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

Abstract

Abstract. 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 subalpine 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 δ 13 C POC and δ 15 N 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 The decrease in δ 15 N NO 3 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.

Denitrification alternates between a source and sink of nitrous oxide in the hypolimnion of a thermally stratified reservoir

“…One factor that may explain differences in denitrification rates among these studies is the thickness of the hypolimnion. While it is frequently assumed that most denitrification in reservoirs is sedimentary (Wall et al 2005;David et al 2006), several studies have documented denitrification in hypoxic water columns (Terai and Yoh 2001;Bratkic et al 2012;Grantz et al 2012) and the importance of water column denitrification will increase with the ratio of water volume to benthic surface area. The hypolimnion in Harsha Lake was hypoxic, bordering on anoxic near the sedimentwater interface (dissolved oxygen , 0.02 mg L 21 ), and the mid-water column N 2 O spike in June suggests the hypolimnion supported denitrification while NO { 3 was available.…”

Temporal variation and stoichiometric ratios of organic matter remineralization in bottom waters of the northern Gulf of Mexico during late spring and summer

“…In addition, the model-simulated DSi/DP was 36 (629), much higher than the initial value of 16. These discrepancies may be explained by adjacent high Si water input from coastal wetlands [Huang et al 2012a].…”

“…While the controlling factors on the size of the hypoxic area have been examined in previous studies [i.e., Wiseman et al, 1997;Rabalais et al, 1999;Bianchi et al, 2010], it remains enigmatic that the fisheries landings in Louisiana and Gulf-wide have increased during 1985-2007 and did not experience much mortality associated with the enlarged hypoxia [Bianchi et al, 2008], which seems relevant to broader issues of management and is certainly outside the scope of this study. The development of hypoxia has attracted much attention from the carbon cycle community [i.e., Wang et al, 2004;Cai and Lohrenz, 2010;Huang et al, 2012a] due to influence of the carbon cycle on the consumption of water column O 2 . For example, strong summer water column stratification and rapid subsurface oxygen consumption lead to significant buildup of DIC and decreases in both pH and carbonate saturation states in bottom waters, as well as subsurface DIC accumulation.…”

Oxygen utilization rate (OUR) underestimates ocean respiration: A model study

“…The overall performance to reproduce large-scale patterns of abiotic as well as biotic tracers is usually taken as evidence that a model reasonably represents the real ocean [e.g., Dutay et al, 2002;Najjar et al, 2007;Duteil et al, 2012;Kriest et al, 2010Kriest et al, , 2012. Should a concept like OUR not perform as expected in such a system, this is strong evidence for this concept to be problematic also in reality.…”