Mesophotic coral reef
A mesophotic coral reef or mesophotic coral ecosystem , originally from the Latin word meso and photic, is characterized by the presence of both light-dependent coral and algae, and organisms that can be found in water with low light penetration. Mesophotic coral ecosystems occur at depths beyond those typically associated with coral reefs as the mesophotic ranges from brightly lit to some areas where light does not reach. Mesophotic coral ecosystem is a new, widely adopted term used to refer to mesophotic coral reefs, as opposed to other similar terms like "deep coral reef communities" and "twilight zone", since those terms sometimes are confused due to their unclear, interchangeable nature.
Many species of fish and corals are endemic to the MCEs making these ecosystems a crucial component in maintaining global diversity. Recently, there has been increased focus on the MCEs as these reefs are a crucial part of the coral reef systems serving as a potential refuge area for shallow coral reef taxa such as coral and sponges. Advances in recent technologies such as remotely operated underwater vehicles and autonomous underwater vehicles have enabled humans to conduct further research on these ecosystems and monitor these marine environments.
Mesophotic coral reefs also serve as a transition zone between shallow and deep-sea environments which creates a specialized and unique biodiversity for the MCEs. They normally occur between 30 and 40 meters up to 150 meters in tropical and subtropical water. Mesophotic community zonation seems to firstly be driven by light and temperature, which broadly corresponds to the upper mesophotic at 30 m to 60 m in depth and the lower mesophotic at 60 m to 150 m. In some areas such as the American Samoa mesophotic reefs contain 80% of corals while the rest inhabits the shallow reefs. They are distributed globally but remain mostly understudied. Basic information on the taxonomic composition, depth range, habitat preferences, and abundance and distribution of the mesophotic coral ecosystem is limited.
Species
The most common species at the mesophotic level are corals, sponges and algae. The corals ranges can overlap with deep-water coral but are distinguished by the presence of zooxanthellae and their requirement for light. The species found in the mesophotic coral reefs were previously believed to be an extension of the shallow water coral ecosystem and a crossover of coral species between the two is common. MCE studies reveal that there are various habitats and communities of corals, sponges and fishes, which are significantly different from their shallow-water counterparts. In the red sea, two species of Alveopora are exclusively found in the mesophotic region. Other unique species that were only found in mesophotic coral reefs are Symbiodinium, Brachyuran crabs, Porites astreoides, Acropora tenella.Other species are found in both shallow reefs and mesophotic reefs, however some are more numerous in the MCEs and some have increased fecundity. Many coral taxa appear to be endemic to mesophotic regions as they are outcompeted in shallow water reefs by faster photosynthetic taxa. Species such as move across the mesophotic and shallow zones daily. Their original habitats are in the mesophotic zone however their food is hunted and captured in shallow zones. The mesophotic coral reef serves as an area for many species of corals that are more sensitive to high sea water temperatures lowering their chances to suffer from coral bleaching. The oldest known mesophotic coral ecosystems have been described from the Silurian of Sweden, such ecosystems are also known from Devonian. Oldest scleractinian-dominated mesophotic ecosystems are known from the Triassic.
Due to recent anthropogenic impacts, it was thought that the MCEs are less affected by human development and climate change, and could be used as a source for reseeding shallow water coral species. However, recent analyses show that mesophotic ecosystems face significant impacts from climate change.
