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The water in the Baltic Sea is [[Brackish water|brackish]], meaning it has a low salinity. Each year, approximately 550 cubic kilometers of freshwater from [[Rain|rainfall]] and [[River|rivers]] within its drainage basin flow into the Baltic Sea. However, only about 100 km³ of water [[Evaporation|evaporates]] from the Baltic Sea into the atmosphere annually. This results in an excess of approximately 450 km³ of [[fresh water]] each year, which accounts for about two percent of the total volume of the Baltic Sea. To maintain a stable water level over the long term, the excess water flows out of the Baltic Sea through the [[Danish straits]] into the [[North Sea]]. Periodic counterflow from the North Sea to the Baltic Sea prevents the Baltic from gradually turning into a freshwater basin.<ref name="Ilmatieteen laitos">{{Cite web |last= |date= |title=Itämeren muoto, ala ja tilavuus |url=http://ilmatieteenlaitos.fi/itameren-muoto-ala-ja-tilavuus |access-date=2023-07-24 |publisher=[[Finnish Meteorological Institute]] |language=fi}}</ref>
The water in the Baltic Sea is [[Brackish water|brackish]], meaning it has a low salinity. Each year, approximately 550 cubic kilometers of freshwater from [[Rain|rainfall]] and [[River|rivers]] within its drainage basin flow into the Baltic Sea. However, only about 100 km³ of water [[Evaporation|evaporates]] from the Baltic Sea into the atmosphere annually. This results in an excess of approximately 450 km³ of [[fresh water]] each year, which accounts for about two percent of the total volume of the Baltic Sea. To maintain a stable water level over the long term, the excess water flows out of the Baltic Sea through the [[Danish straits]] into the [[North Sea]]. Periodic counterflow from the North Sea to the Baltic Sea prevents the Baltic from gradually turning into a freshwater basin.<ref name="Ilmatieteen laitos">{{Cite web |last= |date= |title=Itämeren muoto, ala ja tilavuus |url=http://ilmatieteenlaitos.fi/itameren-muoto-ala-ja-tilavuus |access-date=2023-07-24 |publisher=[[Finnish Meteorological Institute]] |language=fi}}</ref>


The saltwater inflow, known as the major Baltic inflow (MBI), is said to occur when there is a strong overflow of saline water from the North Sea to the Baltic Sea over the cross-sections of the [[Darss Sill]] ([[Great Belt|Belt Sea]]) and the [[Drogden Sill]] ([[Øresund|The Sound]]). Such an flow almost entirely destroys the salinity stratification in the areas of the sills for several days. Typically, the inflow event must last for at least five days to be classified as an MBI. During extremely powerful events, the Baltic Sea receives over 100 km³ of very saline water from the ocean, whereas during weaker MBIs, the volume is less than 100 km³, averaging around 70 km³.<ref name=":1">{{Cite journal |date=1992-12-01 |title=Characteristics of major Baltic inflows—a statistical analysis |url=https://www.sciencedirect.com/science/article/pii/027843439290060W |language=en |volume=12 |issue=12 |pages=1375–1400 |doi=10.1016/0278-4343(92)90060-W |issn=0278-4343}}</ref>
The saltwater inflow, known as the major Baltic inflow (MBI), is said to occur when there is a strong overflow of saline water from the North Sea to the Baltic Sea over the cross-sections of the [[Darss Sill]] ([[Great Belt|Belt Sea]]) and the [[Drogden Sill]] ([[Øresund|The Sound]]). Such an flow almost entirely destroys the salinity stratification in the areas of the sills for several days. Typically, the inflow event must last for at least five days to be classified as an MBI. During extremely powerful events, the Baltic Sea receives over 100 km³ of very saline water from the ocean, whereas during weaker MBIs, the volume is less than 100 km³, averaging around 70 km³.<ref name=":1">{{Cite journal |date=1992-12-01 |title=Characteristics of major Baltic inflows—a statistical analysis |url=https://dx.doi.org/10.1016/0278-4343%2892%2990060-W |language=en |volume=12 |issue=12 |pages=1375–1400 |doi=10.1016/0278-4343(92)90060-W |issn=0278-4343 |last1=Matthäus |first1=Wolfgang |last2=Franck |first2=Herbert |journal=Continental Shelf Research |bibcode=1992CSR....12.1375M }}</ref>


