ORGANIC-MATTER, IRON AND NUTRIENT TRANSPORT AND NATURE OF DISSOLVED ORGANIC-MATTER IN THE DRAINAGE-BASIN OF A BOREAL HUMIC RIVER IN NORTHERN FINLAND

Organic carbon and iron transport into the Gulf of Bothnia and the sea sonal changes in the nature of dissolved organic matter (DOM) were stu died in 1983 and 1984 at the mouth of the River Kiiminkijoki, which cr osses an area of minerotrophic mires in northern Finland. Organic and inorganic transport within the drainage basin was studied in the summe r and autumn of 1985 and 1986, and information was also acquired on th e nature of DOM by gel filtration in river, brook and peat mining wate r. The river water is organically coloured, with high organic matter, Fe concentrations and transport rates, while the values for phosphorus and nitrogen are low. The organic carbon concentrations increase with discharge with the major organic load restricted to the time of sprin g floods. The results indicate that the dissolved organic carbon (DOC) is mainly of terrestrial origin, leaching mostly from peatlands. Auto chthonous DOC sources seem also to be important in summer. The DOC con centrations decrease under low flow conditions probably mainly due to sedimentation and decomposition of humic substances. The proportion of drifting algae as a particulate organic carbon (POC) source seems to increase in summer. The changes in the ratio of Fe/DOC, the colour of the DOM and the ratio of fluorescence to DOC with discharge give indic ations of the origin, formation, nature and fate of the DOM in the riv er water. Temperature-dependent microbiological processes in the forma tion and sedimentation of Fe-organic colloids seem to be important. Es timates are given for the amounts and transport rates of organic carbo n and Fe discharged into the Gulf of Bothnia by river. High apparent m olecular weight (HAMW) organic colloids are important for the organic, Fe and P transport in the basin. The gel filtration data suggest that the DOM in the water consists mainly of fulvic acids, although humic acids are also important. The apparent molecular weight of the DOM is higher than in the lakes in the area of ombrotrophic peatlands in sout hern Finland. Colour in this humic water increases with DOC and Fe con centrations. The results indicate an increase in the mobilization of H AMW Fe-organic colloids in the peatlands following drainage and peat m ining. The transport of inorganic nitrogen from the peatlands in the a rea and in the river is increasing due to peat mining. The correspondi ng changes in the transport of organic matter, Fe and P are less marke d but point in a similar direction.