The Trophic State Index (TSI) is a classification system designed to rate water bodies based on the amount of biological productivity they sustain. Although the term "trophic index" is commonly applied to lakes, any surface water body may be indexed.
Lake George, New York, an oligotrophic lake
Kurtkowiec Lake, an oligotrophic lake in the Tatra Mountains of southern Poland
Algal bloom in a village river in the mountains near Chengdu, Sichuan, China
The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it is from the start of the chain. A food web starts at trophic level 1 with primary producers such as plants, can move to herbivores at level 2, carnivores at level 3 or higher, and typically finish with apex predators at level 4 or 5. The path along the chain can form either a one-way flow or a food "web". Ecological communities with higher biodiversity form more complex trophic paths.
First trophic level. The plants in this image, and the algae and phytoplankton in the lake, are primary producers. They take nutrients from the soil or the water, and manufacture their own food by photosynthesis, using energy from the sun.
Second trophic level Rabbits eat plants at the first trophic level, so they are primary consumers.
Third trophic level Foxes eat rabbits at the second trophic level, so they are secondary consumers.
Fourth trophic level Golden eagles eat foxes at the third trophic level, so they are tertiary consumers.
