Forsån
Encyclopedia
Forsån is a stream
in southern Stockholm
, Sweden
. It is also known as Stortorpsån and Forsen.
Leaving lake Magelungen calmly by a small sandy beach, Forsån's upper reach passes through a furrow blown in the 1860s to lower the level of the lake resulting in the present wide torrents passing through the bedrock. In contrast, the last 500 metres of the course are characterized by flat shores and stagnant, overgrown waters. The stream has a vigorous population of Signal crayfish
and is the only winter habitat in Stockholm for White-throated Dipper
.
is located in the area which is also crosses by two major traffic routes and one railway.
and 30 tons of nitrogen
to Forsån, the contribution from the catchment area itself, 10 kg and 40 kg respectively, insignificant in comparison. No estimates of levels of metals brought to the stream has been done, but low or moderate levels of common metals were reported in river sediments in 1997, except for very high levels of lead
next to the road Magelungsvägen.
such as Hippeutis complanatus
and bivalve Swan mussel
; and during the preceding decades two species of limnetic
gastropods were documented. Larvae of White-legged Damselfly
and Blue-tailed Damselfly
were recorded in summer 2000. Signal crayfish
and Trout
have been introduced in the stream, of which the former succeeded in establishing while Roach
is believed to be unfavourable to the latter. The white water in makes a section of the stream ice-free during winters which attracts White-throated Dipper
and Common Kingfisher. In 1996, traces of Beaver
were reported.
Stream
A stream is a body of water with a current, confined within a bed and stream banks. Depending on its locale or certain characteristics, a stream may be referred to as a branch, brook, beck, burn, creek, "crick", gill , kill, lick, rill, river, syke, bayou, rivulet, streamage, wash, run or...
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....
. It is also known as Stortorpsån and Forsen.
Leaving lake Magelungen calmly by a small sandy beach, Forsån's upper reach passes through a furrow blown in the 1860s to lower the level of the lake resulting in the present wide torrents passing through the bedrock. In contrast, the last 500 metres of the course are characterized by flat shores and stagnant, overgrown waters. The stream has a vigorous population of 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...
and is the only winter habitat in Stockholm for White-throated Dipper
White-throated Dipper
The White-throated Dipper , also known as the European Dipper or just Dipper is an aquatic passerine bird found in Europe, Middle East, Central Asia and the Indian Subcontinent. The species is divided into several subspecies, based primarily on colour differences, particularly of the pectoral band...
.
Catchment area
Approximately half of the catchment area is occupied by contaminated ground and polluting operations in the Larsboda industrial area; with the remaining area composed of parks and green spaces. One minor allotment-gardenAllotment (gardening)
An allotment garden, often called simply an allotment, is a plot of land made available for individual, non-professional gardening. Such plots are formed by subdividing a piece of land into a few or up to several hundreds of land parcels that are assigned to individuals or families...
is located in the area which is also crosses by two major traffic routes and one railway.
Environmental influence
The inflow from Magelungen brings approximately 2 tons of phosphorusPhosphorus
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...
and 30 tons 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...
to Forsån, the contribution from the catchment area itself, 10 kg and 40 kg respectively, insignificant in comparison. No estimates of levels of metals brought to the stream has been done, but low or moderate levels of common metals were reported in river sediments in 1997, except for very high 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...
next to the road Magelungsvägen.
Fauna
Samples of bottom fauna taken in 1998 documented the presence of nationally rare species of gastropodsGastropoda
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...
such as Hippeutis complanatus
Hippeutis complanatus
Hippeutis complanatus, or the flat ram's-horn snail, is a species of minute air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk or micromollusk in the family Planorbidae, the ram's horn snails....
and bivalve Swan mussel
Swan mussel
The swan mussel, Anodonta cygnea, is a large species of freshwater mussel, an aquatic bivalve mollusc in the family Unionidae, the river mussels....
; and during the preceding decades two species of limnetic
Limnetic zone
The limnetic zone is the well-lit, open surface waters in a lake, away from the shore. The vegetation of the littoral zone surrounds this expanse of open water and it is above the profundal zone....
gastropods were documented. Larvae of White-legged Damselfly
White-legged Damselfly
The White-legged Damselfly or Blue Featherleg, Platycnemis pennipes, is a damselfly of slow-flowing, muddy waters. It occurs from the Atlantic to Siberia and is often abundant throughout its range.-Morphology:P. pennipes is about long...
and Blue-tailed Damselfly
Blue-tailed Damselfly
The Blue-tailed Damselfly, Ischnura elegans, is a European damselfly.Adult male Blue-tailed Damselflies have a head and thorax patterned with blue and black. They have a largely black abdomen with very narrow pale markings where each segment joins the next...
were recorded in summer 2000. 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...
and Trout
Trout
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...
have been introduced in the stream, of which the former succeeded in establishing while 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 be unfavourable to the latter. The white water in makes a section of the stream ice-free during winters which attracts White-throated Dipper
White-throated Dipper
The White-throated Dipper , also known as the European Dipper or just Dipper is an aquatic passerine bird found in Europe, Middle East, Central Asia and the Indian Subcontinent. The species is divided into several subspecies, based primarily on colour differences, particularly of the pectoral band...
and Common Kingfisher. In 1996, traces of Beaver
Beaver
The beaver is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, North American Beaver and Eurasian Beaver . Beavers are known for building dams, canals, and lodges . They are the second-largest rodent in the world...
were reported.