Dissolved organic matter in large lakes: a key but understudied component of the carbon cycle

The world's largest five freshwater lakes include tropical meromictic, temperate weakly stratified, and temperate holomictic lakes ranging in age from twenty-five million to similar to 10,000 years old. They contain > 50% of Earth's surface liquid freshwater and 15% of Earth's lake-water dissolved organic carbon. Large lakes, however, are understudied in terms of carbon cycling, including the roles of dissolved organic matter (DOM) as dissolved organic carbon (DOC), as chromophoric dissolved organic matter (CDOM), and at the compound class or molecular level. Existing data show that the five largest lakes have lower DOC and CDOM concentrations and clearer water than the global lake median. In Baikal, Superior, and Michigan, however, CDOM is the main non-water absorber of visible light. DOM in these three lakes contain a large proportion of reworked, terrestrially-derived organic matter. Isotopic data for Lake Superior show that its DOM consists of carbon converted into organic matter in the previous decade, though it is difficult to resolve if this organic matter is terrestrial or autochthonous material. No DOC or CDOM characterization or reactivity data exist for the tropical large lakes. Large lakes seem to fall on a continuum with smaller lakes and ocean water, in which water residence time (WRT) and DOC age are linearly related and DOC concentration is negatively correlated with log(WRT). However, existing data are sparse and the mechanisms behind these trends remain to be determined.