Karel Van den Meersche

Quantifying Food Web Flows Using Linear Inverse Models

The quantitative mapping of food web flows based on empirical data is a crucial yet difficult task in ecology. The difficulty arises from the under-sampling of food webs, because most data sets are incomplete and uncertain. In this article, we review methods to quantify food web flows based on empirical data using linear inverse models (LIM). The food web in a LIM is described as a linear function of its flows, which are estimated from empirical data by inversemodeling. The under-sampling of food webs implies that infinitely many different solutions exist that are consistent with a given data set. The existing approaches to food web LIM select a single solution from this infinite set by invoking additional assumptions: either a specific selection criterion that has no solid ecological basis is used or the data set is artificially upgraded by assigning fixed values to, for example, physiological parameters. Here, we advance a likelihood approach (LA) that follows a different solution philosophy. Rather than singling out one particular solution, the LA generates a large set of possible solutions from which the marginal probability density function (mPDF) of each flow and correlations between flows can be derived. The LA is exemplified with an example model of a soil food web and is made available in the open-source R-software. Moreover, we show how stoichiometric data, stable isotope signatures, and fatty acid compositions can be included in the LIM to alleviate the under-sampling problem. Overall, LIM prove to be a powerful tool in food web research, which can bridge the gap between empirical data and the analysis of food web structures.

Sediment and carbon fluxes along a longitudinal gradient in the lower Tana River (Kenya)

We estimated annual fluxes of suspended matter and different carbon (C) pools at three sites along the lower Tana River (Kenya), based on monthly sampling between January 2009 and December 2011. Concentrations of total suspended matter (TSM), particulate organic carbon (POC), dissolved organic carbon (DOC), and dissolved inorganic carbon (DIC) were monitored, as was the stable isotope composition of the carbon pools. Both TSM and POC concentrations showed strong seasonality, varying over several orders of magnitude, while DOC and DIC concentrations showed no seasonal variations. A strong shift in the origin of POC was observed, which was dominated by C3-derived C during dry conditions (low δ13CPOC between −28‰ and −25‰), but had significant C4 contributions during high-flow events (δ13CPOC up to −19.5‰). Between Garissa and the most downstream sampling point, a clear decrease in suspended matter and organic C fluxes was observed, being most pronounced during high-discharge conditions: on an annual basis, fluxes of TSM, POC, and DIC decreased by 34% to 65% for the 3 year study period. Our results suggest that floodplains along the lower Tana River could play an important role in regulating the transport of suspended matter and organic C. A comparison of current flux estimates with data collected prior to the construction of several hydropower dams reveals that the sediment loading is reduced during low discharge conditions.