupapa, Moloka‘i. In contrast, the wave-protected south coast has thick beds of fragile branching corals (photo on right taken at Pālā‘au).
In extreme situations, wave impact retards or prevents establishment of reef coral communities. The extreme North Pacific swell that strikes the north coast of Moloka‘i during the winter months has enough energy to remove boulders the size of trucks (Figure 10). Such waves can fragment and abrade all except the most wave-resistant coral species, which persist in a few locations on outcrops and in protected recessed areas where wave force and abrasion are minimized. The elongate east to west orientation of Moloka‘i shields much of the south reef from the North Pacific Swell. North Pacific swell does wrap around the extreme east and west ends of the island, influencing reef development in these areas (Storlazzi et al., 2001) The southeast coast from Kamalo to Halawa Point is severely impacted by the large Northeast Trade Wind Swell passing through the Pailolo Channel. The south coast falls into the "wave shadow" of surrounding islands, but is potentially vulnerable to the infrequent storm waves associated with hurricanes or "Kona" storms from the south. Hurricane Iwa caused extensive damage to the Kaunakakai wharf during late November 1982 and presumably damaged the surrounding fragile reefs (Joe Reich, personal communication). A paradox is that such extreme events can also remove years of accumulated sediments and thereby rejuvenate coral growth on a reef.
During 1969-1970 a large aggregation estimated to consist of 20,000 Acanthaster planci occurred off south Moloka‘i (Branham et al. 1971). They were feeding selectively on Montipora capitata. The major starfish infestation occurred on the rich coral reefs between Kawela and Kamalo. The proportion of dead coral colonies, however, did not increase appreciably during the time of observation. At that time the coral cover in the area was estimated visually and was reported to be approximately 90% Porites compressa and about 5% Montipora capitata. The area of uniform coral cover was reported to be approximately 1 km wide and extended to depths of 30m where the bottom become a sandy slope. This description generally fits conditions that exist today, so major changes have not occurred in the reefs in the past 30 years. The State of Hawai‘i Department of Fish and Game undertook extensive surveys and eradication efforts over the next few years (Onizuka 1979). Divers killed a total of approximately 26,000 starfish between 1970 and 1975 by injecting them with ammonium hydroxide. Additional surveys were conducted throughout the State of Hawai‘i, but no other infestations have been detected.
The introduction of mangroves along the south shore and subsequent formation of thick mangrove forests to the west of Kaunakakai is of interest to ecologists. The impact of the mangroves on shoreline processes and the ecology of the inshore area is undergoing further study at present.
Figure 11. Adult Acanthaster planci or "Crown of Thorns Starfish" off Kamiloloa, Moloka‘i feeding on the coral Montipora capitata.
subsistence fishing is the major activity on Moloka‘i reefs (Baker, 1987). A variety of techniques ranging from trolling, bottom fishing, netting and spearing are utilized. Gathering of limu (seaweed), shellfish and crustaceans is widely practiced. A limited amount of commercial catch is sold locally with some being air flown to Honolulu markets. Local inhabitants use the reefs for recreational swimming and surfing. Only one commercial dive tour company is in operation. Fishing charters operate from Kaunakakai wharf.
The southeast region of Moloka‘i has experienced extensive environmental change throughout its history of human habitation. This kona district sustained the largest population on the island of Moloka‘i due to its rich alluvial soils conducive to agricultural activities.
Hawaiians on all the main Hawaiian Islands modified the landscape with agriculture and harvesting practices. Low-land forests were cleared and native vegetation replaced with Polynesian introductions. Slash and burn techniques were used to clear lands for taro and other crops. Fire was also used to encourage the growth of pili grass used in house thatching. Terracing of slopes and diversion of streams for wetland taro farming was extensive. These terrigenous modifications also affected the nearshore environment altering the rates and patterns of sediment distribution. The effect was thought to be minimal during early time periods due to low levels of population.
Moloka‘i was first settled in approximately 600 BC. The first inhabitants settled on the eastern end and eventually moved westward on the south coast. Between 600 and 1000 BC there is no archeological evidence of large settlements, only of widely scattered residences. The population is believed to be small up to this time.
