Located at 32°N, Bermuda’s coral reefs are unique as they are the most northern massive coral reefs in the Atlantic Ocean. Bermuda is a volcanic island, but you wouldn’t know that by looking at it. While most of the massive seamount is volcanic Basalt, the upper crust that we live and dive on is all limestone. The rock that we see and know as Bermuda was all built by corals and other organisms that are able to fix calcium carbonate from seawater. As a seamount, Bermuda rises steeply up from the depths. The coral reefs of Bermuda created both a fixed point for reference and a navigational hazard for early mariners. Bermuda’s reefs capture the attention of wreck divers; over 300 ships have wrecked.
Corals thrive in clean clear warm water and prefer temperatures of 25-29°C (77-85° F). The small islands totaling 21 square miles of Bermuda are surrounded by the warm waters of the Gulf Stream. Approximately 570 miles east of North Carolina, Bermuda is far from major sources of pollution. The constant supply of clean warm water allows corals to proliferate much farther north then they otherwise would be able to live. Offshore average monthly water temperatures range from 18°C (68°F) in the winter to 28°C (83°F) in the summer. Inshore waters average a few degrees cooler in the winter and warmer in the summer.
Due to lower winter temperatures than in the Caribbean, Bermuda's reefs have only about 1/3 of the species of hard corals found in the Caribbean. However, those species that can tolerate the lower winter temperatures have a competitive advantage and do very well. Bermuda has high rates of coral coverage. The Gulf Stream water that bathes the island is low in pollution and nutrients and unlike many places in the Caribbean, Bermuda’s coral reefs remain healthy.
Bermuda's reefs can be divided into characteristic zones that experience different sets of environmental conditions (Figure 1, Table 1). On the south shore, the seamount falls off quickly, on the north side of the island, there is a large lagoon (Figure 1). From shore seaward, Bermuda has Lagoon, Rim and Terrace Reefs.
Lagoonal Reefs tend to be shallow with lower visibility due to more sediment in the water and higher wave action. These reefs are dominated by soft corals such as sea fans. They also have more species of delicate branching corals. The structure of these corals allows them to shed sedimentation and in the case of soft corals, bend with waves and currents. Corals in the Rim Reef bear the brunt of whatever waves Atlantic Storms kick up. Rim reefs have much better visibility are dominated by massive boulder shaped corals such as brain corals and star corals. As the seamount starts to fall away from the Rim Reefs, the Terrace Reefs begins. With open ocean water and low waves due to the depth, Terrace Reefs have the best visibility and like the rim reefs, are dominated by brain and star corals.
Due to their health and northerly location, Bermuda’s reefs serve scientists watching global climate change like a Canary in a Coal mine. Global water temperature change, seawater rise, and ocean acidification may all affect corals, and those at the edge of their physiological temperature limit, such as here in Bermuda, are likely to be effected first.
Coral Reefs have many benefits – the obvious one is that they created the part of Bermuda that we live on, dive on, the rock that we build our houses out of and the beautiful south shore beaches. The reefs also buffer the mid-ocean island from waves. Reefs are biological hot spots and support a diversity of life; they are often compared with rain forests. Highly productive, reefs provide a habitat for many commercially important fish and shellfish. More recently, coral reefs ecosystems have started to be investigated for pharmaceuticals.
The Bermuda government has been fairly progressive in its legislation to protect Bermuda’s reefs. Conservation has a long history in Bermuda, the first conservation laws in the New World were past here in 1616 and 1620 to protect cahows and turtles. All corals are protected in Bermuda and there are fishing restrictions as well. Bermuda has 29 marine protected areas for divers to enjoy, many of them centered on wrecks or especially nice dive sites. Each of these sites has an established mooring which eliminates the need to anchor and potentially damage the corals.
Coral reefs are massive structures capable of creating islands, but what exactly are corals? Animal, vegetable or mineral? When corals washed ashore, people though they were some sort of natural rock formation. Later, when living coral was examined, it was classified as a plant. Aristotle classified corals as “flower animals” – this is the literal translation of class Anthozoa which contains corals and their relatives such as sea anemones. Zoologist Sir Maurice Yonge was the first to observe nocturnal feeding of hard corals.
Corals thrive in clean low nutrient waters – so how can they live in such impoverished conditions? Corals have algae symbionts called zooxanthellae. Those that have lost their symbionts have “bleached”. Bleaching does not necessarily kill corals, but it can if they are already stressed. Healthy corals often regain their symbionts after a bleaching event. In return for a safe place to live, the algae symbionts produce energy for the coral. Corals are animals, they lay down a mineral skeleton and have plant symbionts; one could argue that they are a bit of everything.
Corals are massive and can form massive structures, but an individual coral animal is very small. Coral polyps look a lot like small sea anemone polyps. The living coral of most species is a colony of many tiny individuals. These individuals live on the thin delicate top of the calcium carbonate coral skeleton. Since only the top layer is alive, corals are delicate and can be damaged by contact.
The coral reefs in Bermuda are healthy and thriving and contain many of the species found in the Caribbean. There are swim throughs, caves, wrecks, colorful fish and invertebrates and the visibility is generally very good.