| الملخص: | The San Francisco Estuary (SFE) ecosystem receives anthropogenic ammonium (NH<sub>4</sub>) from agricultural runoff and sewage treatment plants and has low chlorophyll levels. As observed in other aquatic systems, NH<sub>4</sub> at concentrations < 4 µmol/L inhibits nitrate (NO<sub>3</sub>) uptake by phytoplankton and can reduce the frequency with which phytoplankton assimilate all available inorganic nitrogen (i.e., NO<sub>3</sub> and NH<sub>4</sub>); paradoxically, elevated NH<sub>4</sub> can reduce the chances of phytoplankton blooms in some high NH<sub>4</sub> ecosystems. For blooms to occur, NH<sub>4</sub> must first be reduced to non-repressive levels, then NO<sub>3</sub> uptake can occur and is accompanied by more rapid carbon (C) uptake and chlorophyll accumulation. The consequence of this sequence is that when NO<sub>3</sub> uptake, C uptake, or chlorophyll concentrations are plotted against ambient NH<sub>4</sub>, a rectangular hyperbola results. Here, these relationships are statistically described for a variety of SFE field data, and their management applications are discussed. These relationships enable ambient NH<sub>4</sub> to be used to predict both the likelihood of blooms and to investigate historical changes in productivity when no rate measurements were made. We apply the statistical relationship to a 40-year time series from the SFE during which there was an ecosystem-scale change in the estuarine foodweb with a drastic decline in pelagic fishes (the pelagic organism decline) and suggest that this period aligned with the lowest annual primary production and highest NH<sub>4</sub>. The relationship may be generalizable to other high-nitrogen, low-growth systems and aid nutrient management decisions, assuming potential limitations are considered.
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