Between 1000 and 1400 BC major landscape changes occurred. Terracing for agricultural purposes expanded. Over 50 fishponds were completed during this time period. They ranged in size from a few acres to several hundreds of acres. Depths ranged from 1 to 10 meters.
Figure 12. The south coast of Moloka‘i with fishponds (top) and as it existed before fishponds were constructed (bottom). From Roberts (2000).
The extent of these fishponds was great, covering and modifying large sections of the coastline (Figure 12: Roberts 2000). Both down-slope and along-shore sediment transport patterns were altered. Many of these fishponds formed catchment basins for terrigenous sediments. Sediment transport along shore is dependent on wind strength and direction, nearshore currents and fishpond size and shape.
The population of the Hawaiian Islands grew rapidly with estimates by early Europeans of several hundred thousand. As population expanded so did the demand for resources and the accompanying environmental changes.
Modification of the environment accelerated with the arrival of the Europeans beginning in the 1770’s with Capts. Vancouver and Cook. With the arrival of foreigners came private landownership and its associated problems.
The primary factor responsible for changes to the environment was the introduction of ungulates. Goats were first left on Ni‘ihau as a food source by Capt. Cook in 1778. In close succession, they were left on several other islands by Capts. Vancouver and La Perouse. Horses, cattle, pigs and sheep joined them shortly and they thrived and multiplied. Along with these introductions, Moloka‘i had deer transported from Japan as a gift to Kamehameha V from the Duke of Edinburgh in 1870. With protection under the kapu system the herd quickly increased.
Since native Hawaiian plants had not adapted to pressure from grazing animals, a vegetation shift to introduced, alien species occurred. In absence of natural defenses populations of endemic plants quickly declined. Vast areas were left barren due to compaction of soils that increased runoff and accelerated erosional processes. Loss of nutrient rich topsoil exacerbated the problem, inhibiting recovery of the system. As a result of deforestation, perennial streams became ephemeral or intermittent altering the microclimates in the region.
Another major alteration of the forests were a result of the lucrative sandalwood trade. Along with direct harvest of adult trees, saplings were destroyed by gatherers to prevent future exploitation of labor.
Clearing of land for sugarcane plantations destroyed much of the remaining native vegetation. Introduced insects and disease made agriculture difficult long after the sugar industry was abandoned. This further accelerated erosion of soils and deposition onto the coastal reefs.
The last 100 years have seen rehabilitation efforts with varying degrees of success. The introduction of mangroves (Rhizophora mangle) in the early 1900’s was an effort by the sugar companies to stabilize the increasing mudflats in south Moloka‘i.
During this same period, the largest private landowner, Moloka‘i Ranch attempted to limit the number of feral animals and alternate grazing lands.
The Moloka‘i Forest Reserve was created in 1912 in a collaborative effort by private landowners and the government to allow recovery of overgrazed lands. Fencing to block access of some of the southern watersheds to feral animals was the focus of this rehabilitation effort (Roberts 2000).
Although recent efforts at restoration of fishponds have been made, very few are candidates for restoration due to severe eutrophication, which has filled many ponds with silt and the overgrowth of the invasive mangrove.
Due to high nutrient levels east of Kaunakakai, alien algae have invaded reefs once populated by native species.
Large quantities of terrigenous silt continue to enter the nearshore reefs following high rainfall events. Benthic sediments are continually resuspended with currents and waves in some regions.
subsistence fishing is an extremely important economic activity to the local population (Baker, 1987). Small commercial fishing operations and fishing/dive charters exist on Moloka‘i, but are not a major economic feature.
The main factors that determine the distribution of fishing are the physical reef environment and the locally established territoriality. The south shore of Moloka‘i’s fringing reef is composed of channels and deeper areas that fish can use to transit from deep to inshore waters. Except at extremely low tide, fish can move freely from the reef flat to the reef crest and offshore waters without physical barriers. Reef topography however, does physically restrict certain types of fishing. Throw nets, surround and gill nets are mainly used on the shallow reef flat due to their ineffectiveness in deeper waters. Range and accuracy of fishing gear must increase in deeper waters to catch the bigger and faster fishes in this area. Traps constructed on the reef flat to corral fishes (bull pen) must be placed in areas devoid of coral heads and loose rubble.
