Mixotrophic organisms utilize both photosynthetic and phagotrophic nutritional modes and are increasingly recognized as important contributors to aquatic food webs. Dinobryon, a widespread freshwater genus of mixotrophs had dramatic seasonal and annual variation in Lake Lacawac, a mesotrophic kettle lake in northeastern Pennsylvania. Although light is required for population growth in Dinobryon, it does not appear to control vertical distribution patterns observed during a 3-year survey. One year, the maximum annual abundance was observed close to the surface, while the following year the maximum annual abundance occurred at 4 m depth where light levels were <2% of surface irradiance. The depth of maximum Dinobryon abundance during the second year overlapped with an oxygen maximum at 4 m, which strongly suggests effective photosynthesis at a relatively low light level. Analysis of the distribution data suggests that temperature may be a primary driver of Dinobryon spp. distribution in the water column of this lake. Although annual variability in absolute abundance was high, peak Dinobryon biomass always occurred within a narrow range of temperature (10°-16°C). This distributional study was validated by an early spring field experiment where temperature was raised with water heater to 16° C. The abundance of Dinobryon increased two-fold at the higher temperature. A novel laboratory instrument was developed and constructed that established a vertical temperature gradient with even lighting to test the ability of Dinobryon to congregate within that specific temperature range independent of light intensity. Dinobryon abundance in the column was maximum within the same range of water temperatures observed in the field study. Because lighting was even across the vertical water column, any effect of light attenuation in the lake distribution was eliminated and accumulation due to temperature effects was confirmed. With climate change becoming more apparent, it is clear that understanding temperatures role in the distribution of the aquatic food web is timely and important. / Biology