Reef builders
Corals are the main components of any coral reefs including ones found in the mesophotic region. Other organisms also contribute to the biodiversity found in these reefs such as macroalgae and sponges.Macroalgae
Macroalgae, more commonly referred to as seaweed, are species of macroscopic, multicellular, marine algae. They include Rhodophyta, Phaeophyta and Chlorophyta macroalgae. Many of these species also play the role of capturing carbon and producing much of the world's oxygen. Their depth distribution in coral reefs including mesophotic coral reefs rely on the availability of carbon, nitrogen and photosynthetic potential. Macroalgae competition with existing coral communities creates coral bleaching. Their distribution in mesophotic coral reefs are determined by the temperature gradients where they prefer warmer temperatures.Sponges
Sponges are the second most dominant taxon found in many coral reefs after corals. Sponges are largely dependent on planktonic food resources, both dissolved and particulate as water flows through their bodies providing them food and oxygen. They are able to create benthic diversity through filter feeding, creating essential habitat for many species of invertebrates and fish. Sponges are primarily mixotrophic or heterotrophic. They prefer cold water environments, making them crucial inhabitants of the lower mesophotic coral reefs. MCEs contain many sponge species that differ to shallow reefs and that remain undescribed.Community ecology
Distinct bathymetric zonation patterns occur in the mesophotic coral reef. Corals are the dominant species and provide the most cover in the most upper regions of the mesophotic area. In some coral reefs, the coral cover decreases with depth while sponge cover increases. In depths of over 50 meters, sponge become the dominant species however dense community of corals such as the scleractinian corals can be found at depths of 60 meters.Coral cover begins to decline in the majority of mesophotic coral reefs at depths of 90 m as sunlight is low in these regions, limiting coral and macroalgae growth. The distribution of corals is also limited because of the changes in irradiance with depth and daily temperature fluctuations that can reach up to 4 °C.
Temperature
Temperature is important for coral growth especially in the mesophotic region as sunlight does not penetrate fully. The ideal temperature for coral reefs globally is between 23 and 29 °C. On some instances, some corals can endure temperatures of up to 40 °C. Corals and coral reefs are usually unable to grow in temperatures that are below 18 °C. Temperatures between 15 and 16 °C are considered to be the limit for coral reefs to survive. Prolonged exposure to these temperatures can lead to mortality for most corals. A specific type of coral was discovered to be able to survive in temperatures as low as 13 °C.In high temperatures with high solar irradiance, coral bleaching usually occurs which is a phenomenon where corals releases their living in their tissues draining the corals of its colors. They become white due to loss of the zooxanthellae and photosynthetic pigments which causes them to be under extreme amounts of stress and exposes them to high mortality rates.
Temperatures from the highest depths of the mesophotic coral reef differs from surface temperature of around 5 °C. The temperatures found at mesophotic coral reefs can vary due to surface temperature and events such as hurricanes and internal waves. Internal waves can induce fluctuations of the thermocline causing temperature in mesophotic coral reefs to vary from 10 to 20 °C.
Research on MCEs
Research on mesophotic coral reefs have been limited until the 20th century because of the difficulty of conditions to observe them. A study in 2017 showed that a majority of research done on MCEs have been completed since 2010 and 15% of total studies being completed in 2016 alone. The same study suggests that we know less about mesophotic coral reefs than we originally thought as 57% of research have been conducted on reefs in the Atlantic region while Pacific mesophotic coral reefs remain understudied. There has been little to no research conducted in the regions of South East Asia and India. Research on these coral reefs remain currently highly biased on location and region while not representing oceans globally.The best studied mesophotic coral reefs in the world are the northern coast of Jamaica, the Bahamas, the northern Gulf of Mexico, and Puerto Rico. The best studied area in non-Atlantic oceans are the Marshall Islands, the main Hawaiian Islands, Johnston Atoll, and the northern Red Sea. In 1973, biologist Thomas J. Goreau found that zooxanthellate coral species were common at the surface of Jamaican MCEs. Below 50 m, coral species diversity declined rapidly and sponges are more commonly found. Zooxanthellate corals, plate-like colonies of Agaricia and Leptoseris were the most common and were found up 99 m in depth. Other conducted studies in the following years found that Caribbean MCE's also possesses similar geomorphology and specie distribution as Goreau's discovery in 1973. Later research has proven that MCEs have many species that are endemic to the mesophotic region and require these depths and temperatures to reproduce.
Previous research has hypothesized that mesophotic coral reefs serves as a refuge for shallow reef species as they are less easily disturbed due to their depths. This hypothesis is argued to be false as the same disturbances that impact shallow reef species also impact mesophotic coral reefs. These impacts include hurricanes, thermal stress, pollution, sedimentation, and eutrophication resulting in significant mortality rates for all coral species.