Biogenic reefs constructed by the tube-building ecosystem engineer Lanice conchilega (Terrebilidae, Polychaeta) have profound structuring impacts on the benthic environment in that they alter the biogeochemical and physical properties of the sediment. This study provides new insights into the functioning and effects on food webs of L. conchilega reefs in intertidal sediments using linear inverse models to quantify the carbon flows in the food webs in the presence and absence of the tubeworm. The inverse food web models were based on an empirical dataset from 2 study sites, which provided biomass and stable isotope data, and information on general physiological constraints from the literature. Results of the model showed that the carbon input into reef food webs (mean ± SE; 191 ± 50 mmol C m-2 d-1) is ca. 40 times higher compared to bare sand areas (5 ± 2 mmol C m-2 d-1) and is mainly derived from organic matter (OM) in the water column. Most of the OM input towards these reefs is consumed by suspension-feeding macrofauna, particularly L. conchilega; however, the worm is not an important source of carbon for other macrofaunal organisms. The ratio of OM input to primary production indicates that the OM needs to be produced in an area at least 15 times larger than the reef area, demonstrating significant OM ‘focussing’ within the reef food web. The reef structures created by L. conchilega act as a trap for OM, resulting in an overall higher macrofaunal biomass and much more diverse food webs than in the absence of the tubeworm.