Refractory, semi-labile and labile fractions were identified in the total DOC pool.
Refractory DOC is the most abundant fraction irrespective of the DOC source (marine vs. terrestrial).
Marine DOC in the Baltic Sea is more bioavailable and more reactive than terrestrial DOC.
Biomineralization of marine DOC consumes 30–50% more O2 than biomineralization of terrestrial DOC.