Reefs which form close to land, following the contours of the coastline are called fringing reefs.
Several of the bays and lagoons in the Dutch Caribbean contain examples of one or more of the three threatened marine ecosystems: coral reefs, seagrass beds and mangroves. Coral reefs at the entrance of a bay serve as a protection against high seas. The seagrass beds form a critical nursery ground for countless species of coral reef fish and invertebrates and a foraging ground for green turtles. In some bays, cushion stars and conch can still be found amongst the seagrasses as well as fields of pulsating Upside-down Jellyfish (Cassiopea spp.) or ‘Cassiopeia’. The mangroves provide a safe haven and roosting place for herons, egrets, stilts, terns, flamingos, frigate birds, endangered ospreys and pelicans. They are also important nesting and breeding grounds for various species of birds and – between the roots of the mangroves – fish and other marine life.
For someone who has never been snorkeling or diving over a tropical coral reef, it is almost impossible to imagine the sheer restless energy, the breathtaking colour and the beauty of a reef. Waves crash, currents drift, tides ebb and flow, drawing a ceaseless rain of food and fresh seawater across the reef. There are fantastically shaped corals from dome-like brain corals to mountainous pagodas, prickly tables and perfect spheres. Flitting and darting amongst the corals are numberless vividly coloured reef fish.
Types of reef
Reefs which form close to land, following the contours of the coastline, are called fringing reefs. Where the reef is separated from land by a lagoon or open water, it is called a barrier reef. Coral reef atolls are almost exclusively a feature of the Pacific, where they form rings of coral up to several kilometers in diameter around open sea or sandy islands.
The coral reefs of the Dutch Caribbean are very diverse. Bonaire and Curaçao have fringing coral reefs which encircle their islands.
Starting in very shallow water, the reefs slope gently seaward to around 30 feet (10m) before dropping down to depths in excess of 200 feet (60m). St Maarten has patch/barrier reefs whilst the reefs on Saba are found on the top of underwater seamounts or pinnacles, which make for spectacular diving. St Eustatius by contrast has very well developed patch reefs.
Despite enormous differences between coral reefs, they all have the same basic needs: warm, clear, sunlit water with an absence of sediment, which could choke and kill them, and a hard substrate on which to grow. They are only found in tropical seas where the year-round water temperature is between 18 – 30’C, or broadly speaking between 30’ north and 30’ south of the equator.
Building a coral reef
Coral reefs are painstakingly constructed by a humble group of invertebrate animals belonging to the cnidarian family. Scleractinian (stoney) corals are closely related to anemones but different from them in that they form huge interlinked colonies consisting of hundreds and thousands of individual coral polyps. Coral polyps are incredibly delicate and very slow growing. The fastest species of branching coral only grows at 10cm a year; other corals take years to grow a few millimeters. Coral colonies can reach immense sizes and are tremendously long-lived. There are colonies alive today which are believed to be over 700 years old.
Stoney corals have developed the remarkable ability to extract calcium carbonate from sea water. This they use to build complex limestone homes which form the solid framework, or matrix, of the reef.
Around this framework there co-exists a complex web of life. Calcareous algae help to build the reef up whilst animals like boring sponges, worms, grazing fish and urchins work to erode it away. In their eagerness to graze on algal (plant) turf, parrotfish recycle huge amounts of reef: up to 75% of their stomach content is ground-up coral reef. Still other creatures find shelter on the reef and for them it is their home. This multitude of life is what we know of as the coral reef.
But what has long fascinated scientists is how corals could build such immense cities of life in the aquatic equivalent of a desert. Oceanic water is remarkably low in food (plankton) and nutrients.
Seawater is rich in dissolved calcium carbonate. Coral polyps are able to take this up and secrete it beneath themselves as microscopic crystals. The polyps of each type of coral build according to their own innate plan. And even though coral heads are incredibly plastic, forming radically different shaped colonies according to the amount of wave action, current and light, their polyps always build identical limestone homes.
The value of reefs
In part, coral reefs are valuable because they protect tropical shorelines from damage by storm waves. A recent World Bank study suggests that each square meter of coral reef in the Caribbean protects $ 47,000 in property. They provide hundreds of thousands of people with income and food from fishing. A coral reef in prime condition can produce over 20 tons of protein per square kilometer a year. And tourism is a major source of income for many tropical island nations. Caribbean reefs alone are worth around $ 140 billion per year in tourism dollars.
At the global level, scientists believe that by taking up calcium carbonate from seawater, corals create carbon sinks which lessen the impact of the green house gasses involved in global warming. Coral reefs are home to such a vast array of animals and plants that only a small fraction of them have so far been described. More and more medical compounds are being isolated from coral reef creatures which can be used to combat HIV, cancer and other diseases whilst sterilized coral skeleton is already being used as a bone substitute for implants.
But most of all, coral reefs give us an unparalleled opportunity to marvel at the wonder of life and the beauty and complexity of the world we live in.
Tropical dry forests are one of the most threatened parts of the earth’s environment. This forest type typically experiences an annual hard dry season. The average rainfall is sufficient enough to promote growth of trees, but these tree and plant species must be able to withstand periods of low precipitation and moisture. During the driest months, these species will drop their leaves much in the same manner that northern deciduous forest species lose their leaves in the fall and winter. Dry forests occur most commonly on low islands or on the lee side of mountainous islands and on coastal areas of low relief.
