The marine ecosystem formed by giant brown algae that provides habitat for sea otters is a kelp forest.
Kelp forests grow in cool, nutrient-rich coastal waters where large brown algae anchor to rocky seafloors. Giant kelp, including Macrocystis species, can form a canopy near the surface while creating shaded layers below, much like the vertical structure of a terrestrial forest. The ecosystem supports fish, snails, sea urchins, abalone, seals, sea lions, birds, and many smaller organisms.
Sea otters are especially important in North Pacific kelp forests because they eat sea urchins. By limiting urchin numbers, otters reduce grazing pressure on kelp and help preserve the forest's physical structure. This predator-driven relationship is a classic example of a trophic cascade, although other ecosystems may rely on different predators.
Kelp forests are not coral reefs, mangrove forests, or seagrass meadows. Coral reefs are built by animals, mangroves are rooted trees in coastal wetlands, and seagrass meadows consist of flowering plants. Kelp is algae, making kelp forest the precise answer.