Major Baltic inflows primarily occur during winter and early spring because weather conditions are most favorable for the inflow of saline water. First, the Baltic Sea must experience easterly and southeasterly winds for about 20 to 30 days, reducing precipitation within the Baltic Sea drainage basin, enhancing outflow from the basin, and causing a drop in the sea level.<ref name=":0">{{Cite journal |date=2022-02-16 |title=Salinity dynamics of the Baltic Sea |url=https://esd.copernicus.org/articles/13/373/2022/ |language=English |volume=13 |issue=1 |pages=373–392 |doi=10.5194/esd-13-373-2022 |issn=2190-4979}}</ref> Before the arrival of the saline inflow, the Baltic Sea's surface level is typically around 26 centimeters lower than usual.<ref name=":1">{{Cite journal |date=1992-12-01 |title=Characteristics of major Baltic inflows—a statistical analysis |url=https://www.sciencedirect.com/science/article/pii/027843439290060W |language=en |volume=12 |issue=12 |pages=1375–1400 |doi=10.1016/0278-4343(92)90060-W |issn=0278-4343}}</ref> Following this, a roughly one-month period of westerly winds begins, during which the sea level in the [[Kattegat]] rises, and pressure differences force the saline water from the [[North Sea]] through the narrow [[Danish straits]] into the Baltic Sea.<ref name=":0" /> Throughout the entire inflow process, the Baltic Sea's water level rises on average by about 59 cm, with 38 cm occurring during the preparatory period and 21 cm during the actual saline inflow. The MBI itself typically lasts for 7–8 days.<ref name=":1" />
Major Baltic inflows primarily occur during winter and early spring because weather conditions are most favorable for the inflow of saline water. First, the Baltic Sea must experience easterly and southeasterly winds for about 20 to 30 days, reducing precipitation within the Baltic Sea drainage basin, enhancing outflow from the basin, and causing a drop in the sea level.<ref name=":0">{{Cite journal |date=2022-02-16 |title=Salinity dynamics of the Baltic Sea |url=https://esd.copernicus.org/articles/13/373/2022/ |language=English |volume=13 |issue=1 |pages=373–392 |doi=10.5194/esd-13-373-2022 |issn=2190-4979 |last1=Lehmann |first1=Andreas |last2=Myrberg |first2=Kai |last3=Post |first3=Piia |last4=Chubarenko |first4=Irina |last5=Dailidiene |first5=Inga |last6=Hinrichsen |first6=Hans-Harald |last7=Hüssy |first7=Karin |last8=Liblik |first8=Taavi |last9=Meier |first9=H. E. Markus |last10=Lips |first10=Urmas |last11=Bukanova |first11=Tatiana |journal=Earth System Dynamics |bibcode=2022ESD....13..373L }}</ref> Before the arrival of the saline inflow, the Baltic Sea's surface level is typically around 26 centimeters lower than usual.<ref name=":1">{{Cite journal |date=1992-12-01 |title=Characteristics of major Baltic inflows—a statistical analysis |url=https://dx.doi.org/10.1016/0278-4343%2892%2990060-W |language=en |volume=12 |issue=12 |pages=1375–1400 |doi=10.1016/0278-4343(92)90060-W |issn=0278-4343 |last1=Matthäus |first1=Wolfgang |last2=Franck |first2=Herbert |journal=Continental Shelf Research |bibcode=1992CSR....12.1375M }}</ref> Following this, a roughly one-month period of westerly winds begins, during which the sea level in the [[Kattegat]] rises, and pressure differences force the saline water from the [[North Sea]] through the narrow [[Danish straits]] into the Baltic Sea.<ref name=":0" /> Throughout the entire inflow process, the Baltic Sea's water level rises on average by about 59 cm, with 38 cm occurring during the preparatory period and 21 cm during the actual saline inflow. The MBI itself typically lasts for 7–8 days.<ref name=":1" />
[[File:Major_Baltic_Inflows_(Mohrholz_2018).svg|lang=en|thumb|407x407px|The two principal time series of MBIs in the Baltic Sea. The newer DS5 time series (green bars above, Mohrholz 2018) is based on observations from the Darss Sill and includes all saltwater inflow events lasting over 5 days. The older FM96 time series (blue bars below, Fischer and Matthäus, 1996) has data gaps after 1976 when the [[Lightvessel Gedser Rev|l/v Gedser Rev]] ended salinity measurements. The newer time series shows cyclical variations in the occurrence of saline inflows at approximately 30-year intervals.]]
[[File:Major_Baltic_Inflows_(Mohrholz_2018).svg|lang=en|thumb|407x407px|The two principal time series of MBIs in the Baltic Sea. The newer DS5 time series (green bars above, Mohrholz 2018) is based on observations from the Darss Sill and includes all saltwater inflow events lasting over 5 days. The older FM96 time series (blue bars below, Fischer and Matthäus, 1996) has data gaps after 1976 when the [[Lightvessel Gedser Rev|l/v Gedser Rev]] ended salinity measurements. The newer time series shows cyclical variations in the occurrence of saline inflows at approximately 30-year intervals.]]


