Nature and Environment Education - SCHOOL PROGRAMS - CORAL REEF

Reef type (Why is diving so easy and accessible on Bonaire?)

The reefs around Bonaire are all of the fringing type, that is, they line the coast like a narrow fringe or ribbon. In fact it would be more accurate to say that Bonaire is surrounded by one almost continuous fringing reef. The fringing reef of Bonaire is not separated from shore by a lagoon area as is common for fringing reefs. In many places the reef starts right at the shoreline, almost immediately below the water surface. This feature explains why diving is so easy on Bonaire, why the reefs are so easily accessible, and, at the same time, why they are so vulnerable.

The coast

In the southern part of the island and along Klein Bonaire we find mostly a low coast consisting of a rubble or sand beach, with or without beachrock formations. The only exceptions are the Belnem area and the west point of Klein Bonaire, where a low cliff coast occurs. The northern part of the island has predominantly a cliff coast, interrupted by some rubble beaches. Limestone terraces of different elevation, representing fossil reefs formed when the sea level was different from the present or when the land mass was uplifted, can be found all along the coast in the northern part of the island. You can see a good example of the terraces when you drive along the scenic road towards Goto. The road is situated on the lower terrace, which forms a low cliff coast until you get to “1000 Steps”. On your right is a “solution notch”, corresponding with a previous sea level. A similar solution notch is formed in the lower terrace at the present sea level. These notches are formed primarily through bio-erosion by boring and grazing organisms. Rubble beaches occur either where the lower cliff has been eroded away, or where coral rubble has closed off an inner bay (for example Goto, Slagbaai, Bartol).

Along the north coast, where the limestone cliff is exposed to wind and waves, you can see another phenomenon: the formation of so-called sawah benches in the center of the solution notch. What happens here is that calcareous algae and Vermitids (worm-shells) that thrive in the high energy surf zone, have capped the limestone with a hard, impenetrable layer, thus preventing erosion. You will find a good example of sawah benches in Boca Cocolishi in Washington-Slagbaai Park.

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Reef profile (What are the different types of reefs around Bonaire? Where can they be found? Where can I find a sheer wall? A double reef system? Spurs and grooves?)

The profile of the reefs along the leeward shore of Bonaire and along Klein Bonaire is fairly consistent. A shallow and narrow reef terrace slopes down gently from the shoreline to a depth of about 30 ft, where it begins to drop steeply - typically at an angle of 45° - to depths of 130 ft and over. At the lower end of the reef we usually find a sand terrace or sand slope. The depth at which the reef begins to drop steeply, 30 ft, is commonly called the drop-off. It is clearly visible from shore or from a boat as the transition between the turquoise and blue water. The steep drop of the reef is referred to as the reef slope. The shallow terrace is usually narrow and the reef slope is steep, so that deep water - 200 ft and over - is found quite close to shore.

Although this profile of shallow terrace, drop-off and reef slope provides an adequate characterization of the reef structure anywhere in Bonaire, there are obviously variations to the general theme.

• The terrace. The shallow terrace can be as narrow as 70 ft or occasionally exceed 500 ft in width. The shallow terrace is relatively wide in the south and along the west coast of Klein Bonaire. It is narrow along the north coast of Klein Bonaire and along the central-western part of the leeward coast of the main island.
  The structure of the terrace differs from the normal, gently sloping pattern at Rappel and Slagbaai South. Here you will see a depression in the center of the terrace - almost like a channel - followed by a rim or lip towards the dropoff, that may rise up to 10 ft above the terrace.

• The drop-off. The drop-off occurs typically at a depth of 30 ft, except along the east coast of Klein Bonaire, where it starts at 5 to 10 ft. Between Carl’s Hill and Jerry’s Jam on Klein Bonaire, the drop-off is virtually non-existent as the reef slopes down from the Elkhorn formations at the top almost without a noticeable change in angle.

• Walls. Bonaire is not well-known for wall diving. Yet a number of vertical slopes exist: at Carl’s Hill, Cliff, Small Wall, Rappel, La Dania’s, and Wayaka.

• Double reef. In most places the reef slope continues below 130 ft, with the second terrace or sand slope occurring between 170 and 270 ft. However, where the reef slope ends more shallow and a sand terrace begins around 100 ft, a second reef has formed, separated from the first reef by a sand channel. This is the case in the south, between Belnem and Pekelmeer, known as the “Alice in Wonderland double reef complex” (Van ’t Hof, 1983). The width of the sand channel and depth at which it occurs vary considerably, and the second reef may be continuous or intersected by sand rivers. A small stretch of double reef has also developed at 18th Palm.

• Buttresses. At various locations the reef slope has an undulating nature, caused by outward projecting ridges alternated by sandy valleys. These ridges, that run down steeply from the drop-off, are called buttresses, as they seem to “buttress” the reef terrace above. Buttress formation is well-known from the reefs of Jamaica and the Cayman Islands, where neighboring buttresses have sometimes joined and formed tunnels. Buttresses are found on Bonaire at several locations on Klein Bonaire and in the north. The most pronounced buttress formation can be seen at La Dania’s, north of Slagbaai, at Forest, and Ebo’s Reef.

• Spurs and grooves. Ridges alternated with sand channels, oriented perpendicular to the direction of the wave front, may develop in shallow water with high wave energy. They are most characteristic of the windward shore of the island, but they can also be found at Playa Benge and in the southern part of Boca Bartol.

