The role of catchment soils and land cover on dissolved organic matter (DOM) properties in temperate lakes

English. This work was supported by Estonian Ministry of Education andResearch through the institutional project IUT 21-2 and the grantPUT1598, by the European Regional Development Fund through thescholarship programme Dora Plus. The Estonian Ministry ofEnvironment and Estonian Environment Agency supported data collection in the national monitoring programme.

English. Dissolved organic matter (DOM) is a critical component in freshwater ecosystem functioning. The main sourcesof DOM in lakes are allochthonous inputs from the catchment and autochthonous in-lake production. This studyfocused on the role of catchment characteristics on the qualitative and quantitative properties of DOM in smalltemperate lakes along a gradient of alkalinity. We examined DOM properties based on the optical absorbanceand fluorescence measurements of water from 34 Estonian lakes. The content and composition of DOM werehighly diverse in the lakes studied, e.g. the dissolved organic carbon (DOC) concentrations varied from 3.2 to53.0 mg L−1. Land cover, soil, and catchment hydrology and geology had substantial effects on DOM in lakes.Stock of soil organic carbon (SOC) in the catchment and water exchange rate (a descriptor of catchment hy-drology, reciprocal of water residence time) had major positive effects on DOC concentrations. The aromaticityand molecular weight of DOM, i.e. the relative abundance of humic substances, and the dominance of al-lochthonous DOM increased with the drainage ratio (catchment area/lake area) and the percentages of bogs, andDystric and Fibric Histosols (peat soils in transitional mires and bogs, respectively) in the catchments.Dominance of non-humic over humic substances and autochthonous over allochthonous DOM in lakes corre-sponded to calcareous catchments and higher percentages of Gleyic Rendzinas (thin soils on calcareous rock),Sapric Histosols (peat soils in mires) and open spaces (areas with little vegetation). Our results showed that soilvariables had in general a greater effect than land cover and were more informative for describing the role ofcatchment characteristics on DOM in lakes. Patterns in DOM quantity and quality found in our study weresimilar to patterns found in other temperate lakes; therefore, our results have important implications for un-derstanding catchment-lake interactions across the temperate region.