== Occurrence of MBIs ==
== Occurrence of MBIs ==
The formation of an MBI requires specific, relatively rare weather conditions. Between 1897 and 1976, approximately 90 MBIs were observed, averaging about one per year.<ref name=":1">{{Cite journal |date=1992-12-01 |title=Characteristics of major Baltic inflows—a statistical analysis |url=https://www.sciencedirect.com/science/article/pii/027843439290060W |language=en |volume=12 |issue=12 |pages=1375–1400 |doi=10.1016/0278-4343(92)90060-W |issn=0278-4343}}</ref> Occasionally, there are even multi-year periods without any MBIs occurring. Large inflows that effectively renew the deep basin waters occur on average only once every ten years.{{According to whom|date=July 2023}}
The formation of an MBI requires specific, relatively rare weather conditions. Between 1897 and 1976, approximately 90 MBIs were observed, averaging about one per year.<ref name=":1">{{Cite journal |date=1992-12-01 |title=Characteristics of major Baltic inflows—a statistical analysis |url=https://dx.doi.org/10.1016/0278-4343%2892%2990060-W |language=en |volume=12 |issue=12 |pages=1375–1400 |doi=10.1016/0278-4343(92)90060-W |issn=0278-4343 |last1=Matthäus |first1=Wolfgang |last2=Franck |first2=Herbert |journal=Continental Shelf Research |bibcode=1992CSR....12.1375M }}</ref> Occasionally, there are even multi-year periods without any MBIs occurring. Large inflows that effectively renew the deep basin waters occur on average only once every ten years.{{According to whom|date=July 2023}}


Very large MBIs have occurred in 1897 (330 km<sup>3</sup>), 1906 (300 km<sup>3</sup>), 1922 (510 km<sup>3</sup>), 1951 (510 km<sup>3</sup>), 1993/94 (300 km<sup>3</sup>),<ref name="SMHI 2014">{{Cite web |date=2014-01-15 |title=Inflows to the Baltic |url=http://www.smhi.se/en/theme/inflows-to-the-baltic-1.13097 |access-date=2023-07-24 |publisher=[[Swedish Meteorological and Hydrological Institute]]}}</ref> and 2014/2015 (300 km<sup>3</sup>).<ref name="YLE 5.8.2016">{{Cite web |last= |date=2016-08-05 |title=Itämeren suolapulssin liikkeet ihmetyttävät tutkijoita |url=http://yle.fi/uutiset/3-9069768 |access-date=2023-07-24 |publisher=[[Yle]] News |language=fi}}</ref> Large MBIs have on the other hand been observed in 1898 (twice), 1900, 1902 (twice), 1914, 1921, 1925, 1926, 1960, 1965, 1969, 1973, 1976, and 2003.<ref name="SMHI 2014" /> The MBI that started in 2014 was by far the third largest MBI in the Baltic Sea. Only the inflows of 1951 and 1921/1922 were larger than it.<ref name=":2">{{Cite journal |date=2018-04-12 |title=Recently Accelerated Oxygen Consumption Rates Amplify Deoxygenation in the Baltic Sea |url=https://onlinelibrary.wiley.com/doi/abs/10.1029/2017JC013686 |language=en |volume=123 |issue=5 |pages=3227–3240 |doi=10.1029/2017JC013686 |issn=2169-9275}}</ref>
Very large MBIs have occurred in 1897 (330 km<sup>3</sup>), 1906 (300 km<sup>3</sup>), 1922 (510 km<sup>3</sup>), 1951 (510 km<sup>3</sup>), 1993/94 (300 km<sup>3</sup>),<ref name="SMHI 2014">{{Cite web |date=2014-01-15 |title=Inflows to the Baltic |url=http://www.smhi.se/en/theme/inflows-to-the-baltic-1.13097 |access-date=2023-07-24 |publisher=[[Swedish Meteorological and Hydrological Institute]]}}</ref> and 2014/2015 (300 km<sup>3</sup>).<ref name="YLE 5.8.2016">{{Cite web |last= |date=2016-08-05 |title=Itämeren suolapulssin liikkeet ihmetyttävät tutkijoita |url=http://yle.fi/uutiset/3-9069768 |access-date=2023-07-24 |publisher=[[Yle]] News |language=fi}}</ref> Large MBIs have on the other hand been observed in 1898 (twice), 1900, 1902 (twice), 1914, 1921, 1925, 1926, 1960, 1965, 1969, 1973, 1976, and 2003.<ref name="SMHI 2014" /> The MBI that started in 2014 was by far the third largest MBI in the Baltic Sea. Only the inflows of 1951 and 1921/1922 were larger than it.<ref name=":2">{{Cite journal |date=2018-04-12 |title=Recently Accelerated Oxygen Consumption Rates Amplify Deoxygenation in the Baltic Sea |url=https://onlinelibrary.wiley.com/doi/abs/10.1029/2017JC013686 |language=en |volume=123 |issue=5 |pages=3227–3240 |doi=10.1029/2017JC013686 |issn=2169-9275 |last1=Meier |first1=H. E. Markus |last2=Väli |first2=Germo |last3=Naumann |first3=Michael |last4=Eilola |first4=Kari |last5=Frauen |first5=Claudia |journal=Journal of Geophysical Research: Oceans |bibcode=2018JGRC..123.3227M }}</ref>


