Sedentary herbivores may improve the food resources available to them by ‘gardening’, and most obviously by fertilising primary producers with excreted nutrients such as nitrogen. In five English lakes, spanning a gradient of nutrient availability, we predicted that fertilisation of the larval retreat by the littoral, gallery-building caddisfly Tinodes waeneri would result in: (a) a distinct algal assemblage from that in the background epilithon, and that (b) the difference would be greatest in the least productive lakes (where the importance of the nutrient subsidy from larvae should be greatest). Classes of algae present in samples of galleries and epilithon were investigated using chlorophyll and carotenoid pigment analysis and diatoms were identified. Galleries were characterised by a greater content of pigments indicative of diatoms, including fucoxanthin, than the background epilithon (which contained a higher proportion of chlorophyte algae). Redundancy analysis (RDA) of diatom counts indicated a clear separation between gallery and epilithic assemblages in all lakes, supporting hypothesis (a). Furthermore, in agreement with hypothesis (b), the assemblage of diatoms on galleries was most similar to that of the epilithon in the more productive lakes, with the greatest divergence in Windermere and Coniston, mainly due to a much greater relative proportion of Gomphonema in the epilithon than in the galleries, suggesting that fertilisation had a greater impact where background nutrient concentrations were low. Lakes in the RDA triplot were arranged in order of productivity along axis 1, with gallery assemblages in each case located towards the more productive end relative to the epilithon. This sedentary, retreat-building grazer, probably in common with many other taxa with such traits in aquatic ecosystems, improves its own food resources by gardening. Larvae can modify algal assemblages in their feeding patches by weeding and fertilisation. They create new surfaces for growth of the biofilm via continually adding to the front of their silken galleries and harvesting the biofilm (dominated by diatoms) that has developed on older parts of their galleries to the rear. It is also feasible that the physical structure of the gallery (separately from the effects of nutrients) plays some role in increasing the availability of algae within the territory. This behaviour is likely to extend the range of conditions under which such species can persist, raise the carrying capacity of their habitat, and has possible wider ecosystem consequences for productivity and nutrient cycling.