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 ~ 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.
Bibliographical noteFunding Information:
This work was funded in part by a sabbatical leave to ECM from the University of Minnesota Duluth. Additional funding came from the Minnesota Environment and Natural Resources Trust Fund as recommended by the Legislative-Citizen Commission on Minnesota Resources (LCCMR) and the National Science Foundation (Grant OCE 0825600 to ECM). The authors thank Dr. Penny Johnes and the organizers and participants of the “Dissolved organic matter in freshwaters: nature, origins and ecological significance meeting” (Royal Society, January 20 & 21, 2020) for encouragement and discussion that led to this paper. Thanks also to Robert Sterner (Large Lakes Observatory) and 2 anonymous reviewers whose comments significantly improved the manuscript.
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