Trekanten is a small 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-central 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....

. South of it is the lake Långsjön

Långsjön

Långsjön is a lake in southern Stockholm, Sweden.The lake is situated in an old residential neighbourhood located between the municipalities of Stockholm and Huddinge and most of the shoreline is private property. Water level is controlled by a sluice in the north-western end of the lake where...

With a small drainage area with no major feeders and originally lacking natural beauty, Trekanten is today located in a park furnished with an artificial bathing beach and various planted-out fish species. As such it is considered as of great recreational value and is popular for bathing and fishing. In the early 1980s, potable water was begun to be poured into the lake while water from the bottom of the lake was pumped out, operations resulting in a significant reduction in levels of 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...

 but which failed to affect levels of nutrients and oxygen depletion in bottom layers. Levels of lead

Lead

Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...

 and copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 are among the highest documented in any lake in Stockholm. Whilst the lake flora and fauna is roughly representative for Stockholm, the lake forms an important natural link between the bays Årstaviken and Vinterviken.

Catchment area

Along the shores of the lake is an attractive park created on a flat terrain. Leading up to Nybohov on the southern side of the lake is a fault bluff with pines and 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...

 trees, including hazel

Hazel

The hazels are a genus of deciduous trees and large shrubs native to the temperate northern hemisphere. The genus is usually placed in the birch family Betulaceae, though some botanists split the hazels into a separate family Corylaceae.They have simple, rounded leaves with double-serrate margins...

. On the eastern shore is a bathe next to the Liljeholmen

Liljeholmen

Liljeholmen is a district in South Stockholm, organized into the borough with the same name.In 1860 Liljeholmen became the first suburb outside Stockholm city limit...

 metro station, a bus terminal, the light rail Tvärbanan

Tvärbanan

Tvärbanan is a light rail line in the suburbs of Stockholm, Sweden. Its name literally translated into English is Crossways line. It links together many bus and rail lines crossways through its connections with the southern and western subway branches of the Stockholm Metro and the Stockholm...

, newly added flats, and some older buildings. Several residential areas surround the lake, including Gröndal

Gröndal

Gröndal is also the official Swedish name for the Viherlaakso district in the Finnish city of Espoo.Gröndal is a district in South Stockholm, organized into Liljeholmen borough....

.

About 30 per cent of the catchment area is settled land and two major traffic routes passes through the area.

Environmental influence

In an old industrial area north of the lake is a timber trade, the only remaining polluting operation in the catchment area. About 60 kg of 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...

 is brought to the lake annually, of which more than half is released from lake sediments and the rest is derived 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...

 (also contributing some 260 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...

). Historically, the lake was flanked by small scale industries such as tanneries

Tanning

Tanning is the making of leather from the skins of animals which does not easily decompose. Traditionally, tanning used tannin, an acidic chemical compound from which the tanning process draws its name . Coloring may occur during tanning...

, dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

, and creosote

Creosote

Creosote is the portion of chemical products obtained by the distillation of a tar that remains heavier than water, notably useful for its anti-septic and preservative properties...

 works and until the early 1960s sewage and overflow water was guided into the lake. During the past 20 years, however, airing of bottom layers coupled with addition of potable water have considerably improved the state of the lake. While the affected terrain has been restored since the operations were discontinued, in 1997 low levels of arsenic

Arsenic

Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...

 and increased levels of DDT

DDT

DDT is one of the most well-known synthetic insecticides. It is a chemical with a long, unique, and controversial history....

 were documented.

Though stormwater

Stormwater

Stormwater is water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt that enters the stormwater system...

 from parts of the traffic route Essingeleden

Essingeleden

Essingeleden is a motorway that goes from Solna to Stockholm, Sweden, crossing the westmost parts of central Stockholm, by going over Kungsholmen, Lilla Essingen, and Stora Essingen....

 has been redirected elsewhere, water from a 300 metres long viaduct used by some 1200,000 cars daily is still brought into the lake via a water treatment plant lessening oil levels with some 10 per cent and heavy metals

Heavy metals

A heavy metal is a member of a loosely-defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic weight,...

 with 9-14 per cent. Water from 200 metres of the Liljeholmsvägen traffic route (45,000 vehicles/day) is still led untreated into the lake. Thus, while surrounding blocks of flats are believed to contribute with most of the phosphorus and nitrogen, an estimated fourth of the zinc

Zinc

Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

 brought to the lake is believed to come from car tyres and about 18 per cent from settlements, while copper roofs, formerly abundant around the lake but mostly replaced by asphalt today, are thought to cause most of the copper.

Flora and fauna

In late summer, 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...

 stock is 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...

 paired equal levels of 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, cyanobacteria, and a species of eutroph

Eutrophication

Eutrophication or more precisely hypertrophication, is the movement of a body of water′s trophic status in the direction of increasing plant biomass, by the addition of artificial or natural substances, such as nitrates and phosphates, through fertilizers or sewage, to an aquatic system...