Lacking traditional fishing rights, Moloka’i relies heavily on local sanctions and traditions. Increased competition with subsistence fishermen from recreational fishermen has resulted in a decline in fisheries resources. The large schools of moi, mullet and akule once inhabiting the reef flats have diminished due to an increase in fishermen and lack of restrictions. This decline has been attributed to commercial and sport fishermen from nearby islands. Some local subsistence fishermen have acted upon their hostility towards these outsiders (Baker, 1987).
No Marine Life Conservation Districts exist on Moloka‘i. A portion of Kaunakakai harbor is restricted to certain types of fishing activity. Authority for managing the marine resources within three miles (4.8 km) of Moloka‘i lies with the Division of Aquatic Resources, Department of Land and Natural Resources. All laws pertaining to the management of state marine resources apply (see pamphlet "Hawai‘i Fishing regulations, September 1999", 51 pp. available from Division of Aquatic Resources, Department of Land and Natural Resources, Kalanimoku Building, 1151 Punchbowl St., Rm. 330, Honolulu, Hawai‘i).
The major concern is improper land use practices that have led to accelerated erosion and increased nutrient input. From a historical perspective the problem was overgrazing by cattle, sheep and horses. Overgrazing led to loss of vegetation and extremely high erosion rates.
The introduction of plantation agriculture (sugarcane and pineapple) led to vast tracts of land under cultivation that were subject to erosion during heavy rains. Large-scale agriculture has now been replaced by small-diversified operations that tend to reduce the erosional process. In recent decades, populations of feral ungulates (goats, pigs and deer) have grown out of control and are denuding many areas of vegetation. Construction of the solid-fill Kauankakai causeway appears to have blocked shoreline transport of sediments and caused an accumulation to the east of the causeway. In the past, dredging of reefs in the area of Kamalo and Pukoo impacted reefs in the area.
Fishing pressure is relatively light compared to some of the other islands, but the local population depends heavily on this resource for subsistence fishing. Some local inhabitants are concerned about overfishing by boats that come to Moloka‘i from other islands and harvest using extremely long stretches of gill nets. It is important to them that the reefs are healthy and not overfished.
Impact of land-derived sedimentation on the coral reefs of south Moloka‘i has been and continues to be the major environmental concern on Moloka‘i. A related issue is the role of the Kaunakakai causeway in blocking the shoreline transport of sediment, nutrients and other land-derived materials with consequent degradation of the reefs. Sediment from prior land use practices are discharged onto the reefs during infrequent but intense rainfall events. The impact of sediment on the reefs and the understanding of sediment dynamics are important to managing the condition of the reefs. Most coral have adapted to tolerate low levels of sedimentation. High wave energy regions can flush sediment away from coral colonies. Some corals can move particles away from the colony using their tentacles. Others produce mucous to shed silt from their tissues. Still others are known to efficiently ingest sediment (Anthony 2000). Yet, large amounts of sedimentation can be debilitating to corals. Both short-term and long-term lethal and sub-lethal effects to corals have been reported. Adverse effects can occur at all life stages.
Figure 13. Large sediment plumes form along the south coast of Moloka‘i after heavy rains.
Coral survival is affected by particle size, sediment type, and intensity and duration of the event. Habitat location has been demonstrated to be a dominant influence in coral tolerance to sedimentation. Species of corals found near the coast have greater ability to remove particles than species found in deeper, less turbid waters. Species with smaller polyp size are known to be less capable in particle removal (Te 2001).
Water motion transports particles across coral colonies. Transported particles can cause damage to the tissues of corals through physical abrasion. Scratches and lesions result from continued exposure to sediment. This can result in a change in morphology from foliose or lobate forms which foster sediment accumulation, to branching, vertical morphologies less prone to sediment retention.
Coral mortality can occur from smothering by sediments over short periods of time. Burial suffocates corals by preventing nutrient and oxygen delivery to tissues. It also blocks out sunlight critical to coral survival. When limited resources are diverted to repair and survival, other metabolic processes may suffer. Energy previously expended for growth and reproduction now shift to deal with life threatening priorities. Polyp and tentacle retraction and extension has been reported in some species of corals when exposed to high levels of sedimentation. Mesenterial filaments used primarily in digestion and defense in corals are extruded in turbid conditions (Brown and Howard 1985). Continuous exposure to high levels of sediments can result in tissue regression and eventually a total shutdown response.
