Living things can not function without energy. They must use energy to do the work required for life. While living things can not create energy, they can transform one kind of energy to another kind. In the ocean, the cycle of energy transformation starts with the producers. Producers capture energy from light or chemicals and transform it into organic molecules – food. Organisms that can not produce their own food must get their nutrients from these producers. These organisms are called consumers. Herbivores are organisms that eat plants. Carnivores are organisms that eat other animals and Omnivores are organisms that eat both plants and animals. Scavengers are organisms that eat dead organisms. Together, all of these organisms form a food web. A food web represents the transfer of food and energy when one organism consumes another.
Tiny animals known as zooplankton eat plants and microscopic plants known as phytoplankton. Invertebrates such as mussels and clams eat the zooplankton. Fish and other vertebrates such as cod eat the invertebrates. Larger vertebrates like an Orca eat the fish. This is just one example of a food chain. Many organisms have more than one food source. Therefore, there are innumerable unique food chains present in any given ecosystem. And when these food chains intersect and overlap, they are form food webs. It is important to note that only about 10% of the energy of an organism is actually passed to the next organism in the food chain. Much of the energy an organism has is lost as heat as the organism works to survive. Therefore, organisms at the end of a food chain must eat more in order to get the same amount of energy as organisms lower on the food chain.
However, the food webs don’t end with the largest animal. In nature, when the organism at the end of the food chain dies, organisms known as decomposers break down its body. These decomposers and bacteria break the dead animal down into inorganic nutrients which will eventually be used by the producers at the beginning of the food chain. Everything flows in a cycle! All organisms are dependent upon one another to maintain the energy cycles within an ecosystem.
Matter in the form of nutrients also moves through the organisms in an environment. The elements are cycled through the water cycle, which you have already studied, the carbon, oxygen, phosphorus, nitrogen, and sulfur cycles.
Carbon cycle: occurs as photosynthesizing plants and algae take in carbon, in the form of carbon dioxide (an inorganic compound), and produce carbohydrates (organic compounds); animals take in carbon compounds when they eat plants, then they give off this carbon, again in the form of carbon dioxide, as a waste product of cellular respiration; the carbon once again becomes available to plants for photosynthesis.
Oxygen cycle: releases this crucial element as a by-product of photosynthesis from algae and plants; oxygen is taken up by animals and plants for cellular respiration; the carbon and oxygen cycles together are called the carbon-dioxide–oxygen cycle, which shows how plants and animals are dependent upon one another.
Nitrogen cycle: makes this element available through a natural decay process, for the manufacture of proteins; nitrogen moves from dead matter in the sea back into marine plants through a series of chemical reactions controlled by bacteria; one product of the nitrogen cycle is nitrate, which is produced through the action of bacteria on ammonia; nitrates are then absorbed by plants to synthesize proteins.
Sulfur cycle: recycles this important element for protein molecules; sulfur moves from dead matter in the sea back into marine plants through a series of chemical reactions controlled by bacteria; first, bacterial reaction breaks down wastes into hydrogen sulfide (H2S); next, bacteria convert H2S into sulfur, not yet suitable for absorption by plants; other bacteria convert the sulfur into sulfate (SO4), which is absorbed by algae to make proteins; animals get their sulfur when they eat plants and other animals.
Phosphorus cycle: supplies phosphorus to two of the most important molecules in living things, ATP and DNA; living things need phosphorus for growth and energy; phosphorus moves from dead matter in the sea back into marine plants through a series of chemical reactions controlled by bacteria; this produces phosphate (PO4), which is absorbed by algae and plants to make DNA and ATP; animals get their phosphorus when they eat plants, etc. The droppings, or guano, of many sea birds, is also rich in phosphorus and is harvested and sold commercially as fertilizer.