Lakes and reservoirs often experience sudden increases in macrophyte biomass. These changes have the potential to alter the abiotic environment in ways that affect other species. The American lotus (Nelumbo lutea) is expanding its distribution across portions of North America. N. lutea forms a dense canopy over the shallow margins of lakes and may influence the abiotic properties of the littoral zone, but this hypothesis has not been tested. Here we investigate the effects of a growing bed of N. lutea on water quality in Pymatuning Reservoir. In summer 2007 we deployed remote logging sensors to measure dissolved oxygen and temperature inside of a N. lutea bed and in adjacent open water. Dissolved oxygen concentrations were consistently lower in the lotus bed than in open water and oxygen concentrations within the N. lutea bed declined over the course of the summer. In Jul. and Aug. the N. lutea beds experienced hypoxia while oxygen concentrations remained high in open water. Low dissolved oxygen levels in N. lutea beds may affect the distribution of fish and invertebrates and alter biochemical processes in the epibenthos.