High-resolution diatom stratigraphies from mid-Holocene sediments taken from fringe and central locations in Moss Lake, a small lake in the foothills of the Cascade Range, Washington, have been analyzed to investigate the impacts (and duration) of tephra deposition on the aquatic ecosystem. Up to 50 mm of tephra was deposited from the climactic eruption of Mount Mazama 7958–7795 cal yr BP, with coincident changes in the aquatic ecosystem. The diatom response from both cores indicates a change in habitat type following blanket tephra deposition, with a decline in tychoplanktonic Fragilaria brevistriata and Staurosira venter and epiphytic diatom taxa indicating a reduction in aquatic macrophyte abundance. Additionally, the central core shows an increase in tychoplanktonic Aulacoseira taxa, interpreted as a response to increased silica availability following tephra deposition. Partial redundancy analysis, however, provides only limited evidence of direct effects from the tephra deposition, and only from the central core, but significant effects from underlying environmental changes associated with climatic and lake development processes. The analyses highlight the importance of duplicate analyses (fringe and central cores) and vigorous statistical analyses for the robust evaluation of aquatic ecosystem change.