Water Clarity

Have you ever wondered why the water around the Virgin Islands is so clear? You may think that the reason for this is that the ocean here is less polluted than off the coast of the United States or Europe. This may be part of the story, but even in the most remote and unspoiled regions of the north the oceans are not nearly as clear as some of the more developed bays of St. John.

Plankton
The real answer to the question has to do with an entity called plankton which The Concise Columbia Electronic Encyclopedia defines as: "very small to microscopic plants and animals that have little or no power of locomotion and drift or float in surface waters."

If you've ever looked carefully through the water using a dive mask you've probably seen the tiny, odd-shaped particles suspended in the water and drifting about at the mercy of the smallest currents. These particles are plankton that happen to be big enough to be seen by the naked eye.

There are two classes of plankton, plant plankton, called phytoplankton and animal plankton, called zooplankton.

Phytoplankton
Phytoplankton are probably the most important life forms on the planet. To begin with, they provide food for all other life in the ocean. Moreover, because they are part of the plant kingdom, they nourish themselves through the process of photosynthesis, which uses sunlight to turn carbon dioxide and nitrogen into sugars and starches. One of the waste products of this process is oxygen. Phytoplankton, although microscopic in size, are so abundant that they produce the majority of the world's oxygen, without which, life on our planet would not exist as we know it.

Because phytoplankton need sunlight, they must exist close to the surface of the ocean. Zooplankton depend upon phytoplankton for food and form a planktonic layer immediately below the plant plankton.

Water Temperature and Resulting Currents
In colder parts of the world the ocean water is warmer on the bottom of the ocean than on the top, especially in the winter. Nutrients washed down from the land by rivers, as well as waste products of fish and other sea life, tend to settle towards the bottom. Warm water rises, and when the bottom of the sea is warmer than the top, the nutrients are swept towards the surface by rising currents. These nutrients act as fertilizer for the phytoplankton, and also may serve as food for the zooplankton. The presence of these nutrients near the ocean surface creates an abundance of planktonic life. There is so much plankton in these colder waters that the ocean appears murky.

In the tropics the sun constantly warms the ocean surface, which, consequently, is warmer than the bottom. Lacking upward currents, nutrients tend to settle to the bottom of the sea and stay there. Planktonic life is scarce and the tropical waters are clear.

Sediments
Another phenomenon responsible for the clarity of the Virgin Island waters is the absence of major rivers and streams. The relatively small streams, or guts, that drain the mountain valleys of St. John and the rest of the Virgin Islands generally lead to salt ponds or mangrove swamps. This allows silt and sediments carried by the stream to settle in the pond or be filtered by the mangrove roots before entering the sea which minimizes turbidity or water cloudiness.

Seagrass
In addition, offshore coral reefs (that happen to be dependent on clear water for their very survival) protect the shoreline from the full force of ocean swells and thus keep bottom sediments from being too churned up. Undersea grass beds also keep water clear by slowing down bottom currents as well as by stabilizing the sea bottom with their complicated root system.

Environmental Concerns
Human beings can effect the water clarity in negative ways. Runoff from upland development such as dirt roads or excavations for buildings can cause turbidity. Also ineffective waste treatment can result in an undesirable increase in algae causing the water to be cloudy. Turbidity also has the nasty tendency to kill coral and thereby upset the natural balance of the offshore and coastal environment.

rains wash dirt and sediment into the sea causing turbidity

If, however, we act responsibly and keep our ecosystems healthy and in balance with each other, we can continue to enjoy our magnificently clear water for many generations to come.

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