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Primary Producers
As in the oceans, the base of the lake food web consists primarily of phytoplankton rather than plants. Primary producers convert energy from the sun into a form all the other organisms use. They also provide the oxygen for the ecosystem. Phytoplankton, commonly known as algae, are free-floating microorganisms. Lakes also contain vegetation including floating plants such as duckweed, submerged plants and emergent -- semi-aquatic -- plants such as reeds around the edges.
Herbivores
Hundred of creatures of all sizes eat the algae and plants in a lake, and these are the next stage of the food web. They include herbivorous zooplankton, mollusks such as snails, fish, and mammals such as beavers. Frog and toad tadpoles often spend at least part of their life as algae eaters. Some water birds, including ducks, supplement their diet with aquatic vegetation.
Carnivores, Scavengers and Omnivores
The animals that eat the herbivores also vary in size. On the smallest scale are carnivorous zooplankton such as daphnia -- water fleas -- that feed on other zooplankton. There are also a host of larger invertebrates including dragonfly larvae. Most lake fish are carnivores or omnivores, eating invertebrates, amphibians or smaller fish. Mammals such as otters live mostly in and around water bodies, and opportunistic land mammals such as bears and raccoons take lake prey when they can catch it. Most of the water birds, such as herons, that visit lakes eat invertebrates or fish. Water reptiles such as water moccasins are carnivores.
Decomposers
Lakes also contain organisms that break down dead organic material. Most of these organisms are bacteria, some are fungi and some are protoctists. Protoctista is the kingdom that includes phytoplankton and many members of the zooplankton. The role of decomposers in the ecosystem might be invisible, but it is crucial. They make nutrients such as nitrogen available to the primary producers and release carbon dioxide, also essential for the primary producers.