Oikos 116: 427  438, 2007 doi: 10.1111/j.2006.0030-1299.15557.x, Copyright # Oikos 2007, ISSN 0030-1299 Subject Editor: Heikki Seta ¨la ¨, Accepted 27 October 2006 Trophic cul-de-sac, Pyrazus ebeninus, limits trophic transfer through an estuarine detritus-based food web Melanie J. Bishop, Brendan P. Kelaher, Ralph Alquezar, Paul H. York, Peter J. Ralph and C. Greg Skilbeck M. J. Bishop (melanie.bishop-1@uts.edu.au)., B. P. Kelaher, R. Alquezar, P. H. York, P. J. Ralph and C. G. Skilbeck, Inst. for Water and Environmental Resource Management and Dept of Environmental Sciences, Univ. of Technology, Sydney, Broadway, NSW, 2007, Australia. The importance to food-webs of trophic cul-de-sacs, species that channel energy flow away from higher trophic levels, is seldom considered outside of the pelagic systems in which they were first identified. On intertidal mudflats, inputs of detritus from saltmarshes, macroalgae or microphytobenthos are generally regarded as a major structuring force underpinning food-webs and there has been no consideration of trophic cul-de-sacs to date. A fully orthogonal three-factor experiment manipulating the density of the abundant gastropod, Pyrazus ebeninus , detritus and macrobenthic predators on a Sydney mudflat revealed large deleterious effects of the gastropod, irrespective of detrital loading or the presence of predators. Two months after experimental manipulation, the standing-stock of microphytobenthos in plots with high (44 per m 2 ) densities of P. ebeninus was 20% less than in plots with low (4 per m 2 ) densities. Increasing densities of P. ebeninus from low to high halved the abundance of macroinvertebrates and the average number of species. In contrast, the addition of detritus had differing effects on microphytobenthos (positively affected) and macroinvertebrates (negatively affected). Over the two-months of our experiment, no predatory mortality of P. ebeninus was observed and high densities of P. ebeninus decreased impacts of predators on macroinvertebrate abundances. Given that the dynamics of southeast Australian mudflats are driven more by disturbance than seasonality in predators and their interactions with prey, it is likely that Pyrazus would be similarly resistant to predation and have negative effects on benthic assemblages at other times of the year, outside of our study period. Thus, in reducing microphytobenthos and the abundance and species richness of macrofauna, high abundances of the detritivore P. ebeninus may severely limit the flow of energy up the food chain to commercially-important species. This study therefore suggests that trophic cul-de-sacs are not limited to the eutrophied pelagic systems in which they were first identified, but may exist in other systems as well. Ecological studies have long sought to characterize the flow of energy and materials through terrestrial and aquatic systems (Elton 1927, Lindeman 1942, Odum and Odum 1955, Hutchinson 1959, Odum et al. 1962, Hairston and Hairston 1993). Knowledge of food-web structure is critical to the management of anthropogenic disturbances and the conservation of species (Cohen et al. 1993, Duffy 2002). Because interactions among species are highly complex, many studies have at- tempted to gain insight into generalities of web- structure by grouping species into fewer trophic groups. Using the trophic-group approach, the height of the trophic pyramid has been explained both in terms of the attenuation of energy passing from the ‘‘bottom- up’’ (Lindeman 1942) and the ‘‘top-down’’ forcing of consumers, by direct and indirect mechanisms (Slobodkin et al. 1967). Despite much focus on the relative importance of bottom-up and top-down forces in determining food- web structure (Menge 1992, Power 1992), there has been little consideration of trophic cul-de-sacs, species that have large effects on community structure due to their channeling of energy and organic matter away from higher trophic levels. In the Black Sea, the jellyfish 427