• Reef slides. Reef building is a dynamic process. The most important reef building coral, the Boulder star coral, has its greatest abundance and its largest structures in the drop-off zone. Large structures sometimes become too heavy for the substrate they are attached to and topple down the slope. Also, an entire reef section may become detached and slide down, causing a so-called reef slide. Reef slides have occurred in the not too distant past along Klein Bonaire in the No Name beach area. A significant recent slide that lasted for 18 months, took place at Ebo’s Reef in late 1993.

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Coral zonation (Where are various types of hard and soft corals likely to be found?)

Some reef coral species occur at almost any depth, others have a more narrow distribution. For example, you will find Elkhorn coral only in shallow water, while sheet and scroll corals occur on the deep slope only. Such differences in distribution allow us to distinguish different zones on the reef, each of which can be characterized by a dominant species or group of species.

Although the zonation of the reef follows a general pattern, we will also see that many variations exist. We generally distinguish the following zones of the fringing reef in Bonaire:

• Shore zone. This zone occurs close to shore in very shallow, turbulent water. Only the very hardy coral species, such as Knobby brain coral, sometimes together with Lesser starlet coral, Golfball coral and Mustard hill coral, species that can cope with silt, are found here. You will find a typical shore zone mostly in the south and along the exposed side of Klein Bonaire, where the most shallow part of the terrace consists of solid limestone rock.

• Elkhorn zone. Before the waves of the hurricane Lenny hit Bonaire in 1999 the island used to have a zone dominated by Elkhorn coral. This could be found at a depth of 10 ft or less. The strong waves broke all these corals. The good news is that they are coming back. Look closely and enjoy the beauty of the return of these corals.

• Mixed zone. The zone between the Elkhorn zone and the dropoff is most commonly a mixed community of hard and soft corals. Soft corals frequently dominate this zone. Staghorn coral forms patches or even extensive fields here; although much staghorn has perished from disease and storm surge, promising new formations are visible in many places. Patches of Yellow pencil coral are common just above the dropoff zone. Other common coral species in the mixed zone include small, knobby Boulder star coral, Symmetrical brain coral and Grooved brain coral.
  Where the terrace is narrow (such as at most sites between Karpata and Barcadera), Staghorn coral formations are limited or even completely absent in the mixed zone.

• Drop-off zone. This zone, between 25 and 40 ft, is characterized by Boulder star coral. The upper part of this zone may be somewhat barren, while patches of Yellow pencil coral are often common here. Soft corals increase in abundance with decreasing depth.

• Upper reef slope. The upper reef slope, including the area from 40 to about 90 ft, has the greatest diversity of hard coral species. Some 20 species of hard coral can usually be recorded in this zone on a single dive. Large towers of Boulder star coral often occur in the top portion of this zone, creating a high-profile area. Soft corals may be abundant throughout this zone (in the south and along the southwestern tip of Klein Bonaire), or may be more or less restricted to the upper part of this zone in other parts of the islands.

• Lower reef slope. The reef slope below 90 ft is characterized by extensive formations of Sheet and Scroll corals, sometimes together with shingle formations of Boulder star coral at the transition between upper and lower reef slope.

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Distribution of other reef organisms (What other critters live in each reef zone?)

Although hard coral species are commonly used to distinguish reef zones, other reef organisms are obviously also associated with, and contribute to the characterization of, different zones on the reef.

On the shallow terrace you will find an abundance of parrotfish, wrasses and damselfish. Anemones, the Common Caribbean anemone in particular, are common in the terrace community. Associated with the anemones are juvenile wrasses that find shelter among the anemone’s tentacles, and several species of shrimp that have a symbiotic relationship with their hosts. The Lettuce sea slug can be found upon the dead corals. Staghorn rubble is the best place to look for juvenile Damselfish. Coneys are common on the terrace and the schooling behavior of French and Smallmouth grunts can be observed here. Mobs of Blue tangs frequently roam this zone. Wherever the lower terrace is dominated by soft corals, there will be small filefish and trumpetfish hiding among their branches and Basket starfish curled up (by day) in the Sea plumes.

Towards the dropoff zone you are likely to see Schoolmasters, Porgy, Trumpetfish, Yellow goatfish, Spanish hogfish, and, in midwater, schools of Brown chromis. Cleaning stations serviced by juvenile Spanish hogfish and Bluehead wrasses are common here. The Graysby, a small grouper, is most common in the dropoff zone and along the upper slope. Sponges become more abundant in the dropoff zone.

Along the upper reef slope soft corals and sponges are usually well-represented. Fairy basslets find shelter among the large coral heads, schools of Creole wrasse pass overhead and Creole fish are common. Here are also your best chances of seeing the most common of the larger grouper species, the Tiger and Yellowmouth grouper. Towards the lower reef slope, the common soft coral species disappear and black corals, especially Wire coral, become conspicuous. Soft corals that characterize this zone are the Devil’s sea whip - not to be confused with Wire coral - and a close relative, the branched Long sea whip.

If the reef slope is short, the sand plateau at the lower end of the slope will treat you to species such as the Sand tile fish, Garden eels, Cherubfish and Tobacco fish. Interesting enough, some of these species may also be found in shallow rubble habitats.

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