Balancing of organic carbon fluxes into lake sediments from various sources is of basic importance for evaluation and interpretation of sedimentary organic matter records. Limnological and stable carbon isotope investigations are coupled as two separate tools to approach the organic carbon cycle of Lake Holzmaar and Lake Stechlin. Main organic carbon sources at our sampling sites comprise primary production, macrophytes and terrestrial plants. Our sampling strategy included these compartments if necessary in its seasonal cycle. Both approaches give similar results concerning the origin and composition of sedimentary organic matter. The dominant source is phytoplankton material, both macrophytes and terrestrial plants are less important. For Lake Stechlin sediments 70% of organic carbon originates from primary production. Thus interpretation of variations in quantity and d13C-values in sedimentary organic matter must focus on primary production and its intraannual and interannual variability.
Basing on the results of this concept we obtain a model of the limnological carbon cycle that can be compared with sedimentary records. d13C-values of bulk organic carbon give hints for paleoenvironmental changes because they may represent changes of carbon sources.