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

Kaire Toming, Lea Tuvikene, Sirje Vilbaste, Helen Agasild, Malle Viik, Anu Kisand, Tõnu Feldmann, Tõnu Martma, Roger I. Jones and Tiina Nõges

Limnol. Oceanogr., 58(4), 2013, 1259-1270 | DOI: 10.4319/lo.2013.58.4.1259

ABSTRACT: We traced the origin of dissolved organic matter (DOM) in the large, shallow, eutrophic Lake Võrtsjärv in Estonia. Allochthonous DOM (Al-DOM) had higher δ13C values than autochthonous DOM (Au-DOM). The δ13C of inflow DOM varied from −28.2‰ to −25.4‰ (mean −26.7‰) and in-lake DOM varied from −28.4‰ to −26.1‰ (mean −27.2‰). Low stable isotope (SI) signatures of Au-DOM were caused by relatively 13C-depleted values of its precursors (mainly phytoplankton) with mean δ13C of −28.9‰. SI signatures of dissolved inorganic carbon (DIC) in the inflows and in the lake were also relatively low (from −15.1‰ to −3.28‰). 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 Võrtsjärv 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.

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