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Phytoplankton nitrate reductase activity in the Pamlico River estuary, North Carolina, paralleled the development of a winter dinoflagellate bloom. High enzyme activity was associated not only with high algal biomass, primarily of Peridinium triquetrum, but also with low nitrate concentrations in the typically nitrate-rich waters. Nitrate was an important source of nitrogen for the bloom organisms in spite of the abundance of ammonium. The initiation and cessation of nitrate assimilation seemed closely related to the appearance and disappearance of the bloom. Although nitrate metabolism may be an important factor in bloom timing, temperature, salinity, and flushing of the system are also involved. In situations where nitrogen may be an important factor in the initiation and maintenance of phytoplankton blooms, and where more than one nitrogen source is available, it would be of great value to know the principal form being utilized by the organisms. Problems of this nature have led to the application of the nitrate reductase enzyme assay, which determines the ability of the organism to reduce nitrate for ultimate incorporation into cellular protein. Since nitrate reductase is induced by nitrate and repressed during ammonium assimilation, Eppley et al. (1969) proposed that its assay might be a valuable tool for indicating the nitrogen source for phytoplankton. High nitrate reductase activity would indicate that nitrate was the primary nitrogen source, low or nonexistent activity that another form of nitrogen was being utilized or that the nitrogen source was exhausted. EIolmes et al. (1967) had successfully applied this technique to a red-water dinoflagellate bloom off the southern coast of California. The seasonal phytoplankton cycle of the Pamlico River estuary in North Carolina usually includes a pronounced dinoflagellate bloom from December through April |
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