Fixed-depth subthermocline phytoplankton production estimates were compared to variable-depth estimates calculated along the simulated trajectory of an internal wave. The mean of 5000 Monte Carlo simulations, varying wave phase and amplitude, was not significantly different than the fixed-depth estimate for average Lake Michigan internal waves. Differences were significant, however, for wave amplitudes >5 m but only at some depths. Differences between the two estimates were related to differences in irradiance received and the portion of the photosynthesisirradiance (P-I) curve controlling production. Oscillating communities always receive more irradiance than fixed-depth communities and the magnitude of this increase is related to the extinction coefficient and the amplitude of the internal waves. Production was also estimated along an individual isotherm trajectory (isotherm-derived) and compared to fixed-depth production. Larger differences between these isotherm-derived and fixed-depth estimates were noted in some cases and were related to differences in the mean isotherm depth and the sampled (fixed) depth. If one accounts for the trajectory of the sampled community, fixed-depth estimates are reliable; however, if the trajectory is unknown or unaccounted for, any individual fixed-depth production estimate may not adequately measure in situ production.