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Abstract

The identification and quantification of organic compounds present in the environment is
a major area of application in modern analytical chemistry. However, it is still scarcely
recognized that, in addition to the chemical identity and concentration of organic
compounds, there is more information to be found about their source and fate in the
environment from their isotope composition. Isotope ratio mass spectrometry (IRMS)
following on-line combustion (C) of compounds separated by gas chromatography (GCC-
IRMS) has been commercially available only since 1990. Our understanding of the
isotope composition of organic compounds in different studies is therefore still somewhat
limited.
In this doctoral thesis, organic geochemical biomarkers, combined with compound
specific isotope composition, has been used to determine the sources and transformation
pathways of organic matter (OM) in the anoxic, eutrophic alpine Lake Bled. The stable
isotope approach was further used to identify the sources of polycyclic aromatic
hydrocarbons (PAHs) in sediments. The determination of sources and transformation of
OM has been based on the composition of lipids. Importantly, among the available
molecular markers, lipids are used mainly because of their source specificity and higher
resistance to bacterial degradation than other classes of organic compounds. Examination
of lipid biomarkers in particulate organic matter (POM) and suspended trap material in
the water column indicates whether fatty acids (FA) of algal origin were abundant in the
epilimnion while, in the anoxic zone, POM and trap material were enriched in bacterial
FA. The lowest isotopic composition of carbon (δ13C) value of –51.7‰ was observed in
cis-11-octadecenoic acid (18:1n-7) in trap material and was the only FA linked to
methanotrophic bacteria. In addition, zooplankton left a marked imprint on particulate
lipids and trap material at 12 m based on the predominance of octadecanoic acid (n-C18:0)
over hexadecanoic acid (n-C16:0), short–chain, even-carbon n-alkenols, the high
proportion of cholest-5-en-3β-ol (44.8% of total sterol concentration (TST)), a
cholesterol/phytosterol ratio of 0.49 and isotopic composition of particulate nitrogen
(δ15NPN) values of 6.8 and 11.7‰. The sterol distribution reflects a primarily plankton
source, while 24-ethylcholesta-5,22E-dien-3β-ol (stigmasterol) and 24-methylcholest-5-
en-3β-ol (campesterol) are both of allochthonous origin, coming from terrestrial plants.
Lipids in sediments from two different locations in western basin and Zaka Bay were
also mainly of autochthonous origin, however their distribution and isotope composition
differ significantly from those in POM and sedimentary trap material collected 2 m above
the sediment in western basin. These results indicate that some biomarkers have different
source organisms that produce the same molecule. Thus, isotope composition should be
used as a complementary tool for source identification. It is interesting to note that source
identification based only on lipid distribution could lead to incorrect conclusions. Low
δ13C values for long-chain n-alkanes, n-alcohols and FAs and, especially, those
determined in C29 sterols, indicate that these compounds were derived mainly from
microalgae and bacteria and not from higher plants or other terrigenous OM, as
previously reported. The δ13C values observed in long-chain FAs and, especially, sterols,
were lower in the western basin sediments comparing to that in Zaka Bay, indicating the
pronounced contribution of an anaerobic, bacterial source. On the other hand, the δ13C
values obtained for short-chain FAs in both environments and for short-chain n-alkanes in
Zaka Bay indicate a larger contribution of more refractory terrestrial OM. δ13C values of
other lipids were similar in the two sediments showing that identification of their sources
did not results from different depositional regimes.
It is believed that the presence of more labile autochthonous OM could also influence
the methanogenesis pathway in sediments. Acetate fermentation appears to be associated
with more labile OM, whereas reduction of carbon dioxide (CO2) to methane utilizes
more refractory OM. However, the biogeochemical processes and structure of the
archaeal community determined in Lake Bled sediments do not support this assumption.
It was found that hydrogenotrophic methanogenesis is the dominant pathway in the
sediment of alpine Lake Bled, despite the low temperature and prevalence of “fresh”
autochthonous derived OM.
Finally, molecular and stable isotope analyses were combined to trace and identify the
sources of PAH in sediments. The concentrations of PAH in surface sediments at the two
locations (western basin and Zaka Bay) were comparable and higher than those found in
previous studies, reaching 4230 and 4380 ng g-1. It was found that retene (Re) and
perylene (Per) are both of mainly natural origin in Zaka Bay while, at station D in western
basin, the value of δ13C determined at a depth of 12–14 cm in the 1950s indicates that Re
is of pyrolytic origin. The distribution of δ13C values of other individual PAHs showed
that the PAH input to lake sediments was of pyrolytic origin, probably dominated by coal
and later, in the 1950s, also by wood burning. The influence of PAH originating from
vehicle emissions could be seen at the depth of 12–14 cm in western basin and Zaka Bay.
This depth corresponds to the period 1953–1961.
The doctoral thesis clearly indicates that the origin of OM and pollutants in aquatic
environments should be verified by stable isotope geochemistry.