Previously, it was believed that there had been a genuine decline in the number of MBIs after 1980, but recent studies have changed our understanding of the occurrence of saline inflows.<ref name=":0">{{Cite journal |date=2022-02-16 |title=Salinity dynamics of the Baltic Sea |url=https://esd.copernicus.org/articles/13/373/2022/ |language=English |volume=13 |issue=1 |pages=373–392 |doi=10.5194/esd-13-373-2022 |issn=2190-4979}}</ref><ref name=":3">{{Cite journal |date=2018 |title=Major Baltic Inflow Statistics – Revised |url=https://www.frontiersin.org/articles/10.3389/fmars.2018.00384 |volume=5 |doi=10.3389/fmars.2018.00384/full |issn=2296-7745}}</ref> Especially after the lightship [[Lightvessel Gedser Rev|Gedser Rev]] discontinued regular salinity measurements in the Belt Sea in 1976, the picture of the inflows based on salinity measurements remained incomplete. At the [[Leibniz Institute for Baltic Sea Research]] ([[Warnemünde]], [[Germany]]), an updated time series has been compiled, filling in the gaps in observations and covering Major Baltic Inflows and various smaller inflow events of saline water from around 1890 to the present day. The updated time series is based on direct discharge data from the Darss Sill and no longer shows a clear change in the frequency or intensity of saline inflows. Instead, there is cyclical variation in the intensity of MBIs at approximately 30-year intervals.<ref name=":3" /><ref>{{Cite web |title=Major Baltic Inflow statistics - IOW |url=https://www.io-warnemuende.de/major-baltic-inflow-statistics-7274.html |access-date=2023-07-23 |website=www.io-warnemuende.de |language=en}}</ref>
Previously, it was believed that there had been a genuine decline in the number of MBIs after 1980, but recent studies have changed our understanding of the occurrence of saline inflows.<ref name=":0">{{Cite journal |date=2022-02-16 |title=Salinity dynamics of the Baltic Sea |url=https://esd.copernicus.org/articles/13/373/2022/ |language=English |volume=13 |issue=1 |pages=373–392 |doi=10.5194/esd-13-373-2022 |issn=2190-4979 |last1=Lehmann |first1=Andreas |last2=Myrberg |first2=Kai |last3=Post |first3=Piia |last4=Chubarenko |first4=Irina |last5=Dailidiene |first5=Inga |last6=Hinrichsen |first6=Hans-Harald |last7=Hüssy |first7=Karin |last8=Liblik |first8=Taavi |last9=Meier |first9=H. E. Markus |last10=Lips |first10=Urmas |last11=Bukanova |first11=Tatiana |journal=Earth System Dynamics |bibcode=2022ESD....13..373L }}</ref><ref name=":3">{{Cite journal |date=2018 |title=Major Baltic Inflow Statistics – Revised |volume=5 |doi=10.3389/fmars.2018.00384 |issn=2296-7745 |doi-access=free |last1=Mohrholz |first1=Volker |journal=Frontiers in Marine Science }}</ref> Especially after the lightship [[Lightvessel Gedser Rev|Gedser Rev]] discontinued regular salinity measurements in the Belt Sea in 1976, the picture of the inflows based on salinity measurements remained incomplete. At the [[Leibniz Institute for Baltic Sea Research]] ([[Warnemünde]], [[Germany]]), an updated time series has been compiled, filling in the gaps in observations and covering Major Baltic Inflows and various smaller inflow events of saline water from around 1890 to the present day. The updated time series is based on direct discharge data from the Darss Sill and no longer shows a clear change in the frequency or intensity of saline inflows. Instead, there is cyclical variation in the intensity of MBIs at approximately 30-year intervals.<ref name=":3" /><ref>{{Cite web |title=Major Baltic Inflow statistics - IOW |url=https://www.io-warnemuende.de/major-baltic-inflow-statistics-7274.html |access-date=2023-07-23 |website=www.io-warnemuende.de |language=en}}</ref>


