Impact of Coral Reefs

photo of coral that has been bleached

Coral reef ecosystems are complex, dynamic, and sensitive systems. Although they are geologically robust and have persisted through major climactic shifts, they are however, sensitive to small environmental perturbations over the short-term.

Natural and Anthropogenic Influences

Slight changes in one component of the ecosystem affect the health of other components. Changes may be attributed to a number of causes but generally fall into two categories, natural disturbances and anthropogenic disturbances. Distinguishing between natural and anthropogenic disturbance is not always simple because the impacts of human actions may not be seen until well after the action has occurred or may not be seen until it is coupled with a natural disturbance. Also, some events that appear to be natural may have been influenced by human actions. Impacts may be direct or indirect and may be compounded where several occur. For these reasons, it is often difficult to make cause-and-effect linkages when reef degradation is observed. Anthropogenic influences and natural variability in coral reef ecosystems are discussed below with emphasis on changes or effects seen in the coral reefs of the National Marine Sanctuary System.

Natural Disturbances

Coral reef ecosystems are naturally variable and experience natural disturbances that vary on both temporal and spatial scales. Natural disturbance events that affect coral reefs include, tropical storms, outbreaks of a coral predators, disease, extended periods of elevated or low water temperatures, and extremely low tides. Although these events disturb the reefs and may kill a significant amount of coral, they are part of a natural cycle that reefs experience and the reef ecosystem may benefit in other ways. The destruction caused by a hurricane, for example, opens space for reef organisms that had been excluded by larger and longer lived corals. Hurricanes also flush out accumulated sediment within the reef and create more substrate for organisms to settle and grow on. A healthy reef ecosystem will eventually recover from natural disturbance events. However, when these natural disturbances occur to a reef system that has been impacted by human activities, the reef system may have a reduced or even no capacity to rebound. A natural disturbance acting synergistically with accumulated human impacts may result in destruction that is not reversed in the same time frame it naturally would occur.

In the last two decades, coral reefs in the Caribbean and around the world have experienced major natural disturbances. These natural events may have been influenced by human activities. During the early 1980s a water borne pathogen was carried throughout the Caribbean Sea. This pathogen caused a massive die-off of the long-spined sea urchin Diadema antillarum. At some locations the population of Diadema was reduced by 95 percent. Diadema is an important algae grazer, so its reduction resulted in increasing algal growth throughout reefs in the Caribbean. The resulting increased growth of algae along with incidents of coral diseases, nutrient inputs, and over harvesting of fish has resulted in many Caribbean reefs shifting from coral to algal dominance. The pathogen that caused this massive die-off has never been identified. However, it first appeared near the Panama Canal leading some to speculate that it may have been introduced by shipping through the Panama Canal.

Both the Florida Keys and Flower Garden Banks National Marine Sanctuaries experienced a near loss of Diadema throughout their reefs during the early 1980s. In the Florida Keys, the loss of this important herbivore along with diseases, overfishing, and pollution has been implicated in the documented decline in coral cover and increase in macroalgae on the reefs. After the die-off of Diadema on reefs of the Flower Garden Banks, there was a short-term increase in algae that only lasted a year. No decline in coral cover or long-term increase in macroalgae as a result of losing Diadema was observed. It has been suggested that these reefs have been minimally impacted by human activities so even though a major herbivore was significantly reduced, other organisms like herbivorous fish have filled the role of the algae eating Diadema.

Throughout 1997-98, a widespread coral bleaching event that affected all of the major tropical oceans of the world occurred. This event prompted the International Society for Reef Studies to issue a report on the status of bleaching around the world. Coral bleaching is a general response to stress on corals that affects the symbiotic relationship between the coral and its algae. When corals bleach they lose their symbiotic algae and/or the algae lose pigments resulting in the coral becoming pale. Corals can survive short bleaching events, but if the stress is intense or prolonged, corals may eventually die. Reports of the 1997-98 bleaching coincided with elevated water temperatures. The elevated water temperatures have been linked to the 1997-1998 El Nino event. There is no evidence that human actions influenced this event. Nevertheless, the worldwide impact this El Nino had on coral reefs illustrates how susceptible corals are to changes in water temperature. Therefore, we need to minimize the impact of human activities that may cause global warming.

