| Summary: | The estuarine behavior of organic carbon (OC) and trace elements (TE) was studied for the largest European sub-Arctic river, which is the Severnaya Dvina; this river has a deltaic estuary covered in ice during several hydrological seasons: summer (July 2010, 2012) and winter (March 2009) baseflow, and the November-December 2011 ice-free period. Colloidal forms of OC and TE were assessed for three pore size cutoffs (1, 10, and 50 kDa) using an in situ dialysis procedure. Conventionally dissolved (< 0.22 mu m) fractions demonstrated clear conservative behavior for Li, B, Na, Mg, K, Ca, Sr, Mo, Rb, Cs, and U during the mixing of freshwater with the White Sea; a significant (up to a factor of 10) concentration increase occurs with increases in salinity. Si and OC also displayed conservative behavior but with a pronounced decrease in concentration seawards. Rather conservative behavior, but with much smaller changes in concentration (variation within +/- 30%) over a full range of salinities, was observed for Ti, Ni, Cr, As, Co, Cu, Ga, Y, and heavy REE. Strong non-conservative behavior with coagulation/removal at low salinities (< 5 parts per thousand) was exhibited by Fe, Al, Zr, Hf, and light REE. Finally, certain divalent metals exhibited non-conservative behavior with a concentration gain at low (similar to 2-5 parts per thousand, Ba, Mn) or intermediate (similar to 10-15 parts per thousand, Ba, Zn, Pb, Cd) salinities, which is most likely linked to TE desorption from suspended matter or sediment outflux. The most important result of this study is the elucidation of the behavior of the 'truly' dissolved low molecular weight LMW < 1 kDa fraction containing Fe, OC, and a number of insoluble elements. The concentration of the LMW fraction either remains constant or increases its relative contribution to the overall dissolved (< 0.22 mu m) pool as the salinity increases. Similarly, the relative proportion of colloidal (1 kDa-0.22 mu m) pool for the OC and insoluble TE bound to ferric colloids ...
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