== Effects on the state of the Baltic Sea and its ecosystem ==
== Effects on the state of the Baltic Sea and its ecosystem ==
Major Baltic inflows (MBIs) are the only natural phenomenon capable of oxygenating the deep saline waters of the Baltic Sea, making their occurrence crucial for the ecological state of the sea.<ref name=":2" /> The salinity and oxygen from MBIs significantly impact the Baltic Sea's [[ecosystems]], including the reproductive conditions of marine fish species such as [[cod]], the distribution of freshwater and marine species, and the overall [[biodiversity]] of the Baltic Sea.<ref>Climate Change in the Baltic Sea. [https://helcom.fi/wp-content/uploads/2021/09/Baltic-Sea-Climate-Change-Fact-Sheet-2021.pdf 2021 Fact Sheet]. Baltic Sea Environment Proceedings n°180. HELCOM/Baltic Earth 2021.</ref>
Major Baltic inflows (MBIs) are the only natural phenomenon capable of oxygenating the deep saline waters of the Baltic Sea, making their occurrence crucial for the ecological state of the sea.<ref name=":2" /> The salinity and oxygen from MBIs significantly impact the Baltic Sea's [[ecosystems]], including the reproductive conditions of marine fish species such as [[cod]], the distribution of freshwater and marine species, and the overall [[biodiversity]] of the Baltic Sea.<ref>Climate Change in the Baltic Sea. [https://helcom.fi/wp-content/uploads/2021/09/Baltic-Sea-Climate-Change-Fact-Sheet-2021.pdf 2021 Fact Sheet]. Baltic Sea Environment Proceedings n°180. HELCOM/Baltic Earth 2021.</ref>


The heavy saline water brought in by MBIs slowly advances along the seabed of the [[Baltic Proper]] at a pace of a few kilometers per day, displacing the deep water from one basin to another.<ref name="Ymparisto.fi 12.2.2015">{{Cite web |last= |date=2015-02-12 |title=Arandan Itämeren seurantamatka: suolapulssi etenee eteläisellä Itämerellä, Suomenlahden ja Selkämeren fosforipitoisuus viime talvea korkeampi |url=https://www.syke.fi/fi-FI/Tutkimus__kehittaminen/Vesi/Arandan_Itameren_seurantamatka_suolapuls(32743) |access-date=2023-07-24 |publisher=[[Finnish Environment Institute]] |language=fi}}</ref> Although some oxygen is transported from the North Sea to the Baltic Sea, only a small portion of the oxygen responsible for renewing the deep basins originates from the Baltic Sea entrance area. In the southwestern basins of the Baltic Sea ([[Arkona Sea|Arkona sea]], [[Bornholm Basin|Bornholm basin]]), there is already oxygen present in the water column, and the role of the saltwater pulse is to entrain and direct it towards the deep basins of the sea.<ref>{{Cite journal |last=Neumann |first=Thomas |last2=Radtke |first2=Hagen |last3=Seifert |first3=Torsten |date=2017-01-25 |title=On the importance of Major Baltic Inflows for oxygenation of the central Baltic Sea: IMPORTANCE OF MAJOR BALTIC INFLOWS |url=http://doi.wiley.com/10.1002/2016JC012525 |journal=Journal of Geophysical Research: Oceans |language=en |volume=122 |issue=2 |pages=1090–1101 |doi=10.1002/2016JC012525}}</ref>
The heavy saline water brought in by MBIs slowly advances along the seabed of the [[Baltic Proper]] at a pace of a few kilometers per day, displacing the deep water from one basin to another.<ref name="Ymparisto.fi 12.2.2015">{{Cite web |last= |date=2015-02-12 |title=Arandan Itämeren seurantamatka: suolapulssi etenee eteläisellä Itämerellä, Suomenlahden ja Selkämeren fosforipitoisuus viime talvea korkeampi |url=https://www.syke.fi/fi-FI/Tutkimus__kehittaminen/Vesi/Arandan_Itameren_seurantamatka_suolapuls(32743) |access-date=2023-07-24 |publisher=[[Finnish Environment Institute]] |language=fi}}</ref> Although some oxygen is transported from the North Sea to the Baltic Sea, only a small portion of the oxygen responsible for renewing the deep basins originates from the Baltic Sea entrance area. In the southwestern basins of the Baltic Sea ([[Arkona Sea|Arkona sea]], [[Bornholm Basin|Bornholm basin]]), there is already oxygen present in the water column, and the role of the saltwater pulse is to entrain and direct it towards the deep basins of the sea.<ref>{{Cite journal |last1=Neumann |first1=Thomas |last2=Radtke |first2=Hagen |last3=Seifert |first3=Torsten |date=2017-01-25 |title=On the importance of Major Baltic Inflows for oxygenation of the central Baltic Sea: IMPORTANCE OF MAJOR BALTIC INFLOWS |url=http://doi.wiley.com/10.1002/2016JC012525 |journal=Journal of Geophysical Research: Oceans |language=en |volume=122 |issue=2 |pages=1090–1101 |doi=10.1002/2016JC012525}}</ref>


