Blelham Tarn
{{Short description| A lake in Cumbria, England}}
{{Use dmy dates|date=April 2022}}
{{Infobox body of water
| name = Blelham Tarn
| image = Boathouse on Blelham Tarn - geograph.org.uk - 617279.jpg
| alt = A derelict boathouse extending out onto a lake
| caption = The boathouse on the tarn in 2007
| image_bathymetry =
| alt_bathymetry =
| pushpin_map = United Kingdom Lake District#United Kingdom South Lakeland
| pushpin_label_position =
| pushpin_map_caption = Location in the Lake District National Park##Location in South Lakeland, Cumbria
| caption_bathymetry =
| location = Cumbria
| coords = {{Coord|54|23|44|N|2|58|41|W|region:GB_type:waterbody|display=inline,title}}
| type = natural
| inflow =
| rivers =
| outflow = Blelham Beck
| catchment =
| basin_countries =
| agency =
| designation =
| length = {{convert|0.66|km|abbr=on}}
| width = {{convert|0.257|km|abbr=on}}[http://www.f22.org.uk/lakes/Blelham_Tarn/index.htm Blelham Tarn]
| area = {{convert|11|ha|abbr=on}}
| depth = {{convert|6.8|m|abbr=on}}
| max-depth = {{convert|14.5|m|abbr=on}}
| volume = {{convert|717829|m3|acre.ft|sigfig=3}}{{cite web|title=Blelham Tarn Water Body ID 29270|url=https://eip.ceh.ac.uk/apps/lakes/detail.html#wbid=29270|website=UK Lakes Detail|publisher=NERC Centre for Ecology and Hydrology|accessdate=23 May 2016|url-status=dead|archiveurl=https://web.archive.org/web/20160611204850/https://eip.ceh.ac.uk/apps/lakes/detail.html#wbid=29270|archivedate=11 June 2016}}
| shore =
| elevation = {{convert|47|m|abbr=on}}
| frozen =
| website =
| reference =
}}
Blelham Tarn is a large valley tarn in the Lake District of England, to the north of the hill Latterbarrow. The settlements of Outgate, Low Wray and High Wray are close by. The tarn is drained to the northeast by the short Blelham Beck into Windermere. This beck was previously straightened and lowered.[https://archive.today/20141125211920/http://www.sssi.naturalengland.org.uk/Special/sssi/unit_details.cfm?situnt_id=1010219 Blelham Tarn & Bog - Unit 2]
Fish species in the tarn include brown trout, eel, perch, pike and roach,[http://www.thecumbriadirectory.com/Cumbria_Countryside/Tarns/tarn_view.php?tarn=blelham_tarn Blelham Tarn] much of the tarn shore is reedbed and waterfowl present can include great crested grebe, whooper swan and golden-eye.
The tarn is regularly monitored by the United Kingdom Lake Ecological Observatory Network and is characterised as eutrophic and monomictic and has suffered from agricultural water pollution[https://webarchive.nationalarchives.gov.uk/ukgwa/20091003222146mp_/http://www.sssi.naturalengland.org.uk/Special/sssi/unit_details.cfm?situnt_id%3D1000038 Blelham Tarn & Bog - Unit 6] with large quantities of blue-green algae in the summer. The lake temperature at various depths varied over the period July 2012 to November 2014 between 2 and 25 Celsius as the air temperature (3 m above the surface) varied between -3 and 22 Celsius. Over the same period the pH varied from 6.4 to 9.8 and the [[Oxygenation (environmental)|
dissolved oxygen]] ranged from 7 to 14 mg/L.{{Cite web |url=http://data.ecn.ac.uk/ukleon/results.asp |title=Data from the UK Lake Ecological Observatory Network |access-date=25 November 2014 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304132052/http://data.ecn.ac.uk/ukleon/results.asp |url-status=dead }}
Blelham Tarn and Bog, with a total area of 49 hectares, is designated a site of special scientific interest[http://www.sssi.naturalengland.org.uk/Special/sssi/unitlist.cfm?sssi_id=1002887 SSSI units for Blelham Tarn & Bog]{{dead link|date=November 2016 |bot=InternetArchiveBot |fix-attempted=yes }} and Blelham Bog is designated a National Nature Reserve[https://www.gov.uk/government/publications/cumbrias-national-nature-reserves/cumbrias-national-nature-reserves Cumbria's National Nature Reserves ] The bog contains various species of sphagnum moss, bog myrtle, cotton-grass and the white-beaked sedge; and rare caddis-flies and vertigo lilljeborgi.