Which shallow coastal habitat is dominated by flowering plants that grow underwater?

The shallow coastal habitat dominated by underwater flowering plants is a seagrass meadow. Seagrasses are true marine plants, not seaweeds: they have roots, stems, leaves, flowers and seeds, and they grow rooted in sand or mud beneath clear, sunlit water.

Most meadows occur in sheltered bays, lagoons, estuaries and shallow continental-shelf waters. Their leaves slow currents and waves, helping trap sediment and protect shorelines. The plants also absorb nutrients and carbon, while their dense growth provides food and shelter for fish, crabs, shrimp, seahorses and young marine animals. Dugongs, manatees and green turtles are among the best-known grazers.

Seagrass meadows are often confused with kelp forests because both are underwater habitats, but kelp is a large brown alga rather than a flowering plant. Coral reefs are built by animals, while salt marshes are mostly rooted plants growing in the intertidal zone above or near the waterline. Seagrasses evolved from land plants that returned to the ocean roughly 75–100 million years ago.