A botanical term referring to a plant community or formation dominated by char or characeae (a family of algae), or a similar ecological grouping.
From Latin 'character' plus the suffix '-etum' (denoting a plant community or formation). This is a highly technical ecological classification term.
Characetums are essentially underwater ecosystems dominated by stoneworts—these ancient plants have been algae for over 400 million years and are so algebraically complex they're more related to land plants than other algae.
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