Patterns in Anthropogenic Nitrogen and Water Quality Leading to Phytoplankton Blooms in Urban Estuaries

Abstract

The San Francisco Estuary (SFE) ecosystem receives anthropogenic ammonium (NH4) from agricultural runoff and sewage treatment plants and has low chlorophyll levels. As observed in other aquatic systems, NH4 at concentrations < 4 µmol/L inhibits nitrate (NO3) uptake by phytoplankton and can reduce the frequency with which phytoplankton assimilate all available inorganic nitrogen (i.e., NO3 and NH4); paradoxically, elevated NH4 can reduce the chances of phytoplankton blooms in some high NH4 ecosystems. For blooms to occur, NH4 must first be reduced to non-repressive levels, then NO3 uptake can occur and is accompanied by more rapid carbon (C) uptake and chlorophyll accumulation. The consequence of this sequence is that when NO3 uptake, C uptake, or chlorophyll concentrations are plotted against ambient NH4, 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 NH4 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 NH4. The relationship may be generalizable to other high-nitrogen, low-growth systems and aid nutrient management decisions, assuming potential limitations are considered.