Ältasjön is a lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 in southern Stockholm

Stockholm

Stockholm is the capital and the largest city of Sweden and constitutes the most populated urban area in Scandinavia. Stockholm is the most populous city in Sweden, with a population of 851,155 in the municipality , 1.37 million in the urban area , and around 2.1 million in the metropolitan area...

, Sweden

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

, located north-east of Lake Flaten

Flaten

Flaten is a lake in southern Stockholm, Sweden, located just north of Lake Drevviken. The name is also used for the surrounding area and the nature reserve created there in 2007....

 and named for the vicinity to the urban district Älta

Älta

Älta is a locality situated in Nacka Municipality, Stockholm County, Sweden with 9,274 inhabitants in 2005....

.

Ältasjön, of which the easternmost fourth belongs to the Stockholm Municipality

Stockholm Municipality

Stockholm Municipality or the City of Stockholm is a municipality in Stockholm County in east central Sweden. It is the largest of the 290 municipalities of the country in terms of population, but one of the smaller in terms of area, making it the most densely populated...

 and the rest to Nacka Municipality

Nacka Municipality

Nacka Municipality is a municipality in Stockholm County in east central Sweden. Its seat is located at Nacka. The municipality is situated just east of the capital Stockholm and the western parts are considered a suburban part of the Stockholm urban area.The present municipality was created in...

, is the uppermost lake in the Sicklaån water system which also include Ulvsjön

Ulvsjön

Ulvsjön is a lake of Södermanland, Sweden....

, Söderbysjön

Söderbysjön

Söderbysjön is a lake of Södermanland, Sweden....

, Dammtorpssjön

Dammtorpssjön

Dammtorpssjön is a lake of Södermanland, Sweden....

, Källtorpssjön

Källtorpssjön

Källtorpssjön is a lake of Södermanland, Sweden....

, Järlasjön

Järlasjön

Järlasjön is a lake of Södermanland, Sweden....

, and Sicklasjön

Sicklasjön

Sicklasjön is a lake in eastern central Stockholm, Sweden. It is bordering the municipalities of Stockholm and Nacka and is named for the vicinity to the urban district Sickla....

. 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

Bathing

Bathing is the washing or cleansing of the body in a fluid, usually water or an aqueous solution. It may be practised for personal hygiene, religious ritual or therapeutic purposes or as a recreational activity....

, camping

Camping

Camping is an outdoor recreational activity. The participants leave urban areas, their home region, or civilization and enjoy nature while spending one or several nights outdoors, usually at a campsite. Camping may involve the use of a tent, caravan, motorhome, cabin, a primitive structure, or no...

, bird-watching, and water-skiing and is considered as of great recreational value. High levels of chlorophyll

Chlorophyll

Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...

 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.

Catchment area

Approximately half of the catchment area is occupied by the settlements and roads of Älta, located within Nacka, with the remaining area composed of forests, wetlands, and smaller patches of open terrain. In the northern part, forming the southern part of the open-air area, is a deciduous

Deciduous

Deciduous means "falling off at maturity" or "tending to fall off", and is typically used in reference to trees or shrubs that lose their leaves seasonally, and to the shedding of other plant structures such as petals after flowering or fruit when ripe...

 hardwood

Hardwood

Hardwood is wood from angiosperm trees . It may also be used for those trees themselves: these are usually broad-leaved; in temperate and boreal latitudes they are mostly deciduous, but in tropics and subtropics mostly evergreen.Hardwood contrasts with softwood...

 forest. South of the lake is the traffic route Tyresövägen receiving some 20,000 vehicles per day, and west of the road is a speedway track

Motorcycle speedway

Motorcycle speedway, usually referred to as speedway, is a motorcycle sport involving four and sometimes up to six riders competing over four anti-clockwise laps of an oval circuit. Speedway motorcycles use only one gear and have no brakes and racing takes place on a flat oval track usually...

.

Environmental influence

The lake receives about 200 kg phosphorus

Phosphorus

Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...

 annually of which half comes from surface runoff

Surface runoff

Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...

 and the rest is released from sediments. Surface runoff also adds 1,300 kg of nitrogen

Nitrogen

Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...

, most of which comes from the settlements of Älta. The lake used to be the recipient for poorly treated waste water from some 1,000 households today connected to the municipal sewers.

Flora and fauna

In the early 20th century, phytoplankton

Phytoplankton

Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

 was dominated by green algae

Green algae

The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...

