SEASONAL-CHANGES IN IRON TRANSPORT AND NATURE OF DISSOLVED ORGANIC-MATTER IN A HUMIC RIVER IN NORTHERN FINLAND

Changes in the concentration of iron and dissolved organic matter (DOM), and in the colour and fluorescence properties in the River Kiiminkijoki were investigated as functions of the seasonal flow regime over a two‐year period. The iron concentration in filtrates and the ratio of Fe to DOC in the river increased under low flow conditions and decreased during the flood periods. The colour of the dissolved organic matter increased with increasing iron content, the effect being more pronounced during the warm period of the year than in winter. The ratio of fluorescence to DOC increased during the warm period of the year but not in winter, and decreased rapidly with discharge at the beginning of the flood period in autumn. The results give indications of the origin, formation, nature, and fate of the DOM in the river water. Temperature‐dependent microbiological processes in the formation and sedimentation of iron‐organic colloids seem to be important. Estimates are given for the amounts and transport rates of iron discharged into the Gulf of Bothnia by the river. Copyright © 1990 John Wiley & Sons, Ltd