Editing Kattegat (section)

== Biology ==

The [[salinity]] in the Kattegat has a pronounced two-layer structure. The upper layer's salinity is between 18‰ and 26‰, while that of the lower layer (separated by a strong [[halocline]] at around {{convert|15|m|abbr=on}}) has a salinity between 32‰ and 34‰. The lower layer consists of inflowing seawater from the Skagerrak, with a salinity on par with most other coastal seawaters, while the upper layer, consisting of inflowing seawater from the Baltic Sea, has a much lower salinity, comparable to [[brackish water]], but still a great deal higher than the rest of the Baltic. These two opposing flows transport a net surplus of {{convert|475|km3|cumi|abbr=on}} of seawater from the Baltic to the Skagerrak every year.<ref>{{cite web|url=http://www2.dmu.dk/1_viden/2_miljoe-tilstand/3_vand/4_eutrophication/hydrography.asp |title=Hydrography |author=Ærtebjerg, G., Andersen, J.H. and Schou Hansen |publisher=Danish Environmental Protection Agency and National Environmental Research Institute |website=Nutrients and Eutrophication in Danish Seawaters |date=2003 |access-date=30 January 2016}}</ref> During strong winds, the Kattegat's layers are completely mixed in places such as the [[Great Belt]], making the  overall salinity highly variable. This leads to some distinctive conditions for [[sealife]] in the area.<ref>{{cite book|url=https://books.google.com/books?id=csLKtvZNV98C&pg=PA74 |title=Physical Oceanography of the Baltic Sea|author=Matti Leppäranta and Kai Myrberg|date=2009 |publisher=Springer-Praxis |access-date=30 January 2016 |pages=72–74|isbn=9783540797036}}</ref>

[[Cold seep]]s, known locally as ''bubbling reefs'' ({{langx|da|boblerev}}), can be found in the northern Kattegat. Unlike those in most other places (including the North Sea and Skagerrak), the Kattegat's cold seeps are at relatively shallow depths, generally from {{convert|0|to(-)|30|m|ft|abbr=on|-2}} below the surface.<ref name=Jensen1992>{{cite journal|author1=Jensen |author2=Aagaard |author3=Burke |author4=Dando |author5=Jørgensen |author6=Kuijpers |author7=Laier |author8=O'Hara |author9=Schmaljohann | year=1992 | title="Bubbling reefs" in the Kattegat: Submarine landscapes of carbonate-cemented rocks support a diverse ecosystem at methane seeps | journal=Marine Ecology Progress Series | volume=83 | pages=103–112 | doi=10.3354/meps083103|bibcode=1992MEPS...83..103J |doi-access=free }}</ref><ref name=HelcomCS>{{cite web|url=http://helcom.fi/Red%20List%20of%20biotopes%20habitats%20and%20biotope%20complexe/HELCOM%20Red%20List%201180%20Submarine%20structures%20made%20by%20leaking%20gases.pdf | title=Red List – Submarine structures made by leaking gases | publisher=[[HELCOM]] | year=2013 | access-date=16 June 2017}}</ref> The seeps rely on methane deposited during the [[Eemian]] period and during calm weather the bubbles can sometimes be seen on the surface of the water.<ref name=HelcomCS/> Carbonate [[Cementation (geology)|cementation]] and [[lithification]] form slaps or pillars up to {{convert|4|m|ft|abbr=on}} tall and support a rich [[biodiversity]].<ref name=Jensen1992/><ref name=HelcomCS/> Due to their distinct properties, the Kattegat bubbling reefs are under protection and are recognised by the [[European Union]] (EU) as a [[Natura 2000]] habitat (type 1180).<ref>{{cite web| title=Proposed measures for fisheries management in Natura 2000-sites in the Danish territorial area of the Kattegat and Samsø Belt | url=https://naturerhverv.dk/fileadmin/user_upload/NaturErhverv/Filer/Fiskeri/Natura_2000_hav/Rev/Forslag_til_fiskeriregulering_Kattegat.pdf | series=Meeting at the AgriFish Agency, Copenhagen | publisher=[[Minister for Food, Agriculture and Fisheries (Denmark)]] | date=20 March 2012 | access-date=16 June 2017}}</ref>

===Ecological collapse===
The Kattegat, characterised by widespread [[oceanic anoxic event|anoxia]],<ref name="DruonEtAl2004">{{cite journal |last1=Druon |first1=Jean-Noël |last2=Schrimpf |first2=Wolfram |last3=Dobricic |first3=Srdjan |last4=Stips |first4=Adolf |date=19 May 2004 |title=Comparative assessment of large-scale marine eutrophication: North Sea area and Adriatic Sea as case studies |url=https://www.int-res.com/articles/meps2004/272/m272p001.pdf |journal=Marine Ecology Progress Series |volume=272 |pages=1–23 |doi=10.3354/meps272001 |bibcode=2004MEPS..272....1D |access-date=10 August 2023}}</ref> was one of the first marine [[dead zone (ecology)|dead zone]]s to be noted in the 1970s, when scientists began to study the effects of heavy industry on the natural world.<ref>{{cite web|url=http://www.unep.org/Documents.Multilingual/Default.asp?DocumentID=486&ArticleID=5393&l=en|title=Further Rise in Number of Marine 'Dead Zones'|publisher=[[UNEP]]|date=19 October 2006|access-date=30 January 2016|archive-date=4 October 2009|archive-url=http://arquivo.pt/wayback/20091004170617/http%3A//www.unep.org/Documents.Multilingual/Default.asp?DocumentID%3D486%26ArticleID%3D5393%26l%3Den|url-status=dead}}</ref><ref>{{cite book|url=https://books.google.com/books?id=9fEKAAAAQBAJ&pg=PA4 |title=Introduction to Marine Biology |author=Karleskint, Turner and Small |publisher=Brooks/Cole |page=4|date=2013 |edition=4|isbn=978-1285402222 }}</ref> Since then,  research has leant much insight into processes like [[eutrophication]], and how to deal with it. Since the first Action Plan for the Aquatic Environment in 1985, Denmark and the EU have begun costly, far-reaching domestic projects  to stop, repair and prevent these environmentally destructive and economically damaging processes<ref>{{Cite book |doi=10.1029/CE052p0155|chapter = Benthic macrofauna and demersal fish|title = Eutrophication in Coastal Marine Ecosystems| volume=52| pages=155–178|series = Coastal and Estuarine Studies|year = 1996|last1 = Hagerman|first1 = Lars| last2=Josefson| first2=Alf B.| last3=Jensen| first3=Jørgen N.| isbn=978-0-87590-266-1}}</ref> and are now busy implementing the fourth Action Plan. The action plans sum up a broad range of initiatives and include the so-called Nitrate Directives.<ref>[http://www.mst.dk/English/Agriculture/nitrates_directive/implementation_in_denmark/ Implementation of the Nitrates directive in Denmark] {{Webarchive|url=https://web.archive.org/web/20131203112345/http://www.mst.dk/English/Agriculture/nitrates_directive/implementation_in_denmark/ |date=3 December 2013 }} Danish Ministry of the Environment</ref> The action plans have generally been viewed as a success, although the work is not finished and not all goals have been met yet.<ref>{{cite web|url=http://politiken.dk/debat/analyse/ECE1422611/vandmiljoeplaner-har-vaeret-en-succes/|title=Action Plans for the Aquatic Environment have been a success|newspaper=Politiken|author1=Jesper H. Andersen |author2=Jacob Carstensen |date=25 October 2011|language=da|access-date=24 November 2014}}</ref>