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

• Published in: Journal of Marine Science and Engineering
• Main Authors: Richard C. Dugdale, Alexander E. Parker, Frances P. Wilkerson
• Format: Article
• Language: English
• Published: MDPI AG 2024-11-01
• Subjects: ammonium; estuary; anthropogenic; primary production; phytoplankton
• Online Access: https://www.mdpi.com/2077-1312/12/11/2029

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.