Nitrate concentration declined slightly in both treatment over the course of this study (Fig. 3). An increase in NO3-N concentration was not detected following any of the TAN spike events.Detection of an increase in NO3-N following a TAN spike may have been missed because nitrate was assayed less frequently, but also because of the low concentration of the TAN spike. Bacteria also can consume nitrate and cause it not to accumulate (Choi et al., 2008).Additionally, nitrate likely was used as the nitrogen source by phytoplankton since TAN concentrations generally were low and feed additions were limited. Phytoplankton uptake ammonia preferentially, but utilize nitrate when ammonia concentration is very lowor absent (Reynolds, 1984). The low ammonia and high nitrate concentrations present during this study, the observed 5–18% decline in nitrate concentration, and the high chlorophyll a concentrations support phytoplankton utilization of nitrate.