25th International Meeting on Organic Geochemistry IMOG 2011

P-410
Carbon cycling in lacustrine food webs
J. Marieke Lammers 1 , Carsten J. Schubert 2 , Jack J. Middelburg 1 , Jaap S. Sinninghe
Damsté 1,3 , Gert-Jan Reichart 1
1 Universiteit Utrecht, Utrecht, Netherlands, 2 EAWAG, Kastanienbaum, Switzerland, 3 Netherlands Institute for
Sea Research, Den Burg, Netherlands (corresponding author:M.Lammers@geo.uu.nl)
Heterotrophs in freshwater systems rely on three
major sources of carbon/energy: (1) local primary
production, (2) terrestrial input from plants and soils
and (3) methanotrophic biomass (depending on either
locally produced or imported methane). The
availability of these three energy sources has major
consequences for food web functioning and structure,
and determine the carbon fluxes within the lake and
how much carbon is ultimately exported from the lake
to rivers or the atmosphere. Terrestrial
(allochthonous) organic carbon is traditionally
assumed to be aged and thus refractory, yet evidence
indicates that it may fuel parts of the food web [1, 2],
enhancing secondary production [1, 3]. So far, the
secondary producers involved and relative
contributions of in situ produced and allochthonous
organic matter to secondary productivity remains
poorly quantified.
Processing of terrestrial organic matter in lakes is
highly variable through the year [3], and possibly
depending on trophic level [1]. This research,
therefore, focuses on identifying main organic carbon
fluxes and elucidating food web functioning in
freshwater systems with variable trophic levels.
Energy flow at higher food web levels can be studied
using stable isotope analyses [4]. Two frequently
used approaches, sampling primary producers or bulk
particulate organic matter (POM), both have their
inherent problems. Putative primary consumers may
sometimes consume small heterotrophs and bulk
POM comprises not only primary producers but also
bacteria, small heterotrophs and detritus. Compound
specific isotope analysis (CSIA) of lipid biomarkers
allows including the microbial compartment of food
webs [5].
Two Swiss lakes are studied for food web
structure and carbon flows. Lake Lucerne is a prealpine,
oligotrophic lake with limited terrestrial input.
Rotsee, a small eutrophic lake just north of Lucerne,
has a major methane-based productivity. Samples for
characterizing DIC, DOC and POM were collected at
different water depths. Also, samples of soil
surrounding Lake Lucerne were collected. DIC, DOC
and POM concentrations and carbon isotopic
analyses of inflowing streams, lake water and
outflowing streams are used to deduce whether the
lakes are a net carbon source or sink. DOC is the
main coupling between autotrophs and heterotrophic
bacteria. It forms an important step in the food web
and is therefore analyzed for bulk isotopic
composition. POM is often used as a representative
of phytoplankton carbon and is therefore analyzed for
bulk carbon and nitrogen isotopic composition. To
differentiate between algal and bacterial classes,
POM is also analyzed for biomarker presence and
abundance and subsequently for CS-δ 13 C. As a first
order approximation, functional groups are assigned
using analyses of phospholipid-derived fatty acids
(PLFA). More specifically, other lipid biomarkers such
as free fatty acids, sterols and hopanoids are used to
identify and quantify algal and bacterial classes.
Samples of soil surrounding Lake Lucerne are
analyzed similarly to POM samples in order to obtain
isotopic signature and compositional information on
allochthonous material entering the lake.
References
[1] Cole et al. (2006) Ecology Letters 9: 558-568
[2] Cole et al. (2007) Ecosystems 10: 171-184
[3] Pace et al. (2004) Nature 427: 240-243
[4] Peterson and Fry (1987) Annu. Rev. Ecol. Syst.
18: 293-320
[5] Middelburg et al. (2000) Limnol. Ocean. 45: 1224-
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