Characterization ascertained from δ13C and Δ14C of particulate organic matter in surface water from a shallow and semi-closed Lake Kiba

Concentrations of stable and radioactive carbon isotopes of organic matter were determined for suspended solids in surface water from the shallow and semi-closed Lake Kiba, Japan during June 2014 - March 2016. Particulate organic carbon (POC) concentrations were 0.44-5.01 mg L-1, and δ13C and Δ14C values were, respectively, -30.3 to -22.8 ‰ and -156 to -33 ‰. The organic matter in suspended solid samples consistently showed depleted 14C, with an average Δ14C value of -81 ± 37‰. The carbon isotopes showed seasonal variation, being higher in summer, and with a positive correlation to POC contents. These results suggest that the POC characteristics are controlled by a mixture of two endmembers, organic matter produced by phytoplankton activity within the lake and the watershed organic matter, under a shallow and semi-closed environment.