Pathological infections and tumors can result from increases in sedimentation. Chemical pollutants associated with sediments have been associated with “black band disease” (Brown and Howard, 1985).
Sediments in the water column cause turbid conditions that reduce available light reaching the benthic substrate where corals reside. Light is vital to corals since they live in a low nutrient environment that is highly competitive. Reef-building corals compete by maintaining a symbiotic relationship with a tiny, single celled algae known as zooxanthellae. This dinoflagellate that resides in the tissues of its coral host, provides energy in the form of carbohydrates to the coral. Corals also supplement this feeding by actively feeding on tiny zooplankton or dissolved organic nitrogen. The zooxanthellae also remove waste products from corals which they then use in photosynthesis. The zooxanthellae benefit by receiving a safe place to reside. They are also provided protection from harmful UV rays by the microsporine-like amino acids that the corals produce that act like a suncreen. Sedimentation restricts photosynthesis and increases respiration rates retarding growth rates in corals (Davies 1991).
Spawning in corals has been linked to light dependent diurnal/nocturnal length and lunar periodicity. Typical spawning patterns can be altered and gametic release curtailed by reduced light levels due to increased turbidity.
Light reductions due to increased sedimentation have been reported to lower calcification rates of corals (Davies 1991). A decline in photosynthetic rates and gross production have also been linked to decreases in light (Edmunds and Davies 1989).
Under stressful conditions, corals will expel their zooxanthellae that give them their coloration. This “bleaching” can occur from changes in temperature, salinity, or light levels. If ambient conditions do not return, mortality can occur in a period as short as a few days (Brown and Howard 1985).
Coral larvae recruit and attach to hard substrate. In areas with high levels of sedimentation, soft, shifting benthos prevents larval survival. An inverse relationship has been established between sediment loading and coral recruitment (Gilmour 1999). Chronic sedimentation will result in “polyp bail-out” where individual polyps will abandon their skeletal structure.
Recent research has identified lethal and sub-lethal effects to corals from substances associated with sediment (Glynn et al. 1989, Te 2001). Some of these effects are due to factors related to sediment runoff. Particles can act as substrate for chemicals. These substances attach to sediment and are carried into the water column, where they can produce adverse secondary effects for corals.
Agriculture and construction can add pesticides, fertilizers, and petroleum products to the oceans. These pollutants carried in sediments can affect settlement, recruitment, and survivorship of coral larvae. Even low levels of these toxins can dramatically affect morphology and physiological processes of corals (Glynn et al. 1986).
Figure 14. Impact of sediment from dredging on the reefs of Kamalō (see Figure 4 for aerial photo).
Kamalō was a prime fishing area with extensive coral cover prior to a series of aborted dredging operations in the area that began in the late 1960‘s. The dredging occurred near the Smith-Bronte landing area on the inner reef flat east of Kamalō near Kalae Loa Harbor. The prevailing westward currents carried silt from the dredge operation down the coast and well past Kamalō (Figure 14). The fine silt covered reefs down-current, killing the coral. The area took on the appearance of a wasteland. Everything was covered with fine silt and the fish left the area (Joe Reich, personal communication). Even after the company went bankrupt and abandoned the dredging operation, the fine sediments continued to remobilize whenever the wind and waves increased. Chronic turbidity and sedimentation prevented any recovery of the reefs for many years. As fine sediments were winnowed out and transported offshore, the area slowly began to improve. Reefs showed signs of recovery by mid 1970‘s. Recovery was well underway by early 1980‘s with full recovery by 1990. The reefs off Kamalō presently appear to be "pristine", but much of the area actually represents a regenerated reef that was heavily damaged by siltation.
Toxicity tests were performed from marine sediment deposits on the south shore of Moloka’i. The sediment from the Kamiloloa region where large amounts of sediment is deposited and re-suspended, had the highest toxic effects on sea urchin fertilization and embryonic development. The sediment tested from Kawela resulted in moderate toxicity to fertilization but not to embryonic development. This is likely due to the type of contaminant. Zinc, lead and silver have strong effects on fertilization but not on embryo development. Sediment samples were not taken from the Kaunakakai area.
