Characterization ascertained from δ 13 C and Δ 14 C of particulate organic matter in surface water from a shallow and semi-closed Lake Kiba

remained about double the national standard of lake class 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 δ 13 C and ∆ 14 C values were, respectively, –30.3 to –22.8 ‰ and –156 to –33 ‰. The organic matter in suspended solid samples consistently showed depleted 14 C, with an average ∆ 14 C 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.