Semi-annual carbon and nitrogen isotope variations in the water column of Lake Bled, NW Slovenia

The variability in the stable isotope signature of carbon and nitrogen in particulate organic matter and dissolved species in the water column of the mesotrophic sub-alpine Lake Bled in NW Slovenia has been determined. After the algae bloom from August to December in 2008, samples were taken from the deepest part of the lake which develops an anoxic hypolimnion for most of the year. C/N molar ratios and delta C-13(POC) and delta N-15(PN) values suggest an autochthonous source for particulate organic matter (POM). According to the isotope model, autochthonous carbon accounted for a major part of the particulate organic carbon (POC), ranging from 86% to 96% in September and October, while in December the proportion of allochthonous carbon was more pronounced, ranging from 57% to 59%. Low delta C-13(POC) and delta N-15(PN) values (from -36 to -33 parts per thousand and from 0.8 to 1.8 parts per thousand), observed below 24 m in August and September, indicate the bacterial origin of POM, mainly from methanotrophic bacteria. delta N-15(NO3) and delta N-15(PN) values decreased with depth. The relations between delta N-15(PN) and NO3- and NH4+ concentrations suggest that NH4+ is the main assimilation species for nitrogen in POM. Nitrification was active between 12 and 18 m deep in September and October, indicated by increased NO3- concentrations and decreased delta N-15(NO3) values. The correlation between nitrate concentrations and delta N-15(NO3) values suggests active water column denitrification in October 2008. The decrease in delta(NNO3)-N-15 values observed in December could be explained by degradation of organic matter, followed by nitrification of the degradation products. During our sampling period, there was no evident influence of sewage, agriculture, or atmospheric deposition on the nitrogen balance in the lake.