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The flavonoid content in leaves of Zostera marina and the endangered Zostera noltii, including mono- and disulphated flavonoids, from different sample localities were characterized. Seasonal variation of both individual and total flavonoid, as well as rosmarinic acid concentration were revealed. Minor amounts of luteolin 7-(6″-malonyl)glucoside ( 6 ) and apigenin7-(6″-malonyl)glucoside ( 11 ) were identified in Z. noltii for the first time. The total flavonoid content was found to be higher in Z. noltii than in Z. marina at most of the examined localities, and the qualitative flavonoid content was somewhat different in the two species. The quantitative variation of flavonoids and rosmarinic acid was found to be relatively consistent from year to year in Z. marina during a period of three years. The two species appeared though to have a different flavonoid production in the various seasons at the West coast. While Z. marina had the highest content in young leaves in May or June, with a markedly decrease from June to September and the lowest measured content in February, Z. noltii had the lowest measured flavonoid content in May/June followed by an increase from June to September and the highest measured content during wintertime in February. The observed seasonal differences may be related to the fact that Z. noltii is considered a perennial, thermophilous species, and the increasing flavonoid production during the colder seasons from September to March/April in Norway may serve as a protective function. [ABSTRACT FROM AUTHOR]