Experiments utilizing epilimnetic water were conducted to determine the rate of am- monium regeneration, due to zooplankton excretion and microbial mineralization processes, in rela- tion to the rate of inorganic nitrogen assimilation by phytoplankton. The euphotic zone of dimictic, meso-oligotrophic Castle Lake is characterized by a rapid depletion of both nitrate and ammonium soon after spring thaw. External inputs of nitrogen are minimal during this period and levels of inorganic nitrogen remain low until fall overturn. The rate of ammonium assimilation was high relative to nitrate assimilation, and was significantly correlated with the rate of ammonium regeneration. The importance of this rapid turnover of nitrogen to phytoplankton growth is consistent with the results of previous Castle Lake studies, and with the current conceptual model of primary production in the mixed layer of nitrogen-deficient marine ecosystems.
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