, diatom

Diatom

Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...

s, and carapace

Carapace

A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

 flagellate

Flagellate

Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Flowering plants do not produce flagellate cells, but ferns, mosses, green algae, some gymnosperms and other closely related plants...

s with a smaller amount of cyanobacteria, a normal distribution for lakes rich in nutrients. By 2000, the biomass

Biomass

Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

 was almost exclusively composed of cyanobacteria, most of them non-poisonous "thin filaments" and anabacena the only species being able to fixate nitrogen. Today, the only reminder of the 1990s is the relatively frequent occurrence of the carapace flagellate Ceratium hirundinella. Zooplankton

Zooplankton

Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...

, moderate levels of rotifer

Rotifer

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

s and copepod

Copepod

Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...

s, have shown insignificant variations with time.

An inventory of aquatic plant

Aquatic plant

Aquatic plants are plants that have adapted to living in aquatic environments. They are also referred to as hydrophytes or aquatic macrophytes. These plants require special adaptations for living submerged in water, or at the water's surface. Aquatic plants can only grow in water or in soil that is...

s in 1997 showed the western and southern parts of the lake are dominated by Spiked Water-milfoil

Myriophyllum spicatum

Myriophyllum spicatum is a species of Myriophyllum native to Europe, Asia, and north Africa. It is a submerged aquatic plant, and grows in still or slow-moving water.-Description:...

 and Yellow Water-lily

Nuphar lutea

Nuphar lutea is an aquatic plant of the family Nymphaeaceae, native to temperate regions of Europe, northwest Africa, and western Asia.-Growth:...

, while the eastern part is dominated by Common Club-rush

Schoenoplectus

Schoenoplectus is a genus of about 80 species of sedges with a cosmopolitan distribution. Note that the name bulrush is also applied to species in the unrelated genus Typha...

 and Common Reed. Stratiotes aloides

Stratiotes

Stratiotes is a genus of submerged aquatic plant commonly known as water soldiers. A characteristic of the genus is the habit of the plants rising to the surface at flowering time.-Description:...

 is found near the lake outlet.

The same inventory reported 45 species/taxa

Taxon

|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

 in the lake-bed fauna, which includes all common species, dominated by Trichoptera

Trichoptera

The caddisflies are an order, Trichoptera, of insects with approximately 12,000 described species. Also called sedge-flies or rail-flies, they are small moth-like insects having two pairs of hairy membranous wings...

, Ephemeroptera

Mayfly

Mayflies are insects which belong to the Order Ephemeroptera . They have been placed into an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies...

, Odonata

Odonata

Odonata is an order of insects, encompassing dragonflies and damselflies . The word dragonfly is also sometimes used to refer to all Odonata, but the back-formation odonate is a more correct English name for the group as a whole...

, freshwater Gastropods

Gastropoda

The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca. The class Gastropoda includes snails and slugs of all kinds and all sizes from microscopic to quite large...

, and Leech

Leech

Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways...

es. Perch

Perch

Perch is a common name for fish of the genus Perca, freshwater gamefish belonging to the family Percidae. The perch, of which there are three species in different geographical areas, lend their name to a large order of vertebrates: the Perciformes, from the Greek perke meaning spotted, and the...

, Roach

Rutilus

Rutilus is a genus of fishes in the family Cyprinidae, commonly called roaches. Locally, the name "roach" without any further qualifiers is also used for particular species, particularly the Common Roach Rutilus (Latin for "shining, red, golden, auburn") is a genus of fishes in the family...

, Northern pike

Northern Pike

The northern pike , is a species of carnivorous fish of the genus Esox...

, Tench

Tench

The tench or doctor fish is a freshwater and brackish water fish of the cyprinid family found throughout Eurasia from Western Europe including the British Isles east into Asia as far as the Ob and Yenisei Rivers. It is also found in Lake Baikal...

, Silver Bream, Ruffe

Ruffe

The Eurasian Ruffe or simply Ruffe is a freshwater fish found in temperate regions of Europe and northern Asia. It has been introduced into the Great Lakes of North America, reportedly with unfortunate results...

