Abstract Earth's lakes at northern latitudes are mostly ice‐covered in winter. When lake water freezes, some organic matter dissolved in the water is excluded from the ice. We performed complementary field sampling and laboratory freeze‐up experiments to explore how freeze‐up may impact the partitioning and composition of dissolved organic matter (DOM) in boreal lakes. We found that 16.2 ± 4.7 of dissolved organic carbon (DOC) was retained in the overlying ice, 81.3 ± 5.7 of DOC was expelled to underlying unfrozen water, and 1.3 ± 0.7 was expelled as flocs. During ice formation, nitrogen (TDN, total dissolved nitrogen), ions (specific conductance), and oxidized and aromatic DOM were preferentially expelled to the underlying water column. The apparent retention factor DOCIce: DOCBefore decreased from clearwater to brownwater lakes, that is, with increasing allochthonous DOC lost from lake ice, indicating that DOM exclusion from the ice cover will become more prevalent as lakes experience browning.
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