Along with other lethal effects of sedimentation, contamination can result in both lethal and sub-lethal effects throughout the food chain. Since these compounds bind to fine-grained particles and can bioaccumulate in biota, they can move rapidly through the food chain from the benthic populations to the top predators, affecting the entire ecosystem. Since these fine-grained particles are not flushed out in this region, they may pose a threat to the biological community between Kaunakakai and Kawela (Carr and Nipper, 2002).
Three species of introduced algae were documented by the University of Hawai’i’s Botany Dept. in 2000 on the south shore of Moloka‘i. These alien species included: Acanthophora spicifera, Hypnea musciformis, Gracilaria salicornia and possibly a non native species of Halophila.
The Florida Red Mangrove was introduced in an attempt to stabilize mud shorelines on Moloka‘i. These plants have spread throughout the State of Hawai‘i and are often considered to be an invasive alien species.
Figure 15. Introduced Algae Distribution on Moloka’i (Courtesy of Jennifer Smith, UH Botany Dept.)
Moloka‘i has been called "the last Hawaiian place" because the life style has not been impacted by urbanization and tourist development. Many residents are working to protect Moloka‘i from changes that would alter their way of life. The island is largely rural.
Figure 16. South Moloka‘i fishpond.
Numerous Hawaiian fishponds, loko i‘a exist along the south shore. These are believed to have been built between 1500 - 1800 AD with some possibly dating back to the 13th century. The Hawaiians engineered, created and used sophisticated aquaculture in over 60 fishponds in South Moloka‘i. The loko i‘a were constructed of basaltic and coral boulders to form a semi-circular structure that allowed water and nutrients in but kept the fish within the ponds. The privileged royalty or ali’i were the only ones allowed access to the bounty.
Fifty-two fishponds are reported to exist on the south shore of Moloka‘i (Wyban, 1992). Most are partially filled and in various states of repair. In the 1980’s the Dept. of Land and Natural Resources undertook a study to determine the best use of Moloka‘i’s remaining fishponds. The recommendations included dredging for marinas, and filling to create land for private homes, shopping centers and hotels. At this time Walter Ritte became involved in the restoration of some of these ponds. The non-profit organization, Hui o Kuapa, was created in 1984 for the community to restore the fishponds. Several fishponds were restored including ‘Ualapu’e, Honouliwai, Keawanui, Panahaha, and Kahinapohaku. This raised the level of awareness in the local community. Research and education have continued and resource management values are now dedicated to the preservation of these engineering wonders.
Keawanui (55 acres) and Ualapu’e (22 acres) have been placed on the National Registry of Historical Landmarks. The Kakahaia fishpond has been made an official bird sanctuary for the protection of wetland wildlife. This is a 15 acre freshwater pond with 42 acres of surrounding marsh. Along with the officially restored ponds, several fishponds are currently being used by local residents for various aquaculture projects.
Figure 17. Restored South Moloka‘i fishpond.
Moloka‘i is clearly one of the best locations in the State of Hawai‘i to study the effects of land derived sedimentation on coral reefs. Further, the wide fringing reef along the south coast is the longest and best-developed fringing reef in the main Hawaiian Islands. There are many questions about why and how such a massive reef structure has developed. Thus, it has become a focal point for joint UH-USGS studies of reef dynamics in Hawai‘i. USGS is investigating geologic processes and quantifying the recent growth history and change of the reefs. Thematic mapping is being used to gain insight into reef history, forcing functions and to assess reef condition. Accurate bathymetry and topographic data are also being generated. Sedimentation and geologic processes have been examined.
The Coral Reef Assessment and Monitoring Program have established 3 permanent monitoring sites at two depths to track temporal changes in coral reef communities. In addition, two shallow photoquad sites have been established to monitor growth, mortality and recruitment of corals on the colony level. These sites that were established in 2000 are being monitored on an annual basis. Twenty-eight rapid assessments at four sites were conducted in 2002 to determine spatial differences in community structure.
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Last Updated: 01/16/06
By: Lea Hollingsworth
Hawai‘i Coral Reef Assessment & Monitoring Program
Hawai‘i Institute of Marine Biology
P.O. Box 1346
Kāne‘ohe, HI 96744