Editing Fjord (section)

==Features and variations==
[[File:Weichsel-Würm-Glaciation.png|thumb|Distribution of ice (white) in Europe during the [[last glacial period]]]]

===Hydrology===
During the winter season, there is usually little inflow of water that is fresh. Surface water and deeper water (down to {{cvt|100|m|disp=or||}} or more) are mixed during winter because of the steady cooling of the surface and wind. In the deep fjords, there is still fresh water from the summer with less density than the saltier water along the coast. Offshore wind, common in the fjord areas during winter, sets up a current on the surface from the inner to the outer parts. This current on the surface in turn pulls dense salt water from the coast across the fjord threshold and into the deepest parts of the fjord.<ref name= Skreslet>{{Cite book |title=Fjordene og kyststrømmen |last=Skreslet |first=Stig |publisher=Møre og Romsdal naturvern |year=1980 |place=Åndalsnes; Rauma/Ulvåa på vektskåla |pages=48–54 }}</ref> Bolstadfjorden has a threshold of only {{cvt|1.5|m|||}} and strong inflow of freshwater from [[Vosso]] river creates a brackish surface that blocks circulation of the deep fjord. The deeper, salt layers of Bolstadfjorden are deprived of oxygen and the seabed is covered with organic material. The shallow threshold also creates a strong tidal current.<ref name= "Aarseth" />

During the summer season, there is usually a large inflow of river water in the inner areas. This freshwater gets mixed with saltwater creating a layer of brackish water with a slightly higher surface than the ocean which in turn sets up a current from the river mouths towards the ocean. This current is gradually more salty towards the coast and right under the surface current there is a reverse current of saltier water from the coast. In the deeper parts of the fjord the cold water remaining from winter is still and separated from the atmosphere by the brackish top layer. This deep water is ventilated by mixing with the upper layer causing it to warm and freshen over the summer.<ref>{{cite journal | first1= J.H. | last1= Simpson | first2= T.P.| last2= Rippeth| year= 1993| title= The Clyde Sea – a model of seasonal stratification and mixing| journal= Estuarine, Coastal and Shelf Science| number= 37| pages= 129–144| doi= 10.1006/ecss.1993.1047 }}</ref> In fjords with a shallow threshold or low levels of mixing this deep water is not replaced every year and low oxygen concentration makes the deep water unsuitable for fish and animals. In the most extreme cases, there is a constant barrier of freshwater on the surface and the fjord freezes over such that there is no oxygen below the surface. [[Drammensfjorden]] is one example.<ref name=Skreslet/> The mixing in fjords predominantly results from the propagation of an [[internal tide]] from the entrance sill or internal seiching.<ref>Arneborg, L., Janzen, C., Liljebladh, B., Rippeth, T., Simpson, J. H. & Stigebrandt, A. (2004). Spatial variability of diapycnal mixing and turbulent dissipation rates in a stagnant fjord basin. Journal of Physical Oceanography, 34(7), 1679–1691</ref>

The Gaupnefjorden branch of [[Sognefjord]]en is strongly affected by freshwater as a glacial river flows in. [[Velfjorden]] has little inflow of freshwater.<ref name=":0" />

===Coral reefs===
In 2000, some [[coral reef]]s were discovered along the bottoms of the Norwegian fjords.<ref>[http://www.imr.no/coral/ Institute of Marine Research: Coral reefs in Norway] {{webarchive|url=https://web.archive.org/web/20081011222735/http://www.imr.no/coral/ |date=2008-10-11 }}</ref> These reefs were found in fjords from the north of Norway to the south. The marine life on the reefs is believed to be one of the most important reasons why the Norwegian coastline is such a generous fishing ground. Since this discovery is fairly new, little research has been done. The reefs are host to thousands of lifeforms such as [[plankton]], [[coral]], [[Sea anemone|anemones]], fish, several species of shark, and many more. Most are specially adapted to life under the greater pressure of the [[water column]] above it, and the total darkness of the deep sea.<ref name=natgeofjord/>

New Zealand's fjords are also host to [[deep-water coral]]s, but a surface layer of dark fresh water allows these corals to grow in much shallower water than usual. An underwater observatory in [[Milford Sound]] allows tourists to view them without diving.<ref>Paddy Ryan. ''[http://www.TeAra.govt.nz/EarthSeaAndSky/MarineEnvironments/Fiords/2/en Fiords – Underwater rock walls and basins]'', Te Ara – the Encyclopedia of New Zealand. Updated 21 September 2007. Accessed 2008-04-18.</ref>

