Benthic algae live attached to the bottom of a water body or to living or nonliving objects on the bottom. Planktonic algae live free-floating in the ocean and in the largest to smallest lakes and streams. Algae also occur in such varied places as the surface layers of soils and porous rocks, on the bark and leaves of trees, in snow, hot springs, and in symbiotic association with fungi to form lichens. In fresh water, blooms tied to nutrient enrichment are often a major nuisance. Knowledge of the algae of the United States is not uniform across various groups or environments.
Local floras are available for many places.
Few species are shared between the Atlantic and Pacific coasts. Information about marine microalgae is less accessible. Because no attempt has been made to produce a national flora of freshwater algae in this century, it is not possible to estimate the number of such species. Many groups of algae are cosmopolitan, however, and European monographs and floras can be useful. In general, distribution, status, and trends of algae, even of conspicuous marine algae, are not well established.