Patterns and regulation of dissolved organic carbon: An analysis of 7,500 widely distributed lakes

Dissolved organic carbon (DOC) is a key parameter in lakes that can affect numerous features, including microbial metabolism, light climate, acidity, and primary production. In an attempt to understand the factors that regulate DOC in lakes, we assembled a large database (7,514 lakes from 6 continents) of DOC concentrations and other parameters that characterize the conditions in the lakes, the catchment, the soil, and the climate. DOC concentrations were in the range 0.1–332 mg L21, and the median was 5.71 mg L21. A partial least squares regression explained 48% of the variability in lake DOC and showed that altitude, mean annual runoff, and precipitation were negatively correlated with lake DOC, while conductivity, soil carbon density, and soil C : N ratio were positively related with lake DOC. A multiple linear regression using altitude, mean annual runoff, and soil carbon density as predictors explained 40% of the variability in lake DOC. While lake area and drainage ratio (catchment : lake area) were not correlated to lake DOC in the global data set, these two factors explained significant variation of the residuals of the multiple linear regression model in several regional subsets of data. These results suggest a hierarchical regulation of DOC in lakes, where climatic and topographic characteristics set the possible range of DOC concentrations of a certain region, and catchment and lake properties then regulate the DOC concentration in each individual lake. Dissolved organic carbon (DOC) is a major modulator of the structure and function of lake ecosystems. The DOC pool of lakes consists of both autochthonous DOC (i.e., produced in the lake) and allochthonous DOC (i.e., produced in the catchment), although allochthonous DOC is generally believed to represent the larger fraction of the total DOC in lakes. Due to the dark color of many DOC compounds, DOC affects the thermal structure and mixing depth of lakes (Fee et al. 1996). DOC is an important regulator of ecosystem production, since its absorptive properties impede photosynthesis (Jones 1998), while at the same time, it serves as a substrate for heterotrophic bacteria (Tranvik 1988). These compounded effects of DOC on primary and secondary production result in many lakes being net heterotrophic ecosystems, with the consequence that they export CO2 to the atmosphere (Sobek et al. 2005). Furthermore, DOC can affect the fate of other solutes, such as metals (Perdue 1998) or organic contaminants (Haitzer et al. 1998). Also, DOC is an efficient protector of lake biota from harmful ultraviolet (UV) radiation (Molot et al. 2004), while hormone-like effects of DOC may affect the physiology of aquatic animals (Steinberg et al. 2004). This wide array of effects on lake ecosystems explains the considerable interest that DOC has received during the last decades. Principally, the DOC concentration in lakes is regulated in a hierarchical manner (Mulholland et al. 1990). Allochthonous DOC is leached from terrestrial soils to lakes, and the amount of DOC released from soils is determined by the water yield and by the production of leachable organic carbon in soils. In the lakes, autochthonous DOC is produced and, together with allochthonous 1 To whom correspondence should be addressed. Present address: Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Sciences, Swiss Federal Institute of Technology Zurich (ETH), Universitätsstrasse 16, 8092 Zürich, Switzerland. Tel: +41-44-6328335, fax: +41-44-6331122 (sebastian. sobek@env.ethz.ch).