Combining stable isotope enrichment, compartmental modelling and ecological network analysis for quantitative measurement of food web dynamics

1. Quantitative food web analysis is made difficult by the need for simultaneous measurement of multiple carbon (C) flows. C flow rates can potentially be quantified indirectly using conventional methods such as gut contents analysis, but the involvement ofmany conversion factors and assumptions results in significant errors in the final estimation. Such difficulties have limited quantitative data on foodweb dynamics, restricting community-level hypothesis testing. 2. We conducted laboratory microcosm and field experiments, using combined enriched stable isotope (C), compartmental modelling and ecological network analysis (ENA) as a novel approach to quantifying food web dynamics. Feasibility of this approach was demonstrated via the quantification of C flow, backflow and cycling in a subtropical estuarine sandflat foodweb comprising three trophic levels: microphytobenthos (MPB), three dominantmeiofaunal groups (juvenile polychaetes, harpacticoid copepods and nematodes) and soldier crabs (Mictyris longicarpus). 3. Enriched sodium bicarbonate (NaHCO3) was used to trace C flow. The compartmental modelling software WinSAAM quantified C flow by comparing the temporal patterns of producer and consumer C enrichment. Network analysis software EcoNet was used to assess food web dynamics by providing system indices that quantified the flows and foodweb interactions. 4. This approach allows concurrent quantitative assessment of food web structure and ecosystem functioning. WinSAAM results showed that C return rate from nematodes to MPB was greater than the other two meiofaunal groups and soldier crabs assimilated more C from nematodes than from the other two meiofaunal groups. System indices from EcoNet suggested that indirect flow contributed more than direct flow in this system. Unique capabilities of this approach in food web studies and assumptions underlying the proposed and conventional food web analytical approaches are discussed. 5. Another significant advance of this approach is that it allows sensitive quantification of trophic linkages in a food web without requiring equilibrium to be achieved, thus enabling instantaneous assessment of the relative importance of various C flow pathways. Specific hypotheses about food web dynamics, such as the impact of eutrophication and urbanization, can then be tested quantitatively throughmanipulative experiments by comparing indices reflecting system behaviour.