Coral reefs and seagrass beds

Coral reefs are found in the euphotic zone of continental shelves at depths up to 164 feet (50 meters), in water with a mean temperature averaging above 68 degrees F (20 degrees C), and with a salinity ranging from 32 to 42 practical salinity units. They are found in both tropical and subtropical waters, in latitudes from 25 degrees north to 25 degrees south. Coral reefs have greater taxonomic diversity per area than any other marine environment and are home to approximately 25 percent of all marine life. They have been compared to tropical rain forests in terms of the diversity of life. Exact species counts on coral reefs are unknown, and many coral reef species remain undescribed. Some estimates state that up to 2 million different species live in and around coral reefs. Coral reef ecosystems cover about 0.17 percent of Earth's surface, and are extremely productive, with gross primary productivity estimates of 3.3–1.1 pounds (1,500–500 grams) of coral per square meter per year; this is about 100 times greater than primary productivity in the ocean's pelagic zones.

Coral reefs are most common in the western Pacific, western Atlantic, and Indian Oceans, where waters are oligotrophic. More eutrophic waters, a consequence of upwelling, mean that coral reefs in the eastern Pacific and Atlantic have reduced species richness and are smaller. Mangrove forests and seagrass beds are often adjacent to coral reefs, and the former communities benefit coral reef ecosystems by absorbing nutrients and slowing rates of sedimentation.

Coral reefs are biogenic structures, composed primarily of calcium carbonate. Coral reefs take up over half of all the calcium ions that enter the world's oceans. Reefs are surrounded by biogenous sediments derived, in part, from the breakdown of this skeleton by bioerosion (particularly by boring sponges) and abiotic processes (particularly through wave action). The main taxonomic groups that create reef structures are hermatypic (reef-building) anthozoans, including stony corals and stony hydrozoans, though crustose coralline algae and mollusks are also important contributors to the reef's infrastructure. Other common organisms found in coral reef communities include sponges, gorgonians (sea fans), echinoderms, and fishes.

Hermatypic corals are cnidarians that grow clonally; about 75 percent of them can also reproduce sexually, and release of sperm and eggs is triggered in response to changes in temperature, day length, or lunar cycles. Corals can feed on plankton using their tentacles, which contain pneumatocysts. They can also acquire planktonic food with their mesenterial filaments, which release digestive enzymes and can be everted to absorb food particles that have not entered the gastrovascular cavity. All hermatypic coral polyps also contain endosymbiotic, mutualistic dinoflagellates, termed zooxanthellae, which live between the cells of the polyp's gastrodermis.

The zooxanthellae, which are members of multiple taxonomic groups, are autotrophs that exchange photosynthate and amino acids with their animal hosts in return for protection and nitrogenous waste. Zooxanthellae provide up to 90 percent of their primary productivity to the polyp; this allows the polyp to produce and deposit calcium carbonate, which it embeds in a carbohydrate matrix to form a calyx into which it anchors. Polyps continue to deposit calcium carbonate throughout their lifetimes, allowing the reef's skeleton to continue to grow in size.

Unfavorable environmental conditions can trigger polyps to expel their endosymbionts, a process called bleaching. Such conditions include water temperatures in excess of 95 degrees F (35 degrees C), high light conditions (particularly ultraviolet radiation), or eutrophication. As the mutualism is obligate for many coral species, the polyps often die in the days or months after a bleaching event. Some polyps are able to regain zooxanthellae, and recent studies have shown that zooxanthellae species and genotypes differ in their resistance to stress and likelihood to abandon their host polyps.

Since coral polyps are small (usually less than .12 inch, or 3 millimeters in diameter), and calcium carbonate deposition rates are relatively low, a coral colony grows less than 3.9 inches (10 centimeters) per year. Thus, formation of an entire reef structure takes up to 10,000 years. There are three main types of coral reefs, each of which has its topographic features determined by abiotic factors including light, temperature, the direction and strength of waves and currents, and water depth. Coral reef classification is based on both reef morphology and reef origin.

Types of Reefs

The most common reef type worldwide is the fringing reef, which is found in narrow bands parallel to tropical shorelines. Fringing reefs are found in nearshore habitats with existing hard substrate to support the settlement of a coral's planula larvae, which can metamorphose into polyps after settlement. Regions of a fringing reef include the reef flat (inshore), which extends to the reef crest and drops off to the reef slope, which interfaces with the open ocean. The world's longest coral reef, in the Red Sea, is a fringing reef.