===Skerries===
In some places near the seaward margins of areas with fjords, the ice-scoured channels are so numerous and varied in direction that the rocky coast is divided into thousands of island blocks, some large and mountainous while others are merely rocky points or rock [[reef]]s, menacing navigation. These are called [[skerry|skerries]].<ref name=natgeofjord>{{cite magazine |url=http://education.nationalgeographic.com/education/encyclopedia/fjord/?ar_a=1 |archive-url=https://web.archive.org/web/20121005064635/http://education.nationalgeographic.com/education/encyclopedia/fjord/?ar_a=1 |url-status=dead |archive-date=October 5, 2012 |title=Fjord |magazine=National Geographic |access-date=April 24, 2015}}</ref> The term skerry is derived from the [[Old Norse]] ''{{lang|non|sker}}'', which means a rock in the sea.{{sfn|Moore|1890|p=277}}

Skerries most commonly formed at the outlet of fjords where submerged glacially formed valleys perpendicular to the coast join with other cross valleys in a complex array. The island fringe of Norway is such a group of skerries (called a ''{{lang|no|skjærgård}}''); many of the cross fjords are so arranged that they parallel the coast and provide a protected channel behind an almost unbroken succession of mountainous islands and skerries. By this channel, one can travel through a protected passage almost the entire {{convert|1601|km|abbr=on|0}} route from [[Stavanger (city)|Stavanger]] to [[North Cape, Norway|North Cape]], Norway. The [[Blindleia]] is a skerry-protected waterway that starts near [[Kristiansand (town)|Kristiansand]] in southern Norway and continues past [[Lillesand (town)|Lillesand]]. The [[Sweden|Swedish]] coast along [[Bohuslän]] is likewise skerry guarded. The [[Inside Passage]] provides a similar route from [[Seattle]], [[Washington (state)|Washington]], and [[Vancouver]], [[British Columbia]], to [[Skagway, Alaska|Skagway]], [[Alaska]]. Yet another such skerry-protected passage extends from the [[Straits of Magellan]] north for {{convert|800|km|abbr=on}}.

===Phytoplankton===
Fjords provide unique environmental conditions for [[phytoplankton]] communities. In polar fjords, glacier and ice sheet outflow add cold, fresh meltwater along with transported sediment into the body of water. Nutrients provided by this outflow can significantly enhance phytoplankton growth. For example, in some fjords of the West Antarctic Peninsula (WAP), nutrient enrichment from meltwater drives [[diatom]] blooms, a highly productive group of phytoplankton that enable such fjords to be valuable feeding grounds for other species.<ref>{{Cite journal |last1=Mascioni |first1=Martina |last2=Almandoz |first2=Gastón O. |last3=Ekern |first3=Lindsey |last4=Pan |first4=B. Jack |last5=Vernet |first5=Maria |date=2021-12-01 |title=Microplanktonic diatom assemblages dominated the primary production but not the biomass in an Antarctic fjord |journal=Journal of Marine Systems |language=en |volume=224 |pages=103624 |doi=10.1016/j.jmarsys.2021.103624 |bibcode=2021JMS...22403624M |issn=0924-7963|doi-access=free }}</ref> It is possible that as [[climate change]] reduces long-term meltwater output, nutrient dynamics within such fjords will shift to favor less productive species, destabilizing the food web ecology of fjord systems.

In addition to nutrient flux, sediment carried by flowing glaciers can become suspended in the water column, increasing [[turbidity]] and reducing light penetration into greater depths of the fjord. This effect can limit the available light for [[photosynthesis]] in deeper areas of the water mass, reducing phytoplankton abundance beneath the surface.<ref>{{Cite journal |last1=Kang |first1=Sung-Ho |last2=Kim |first2=Yea-Dong |last3=Kang |first3=Jae-Shin |last4=Yoo |first4=Kyu-Cheul |last5=Yoon |first5=Ho-Il |last6=Lee |first6=Won-Cheol |date=2003 |title=Monitoring on the Marine Environment and Phytoplankton of Kongsfjorden, Svalbard, Arctic |url=https://www.koreascience.or.kr/article/JAKO200312242755678.page |journal=Ocean and Polar Research |volume=25 |issue=2 |pages=213–226 |doi=10.4217/OPR.2003.25.2.213 |issn=1598-141X|doi-access=free }}</ref>

Overall, phytoplankton abundance and species composition within fjords is highly seasonal, varying as a result of seasonal light availability and water properties that depend on glacial melt and the formation of sea ice. The study of phytoplankton communities within fjords is an active area of research, supported by groups such as FjordPhyto, a citizen science initiative to study phytoplankton samples collected by local residents, tourists, and boaters of all backgrounds.

===Epishelf lakes===
An epishelf lake forms when meltwater is trapped behind a floating ice shelf and the freshwater floats on the denser saltwater below. Its surface may freeze forming an isolated ecosystem.

{{Panorama
|image   = File:PanoHardangerfjorden1.jpg
|height  = 150
|alt     = [[Hardangerfjord]] in [[Vestland]], [[Norway]]
|caption = [[Hardangerfjord]] in [[Vestland]], [[Norway]]
}}