The plants that make freshwater lakes and rivers their habitat are often adapted to conditions that are submerged in water but rooted in the mucky bottom or float freely on the surface. As long as temperature, sunlight and nutrients remain available to these aquatic plants, they prosper. Algae are considered plant-like protists, containing green pigments and producing food, just like true plants. Hydroponics, the culture of plants in nutrient-rich water, expands the number of plants that can be grown in media saturated with water.
Tiny plants that produce two-lobed thallus structures (leaf-like appendages) that float upon the still water surface, duckweed (Lemna spp.) produces pale, thread-like roots that dangle downward. Native to temperate regions, once fall frosts occur, the plants sink to the lake bottom to overwinter and float up in spring once temperatures warm.
Considered a noxious weed in tropical and subtropical regions, water hyacinth (Eichhornia crassipe) develops a connected stem-matrix of many plants. Its stems are porous and often swell to form air-filled "bulbs" that facilitate floating on the water surface while many white, lavender and blackish roots dangle below in the water.
Waterweed is a generic name often used on submerged plants with snake-like stems and tiny leaves in the genus Elodea (as well as Hydrilla and Egeria, according to Texas Agrilife Extension). The plants are often rooted in the soil at the bottom of the pond or lake, but fragments of stems readily float and survive as plants. Often, Elodea species are used as freshwater aquarium vegetation.
Sometimes called fairy moss, mosquito fern (Azolla spp.) includes any of eight different species that grow in freshwater bodies not affected by frosty winter temperatures below 23 degrees F. The frilly, sprawling fern-like leaves comprise scales that will shade the water underneath, limiting algae growth. In tropical regions, the mosquito fern (like water hyacinth) can become a noxious weed.