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South Georgia's inshore ecosystem has at least 103 species of algal flora (seaweeds). These have been recorded in sheltered and rocky shores on the northeast coast. Nine are green algae, 35 brown algae and 47 red algae.
Some of the deeper water brown algae are exceptionally large, e.g. the giant kelp reaches at least 40 m in length. This sub-tidal vegetation provides a home for many species of immature fish and invertebrates.
South Georgia has four endemic algae: the one green alga, two brown algae and one red alga. Twelve of the 103 species have been found only in South Georgia and Tierra del Fuego, which lies about 2,150 km to the west. About 25% of the species also occur in the Northern Hemisphere. About 50% are found in other sub-Antarctic islands at similar or more northerly latitudes, and from mainland South America. The others are confined to Antarctic coastal waters, with a few reaching their northernmost limit at South Georgia, e.g. the brown algae.
Many species of microalgae occur in the seas around South Georgia, including diatoms, dinoflagellates and other unicellular forms. This phytoplakton community is abundant and is the basis of the marine food chain of the Southern Ocean.
Plant life on South Georgia is constrained by the island's isolation and its cool summers. Native flora is closely related to that in the Falkland Islands, Tierra del Fuego and southern Patagonia.
As Captain Cook reported there are no trees or shrubs on "Isle Georgia". The dwarf shrub-dominated maritime heath that is abundant in the Falkland Islands and elsewhere in the southern cold temperate zone is not found on South Georgia. The only shrub-like plant is a woody-stemmed herb, commonly called burnet.
Some plant species on South Georgia are found only in the Southern Hemisphere. Some others are found at both poles, for example the alpine cat's tail. Some are found worldwide such as water blinks and brittle-bladder fern. With the exception of the hybrid, no endemic higher plants are known. A few endemic bryophytes (mosses and liverworts) and lichens exist.
Only 25 indigenous higher plants have been recorded on the island. These include one lycopod (clubmoss), six ferns, five grasses, three (or perhaps 4) rushes, one sedge, and nine forbs (non-grass-like herbs). Only six species dominate in distinct communities: greater burnet, Antarctic hairgrass, tufted fescue grass, greater rush, tussac grass and brown rush.
A large numbers of lower plant species flourish on the island. There are around 125 species of mosses, some 80 species of liverworts and about 150 species of lichens. At least 50 species of macro-fungi (toadstools) and about ten macro-algae have been found. Little is known about the microflora (soil fungi, algae, cyanobacteria or bacteria).
There are no emergent plants in the lakes and ponds. However, some species of moss growing at the margin of such water bodies extend into the water for several metres, forming a floating spongy mat in which occasional higher plants become rooted.
Submerged rock, stones and mud to a depth of 1-2 m often have mosses and liverworts growing on them. Several species of moss also grow attached to rock in streams and waterfalls. Shallow muddy bottoms may be covered by various filamentous green algae and gelatinous colonies of the cyanobacterium. Nutrient rich wet areas around penguin rookeries or elephant seal wallows often have a bright green cover of the alga.