Samples of sediment from lakes in Minnesota and Yellowstone National Park (YNP) were analyzed for organic carbon (OC), hydrogen richness by Rock-Eval pyrolysis, and stable carbon- and nitrogen-isotope composition of bulk organic matter. Values of δ¹³C of lake plankton tend to be around –28 to –32 parts per thousand (‰). Organic matter with values of δ¹³C in the high negative 20s overlap with those of organic matter derived from C3 higher terrestrial plants but are at least 10‰ more depleted in ¹³C than organic matter derived from C4 terrestrial plants. If the organic matter is produced mainly by photosynthetic plankton and is not oxidized in the water column, there may be a negative correlation between H-richness...>>MORE