Hydrodynamical events shape lake-type-specific dynamics of manganese in boreal catchment

Abstract

Studies on lakes as modifiers of manganese fluxes in pristine catchments are limited, despite manganese being a sensitive redox-change indicator and participating in the cycling of many key elements. We compiled 26 years of monitoring data from interconnected monomictic humic lake and dimictic clear-water lake to estimate the mean daily lakes' imports, pools and exports of manganese. The humic lake, surrounded by peatlands, exported annually 1.7-fold more manganese per lake area than the clear-water lake, which is enclosed by mineral soils. Hydrodynamical events initiated lake-type-specific manganese refilling and removing periods lasting for months. Both lakes accumulated manganese in the near-bottom water layers during the period of ice-cover. However, the incomplete spring turnover in the humic lake left the manganese in the bottom layers untouched, and manganese was flushed to the clear-water lake mainly with surface water. The manganese that accumulated in the bottom layers was transported to the surface and exported to the clear-water lake only after the autumn turnover. In the clear-water lake, import from the humic lake and the spring turnover filled the surface water pool, which enhanced the export of manganese originating from both the catchment and lake itself. Most of the lake’s manganese was exported during ascending spring flooding while descending flooding enhanced in-lake removal. We conclude that lake-type-specific hydrodynamical events must be considered when using manganese as an indicator of redox-changes. Further research is needed to determine how climate-change induced shifts in the timing of hydrodynamical events affect manganese dynamics and exports in humic and clear-water lakes in boreal catchments.