a.) Choosing organisms from four different trophic levels of this network as examples, explain how energy is obtained at each trophic level. Since energy is a necessary part of how an organism survives, how it obtains its energy is crucial. When you look at an ecosystem and all the organisms that live within it, you can link many species together through their dependence on each other. Scientists will examine these connections to see how they depend on each other, or in other words, where their main source of energy and nutrition is located. Once this step is determined, scientists will assign that group of species to a trophic level; to primary producers, primary consumers, secondary consumers or tertiary consumers. The bottom of the chain and the trophic level on which all others depend are the primary producers. These primary producers are made up of autotrophic organisms, which are able to obtain food and energy sources without consuming organisms or substances taken from other organisms. In Alaska's Arctic lake, one of its main producers is aquatic plants and algae. These aquatic species and algae contain chlorophyll, which means they can use light energy from the sun to synthesize glucose and other organic compounds, which they can use for cellular respiration and building material for growth. In other words, called photosynthesis. Photosynthesis requires light energy, but some autotrophs use chemosynthesis, meaning they can convert nutrients into organic compounds without the presence of light. While the trophic level of primary producers is that of autotrophs, the subsequent remaining levels represent all heterotrophs. Heterotrophs can only obtain their energy by consuming other organisms. In the tropical primary consumer level, these herbivores depend on these primary producers and other plants for their food. An example of a primary consumer is the larvae of chironomids, which is a type of aquatic insect. The next trophic level is made up of secondary consumers. These secondary consumers are also heterotrophs, and these organisms are carnivores that obtain their energy by consuming other herbivores. An organism that belongs to this category is the sculpin, a small fish, which uses organisms such as chironomid larvae to obtain the energy necessary for survival. The fourth and last remaining trophic level is that of tertiary consumers..