Tuttle Creek Reservoir is a reservoir near the eastern edge of the plains phytsiographic province in Kansas. The winds, which are a major climatic feature of the region, and the shallowness of the reservoir make thermal stratification rare. Vertical mixing occurs in all seasons. Because of the turbidity, primary production may be light limited. The relative magnitude of autochthonous and allochthonous organic material is thus of interest. Shipboard incubation of natural water was used to mimic the vertical profile of photosynthetic carbon assimilation. This allowed the collection of sufficient data over the horizontal expanse of the reservoir to assess the magnitude and variation in energy fixation by the phytoplankton.