Editing Baltic Sea (section)

== Definitions ==
[[File:Hel in summer.jpg|thumb|[[Hel Peninsula]]]]
[[File:Belte inter.png|thumb|Danish straits and southwestern Baltic Sea]]
[[File:Karta öfver Åland.png|thumb|[[Åland]] between [[Baltic Proper]] and the [[Gulf of Bothnia]]]]

=== Administration ===
The [[Helsinki Convention on the Protection of the Marine Environment of the Baltic Sea Area]] includes the Baltic Sea and the [[Kattegat]], without calling Kattegat a part of the Baltic Sea, "For the purposes of this Convention the 'Baltic Sea Area' shall be the Baltic Sea and the Entrance to the Baltic Sea, bounded by the parallel of the Skaw in the Skagerrak at 57°44.43'N."<ref>{{cite web|url=http://www.helcom.fi/Documents/About%20us/Convention%20and%20commitments/Helsinki%20Convention/|title=Text of Helsinki Convention|access-date=26 April 2014|archive-url=https://archive.today/20140502201121/http://www.helcom.fi/Documents/About%20us/Convention%20and%20commitments/Helsinki%20Convention/|archive-date=2 May 2014|url-status=dead}}</ref>

===Traffic history===
Historically, the [[Kingdom of Denmark]] collected [[Sound Dues]] from ships at the border between the ocean and the land-locked Baltic Sea, in tandem: in the [[Øresund]] at [[Kronborg]] castle near [[Helsingør]]; in the [[Great Belt]] at [[Nyborg]]; and in the [[Little Belt]] at its narrowest part then [[Fredericia]], after that stronghold was built. The narrowest part of Little Belt is the "Middelfart Sund" near [[Middelfart]].<ref>{{Cite web|url=https://pierer.de-academic.com/66040/Sundzoll|title=Sundzoll|website=Academic dictionaries and encyclopedias|access-date=16 June 2022|archive-date=2 October 2022|archive-url=https://web.archive.org/web/20221002094240/https://pierer.de-academic.com/66040/Sundzoll|url-status=live}}</ref>

===Oceanography===
Geographers widely agree that the preferred physical border between the Baltic and North Seas is the Langelandsbælt (the southern part of the [[Great Belt]] strait near [[Langeland]]) and the [[Drogden]]-Sill strait.<ref>{{cite web|url=http://www.io-warnemuende.de/Antworten_Fragen_zum_Meer.html?frage=49|title=Fragen zum Meer (Antworten) – IOW|website=www.io-warnemuende.de|access-date=31 May 2023|archive-date=20 April 2014|archive-url=https://web.archive.org/web/20140420053829/http://www.io-warnemuende.de/Antworten_Fragen_zum_Meer.html?frage=49|url-status=live}}</ref> The Drogden Sill is situated north of [[Køge Bugt]] and connects [[Dragør]] in the south of [[Copenhagen]] to [[Malmö]]; it is used by the [[Øresund Bridge]], including the ''Drogden Tunnel''. By this definition, the [[Danish straits]] is part of the entrance, but the [[Bay of Mecklenburg]] and the [[Bay of Kiel]] are parts of the Baltic Sea.
Another usual border is the line between [[Falsterbo]], Sweden, and [[Stevns Klint]], Denmark, as this is the southern border of Øresund. It is also the border between the shallow southern Øresund (with a typical depth of 5–10 meters only) and notably deeper water.

===Hydrography and biology===
[[Drogden]] Sill (depth of {{convert|7|m|abbr=on}}) sets a limit to Øresund and [[Darss]] Sill (depth of {{convert|18|m|abbr=on}}), and a limit to the Belt Sea.<ref>{{Cite web |url=https://www.kemi.se/Documents/Publikationer/Trycksaker/PM/PM-9-12-BaltSens.pdf |title=Swedish Chemicals Agency (KEMI): The BaltSens Project – The sensitivity of the Baltic Sea ecosystems to hazardous compounds |access-date=26 April 2014 |archive-url=https://web.archive.org/web/20130530201134/http://kemi.se/Documents/Publikationer/Trycksaker/PM/PM-9-12-BaltSens.pdf |archive-date=30 May 2013 |url-status=dead }}</ref> The shallow [[Sill (geology)|sills]] are obstacles to the flow of heavy salt water from the Kattegat into the basins around [[Bornholm]] and [[Gotland]].

The Kattegat and the southwestern Baltic Sea are well oxygenated and have a rich biology. The remainder of the Sea is brackish, poor in oxygen, and in species. Thus, statistically, the more of the entrance that is included in its definition, the healthier the Baltic appears; conversely, the more narrowly it is defined, the more endangered its biology appears.