The coral reefs of Flower Garden Banks, Gray's Reef, and the Hawaiian Islands Humpback Whale National Marine Sanctuaries were not affected by the major bleaching event in 1997-98. However, The Florida Keys National Marine Sanctuary reported reefs with mild to severe bleaching during 1997 and 1998. In National Marine Sanctuary of American Samoa, bleaching occurred in shallow waters, but this was primarily due to extremely low tides and not elevated water temperature.

The coral reefs within the natural marine sanctuaries all experience some of these natural disturbances. You can read about cycles of disturbance and recovery in National Marine Sanctuary of American Samoa in this month's research feature. Management practices cannot prevent naturally occurring disturbance events. However, the sanctuary program can reduce human impacts and protect the coral reef ecosystem so that it has the capacity to recover from the natural disturbances.

Anthropogenic Influences

A recent World Resources Institute report estimates that nearly 60 percent of the world's reefs are threatened by increasing human activity. The expanding human population and its activities may impact coral reef health in a number of ways. Development, urbanization, and agriculture lead to increases in freshwater runoff, polluted runoff, sedimentation, and nutrient inputs. Growing industry and automobile usage cause an increase in emissions contributing to the green house effect and chemical deposition from air to water. Commercial and private vessel traffic mean the possibility of fuel leaks or spills, vessel groundings, and anchor damage.

Harvest of reef resources is also taking a toll on the health of coral reef ecosystems. Overfishing on reefs leads to an unbalanced ecosystem, allowing more competitive or less desirable organisms to become dominant. Fishing methods such as the use of explosives and poisons severely harm reefs and reef organisms. Harvest of coral skeleton for souvenirs depletes healthy corals or substrate where coral larvae might have settled. Increased tourism in areas of coral reef habitat contributes to increased pressure from scuba diving, recreational fishing, and vessel traffic.

Threats to coral reefs in national marine sanctuaries vary according to the location and uses of the reefs. Examples of the most significant anthropogenic threats to reefs in the sanctuaries are discussed below.

National Marine Sanctuary of American Samoa

In National Marine Sanctuary of American Samoa, two of the most significant anthropogenic threats are overfishing and dynamite fishing. The desire to increase fish catches, especially in an area of increasing depletion, often means the use of more destructive fishing methods. Dynamite fishing not only kills reef fish but indiscriminately kills other reef organisms as well as damaging the physical structure of reefs. Although sanctuary regulations prohibit the use of poisons, explosives, and spearguns, as well as other fishing techniques, the effects of overfishing are still seen in the sanctuary due to insufficient sanctuary enforcement and less stringent regulations outside the sanctuary. A recently established contract with the local Department of Marine and Wildlife Resources and the National Marine Fisheries Service to conduct enforcement at the site may increase regulatory compliance.

Flower Garden Banks National Marine Sanctuary

Although the Flower Garden Banks are surrounded by an area of active offshore hydrocarbon development, the oil industry is not considered a great threat to the health of the banks' coral reef ecosystem. Monitoring studies conducted around oil platforms in place for over 10 years showed that under normal operating conditions, benthic communities have not been significantly altered and contaminants have not accumulated. Large oil leaks or spills have been rare, and small leaks and spills have been shown to have negligible effects. Rather, anchoring by large vessels has long been a threat. The banks offer convenient, shallow anchoring locations for ships waiting for orders, making engine repairs, etc. Several incidences of anchor damage by large vessels have been observed over the past 20 years. An incident in 1983 allowed researchers to quantify the amount of damage caused by one vessel's anchor and anchor chain.

Today, sanctuary regulations prohibit the anchoring or otherwise mooring of vessels greater than 100 feet in length. Anchoring of vessels less than or equal to 100 feet in length where mooring buoys are available is also prohibited. In some cases, vessels over 100 feet may obtain a permit from the sanctuary to use the mooring buoys. There are approximately 15 mooring buoys within the sanctuary maintained by a private company through a contract with the sanctuary. Many of the buoys were originally installed by volunteers prior to sanctuary designation. Mooring buoys along with NOAA nautical charts appropriately marked to notify mariners of the regulations have helped alleviate chronic anchor damage in the sanctuary. However, significant threats still exist because of the lack of information on foreign charts often used on foreign-flagged vessels. Attempts are underway to rectify this problem.