 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...

 (Ceratium hirundinella). Several of the blue green algae present in the lake are potentially poisonous and, notwithstanding non-alarming levels, therefore carefully monitored near the bathe. Lacking soft and shallow bottom areas near the shore, the lake contains only commonplace vegetation, save for the population of Crack Willow

Crack Willow

Salix fragilis is a species of willow native to Europe and western Asia, usually growing beside rivers.It is a medium-sized to large deciduous tree, which grows rapidly to 10–20 m tall, with a trunk up to 1 m diameter and an irregular, often leaning crown. The bark is grey-brown, coarsely...

 and the hybrid between White Willow

White Willow

Salix alba is a species of willow native to Europe and western and central Asia. The name derives from the white tone to the undersides of the leaves....

 and Bay Willow

Bay Willow

Salix pentandra is a species of willow native to northern Europe and northern Asia.It is a large shrub or small tree growing to 14 m tall , usually growing in wet, boggy ground. The leaves are glossy dark green, 5-12 cm long and 2-5 cm broad, with a finely serrated margin...

 found on the southern shore. 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 are dominated by White Waterlily

Nymphaea alba

Nymphaea alba, also known as the European White Waterlily, White Lotus, or Nenuphar, is an aquatic flowering plant of the family Nymphaeaceae....

, Amphibious Bistort

Persicaria

Persicaria is a genus of plants in the family Polygonaceae, collectively known as smartweeds or pinkweeds. The genus was formerly included in the genus Polygonum.The genus includes both annuals and perennials...

, and occasional Curled Pondweed

Potamogeton

Potamogeton is a genus of aquatic, mostly freshwater, plants of the family Potamogetonaceae. Most are known by the common name pondweed, although many unrelated plants may be called pondweed, such as Canadian pondweed...

 (uncommon to Stockholm).

An inventory of lake bed fauna in 1997 produced a list of only 29 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...

 including most common species, mostly 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...

, fireflies

Firefly

Lampyridae is a family of insects in the beetle order Coleoptera. They are winged beetles, and commonly called fireflies or lightning bugs for their conspicuous crepuscular use of bioluminescence to attract mates or prey. Fireflies produce a "cold light", with no infrared or ultraviolet frequencies...

, 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 but no beetle

Beetle

Coleoptera is an order of insects commonly called beetles. The word "coleoptera" is from the Greek , koleos, "sheath"; and , pteron, "wing", thus "sheathed wing". Coleoptera contains more species than any other order, constituting almost 25% of all known life-forms...

s. A rotenone

Rotenone

Rotenone is an odorless chemical that is used as a broad-spectrum insecticide, piscicide, and pesticide. It occurs naturally in the roots and stems of several plants such as the jicama vine plant...

 treatment in 1986 didn't affect the populations of 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...

 and 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...

 still dominating the fauna. 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...

 was introduced shortly after the treatment, and Northern Pike

Northern Pike

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

 and 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...

 is believed to have found their way into the lake by themselves. Crucian carps are decimated by trawling annually and the population of Roach has increased considerably since 2001. Carp, Rainbow Trout

Rainbow trout

The rainbow trout is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America. The steelhead is a sea run rainbow trout usually returning to freshwater to spawn after 2 to 3 years at sea. In other words, rainbow trout and steelhead trout are the same species....

, and Signal Crayfish have been planted-out.

In early summer the lake offers an abundant number of birds including Thrush Nightingale

Thrush Nightingale

The Thrush Nightingale, Luscinia luscinia , is a small passerine bird that was formerly classed as a member of the thrush family Turdidae, but is now more generally considered to be an Old World flycatcher, Muscicapidae...

, Chaffinch

Chaffinch

The Chaffinch , also called by a wide variety of other names, is a small passerine bird in the finch family Fringillidae.- Description :...

, Garden Warbler

Garden Warbler

The Garden Warbler, Sylvia borin, is a common and widespread typical warbler which breeds throughout northern and temperate Europe into western Asia. This small passerine bird is strongly migratory, and winters in central and southern Africa...

, and Blackcap

Blackcap

The Blackcap is a common and widespread sylviid warbler which breeds throughout temperate Europe, western Asia and northwestern Africa, and winters from northwestern Europe south to tropical Africa...

. All birds common to Stockholm, such as Mallard

Mallard

The Mallard , or Wild Duck , is a dabbling duck which breeds throughout the temperate and subtropical Americas, Europe, Asia, and North Africa, and has been introduced to New Zealand and Australia....

, 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:...

, 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...

, and 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...

, are found by the lake, while Heron

Ardea (genus)

Ardea is a genus of herons. Linnaeus named this genus as the Great Herons, referring to the generally large size of these birds, typically 80–100 cm or more in length....

 regularly visits the lake 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...

 fish here during winters. No 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 have been documented by the lake.