, and Crucian Carp

Crucian carp

The crucian carp is a member of the family Cyprinidae, which includes many other fish, such as the common carp, or the smaller minnows. They inhabit lakes, ponds, and slow-moving rivers throughout Europe and Asia. The crucian is a medium-sized cyprinid, which rarely exceeds a weight of over 3.3...

 have been documented in the lake, sample catches typically exceeding excepted levels. Zander

Zander

Zander is a species of fish. The scientific name is Sander lucioperca , and it is closely allied to perch. Zander are often called pike-perch as they resemble the pike with their elongated body and head, and the perch with their spiny dorsal fin. Zander are not, as is commonly believed, a pike and...

 was introduced in the late 1990s. Crayfish plague

Crayfish plague

Crayfish plague, Aphanomyces astaci, is a water mould that infects crayfish, most notably the European Astacus which dies within a few weeks of being infected...

 caused a population of European Crayfish

Astacus astacus

Astacus astacus, the European crayfish, noble crayfish or broad-fingered crayfish, is the most common species of crayfish in Europe, and a traditional foodstuff. Like other crayfish, Astacus astacus is restricted to fresh water, living only in unpolluted streams, rivers and lakes...

 to disappear in 1984, subsequently replaced by Signal Crayfish

Signal crayfish

The signal crayfish, Pacifastacus leniusculus, is a North American species of crayfish. It was introduced to Europe in the 1960s to supplement the Scandinavian Astacus astacus fisheries, which were being damaged by crayfish plague, but the imports turned out to be a carrier of that disease...

 introduced throughout the 1990s. IN 1993 the population of amphibian

Amphibian

Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s included Moor Frog

Moor Frog

The Moor Frog is a slim, reddish-brown, semi-aquatic amphibian native to Europe and Asia. It is a member of the family Ranidae, or "true frogs".-Taxonomy:...

 and Common Toad

Common Toad

The common toad or European toad is an amphibian widespread throughout Europe, with the exception of Iceland, Ireland and some Mediterranean islands...

, of which only the later was found by 1996.

The lake is an important stop-over for many bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

 species, including Tufted Duck

Tufted Duck

The Tufted Duck, Aythya fuligula, is a medium-sized diving duck with a population of close to one million birds.- Description :The adult male is all black except for white flanks and a blue-grey bill. It has an obvious head tuft that gives the species its name.The adult female is brown with paler...

, Common Goldeneye

Common Goldeneye

The Common Goldeneye is a medium-sized sea duck of the genus Bucephala, the goldeneyes. Their closest relative is the similar Barrow's Goldeneye....

, and Common Merganser

Common Merganser

The Common Merganser or Goosander Mergus merganser is a large duck, of rivers and lakes of forested areas of Europe, northern and central Asia, and North America. It eats fish and nests in holes in trees...

; and, more rarely, Slavonian Grebe

Slavonian Grebe

The Horned Grebe or Slavonian Grebe, Podiceps auritus, is a member of the grebe family of water birds. The Slavonian Grebe is an excellent swimmer and diver, and pursues its fish prey underwater...

, and Smew

Smew

The Smew is a small duck, which is somewhat intermediate between the typical mergansers and the goldeneyes . It is the only member of the genus Mergellus; sometimes included in Mergus, this genus is distinct and might actually be a bit closer to the goldeneyes...

. Breeding species attracted to the lake include Eurasian Coot

Eurasian Coot

The Eurasian Coot, Fulica atra, also known as Coot, is a member of the rail and crake bird family, the Rallidae. The Australian subspecies is known as the Australian Coot.-Distribution:...

, Great Crested Grebe

Great Crested Grebe

The Great Crested Grebe is a member of the grebe family of water birds.- Description :The Great Crested Grebe is long with a wingspan. It is an excellent swimmer and diver, and pursues its fish prey underwater. The adults are unmistakable in summer with head and neck decorations...

, and Pochard

Pochard

The Common Pochard, Aythya ferina, is a medium-sized diving duck.The adult male has a long dark bill with a grey band, a red head and neck, a black breast, red eyes and a grey back. The adult female has a brown head and body and a narrower grey bill band. The triangular head shape is distinctive...

; additionally Common Tern

Common Tern

The Common Tern is a seabird of the tern family Sternidae. This bird has a circumpolar distribution, breeding in temperate and sub-Arctic regions of Europe, Asia and east and central North America. It is strongly migratory, wintering in coastal tropical and subtropical regions. It is sometimes...

 is often seen by the lake and occasionally breeds here. Other protected species found by the lake include Common Snipe

Common Snipe

The Common Snipe is a small, stocky wader native to the Old World. The breeding habitat is marshes, bogs, tundra and wet meadows throughout northern Europe and northern Asia...

 and Lesser Spotted Woodpecker

Lesser Spotted Woodpecker

The Lesser Spotted Woodpecker is a member of the woodpecker family Picidae. It is assigned to the genus Dendrocopos ....

.