Marine Biology

ii. Tropical seas

Tropical seas

In the tropical seas, the upper waters are well lighted throughout the year because the sun does not show marked changes in height above the horizon. Light conditions are, therefore, optimal for phytoplankton production. At the same time, the continual input of energy from the sun maintains the surface layers of water at temperatures much higher than those in deeper waters. This means there is a great difference in density between surface and deep waters; hence, mixing does not occur. This thermal stratification extends throughout the year. In the tropical seas, the light conditions are optimal for high productivity. Because the sun’s energy creates a thermal stratification in the water column that prevents mixing and the upward transport of nutrients, however, the productivity is low but constant all year. Tropical seas are very clear and have the deepest compensation depths, but they are that way because there are few phytoplankton in the water column due to the low nutrient content.