Previous studies conducted at Castle Lake, California suggested that phytoplankton growth could be limited by low levels of dissolved inorganic nitrogen during the summer growing season. We have used the isotopes H14CO3-, 13NH4+, 15NO3-, and 13NO3- as tracers to determine the rates of inorganic carbon and nitrogen uptake by the natural phytoplankton community and their response to nitrogen enrichment. Experimental designs followed the procedures of several bioassay techniques which have been developed to investigate algal nutrient deficiency. The results indicate that the phytoplankton were nitrogen deficient throughout the growing season. Ammonium uptake was rate-limited by the external concentration of ammonium. Nitrate uptake was regulated by the availability of nitrate and molybdenum, and potentially by the assimilation of ammonium. Although N-enrichment enhanced photosynthesis in short-term experiments during the early portion of the growing season, the photosynthetic response was initially negative during midseason, and a positive response did not occur until 2-3 days had elapsed. This suggests that a lack of stimulation, or even inhibition, of inorganic carbon uptake upon nitrogen enrichment does not necessarily preclude N-deficiency.