Salt‐induced flocculation of dissolved organic matter and iron is controlled by their concentration and ratio in boreal coastal systems

Rivers and estuaries are important conduits and reactors for organic matter (OM). Despite the substantial export of terrestrial OM by rivers, only a small proportion of this material reaches the open ocean. One potentially important mechanism contributing to the removal of terrestrial dissolved OM (DOM) is flocculation; a process that transforms DOM into larger size fractions that can settle into sediment pools. Here we investigate the role of flocculation in adjacent boreal coastal systems over three consecutive seasons. Flocculation experiments, which include the artificial salting of freshwater DOM to mid (12 psu) and high salinity (25 psu), and a 1:1 mixture of freshwater DOM with their respective saline endmember, highlight a potentially important source of particulate carbon (PC) in boreal estuaries. Along with a 3.5% +/- 1.0% removal of dissolved organic carbon (DOC) and 44% +/- 16% dissolved iron (DFe), the potential for flocculation of these constituents significantly increased with increasing salinity from 12 to 25 psu. The particulate matter (PM) produced by salt-induced flocculation was comparable to in situ PC concentrations (in situ PC = 27.5 mu mol L-1 and flocculated PC = 15.0 mu mol L-1) and stable carbon isotopic signatures (in situ PM = -28.8 parts per thousand and flocculated PM = -28.3 parts per thousand). DFe:DOC and S-r were the only parameters that could significantly explain the degree of carbon and iron flocculation. This demonstrates the importance of DOC, DFe, and optical properties, and the predictive value of DFe:DOC for understanding DOM susceptibility to flocculation and its relevance and contribution to regional and global carbon budgets.