Fate of colloids during estuarine mixing in the Arctic

[摘要] The estuarine behavior of organic carbon (OC) and trace elements (TE) wasstudied for the largest European sub-Arctic river, which is the SevernayaDvina; this river has a deltaic estuary covered in ice during severalhydrological seasons: summer (July 2010, 2012) and winter (March 2009)baseflow, and the November–December 2011 ice-free period. Colloidal forms ofOC and TE were assessed for three pore size cutoffs (1, 10, and 50 kDa)using an in situ dialysis procedure. Conventionally dissolved(< 0.22 μm) fractions demonstrated clear conservativebehavior for Li, B, Na, Mg, K, Ca, Sr, Mo, Rb, Cs, and U during the mixing offreshwater with the White Sea; a significant (up to a factor of 10)concentration increase occurs with increases in salinity. Si and OC alsodisplayed conservative behavior but with a pronounced decrease inconcentration seawards. Rather conservative behavior, but with much smallerchanges in concentration (variation within ±30%) over a full rangeof 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‰) was exhibited by Fe, Al, Zr, Hf, and light REE.Finally, certain divalent metals exhibited non-conservative behavior with aconcentration gain at low (~ 2–5‰, Ba, Mn) or intermediate(~ 10–15‰, Ba, Zn, Pb, Cd) salinities, which is most likelylinked to TE desorption from suspended matter or sediment outflux.

The most important result of this study is the elucidation of the behavior ofthe "truly" dissolved low molecular weight LMW_{< 1 kDa}fraction containing Fe, OC, and a number of insoluble elements. Theconcentration of the LMW fraction either remains constant or increases itsrelative contribution to the overall dissolved(< 0.22 μm) pool as the salinity increases. Similarly,the relative proportion of colloidal (1 kDa–0.22 μm) pool for theOC and insoluble TE bound to ferric colloids systematically decreasedseaward, with the largest decrease occurring at low(< 5‰) salinities.

Overall, the observed decrease in the colloidal fraction may be related tothe coagulation of organo-ferric colloids at the beginning of the mixing zoneand therefore the replacement of theHMW_{1 kDa–0.22 μm} portion by theLMW_{< 1 kDa} fraction. These patterns are highly reproducibleacross different sampling seasons, suggesting significant enrichment of themixing zone by the most labile (and potentially bioavailable) fraction of theOC, Fe and insoluble TE. The size fractionation of the colloidal materialduring estuarine mixing reflects a number of inorganic and biologicalprocesses, the relative contribution of which to element speciation variesdepending on the hydrological stage and time of year. In particular,LMW_{< 1 kDa} ligand production in the surface horizons of themixing zone may be linked to heterotrophic mineralization of allochthonousDOM and/or photodestruction. Given the relatively low concentration ofparticulate versus dissolved load of most trace elements, desorption from theriver suspended material was less pronounced than in other rivers in theworld. As a result, the majority of dissolved components exhibited eitherconservative (OC and related elements such as divalent metals) ornon-conservative, coagulation-controlled (Fe, Al, and insoluble TE associatedwith organo-ferric colloids) behavior. The climate warming at high latitudesis likely to intensify the production of LMW_{< 1 kDa} organicligands and the associated TE; therefore, the delivery of potentiallybioavailable trace metal micronutrients from the land to the ocean mayincrease.

[学科分类] 海洋学与技术