All food chains begin with photosynthetic green plants or producers . Animals feeding on green plants are called primary consumers . At the next level of the food chain are secondary consumers which feed on the primary consumers. There may be a third, fourth, or fifth level of consumers in a food chain. These nutrients are absorbed by green plants and recycled through another food chain.
Most organisms feed on a wide variety of plants and animals.
Thus, food chains become immensely complicated and interweave to form a food web . All living organisms in a food web are interconnected and interdependent. If human activities adversely affect one member of the aquatic ecosystem, the effect often ripples through the food web to other organisms. Bio-accumulation Toxic chemicals often accumulate in animals at the top of the food web. First, contaminants in the water are taken up by small plants and animals.