Understanding how different drivers shape relationships between abundance and body mass (size-spectra) is important for understanding trophic and competitive interactions in food webs, and for predicting the effects of human pressures. Here, we sample seabed communities from small polychaetes (< 0.001g) to large fish (> 1kg) in the Celtic Sea to examine how bottom trawling and primary production affect their size spectra, and to compare these to predictions from a model that couples predator and detritivore communities. Size spectra were not well approximated by linear fits because of truncation of the size spectra of detritivores. Low primary production resulted in lower abundance of benthic fauna. Bottom trawling reduced the abundance of predators and large detritivores, but allowed small detritivores to increase in abundance. These empirical size spectra were partly consistent with predictions from the size spectra model, showing that understanding the structuring of benthic communities requires a consideration of both size and functional group. The findings highlight the need for an ecosystem approach to understanding the effects of exploitation and climate change on marine ecosystems.