Barrier reefs like Australia's Great Barrier Reef are farther offshore, and a lagoon separates the reef from the shoreline. The reef begins with a back-reef slope, which transitions to a reef flat, then reef crest, which absorbs much of the energy from wave impacts. The reef crest ends in the forereef slopes, which trails off to the deep ocean.

Atolls, found mostly in the Pacific and Indian Oceans, consist of a lagoon ringed by a coral reef. The edge of the reef is the reef flat, and the outer slope moves into the open ocean. Atolls are formed around sunken volcanoes, a theory first proposed by Charles Darwin. Fringing reefs first form around islands. As islands slowly subside, reefs gradually gain height through calcium carbonate accretion by hermatypic corals and others. Over long periods of geologic time, the island is replaced by a lagoon.

Interspecific interactions like competition are the primary factors determining community assemblages in coral reefs. Space is at a premium on reefs, and is the primary resource for which organisms struggle. Some scleractinian species actually digest neighboring species with their mesenterial filaments to acquire more space. Bottom-down interactions mediated by herbivory also affect the species composition of reefs. Mutualisms are also common in coral reef systems; these include cleaner symbioses and mutualisms between zooxanthellae and sponges, mollusks, or other cnidarians. Herbivores are important grazers of macroalgae, and reductions in macroalgae densities allow coral polyps and their symbionts to acquire sufficient light.

Coral reefs contribute to local economics by attracting tourism dollars, and they are important sources of organisms for commercially important, as well as sustenance, fisheries. Coral reefs can yield an average of 33,000 pounds (about 15,000 kg) of fish and other seafood per 247 acres (approximately 1 square kilometer) per year. Coral reefs also prevent shoreline erosion and dampen the energy of waves associated with storm activity.

Threats to Coral Reefs

Natural threats to coral reefs include damage from wind and waves associated with hurricanes and cyclones. The slow growth rates of the reef skeleton means that recovery times for reefs can be long. Predation from fishes and invertebrates (polychaetes, mollusks, crustaceans, and echinoderms) is also a threat to coral polyps. Extreme low tides or drops in water level can cause physiological stress to polyps and trigger bleaching. Though coral polyps secrete mucus, this mucus provides only limited protection from desiccation.

In 2022, 91 percent of the reefs surveyed on the Great Barrier Reef were affected by coral bleaching. Worldwide, the most threatened ecosystems are found in southeast Asia. Anthropogenic threats to coral reef ecosystems include overfishing, which can cause trophic cascade effects that allow macroalgae to overgrow coral reefs and trigger bleaching. Fishing practices that use toxins like cyanide can kill the corals' sessile polyps, and dynamite-based fishing or trawling can destroy reef structures. Pollutants can cause eutrophication, can be toxic to the polyps, can trigger bleaching, can reduce water clarity so that zooxanthellae are below their photosynthetic compensation points, or can interfere with episodic sexual reproduction events. Reef harvesting, also called coral mining, damages reefs long-term. Coral is removed and sold as souvenirs and is also used as material for road-fill, bricks, and cement. Approximately 25 percent of coral reefs worldwide are considered damaged beyond repair. According to the Global Coral Reef Monitoring Network, 95 percent of coral reefs throughout the world will experience high thermal stress and potential bleaching in the 2050s.

Sedimentation or siltation is particularly harmful to coral polyps, because it can reduce rates of photosynthesis or actually suffocate polyps. Although viral, bacterial, and fungal diseases occur naturally in coral reef ecosystems, the incidence of disease is heightened by other stressors to corals, including increased ultraviolet radiation or temperature. Finally, the effects of global climate change are particularly problematic for sensitive coral reef communities. Increased oceanic temperatures can contribute to increased bleaching rates and resultant coral reef mortality. Rising levels of atmospheric carbon dioxide diffuse into oceans, where they decrease water pH in a process known as ocean acidification. Carbonate ions combine with carbon dioxide and water, producing bicarbonate while decreasing the relative availability of carbonate. Since hermatypic corals deposit carbonate in their exoskeletons, the ability of reefs to maintain or grow their structures has been impeded.

Sunscreen, or more specifically a common chemical found in the majority of sunscreens—oxybenzone—is significantly contributing to the deterioration and bleaching of coral reefs by breaking the coral down and robbing it of nutrients. Not only are swimmers at beaches to blame for providing increased oxybenzone in the world's oceans, but any person who applies sunscreen then washes it off in the shower or bath contributes to the chemical being added to the ecosystem.