It has been observed that the oxygen supplied by saline inflows is consumed more rapidly in the Baltic Proper than before. In 1993, the oxygen replenishment was depleted in about 17 months, in 2003, it took approximately 13 months, and in 2015, it was exhausted in just six months.<ref name=":2" />
It has been observed that the oxygen supplied by saline inflows is consumed more rapidly in the Baltic Proper than before. In 1993, the oxygen replenishment was depleted in about 17 months, in 2003, it took approximately 13 months, and in 2015, it was exhausted in just six months.<ref name=":2" />


The intense oxygen consumption in the Baltic Sea's seafloor is due to prolonged nutrient loading and the impacts of climate change.<ref name=":0">{{Cite journal |date=2022-02-16 |title=Salinity dynamics of the Baltic Sea |url=https://esd.copernicus.org/articles/13/373/2022/ |language=English |volume=13 |issue=1 |pages=373–392 |doi=10.5194/esd-13-373-2022 |issn=2190-4979}}</ref> Although a single MBI can oxygenate both the deep water [[hydrogen sulfide]] and [[ammonia]], multiple consecutive inflows would be required to raise the oxygen level to a satisfactory level. The so-called "[[oxygen debt]]" was estimated to be about 20 million tons in 2020. This is the amount of oxygen that should be transported to the Baltic Proper via saline inflows to raise its [[oxygen concentration]] to the 3 mL/L level observed after the MBI of 1993. With a single 200 km³ MBI, approximately 2 million tons of oxygen are transported.<ref>{{Cite journal |date=2022-11-01 |title=How oxygen deficiency in the Baltic Sea proper has spread and worsened: The role of ammonium and hydrogen sulphide |url=https://doi.org/10.1007/s13280-022-01738-8 |language=en |volume=51 |issue=11 |pages=2308–2324 |doi=10.1007/s13280-022-01738-8 |issn=1654-7209 |pmid=35737272}}</ref>
The intense oxygen consumption in the Baltic Sea's seafloor is due to prolonged nutrient loading and the impacts of climate change.<ref name=":0">{{Cite journal |date=2022-02-16 |title=Salinity dynamics of the Baltic Sea |url=https://esd.copernicus.org/articles/13/373/2022/ |language=English |volume=13 |issue=1 |pages=373–392 |doi=10.5194/esd-13-373-2022 |issn=2190-4979 |last1=Lehmann |first1=Andreas |last2=Myrberg |first2=Kai |last3=Post |first3=Piia |last4=Chubarenko |first4=Irina |last5=Dailidiene |first5=Inga |last6=Hinrichsen |first6=Hans-Harald |last7=Hüssy |first7=Karin |last8=Liblik |first8=Taavi |last9=Meier |first9=H. E. Markus |last10=Lips |first10=Urmas |last11=Bukanova |first11=Tatiana |journal=Earth System Dynamics |bibcode=2022ESD....13..373L }}</ref> Although a single MBI can oxygenate both the deep water [[hydrogen sulfide]] and [[ammonia]], multiple consecutive inflows would be required to raise the oxygen level to a satisfactory level. The so-called "[[oxygen debt]]" was estimated to be about 20 million tons in 2020. This is the amount of oxygen that should be transported to the Baltic Proper via saline inflows to raise its [[oxygen concentration]] to the 3 mL/L level observed after the MBI of 1993. With a single 200 km³ MBI, approximately 2 million tons of oxygen are transported.<ref>{{Cite journal |date=2022-11-01 |title=How oxygen deficiency in the Baltic Sea proper has spread and worsened: The role of ammonium and hydrogen sulphide |url=https://doi.org/10.1007/s13280-022-01738-8 |language=en |volume=51 |issue=11 |pages=2308–2324 |doi=10.1007/s13280-022-01738-8 |issn=1654-7209 |pmid=35737272 |last1=Rolff |first1=Carl |last2=Walve |first2=Jakob |last3=Larsson |first3=Ulf |last4=Elmgren |first4=Ragnar |journal=Ambio |pmc=9481832 }}</ref>


== References ==
== References ==

Revision as of 23:21, 1 August 2023

The Baltic Sea saltwater inflow, known as the major Baltic inflow (MBI), refers to a significant influx of saline water from the North Sea into the Baltic Sea through the Danish straits. In the Baltic Sea, dense seawater from the North Sea sinks to the bottom and moves along the seabed, displacing the often oxygen-depleted water in the deep basins. Simultaneously, it transports new oxygen-rich water to the deep basins. These inflows are crucial for the Baltic Sea's ecosystem because they alleviate the oxygen depletion that commonly occurs in the deep basins of the poorly mixed sea, and at the same time, they prevent eutrophication caused by internal nutrient loading.

