@phdthesis{944574,
  abstract     = {{European estuaries are characterized by high loads of nutrients and organic matter.  They have therefore the potential to sustain high primary production rates as well as high bacterial activity, with the balance often tipping towards a net heterotrophic ecosystem (respiration exceeds primary production).  Organic matter that is imported from upstream reaches and terrestrial origins is considered allochthonous, while organic matter derived from local, estuarine primary production is autochthonous.  The ecological focus of this thesis is on the balance between autochthonous and allochthonous organic carbon sources for secondary production.  Mesozooplankton, mostly comprising small crustaceans and larval stages of larger organisms between 0.2 and 2 mm length, are an important link in the aquatic food chain, grazing smaller size fractions and serving as food for larger animals, including fish.  We studied the contribution of autochthonous versus allochthonous food sources for mesozoo-
plankton biomass in the Scheldt Estuary, Belgium and the Netherlands (chapter 4). Bacteria use dissolved organic matter (DOM) as a source of energy and biomass.  They have the choice between allochthonous DOM, which is present in high concentrations but is rather refractory, and autochthonous DOM, which has a higher turnover rate and includes many compounds that are readily available for bacterial consumption, but whose concentration is lower and more variable. We studied interactions between bacteria and algae, the producers of autochthonous DOM, in two European estuaries:  the Scheldt Estuary, the Netherlands (chapter 6), and Randers Fjord, Denmark (chapter 5). Besides the ecological aspect of the thesis, focusing on the microbial food web of estuaries, substantial effort has been put into the development of statistical and modeling techniques and their application in an ecological context. Bayesian statistics are fairly new to ecology, and have potential for parameter estimation, especially in non-linear models.  They have been applied to simple food web models in the Scheldt Estuary (chapter 6), and formed the basis for two methodological chapters.  Chapter 2 discusses a method to sample parameter distributions in underdetermined, linear systems, a type of models that is often encountered in food web ecology.
Chapter 3 offers a Bayesian alternative to existing techniques that estimate taxonomic composition in ecological samples from molecular biomarkers, such as pigments or fatty acids. Stable isotopes of C and N have a central role in the unraveling of the estuarine microbial food web. Stable isotope ratios in different food web components are strong indicators for the origin of organic matter, based on the fact that the isotopic signature of allochthonous and autochthonous organic matter is significantly different (chapter 4). When this is not the case, or when one wants to study in detail short-term interactions between food web compartments, stable isotopes can be added deliberately to an experimental setup and food web fluxes can be derived from stable isotope incorporation into different food web components. We measured 13C concentration in polar lipid-derived fatty acids (PLFA) using gas chromatography - combustion - isotope ratio mass spectrometry (GC-c-IRMS). The isotope composition of algae- or bacteria-specific PLFA can be used as a proxy for the isotopic composition of algae and bacteria, respectively, thus avoiding the need for physical separation of these groups.
Calculations and model fitting have been done partly in FEMME, a FORTRAN environment providing tools for ecological modeling (chapter 5), and partly in R, a free software environment for statistical computing and graphics.  The methods developed in the framework of this thesis (chapters 2 and 3) have been published as contributed packages for R, and are available for download on the R website.}},
  author       = {{Van Den Meersche, Karel}},
  keywords     = {{ecological modelling,estuaries,food webs,stable isotopes}},
  language     = {{eng}},
  pages        = {{199}},
  publisher    = {{Ghent University. Faculty of Sciences}},
  school       = {{Ghent University}},
  title        = {{Carbon flows in the planktonic food web of temperate estuaries: a combined approach using stable isotopes, biomarkers and modeling}},
  year         = {{2009}},
}