BankReefs

Bank Reefs

Formation

Bank reefs, sometimes referenced as platform reefs, are built upward from the seafloor by non-photosynthetic calcifiers. A single species often builds these deep-water reefs. Photosynthesis is not typically the primary energy source for these systems, as they exist at depths to which sunlight cannot effectively penetrate. Once the reefs reach an adequate depth, depending on local factors such as water clarity, photosynthetic coral species may also begin colonizing and expanding these reef formations.

Morphology and Ecology

Bank reefs are characterized by coral formations in linear or semi-circular clustered patterns that are often larger than patch reefs, which are similar structures. These coral formations typically consists of three regions classified by location and coral formation type.

The Reef Flat occupies the inshore side of the bank reef. This consists of broken coral skeletons and coralline algae and excludes most other organisms due to the inhospitable, heavy surf that often characterizes this area.

The next adjacent section is the Spur and groove patterned region. Spurs are low ridges of coral. These areas are dominated by fire corals and zoanthids at shallower depths. At around 5-6 feet these species give way to elkhorn, star, and brain corals along with sea fans, seawhips, and sea plumes. Spurs form because bank corals are usually located in areas of up-welling, where the coral structures are able to capture sediments and build mounds where corals attach and grow into bushy, spherical colonies. Grooves are the sandy bottom channels that separate the spurs. These areas consist mostly coarse white limestone sand composed of fragments of shells, coral, and algae plates formed by green calcareous algae. These regions are inhabited by smaller invertebrates that bury into the sand and only venture out at night.

The final section of these coral reef formations is the Forereef. This is the open-sea edge of the bank reef. At its shallowest points this section of the reef is composed primarily of Star coral and inhabited by benthic (residing on rock or sediment) organisms. As depth continues to increase and becomes limiting, coral begins to form plate-like formations that increases surface area and photosynthetic ability. From here the bank reef slopes down into the depths of the ocean until sponges and non-reef building community compose the majority of the ecosystem.

As is typical of most coral reefs, bank reefs have a high species diversity living in and around them. French Angelfish (Pomacanthus paru), Blue Parrotfish (Scarus coeruleus), Queen Parrotfish (Scarus vetula), Queen triggerfish (Balistes vetula), Rock beauties (Holacanthus tricolor), Goatfish (Parupeneus cyclostomus), Porkfish (Anisotremus virginicus), and Snappers (Lutjanus spp.) commonly inhabit these coral formations. Mollusks, decapods, amphipods, and many other invertebrate organisms are also known to reside in the small caves and tunnels that form throughout the Bank reef structure. Bank reefs may also have an abundance of large, sedentary invertebrates like plate-shaped and fan-shaped sponges.

Location

Bank reefs are unique in that they are found at greater depths than other coral formations, typically residing at a depth of 20-60 feet. An example of one such formation is off the central east coast of Florida. Here Oculina varicosa is known to form bank reefs from the sea floor. Carysfort Reef is a notable bank reef off the coast of Key Largo in Florida. The reef is named for a 118-foot long, British sailing vessel called the H.M.S. Carrysford, that wrecked and foundered at this site in 1770. This and other shipwrecks often inspired the construction of lighthouses that are often characteristic of bank reef formations. The Carysfort reef is considered one of the healthier reefs in the Key Largo chain and makes for excellent snorkeling due to its proximity to the clear waters of the Gulf Stream.