Types of lagoons
The type of lagoon depends on the form and origin of the island and often the stage of coral reef development, since it is often coral reefs growing up to the surface that cut off and protect an area of water, making a lagoon. Even a fringing reef may have a depression between the reef crest and the shore, forming a narrow shallow lagoon. A barrier reef can be up to several kilometres offshore, creating a large lagoon which may be tens of metres deep and contain islands and patch reefs. Most atolls are more or less circular in form, with the atoll reef enclosing what can be a very large lagoon. If the reef is not complete, the lagoon may be very open on one or more sides, or it may be joined to the sea by channels or passes through the reef. If the reef is well developed, or the atoll has risen slightly, the lagoon may lose all surface connection with the sea.
Sometimes a lagoon may be made by the shape of an island or a group of nearby islands creating protected areas of water. Shallow coastal lagoons may be created when a low-lying area of water is more or less cut off from the sea by mounds of sand or rubble piled up by storms or waves.
A related type of environment is the estuary, the enlarged area at the mouth of a river where the fresh-water from the river mixes with the salt-water from the sea.
Because lagoon waters are protected they are more accessible to man. They may be very productive, and thus are important fishing areas. At the same time, towns and villages are often built along lagoon shores, and they may be developed as ports. These activities can create important conflicts.
The lagoon environment
Lagoon environments differ in a number of ways from other coastal waters. Where the exchange of water with the ocean is limited, the ocean can no longer exert its stabilizing influence on the temperature. Lagoon temperatures tend to be more variable and more extreme than in the adjacent ocean. The shallow water mass of the lagoon is more easily heated by the sun during the day, and cooled by radiation and evaporation at night. After a long day in the full sun, lagoon temperatures can rise quite high. If the lagoon is deep enough, and the water mixing caused by the wind does not reach to the bottom, then a thermocline may develop. This is when the warm surface water lies on top of the deep cooler water, and the sharp difference in temperature between the two keeps an exchange of water from taking place. If you dive through a thermocline, you can feel the water suddenly get colder. The high and variable temperatures in a lagoon may limit the kinds of plants and animals that can live there.
The salinity, or saltiness, of the open ocean changes very little. However, in a lagoon, the salinity can also be more variable. Heavy rainfall may dilute the water of the lagoon, making it less salty. Water draining off the land,or coming down rivers and streams may also dilute the lagoon. The less the lagoon is connected to the ocean, the more extreme the effect will be. Sometimes the fresh-water will even float in a layer on top of the salt water, and you can see a "blurred" zone where the two mix. If a closed lagoon receives a lot of sun and very little rain, than the opposite may take place. The evaporation of the lagoon water can increase the salinity well above that of the surrounding ocean. Again, the kinds of marine life that can live with wide changes in salinity are limited.
Water movement can also be more limited and more variable in lagoons. Waves may enter the lagoon through passes or channels, or they may occasionally wash over the reef. As the tide rises and falls, there may be flows of water into and out of the lagoon, with very strong currents in the passes. Large lagoons may have currents driven by the prevailing winds. However, in general, lagoons provide a shallow environment protected from the extreme water movement of wave-swept shores.
Lagoons that are open to the sea may have many of the same corals, fish and other sea life as the ocean itself. As the conditions in a lagoon get more variable and more extreme, the number of kinds of things that can live there will get smaller and smaller, although those things that can live there may get to be more common.
The life of lagoons may be more productive and abundant than in the sea outside if there are nutrients that go into it from the land, or if it is able to accumulate nutrients because it is protected. The shallow lagoon bottom is often covered with beds of seagrass or seaweed, and reef corals may grow on the lagoon edges and on patch reefs within it. It may provide important breeding or feeding areas for fish or other animals outside the lagoon as well.
Some lagoons support fisheries that are very important for the island. They may have bait fish that can be used to fish for tuna. Certain lagoons have important beds of pearl oyster. Others may have shellfish that are collected locally for food.
If a lagoon becomes too closed, however, there may not be enough water exchange and the lagoon may become stagnant and support much less life. There may even be no oxygen left on the bottom for animals to live.
Because the lagoon environment is often less stable, there may be big changes in the kinds of plants and animals that grow there. Something may multiply until it seems to be everywhere, then it may die off, to be replaced by something else. These changes may follow the seasons, or they may happen without any apparent reason.
Vulnerability to changes
Lagoons, like the islands of which they are a part, may go through important changes over a long period of time. The coral reef may grow up and cut them off more and more from the ocean. They may gradually fill up with sediment until they become part of the land. If the island is sinking, a lagoon may gradually lose its protective barriers and become part of the ocean shoreline; if the island is rising, the lagoon may be lifted up until its bottom is dry land. Sometimes a natural change may seem sudden, as when a growing reef finally stops the flow of ocean water into a lagoon and its water quality and populations change very quickly, but usually these changes are so slow that we hardly notice them.
When man decides to change a lagoon, he can do it much faster. Often the need for a port or for better boat access means dredging part of the lagoon bottom, clearing coral heads and patch reefs, constructing wharves and other facilities along the shore, and making or enlarging channels or passes. A dredged bottom will never be as productive as the natural one was, and the sediment that is stirred up may smother areas that are not directly affected. The explosives necessary to remove coral heads or enlarge channels also kill many fish. Making a bigger channel may change the patterns of water circulation in the lagoon. Building a causeway between islets may cut off flows of water important to maintaining the quality of the lagoon, and can even block the migration routes by which fish go in or out to feed. All these changes will have their effects on lagoon life and productivity.
Wastes and other runoff from the land may also collect in a lagoon and pollute it. Some islands have had cholera epidemics spread by the pollution of seafood caught in the lagoon. Lagoons are particularly vulnerable to the development of towns or cities around their edge.
Since a lagoon is usually a single system, the whole lagoon is usually affected by damaging activity. It is not usually possible to reserve or protect just one part of it.
Lagoons generally have to be managed as a whole for multiple uses. Each activity has to be developed carefully to be sure it fits with all the other uses.
The way the lagoon system works must be studied in detail before deciding
to make major changes. Since each lagoon is different, it is hard to judge
how one lagoon will behave by comparing it with some place else. Since
most lagoons have many variables and our knowledge of them is limited,
it is hard to predict what effect a change may have. If lagoon productivity
is important for local people, any new inputs or changes must be carefully
controlled. Only in this way can lagoons continue to be useful resources
for island people.
Is there a lagoon on your island? How was it made?
What is the lagoon water like in comparison with the ocean? Warmer? Saltier? Not so clear?
How does water go into and out of the lagoon?
Do you know how the water moves inside the lagoon?
What kinds of plants and animals are common in the lagoon?
Are they always the same, or do they change?
Do you know of fish or animals outside the lagoon that depend on it in some way?
What important things do you take from the lagoon?
Has the lagoon always been the way you know it now?
Can you think of any ways it has changed?
Has the lagoon been affected by man-made changes?
What are they and what have they done to the lagoon?
What needs to be done to manage the lagoon wisely?