The process for MBI formation

The water in the Baltic Sea is brackish, meaning it has a low salinity. Each year, approximately 550 cubic kilometers of freshwater from rainfall and rivers within its drainage basin flow into the Baltic Sea. However, only about 100 km³ of water evaporates from the Baltic Sea into the atmosphere annually. This results in an excess of approximately 450 km³ of fresh water each year, which accounts for about two percent of the total volume of the Baltic Sea. To maintain a stable water level over the long term, the excess water flows out of the Baltic Sea through the Danish straits into the North Sea. Periodic counterflow from the North Sea to the Baltic Sea prevents the Baltic from gradually turning into a freshwater basin.[1]

The saltwater inflow, known as the major Baltic inflow (MBI), is said to occur when there is a strong overflow of saline water from the North Sea to the Baltic Sea over the cross-sections of the Darss Sill (Belt Sea) and the Drogden Sill (The Sound). Such an flow almost entirely destroys the salinity stratification in the areas of the sills for several days. Typically, the inflow event must last for at least five days to be classified as an MBI. During extremely powerful events, the Baltic Sea receives over 100 km³ of very saline water from the ocean, whereas during weaker MBIs, the volume is less than 100 km³, averaging around 70 km³.[2]

Major Baltic inflows primarily occur during winter and early spring because weather conditions are most favorable for the inflow of saline water. First, the Baltic Sea must experience easterly and southeasterly winds for about 20 to 30 days, reducing precipitation within the Baltic Sea drainage basin, enhancing outflow from the basin, and causing a drop in the sea level.[3] Before the arrival of the saline inflow, the Baltic Sea's surface level is typically around 26 centimeters lower than usual.[2] Following this, a roughly one-month period of westerly winds begins, during which the sea level in the Kattegat rises, and pressure differences force the saline water from the North Sea through the narrow Danish straits into the Baltic Sea.[3] Throughout the entire inflow process, the Baltic Sea's water level rises on average by about 59 cm, with 38 cm occurring during the preparatory period and 21 cm during the actual saline inflow. The MBI itself typically lasts for 7–8 days.[2]

The two principal time series of MBIs in the Baltic Sea. The newer DS5 time series (green bars above, Mohrholz 2018) is based on observations from the Darss Sill and includes all saltwater inflow events lasting over 5 days. The older FM96 time series (blue bars below, Fischer and Matthäus, 1996) has data gaps after 1976 when the l/v Gedser Rev ended salinity measurements. The newer time series shows cyclical variations in the occurrence of saline inflows at approximately 30-year intervals.

Occurrence of MBIs

The formation of an MBI requires specific, relatively rare weather conditions. Between 1897 and 1976, approximately 90 MBIs were observed, averaging about one per year.[2] Occasionally, there are even multi-year periods without any MBIs occurring. Large inflows that effectively renew the deep basin waters occur on average only once every ten years.[according to whom?]

Very large MBIs have occurred in 1897 (330 km3), 1906 (300 km3), 1922 (510 km3), 1951 (510 km3), 1993/94 (300 km3),[4] and 2014/2015 (300 km3).[5] Large MBIs have on the other hand been observed in 1898 (twice), 1900, 1902 (twice), 1914, 1921, 1925, 1926, 1960, 1965, 1969, 1973, 1976, and 2003.[4] The MBI that started in 2014 was by far the third largest MBI in the Baltic Sea. Only the inflows of 1951 and 1921/1922 were larger than it.[6]

Previously, it was believed that there had been a genuine decline in the number of MBIs after 1980, but recent studies have changed our understanding of the occurrence of saline inflows.[3][7] Especially after the lightship Gedser Rev discontinued regular salinity measurements in the Belt Sea in 1976, the picture of the inflows based on salinity measurements remained incomplete. At the Leibniz Institute for Baltic Sea Research (Warnemünde, Germany), an updated time series has been compiled, filling in the gaps in observations and covering Major Baltic Inflows and various smaller inflow events of saline water from around 1890 to the present day. The updated time series is based on direct discharge data from the Darss Sill and no longer shows a clear change in the frequency or intensity of saline inflows. Instead, there is cyclical variation in the intensity of MBIs at approximately 30-year intervals.[7][8]

Effects on the state of the Baltic Sea and its ecosystem

Major Baltic inflows (MBIs) are the only natural phenomenon capable of oxygenating the deep saline waters of the Baltic Sea, making their occurrence crucial for the ecological state of the sea.[6] The salinity and oxygen from MBIs significantly impact the Baltic Sea's ecosystems, including the reproductive conditions of marine fish species such as cod, the distribution of freshwater and marine species, and the overall biodiversity of the Baltic Sea.[9]

