Geochemistry of manganese in the Kalix River, northern Sweden

Abstract

Dissolved and suspended Mn in the Kalix River, northern Sweden, were measured weekly over a period of eighteen months. During the same period four lakes in the Kalix catchment were sampled at their outlets and in vertical profiles within the lakes, together with a stream draining a series of mires with shallow lakes. Snow melting in mid-May increased the dissolved Mn concentration in the river tenfold, compared with a concentration of 5 μg L −1 during the winter discharge (January to April). We suggest that the increase was caused by Mn-rich mire water mixing with melting snow and being transported to the river. Large concentrations of dissolved Mn built up in the hypolimnion of the lakes studied during the icecovered period. Break-up of the ice and spring-overturn in June increased the dissolved Mn concentration tenfold in lake discharge and a concomitant peak in the dissolved Mn concentration was observed in the river. Lake-derived Mn was the dominant source for Mn in the river during this time. Suspended Mn in the river was hosted mainly in detrital particles during flood in May. In mid-June, non-detrital suspended Mn started to accumulate and reached a maximum in late July and early August. The Mn Al ratio was 25 times higher during this period than during flood in May, suggesting the precipitation of an Mn-oxyhydroxide phase. The precipitation of the non-detrital Mn-rich phase was correlated in time with increased temperature, increased pH and increased concentration of suspended biogenic particles. The precipitation of dissolved Mn was biologically mediated. Sedimentation and mineralisation of the non-detrital Mn phase in river and lake sediments resulted in a steady increase of the dissolved Mn concentration in the river water during autumn.