Whole-lake mineralization of allochthonous and autochthonous organic carbon in a large humic lake (Ortrasket, N. Sweden)

Organic carbon mineralization was studied. in a large humic lake (Lake Ortr asket) in northern Sweden during a well-defined summer stratification perio d following high water flow during snowmelt. Several independent methods in cluding plankton counts, measurements of bacterioplankton and phytoplankton production, stable isotope monitoring, sediment trapping, and mass balance calculations were used. Total organic carbon mineralization showed a summe r mean of 0.3 g C m(-2) d(-1) and was partitioned about equally between wat er and sediment. In the water column, organic matter was mineralized by bac teria (60%) and protozoan and metazoan zooplankton (30%), as well as by pho tooxidation (10%). Most of the mineralized organic carbon was of allochthon ous origin. Primary production in the lake contributed at most 5% of the to tal organic carbon input and about 20% of the total organic carbon minerali zation. Total carbon mineralization in. the epilimnion and metalimnion agre ed well with an estimate of CO2 evasion from the stratified lake, while CO2 accumulation in the hypolimnion matched the O-2 consumption and resulted i n a very negative delta C-13 of DIC before autumn overturn (-23 parts per t housand). Isotopic compositions of DIC and POC confirmed the dominant influ ence of terrestrial organic input on the cycling of both organic and inorga nic carbon in the lake.