ABSTRACT. Results are presented from a study into the effects of SCUBA diving on water column structure in perennially ice-covered Lake Fryxell, Antarctica. General theoretical predictions are compared with results which assess potential impacts on water quality parameters related to mass water transfer and gas exchange from diving activities. Potential mechanisms of water column disturbance are considered including: 1) mixing due to diver motion, 2) mixing due to the motion of diver-exhaled gases, 3) destabilization of the water column due to dive-associated heat transfer, and 4) changes of water column gas concentrations due to dissolution of diver-exhaled gases. Data (temperature, dissolved oxygen, specific conductance, pH, dissolved organic carbon, salinity) were collected from two control holes and one dive hole immediately before and two days after dive activities. Variability in measured parameters did not differ significantly between holes either before or after diving; and significant differe nces between control holes and dive hole observations, on a depth-by-depth basis, were extremely rare. Pre- and post-dive virus-like particle, bacterial, ciliate, photo- and heterotrophic flagellate densities also did not differ significantly as a result of diving. An apparent lack of long-term trends indicative of water column destabilization over the past 18 years of diving is discussed, along with recent evidence for deep mixing in this lake. Theoretical model results and an analysis of field data suggest that diving impacts on physicochemical properties and microbial distributions in a closed-basin, ice-covered lake occur primarily at spatiotemporal scales smaller than those considered in this study.