Plants sit at the center of the food web, and all other life on Earth depends on them because plants have a unique talent: They can convert sunlight into sugar. Without plants, the sun's energy would go to waste, and life on Earth would end. Aquatic ecosystems are no different, and plants form the basis of the food web, consumed by a variety of animals.
No animal better illustrates the importance of plants to the aquatic food web than krill, which are tiny shrimp-like creatures found throughout the world's oceans, from tropical seas to the frigid Antarctic. According to National Geographic, the average krill is only two inches in size, but the minuscule creatures are one of the most hunted organisms in the sea, feeding hundreds of different species, from small fish to the largest animal on Earth, the blue whale. Krill themselves feed on phytoplankton, microscopic plants found in the ocean, and connect the presence of plants to the survival of some of the ocean's mightiest animals.
Manatees have earned the nickname "sea cow," and it fits. These large, sluggish mammals are voracious consumers of aquatic plants and algae, devouring up to 10 percent of their weight each day. Manatees are found along the East Coast of the Americas and in the Amazon River. Although they are generally slow swimmers, surfacing every few minutes for air, manatees can reach speeds up to 15 miles per hour in short bursts. Valued for their skins, bones and oil, manatees have been hunted to near-extinction and are currently endangered species. More are killed or wounded when struck by fast-moving motorboats or slashed by propellers.
The aquatic weevil is a tiny, amphibious insect that was discovered to be surprisingly useful in controlling harmful algae blooms in ponds and lakes. In the 1980s, Lake Bomoseen in Vermont was choked with a type of pondweed called watermilfoil that was spreading so thick that other life couldn't survive and people couldn't use the lake. When similar problems in other lakes suddenly improved, researchers found that the hero was a tiny weevil that swims from plant to plant, feeding on leaves and stems. The female lays her eggs on the plant, providing a ready food source for the larvae when they hatch. In the fall, the weevils swim to shore to spend the winter. Today, the aquatic weevil is introduced into ponds and lakes to clean up watermilfoil invasions.
Some fish are grazers, feeding on pondweed rather than smaller fish and animals. Although the grass carp begins life as an omnivore, by adulthood, the fish are vegetarians and feed on plants growing in freshwater ponds and streams, munching from the top down to the bottom of the blade. Grass carp can reach impressive sizes, with some weighing 40 pounds in the southern United States, and most fish reaching at least 10 pounds. They are also long-lived, and can survive for as long as 10 years. Because grass carp have a taste for pondweed, they are used to control overpopulated weeds that can damage ponds and lakes. Grass carp are picky, however, and if given their choice of fronds to feast on, will ignore the some of the most destructive species.