When threats of climate change and direct human impacts are considered together, the World Resources Institute estimates that over 85 percent of the world's coral reef ecosystems are threatened. Although about 30 percent of the world's coral reef ecosystems are in Marine Protected Areas, enforcement of protection laws is uneven, and many of the world's most threatened reefs completely lack protection.

Seagrass Beds

Seagrasses are submerged aquatic angiosperms that live in relatively low-current regions within the euphotic zone of estuarine or saline waters. They have a broad thermal tolerance, living in waters with average temperatures ranging from 21 to over 104 degrees F (minus 6 to over 40 degrees C), and can tolerate salinities from 0 to 50 practical salinity units. All reproduce asexually via rhizomatous growth, and this process contributes to most of the growth rate of seagrass populations. Seagrasses, which comprise less than 0.02 percent of the described flowering plant flora, are distributed across two families (Potamogetonaceae and Hydrocharitaceae) and a total of 12 genera. Of these genera, seven are tropical and five are temperate. All but one seagrass species (Enhalus acoroides, the most recent to evolve) utilize hydrophilous pollination from simplified and reduced flowers. The number of seagrass species is estimated to range from 50 to 60. The precise number is still in dispute, as molecular and morphological taxonomists struggle to resolve their data. Other factors contributing to the confusion surrounding seagrass taxonomy is the difficulty in finding reproductive organs, most useful in anatomical classification schemes, for many species, and the fact that many vegetative traits used in classification are highly plastic.

Seagrasses evolved from either terrestrial (marsh) species or freshwater macrophytes, and the sparse fossil record indicates that they invaded coastal marine habitats about 100 million years ago. Biochemical and molecular evidence supports the polyphyletic origin of this evolutionary group. Current levels of population genetic diversity vary widely among species and regions and are affected greatly by seagrass species' extensive rhizomatous (clonal) reproduction.

Habitat requirements restrict the global distribution of seagrass-dominated biomes. Although some abiotic needs are species-specific, all seagrasses require saline waters, substrate in or on which to root, and high levels of light. Salinity levels ideal for seagrass growth range from 10 to 45 practical salinity units, with values above or below this resulting in solute stress, necrosis, and/or greater disease susceptibility, particular to fungal pathogens. Most species root in substrates with high sand or organic contents where they absorb nutrients primarily through their root systems. Areas where particulate matter is fine are less likely to be colonized due to increased sediment turnover rates and decreased light penetration.

Rocky intertidal and subtidal regions are frequently inhabited by members of the genera Phyllospadix, Posidonia, and Thalassodendron, which can attach their rhizomes and roots to rocks in intertidal or subtidal regions. Seagrasses require from 4 to 29 percent of incident radiation to stay above their compensation point, the light level at which the glucose generated via photosynthesis and that used by cellular respiration are in balance. These light levels are higher than those estimated for many shade-adapted terrestrial plants and macroalgae.

Because of their high light requirements, seagrass beds are restricted to the shallow, oligotrophic, littoral portions of continental shelves. They are found along the margins of all Earth's land masses, except Antarctica. Seagrasses are often separated into nine different flora based on their distribution: temperate east Pacific, temperate west Pacific, temperate north Atlantic, temperate south Atlantic, Caribbean, Indo-Pacific, Mediterranean, New Zealand, and South Australia. The species diversity of seagrasses is highest in the waters surrounding the Indo-Pacific region. Biogeographic boundaries between such flora are often sudden, and such shifts are usually determined by changes in water density or nutrient availability. Within a single region, species evenness is usually depressed compared to even the most depauperate of temperate regions, and many stands of seagrasses are composed of only a single species. As in temperate flora and other marine communities (including mangals and coral reef ecosystems), species diversity and evenness declines along a latitudinal gradient from the tropics to the poles.

Seagrass Ecosystems

Seagrass ecosystems are rich in macro- and microalgae, which can be benthic, or use the seagrass organs as a substrate. Epiphytic diversity within seagrass communities can be quite rich, composing up to 45 percent of all algae in that ecosystem, and algal primary productivity can meet or exceed that of the submerged aquatic macrophytes. All major phyla of animals can be found residing in seagrass beds, in infaunal, epifaunal, or motile forms. Experimental manipulations as well as mensurative studies have shown that community composition of seagrasses can be controlled by resource limitation, bottom-down forces, or a combination of these.

