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

[摘要] The variability in the stable isotope signature of carbon and nitrogen inparticulate organic matter and dissolved species in the water column of themesotrophic subalpine Lake Bled in NW Slovenia has been determined. Afterthe algae bloom from August to December in 2008, samples were taken from thedeepest part of the lake which develops an anoxic hypolimnion for most ofthe year. C/N molar ratios and δ¹³C_POC and δ¹⁵N_PN values suggest an autochthonous source for particulateorganic matter (POM). According to the isotope model, autochthonous carbonaccounted for a major part of the particulate organic carbon (POC), rangingfrom 86% to 96% in September and October, while in December theproportion of allochthonous carbon was more pronounced, ranging from 57%to 59%. Low δ¹³C_POC and δ¹⁵N_PN values(from −36 to −33‰ and from 0.8 to 1.8‰), observed below 24 m in August andSeptember, indicate the bacterial origin of POM, mainly from methanotrophicbacteria. δ¹⁵N_NO3 and δ¹⁵N_PN values decreased with depth. The relations between δ¹⁵N_PN and NO₃^- and NH₄⁺ concentrationssuggest that NH₄⁺ is the main assimilation species for nitrogen inPOM. Nitrification was active between 12 and 18 m deep in September andOctober, indicated by increased NO₃^- concentrations and decreasedδ¹⁵N_NO3 values. The correlation between nitrateconcentrations and δ¹⁵N_NO3 values suggests active watercolumn denitrification in October 2008. The decrease in δ¹⁵N_NO3 values observed in December could be explained bydegradation of organic matter, followed by nitrification of the degradationproducts. During our sampling period, there was no evident influence ofsewage, agriculture, or atmospheric deposition on the nitrogen balance inthe lake.

[学科分类] 地球化学与岩石