Contributions of autochthonous and allochthonous sources to dissolved organic matter in a large, shallow, eutrophic lake with a highly calcareous catchment

We traced the origin of dissolved organic matter (DOM) in the large, shallow, eutrophic Lake Vortsjarv in Estonia. Allochthonous DOM (Al-DOM) had higher delta C-13 values than autochthonous DOM (Au-DOM). The delta C-13 of inflow DOM varied from -228.2 parts per thousand to -225.4 parts per thousand (mean -226.7 parts per thousand) and in-lake DOM varied from -228.4 parts per thousand to -26.1 parts per thousand (mean -227.2 parts per thousand). Low stable isotope (SI) signatures of Au-DOM were caused by relatively C-13-depleted values of its precursors (mainly phytoplankton) with mean delta C-13 of -28.9 parts per thousand. SI signatures of dissolved inorganic carbon (DIC) in the inflows and in the lake were also relatively low (from -15.1 parts per thousand to -23.28 parts per thousand). SI values of DOM were lower during the active growing season from May to September and higher from October to April, with the corresponding estimated average proportions of Al-DOM 68% and 81%. The proportion of Al-DOM decreased with increasing water temperature, chlorophyll a, and pH and increased with increasing water level and concentration of yellow substances and DIC. The high proportion of Al-DOM in Vortsjarv shows that, even in this highly productive ecosystem, the labile Au-DOM produced is rapidly utilized and degraded by microorganisms and thus makes a relatively small contribution to the instantaneous in-lake DOM pool.