The heavy saline water brought in by MBIs slowly advances along the seabed of the Baltic Proper at a pace of a few kilometers per day, displacing the deep water from one basin to another.[10] Although some oxygen is transported from the North Sea to the Baltic Sea, only a small portion of the oxygen responsible for renewing the deep basins originates from the Baltic Sea entrance area. In the southwestern basins of the Baltic Sea (Arkona sea, Bornholm basin), there is already oxygen present in the water column, and the role of the saltwater pulse is to entrain and direct it towards the deep basins of the sea.[11]

It has been observed that the oxygen supplied by saline inflows is consumed more rapidly in the Baltic Proper than before. In 1993, the oxygen replenishment was depleted in about 17 months, in 2003, it took approximately 13 months, and in 2015, it was exhausted in just six months.[6]

The intense oxygen consumption in the Baltic Sea's seafloor is due to prolonged nutrient loading and the impacts of climate change.[3] Although a single MBI can oxygenate both the deep water hydrogen sulfide and ammonia, multiple consecutive inflows would be required to raise the oxygen level to a satisfactory level. The so-called "oxygen debt" was estimated to be about 20 million tons in 2020. This is the amount of oxygen that should be transported to the Baltic Proper via saline inflows to raise its oxygen concentration to the 3 mL/L level observed after the MBI of 1993. With a single 200 km³ MBI, approximately 2 million tons of oxygen are transported.[12]

References

  1. ^ "Itämeren muoto, ala ja tilavuus" (in Finnish). Finnish Meteorological Institute. Retrieved 2023-07-24.
  2. ^ a b c d Matthäus, Wolfgang; Franck, Herbert (1992-12-01). "Characteristics of major Baltic inflows—a statistical analysis". Continental Shelf Research. 12 (12): 1375–1400. Bibcode:1992CSR....12.1375M. doi:10.1016/0278-4343(92)90060-W. ISSN 0278-4343.
  3. ^ a b c d Lehmann, Andreas; Myrberg, Kai; Post, Piia; Chubarenko, Irina; Dailidiene, Inga; Hinrichsen, Hans-Harald; Hüssy, Karin; Liblik, Taavi; Meier, H. E. Markus; Lips, Urmas; Bukanova, Tatiana (2022-02-16). "Salinity dynamics of the Baltic Sea". Earth System Dynamics. 13 (1): 373–392. Bibcode:2022ESD....13..373L. doi:10.5194/esd-13-373-2022. ISSN 2190-4979.{{cite journal}}: CS1 maint: unflagged free DOI (link)
  4. ^ a b "Inflows to the Baltic". Swedish Meteorological and Hydrological Institute. 2014-01-15. Retrieved 2023-07-24.
  5. ^ "Itämeren suolapulssin liikkeet ihmetyttävät tutkijoita" (in Finnish). Yle News. 2016-08-05. Retrieved 2023-07-24.
  6. ^ a b c Meier, H. E. Markus; Väli, Germo; Naumann, Michael; Eilola, Kari; Frauen, Claudia (2018-04-12). "Recently Accelerated Oxygen Consumption Rates Amplify Deoxygenation in the Baltic Sea". Journal of Geophysical Research: Oceans. 123 (5): 3227–3240. Bibcode:2018JGRC..123.3227M. doi:10.1029/2017JC013686. ISSN 2169-9275.
  7. ^ a b Mohrholz, Volker (2018). "Major Baltic Inflow Statistics – Revised". Frontiers in Marine Science. 5. doi:10.3389/fmars.2018.00384. ISSN 2296-7745.
  8. ^ "Major Baltic Inflow statistics - IOW". www.io-warnemuende.de. Retrieved 2023-07-23.
  9. ^ Climate Change in the Baltic Sea. 2021 Fact Sheet. Baltic Sea Environment Proceedings n°180. HELCOM/Baltic Earth 2021.
  10. ^ "Arandan Itämeren seurantamatka: suolapulssi etenee eteläisellä Itämerellä, Suomenlahden ja Selkämeren fosforipitoisuus viime talvea korkeampi" (in Finnish). Finnish Environment Institute. 2015-02-12. Retrieved 2023-07-24.
  11. ^ Neumann, Thomas; Radtke, Hagen; Seifert, Torsten (2017-01-25). "On the importance of Major Baltic Inflows for oxygenation of the central Baltic Sea: IMPORTANCE OF MAJOR BALTIC INFLOWS". Journal of Geophysical Research: Oceans. 122 (2): 1090–1101. doi:10.1002/2016JC012525.
  12. ^ Rolff, Carl; Walve, Jakob; Larsson, Ulf; Elmgren, Ragnar (2022-11-01). "How oxygen deficiency in the Baltic Sea proper has spread and worsened: The role of ammonium and hydrogen sulphide". Ambio. 51 (11): 2308–2324. doi:10.1007/s13280-022-01738-8. ISSN 1654-7209. PMC 9481832. PMID 35737272.
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