|Basic Concepts in Environment, Agriculture and Natural Resources Management: An Information Kit (IIRR, 1993, 151 p.)|
|Freshwater and marine ecosystems|
Shallow water immediately along the shore looks green in contrast to that several meters away. There, the water turns blue. As one goes deeper, light is absorbed due to the suspended particles. It is absorbed in decreasing order such that less visible colors result; hence, the blue color.
The green places or those which are illuminated by sunlight are the richest in marine life. They teem with minute plants that larger things depend on.
For example, phytoplanktons are eaten by small animals -- zooplanktons. These animals, in turn, are eaten by larger creatures, such as the whale. When different living things depend upon one another in this way, it is called a food chain. And so, as marine animals depend directly or indirectly upon phytoplankton, it is in those areas rich in plant life that most marine animals are found.
But another essential for plant growth is fertilizer. Fertilizers for plants in the sea are known as nutrient salts and two of the most important nutrients are nitrates and phosphates. Nutrient salts come from decomposing bodies or dead plants and animals which have drifted down -to the bottom of the oceans.
The sunlight that marine plants need is at the upper stratum of the water. However, most of the available fertilizers are near the bottom. Plants can thrive only when these two essentials come together.
This happens on shallow banks where sunlight reaches down to the bottom of the water. It can also happen when wind and temperatures changes stir up the water and mix it from top to bottom. This occurs when two currents meet or when a current reaches a land mass. Water is drawn from deep down to the surface carrying with it fresh nutrients into the sunlit water. The regions of great upwellings of nutrients, such as those off the Peruvian coasts, are the sites of the great fisheries.