The biomes often merge with no clearly defined border, and it is sometime difficult to say precisely where one biome ends and another begins. Boundary areas, however, do have distinctive plant communities, and animals from neighbouring biomes can wander in and out of them.
Various kinds of freshwater bodies such as lakes, rivers, streams, marshes, and marine habitats (oceans) provide good conditions for the growth and existence of aquatic biota, but it is difficult to classify them into biomes.
Generally, however, aquatic biomes can be exemplified by:
(1) ocean (temperatures range from cold to tropical, ecology varies with climate and depth; mainly algae);
(2) shore (subject to a wide variety of environmental changes; algae; grasses);
(3) swamp, bog, marsh (subject to tidal fluctuations if marine; algae, cattails sedges, mosses, mangroves).
The horizontal sequential distribution of the major biomes from the pole to the equator corresponds, remarkably with the sequence of similar biomes from the higher peaks of certain mountains to the sea level.
Although biomes include plants (producers), animals (consumers), and microbes (decomposers), they are commonly named after their vegetational characteristics or dominant groups of plants because these tend to reflect the general climatic features of the area more accurately than animals or decomposers.
Since the consumers depend on the producers for their food and energy, the faunal component of a biome is greatly influenced by the vegetation. However, in some biomes, e.g., grassland, the consumers can determine many of the major features of the biome by their burrowing, grazing, and trampling activities.
The tropical forest biome has the greatest abundance and density of species. About 10 million species are estimated to exist on earth and of these about 5 million occur in this biome alone (see Myers, 1978). This biome is also the least explored one and is now being increasingly subjected to various stresses of disruption and destruction. This could lead to extinction of thousands of species.
Organisms that live in water have to face certain osmotic problems and have developed osmoregulatory devices or organs. Even though the basic principles of ecosystem relations are the same for terrestrial and aquatic habitats, the communities of living things in a stream or lake are decidedly different from those on land. As on land, in aquatic communities too it is the plants that form the basic component of the ecological link.
However, the more uniform aquatic habitats, especially oceans, have somewhat less varied distributions of various kinds of plant communities. Highly distinctive kinds of ecosystems are characteristic of salt lakes, marshes, and subterranean environments.
Various algae are the predominant plants of aquatic ecosystems. Freshwater ecosystems of lakes, ponds, streams and rivers mostly support green algae, blue-green algae, and diatoms. Seaweeds (marine members of green, brown, and red algae) form the major element of sublittoral and littoral coastal vegetation. Freshwater communities of shallow waters include many aquatic angiosperms (e.g., water lilies, duckweeds, pondweeds, etc.) which may be rooted, floating, or semi-aquatic.
The animal communities of freshwater habitats include representatives from almost all phyla and classes of animals (in contrast to life in the open air, to which members of only three phyla are completely adapted). Common freshwater animals include protozoans, flatworms, rotifers, arthropods, snails and fishes.
The oceans constitute the most extensive and the largest abodes of life. Marine biota include species from all the animal phyla and from all except two (Bryophyta and Tracheophyta) of the plant phyla. However, in many marine communities, insects are generally absent (though secondarily marine insects may occur) but various crustaceans such as copepods, ostracods, amphipods, and small shrimps, prawns are common. Unlike freshwater, the Ocean contain many sessile organisms such as sponges, corals and seaanemones. Marine plankton mostly includes diatoms and dinoflagellates, Foraminifera and Radiolaria.
Many species of shellfish (pelecypods), burrowing worms, starfish, snail, and crab are important elements of coastal marine biome. In the oceans, the nektonic animals (swimming forms) are mostly carnivorous, e.g., certain fishes, Whales, squids, and these occur in largest concentrations in the vicinity of their energy sources: over continental shelves and along coastlines.