Ältasjön, of which the easternmost fourth belongs to the Stockholm Municipality and the rest to Nacka Municipality, is the uppermost lake in the Sicklaån water system which also include Ulvsjön, Söderbysjön, Dammtorpssjön, Källtorpssjön, Järlasjön, and Sicklasjön. The lake has a rich bird life and, bordering the Nacka Open-air Area (colloquially referred to as Nackareservatet, "Nacka [Nature] Reserve", but not given the status of a reserve), is used for bathing, camping, bird-watching, and water-skiing and is considered as of great recreational value. High levels of chlorophyll reduces clearity, but, notwithstanding the traffic route passing nearby, metal levels are lower than in any other lake in Stockholm.
The "Save Ältasjön Society", Föreningen Rädda Ältasjön (FRÄS), has been working since 1975 to improve the environmental state of the lake and promote open-air activities in and around it.
An inventory of aquatic plants in 1997 showed the western and southern parts of the lake are dominated by Spiked Water-milfoil and Yellow Water-lily, while the eastern part is dominated by Common Club-rush and Common Reed. Stratiotes aloides is found near the lake outlet.
The same inventory reported 45 species/taxa in the lake-bed fauna, which includes all common species, dominated by Trichoptera, Ephemeroptera, Odonata, freshwater Gastropods, and Leeches. Perch, Roach, Northern pike, Tench, Silver Bream, Ruffe, and Crucian Carp have been documented in the lake, sample catches typically exceeding excepted levels. Zander was introduced in the late 1990s. Crayfish plague caused a population of European Crayfish to disappear in 1984, subsequently replaced by Signal Crayfish introduced throughout the 1990s. IN 1993 the population of amphibians included Moor Frog and Common Toad, of which only the later was found by 1996.
The lake is an important stop-over for many bird species, including Tufted Duck, Common Goldeneye, and Common Merganser; and, more rarely, Slavonian Grebe, and Smew. Breeding species attracted to the lake include Eurasian Coot, Great Crested Grebe, and Pochard; additionally Common Tern is often seen by the lake and occasionally breeds here. Other protected species found by the lake include Common Snipe and Lesser Spotted Woodpecker.