Although many animals use seagrass blades as a substrate, or graze on the seagrasses' epifloral, epifaunal, and periphyton communities, few consume seagrasses directly as a major component of their diets. This is because of the high cellulose content and high carbon to nitrogen ratio in seagrasses and the fact that many are chemically defended. Exceptions to the rule of direct consumption are found in communities which lack keystone predators, where large populations of isopods, amphipods, parrotfishes, or sea urchins may overuse their preferred algal food source and prey switch onto seagrasses. Some invertebrates selectively graze the inflorescences or seeds of seagrasses, with potential impacts on their sexual fitness. Grazing of periphyton, which lack structural or biochemical deterrents to herbivory, actually increases seagrass fitness, as it increases the amount of incident light available for photosynthesis and reduced hydrodynamic drag on seagrass leaves. Larger herbivores, like West Indian manatees (Trichechus manatus), have diets consisting mostly of seagrass species, and other Sirenia like dugongs (Dugong dugon) eat only seagrasses.

Aquatic birds like American widgeons or brant geese might rely on seagrasses for a major part of their diet during migration. After spending their first year of life in the pelagic zone, where isotopic studies have shown them to be omnivorous, green sea turtles (Chelonia mydas) have diets that consist exclusively of seagrass species. Grazing at intermediate levels is also beneficial to seagrass productivity and population growth, because the physiological integration among seagrass clones allows the plant to produce increased standing crops of shoots. Isotopic analyses have revealed that seagrasses are also an important component of detrital food chains in both nearshore (adjacent) and deeper oceanic environments. In fact, more than half of the primary productivity of a seagrass bed is connected to other trophic levels in the food web via detritus. Seagrass leaves themselves are rich in aerenchyma, can float over long distances, and can directly export carbon-rich leaves to adjacent ecosystems.

Seagrass beds are both ecologically and economically important. The extensive root and rhizome systems of seagrasses, as well as their baffle-like blades, allow them to stabilize sediments and improve water quality and clarity. They also do significant carbon and nutrient cycling, sequestering up to 12 percent of the ocean's total carbon and supporting primary productivity levels of as much as 8 grams of coral per square meter per day. Seagrass beds are vital nursery habitats for many organisms, though the number of species that they support and the commercial importance of those species varies by latitude. The hydrodynamic properties of seagrass beds help them entrap planktonic larvae and small nekton.

Seagrass beds also provide shelter from predation for a number of animals, particularly small invertebrates, and are higher than most ecosystems in their levels of secondary productivity. In addition, they are important feeding grounds and nursery habitats for coral reef and mangrove-dwelling fishes; these species often migrate diurnally from reefs to seagrass beds to feed, or they may recruit their offspring into the more quiescent seagrass habitat. Seagrass beds house more species than habitats with similar abiotic conditions that lack seagrasses, even those with extensive macroalgal communities. Rates of predation on both fish and invertebrate species are lower in seagrass beds than on bare substrate. Common taxonomic groups found as juveniles or adults as members of seagrass communities include fishes, crustaceans, mollusks, and polychaetes.

Seagrass meadows are disappearing at a rate similar to that of coral reefs and tropical rainforests. The annual loss of seagrass meadows was at a rate of 1.5 percent per year.

Seagrass species are sensitive to disturbance, especially those that reduce light availability, such as sediment loading and dock building. Eutrophication, in addition to creating resource regimes more conducive to the growth of macroalgae, increases the abundance of epiphytes and phytoplankton, which can reduce light availability. Declines in seagrass can occur in response to ice scouring, storm activity, grazing by waterfowl, or infection (particularly wasting disease, a fungal infection). Recent decades have seen dramatic and worldwide declines in seagrass cover, and Frederick T. Short and Sandy Wyllie-Echeverria estimated that most of these declines can be attributed directly to anthropogenic disturbances. Oil spills can reduce seagrass production, as can changes in salinity because of changes in freshwater input. One example of this occurs because of modifications of water flow through the Everglades, leading to hypersaline conditions in Florida Bay. Mechanical damage from prop scarring or dredging activities harms seagrass beds.

Climate change and associated warming of waters might harm temperate species near the tropical edges of their biogeographic distributions. Sea-level rise might also harm seagrasses by increasing water depth and thus decreasing light availability. Current legislation in the United States requires that seagrass losses be mitigated, under provisions of the Clean Water Act. However, effective restoration of complete seagrass communities that mimic the function of undisturbed communities has proven elusive.

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