We carried out a benthic survey and two experiments in runs at eight sites down the Kakanui River (South Island, New Zealand) during summer low flows, to investigate the interaction between nutrients, periphyton, and macro-grazers. Benthic periphytic biomass was generally low (<20 mg m2 chlorophyll a) at most sites, but high densities of macro-grazers (mainly snails) were observed at six of the eight sites. Chlorophyll a and cellular P concentrations were generally higher on artificial substrates in the first- to third-order tributaries, compared with downstream. Macro-grazer densities (mainly snails) were also highest in the second- and third-order tributaries. Enrichment of patches with N and P did not translate into significant increases in chlorophyll a concentrations. Instead there was a general increase in macro-grazers, and an increase in the relative abundance of Cocconeis placentula. In a second experiment, the chlorophyll a level was five-fold higher on the substrates where macro-grazers were excluded and there was no significant response of chlorophyll a to nutrient addition on these substrates. On the grazed substrates, densities of snails and caddis-larvae were two-fold higher with N+P enrichment. These experiments provided evidence for a tight coupling between first and second trophic levels, and strong grazer control of periphyton, in this river.