Cable damage, similar to anchor damage, is also fairly common in the sanctuary. Cable damage occurs when a slack cables between tugs and barges or other towed vessels contact the reef surface. They topple corals and other upright organisms and can cut into the reef. Though regulations exist that address this problem, identification of responsible parties is often difficult. Other options to reduce such damage involve education and better indication of the reefs on charts.

Florida Keys National Marine Sanctuary

Anthropogenic threats to the reef ecosystem in the Florida Keys National Marine Sanctuary stem primarily from coastal development, overfishing, and inappropriate small vessel use. Large vessel groundings can also quickly devastate large areas of coral reefs. Between 1984 and 1997 seven ships over 200 feet in length grounded in the Florida Keys, destroying over 19 acres of coral reef. Following groundings, sediment is suspended for days, threatening the health of coral and other organisms that survived the initial insult. A system of radar beacons (Racons), navigational aides that allow vessels to identify their precise location relative to the sanctuary's reef tract, have recently been installed as part of the settlement in the Container Ship Houston grounding case. An "area to be avoided" has also been established. It prohibits tank vessels and vessels greater than 50 meters in length from operating in a designated area of coral reef habitat.

Nutrient pollution and sediments from coastal development and farming can block sunlight, smother corals, and impede larval settlement. Nutrient-rich runoff and discharges promote algal blooms and the growth of bottom-dwelling algal competitors. Algal blooms subsequently block sunlight, reducing coral growth. Algal competitors interfere with coral reproduction by competing for substrate. This pollution may also weaken corals and create an environment where coral disease pathogens thrive, ultimately increasing corals' susceptibility to disease. Recent reports suggest that the occurrence of coral disease has increased throughout the Florida Keys.

Water diversion in South Florida is also a problem because of the linkage of Florida Bay to the sanctuary. Water diversion has lead to reduced freshwater flows in the Bay that in turn lead to in an increase in plankton blooms, sponge and seagrass die-offs, and fish kills.

Overfishing, including harvest of lobster and conch, can cause shifts in fish size, abundance, and species composition within the reef community. Multiple jurisdictions regulate fisheries in the Florida Keys. Though established regulations are stringent, they are complex partially due to the multiple jurisdictions and the large diversity of fish species. Enforcement officers patrol the area but more officers are needed as well as outreach programs to keep the public abreast of current regulations.

Gray's Reef National Marine Sanctuary

Anchoring and overfishing currently threaten the health of the Gray's Reef ecosystem. Anchoring harms benthic organisms such as hard corals, sponges, and octocorals. Fish data from the sanctuary shows decreasing trends in fish abundance and diversity, similar to trends seen throughout the region. During its management plan revision process, the sanctuary will consider management tools to more fully protect the organisms of the sanctuary.

Hawaiian Islands Humpback Whale National Marine Sanctuary

Commercial fishing, recreational fishing, destructive fishing practices, and collection of fish for aquaria have led to overfishing and subsequent degradation of Hawaiian coral reefs. Long-term catch trends suggest that there has been approximately an 80 percent decline in the nearshore stocks this century. Overfishing is partially due to an increase in population, improved fishing technology, improved gear, and failure to recognize or follow traditional conservation practices. Additionally, the number of commercial permits issued to collect reef fish increased by 39 percent between 1995 and 1998. The inability to adequately enforce existing regulations and to create new regulations in a timely manner further exacerbates the problem of overfishing. Fisheries issues in the sanctuary are managed by the State of Hawaii within state waters and by the Western Pacific Fisheries Council in Federal waters.

Sediment runoff and pollution and nutrients from agricultural practices also widely impact coral reef habitat in the sanctuary. The effects of sedimentation and nutrification are described above for the Florida Keys National Marine Sanctuary. Sources of sediment on Hawaiian reefs include: improperly managed construction sites; cleared agricultural lands; heavy grazed lands; and eroding stream banks. Nutrients from fertilizers and pollutants such as bacteria from livestock, herbicides, and insecticides enter marine waters in runoff and seepage. The Hawaiian Islands Humpback Whale National Marine Sanctuary is currently working with the State of Hawaii Department of Health and the U.S. Environmental Protection Agency to address the issue of Total Maximum Daily Loads in certain areas within sanctuary boundaries.

The State of Hawaii Department of Land and Natural Resources, Division of Aquatic Resources has jurisdiction over the coral reefs in the sanctuary.