Lake Mariout (Arabic: بحيرة مريوط Boḥēret Maryūṭ , IPA: [boˈħeːɾet mɑɾˤˈjuːtˤ]), also spelled Maryut or Mariut, is a brackish lake in northern Egypt. The lake area covered 200 km² and had a navigable canal at the beginning of the 20th century, but at the beginning of the 21st century, it covers only about 50 km².
The name derives from Mareotis or Marea, the name of the lake in ancient times.
At least 250 years ago, the lake was fresh water, and much of it would dry up during the period just before the Nile flooded again. A storm in 1770 breached the sea wall at Abu Qir, creating a sea-water lake known as Lake Abu Qir. The salt waters were kept separate from Lake Mariout by the canal that allowed fresh water to travel from the Nile to Alexandria. As part of the Siege of Alexandria, on 13 March 1801, the British cut the canal, allowing a great rush of sea water from Lake Abu Qir into Lake Mariout. Lake Abu Qir ceased to exist, and Lake Mariout became brackish instead of fresh. 
It is separated from the Mediterranean Sea by the narrow isthmus on which the city of Alexandria was built. The lake shore is home to fisheries and saltworks. As far back as the early 1900s, it was documented that salt was being refined from the western part of the lake. Some of the marshy areas around the lake have been reclaimed for new building as the city grows and how to manage the pollution that goes into the lake has been considered.
There was a bishopric of Mareotes, in the Roman province of Aegyptus Primus, which was a suffragan of the Metropolitan Archbishop of the Patriarchate of Alexandria, but it faded like most in Roman Egypt, plausibly at the advent of Islam.
Two bishops are historically documented :
- Ischiras (mentioned circa 335)
- Pistos (mentioned in 337)
The diocese was nominally restored in 1933 as Titular bishopric of Mareotes (Latin = Curiate Italian) / Mareoten(sis) (Latin adjective). Although technically a Latin titular bishopric, it has had several Eastern Catholic incumbents, notably of Egypt's native Coptic Catholic Church sui iuris (Alexandrian Rite).
It has had the following incumbents, so far of the fitting Episcopal (lowest) rank:
- Antonios Aziz Mina (2002.12.21 – 2006.01.03) as Bishop of Curia of the Coptic Catholic Patriarchate (Egypt) ([2002.12.19] 2002.12.21 – 2006.01.03), next Eparch (Bishop) of Guizeh of the Copts (Egypt) (2006.01.03 – 2017.01.23)
- Botros Fahim Awad Hanna (2006.09.06 – 2013.04.08) as Bishop of Curia of Egypt of the Copts ([2006.08.31] 2006.09.06 – 2013.04.08), next Eparch (Bishop) of Minya of the Copts (Egypt) ([2013.03.25] 2013.04.08 – ...)
- Cesar Essayan, Conventual Franciscans (O.F.M. Conv.) (2016.08.02 – ...) as (Apostolic Vicar of Beirut (Lebanon).
In 2015 a stele, resembling the Rosetta Stone and dating back some 2,200 years, was discovered in ancient ruins called Taposiris Magna Temple site at Lake Mariout. Measuring 41 inches (105 cm) by 25.6 inches (65 cm) by 7 inches (18 cm), its message commemorates two Ptolemaic pharaohs and Cleopatra I. There are ancient tombs located on the shores of the lake.
The fish species Nile perch lives in the lake, although it principally needs fresh water to live in, and the lake contains some salt. In 1939, a small lake, called the Nozha Hydrodrome was "isolated from Lake Mariout" and this allowed for the Nile perch to flourish there.
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- Michel Lequien, Oriens christianus in quatuor Patriarchatus digestus, Paris 1740, vol. II, coll. 529-530
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