Mangroves are often considered dimly lit, mosquito-ridden forests. However, these ecosystem, full of mosquitoes as they might be, are very important coastal systems. Scientists believe that mangroves export much needed organic compounds which are in short supply on coral reefs. Also, we know that mangroves offer critical nursery areas for the young of many reef fish and roosting and breeding grounds for many aquatic birds.
Uses of mangroves
Mangroves have been highly managed ecosystems since the dawn of time. Until the advent of synthetic lines, the roots of the red mangrove (Rhizophora sp) were routinely used to strengthen natural fiber lines used in fishing. Traditionally they were used much more intensively than today, for example in boat building and basket weaving as well as to produce charcoal.
In fact, looking beyond the mud and the mosquitoes, mangroves are fascinating ecosystems. They form dense, often impenetrable forests, choked with tangled prop roots and bathed in mud. Mangroves have evolved the amazing ability to thrive under extremely harsh conditions. Not only can they tolerate high levels of salt but they are also able to cope with more or less constantly waterlogged soil. Needless to say, there are not many of them; only 40 species have been recorded worldwide. Red mangroves (Rhizophera sp) survive by breathing through lenticels (pores) in their prop roots whilst black mangroves (Avecennia sp) send up a field of pneumatophores (aerial roots) around the trunk of the tree through which they breath. All species of mangrove extrude salt through their leaves, which is why the leaves glisten in sunlight. Living as they do, at the interface between land and water, mangroves provide a valuable service in protecting land from the buffeting power of the sea.
In addition to resident plants and animals like killifish, tarpon (Megalops atlanticus), snook (Centropomus undecimalis), and mangrove snapper (Lutjanus griseus), mangroves provide a safe haven for nearly every kind of reef fish and many invertebrates at some time during their life. Snorkelling through mangroves is like visiting the reef in miniature. The young of cardinalfish (Apogonidae), snapper (Lutjanidae), grouper (Serranidae), wrasse (Labridae), pufferfish (Tetradontidae), boxfish (Ostraciidae), butterflyfish (Chaetodontidae), damselfish (Pomacentridae), scorpionfish (Scorpaenidae) and grunts (Haemulidae) all can be found milling around mangroves, darting in and out of their rootsy home. Some species such as the commercially important spiny lobster (Panulirus sp) use the mangrove too as spawning grounds. In all, between 60 and 80 different species of fish can routinely be found in mangroves.
Mangrove prop roots form veritable oases of life. The roots themselves are smothered in encrusting life including mangrove oysters and barnacles, mussels and anemones, sponges, tunicates, stinging hydroids and worms.
Crabs are a ubiquitous feature of mangrove ecosystems. Grapsid crabs live underwater, emerging at low water to feast on the mangrove forest floor. They are herbivores, gobbling up detritus, leaves and seedlings. Fiddler crabs also forage on the mud but they are deposit feeders, scooping up sediment and sifting through it for edible particles. The males have a huge, brightly coloured claw which is useless for feeding but vital when defending a territory or attracting a mate.
A saliña is a salt pan, salt lake, or salt marsh. They are located in close proximity of the sea. Saliñas are permanently or temporarily flooded, hypersaline and separated from the sea by a coral rubble barrier. They serve as important sites for birds (including flamingos, sandpipers and herons), crabs, mollusks, etc. where they forage, live and sometimes breed. Saliñas are also an important part of the natural rainwater runoff system on the islands and allow for sedimentation to settle before rainwater enters the coastal sea waters. Dirty runoff water going into the sea directly has a negative impact on coral reefs.
A seagrass bed is an ecosystem which often occur alongside coral reefs. Although they are much less charismatic and frequently overlooked, seagrass beds play a vital role in maintaining the health and diversity of adjacent coral reefs. Seagrasses are not one but 50 species of flowering marine plants which usually form dense stands in the shallow, sandy bottom environments of back reef lagoons and semi-enclosed bays. They are bright sunlit fields of waving green fronds and are the favoured habitat of the globally endangered Green Turtle (Chelonia mydas) and Queen Conch (Lobatus gigas). Other common inhabitants are cushion stars, urchins and sea cucumbers. Seagrasses are quite literally a world apart. Scientists have found over 113 species of algae (marine plant) growing on seagrass leaves, not to mention sponges, hydroids, flatworms and tunicates.
One of the most remarkable things about seagrasses is their ability to spread and grow by sending out underground rhizomes (roots). It is also their greatest weakness as it makes them phenomenally vulnerable to trampling and physical damage. Seagrasses trap sediment and bind soil thereby preserving water quality and protecting coastal environments.
Similar to mangroves systems, seagrass beds provide an ideal hiding place for the young of many species of reef fish and other marine life. The Queen Conch (Lobatus gigas) hide for the first year of their life in the sand surrounding seagrass beds before emerging there to forage and live. Adult parrotfish (Scaridae), surgeonfish (Acanthuridae) and porgies (Sparidae) are also frequent visitors, migrating into seagrass environments to feed.