The role of ammonium and nitrate in spring bloom development in San Francisco Bay

• Published in: Estuarine, coastal and shelf science Vol. 73; no. 1; pp. 17 - 29
• Main Authors: Dugdale, Richard C., Wilkerson, Frances P., Hogue, Victoria E., Marchi, Albert
• Format: Journal Article
• Language: English
• Published: London Elsevier Ltd 01.06.2007; Elsevier
• Subjects: ammonium; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; Brackish; Brackish water ecosystems; California; estuary; Fundamental and applied biological sciences. Psychology; nitrate; phytoplankton; San Francisco Bay; Sea water ecosystems; Synecology; INE, USA, California, San Francisco Bay; phytoplankton; ammonium; San Francisco Bay; nitrate; California; estuary; Marine environment; Brackish water environment; Bloom; Ammonium nitrate; Estuaries; Phytoplankton; Nitrates
• ISSN: 0272-7714; 1096-0015
• DOI: 10.1016/j.ecss.2006.12.008

The substantial inventory of nitrate (NO 3) in San Francisco Bay (SFB) is unavailable to the resident phytoplankton most of the year due to the presence of ammonium (NH 4) at inhibitory concentrations that prevents NO 3 uptake. Low annual primary productivity in this turbid estuary is generally attributed to the poor irradiance conditions. However, this may not be the only cause; spring phytoplankton blooms occur irregularly in north SFB only when NH 4 concentrations are low, <4 μmol L −1 and NO 3 uptake by phytoplankton occurs. Field measurements and enclosure experiments confirm the NH 4 inhibition process to be the cause of low NO 3 utilization most of the year. Detailed analysis of spring blooms in three embayments of SFB over 3 years shows a consistent sequence of events that result in bursts of chlorophyll. The first requirement is improved irradiance conditions through stabilization of the water column by stratification or reduced tidal activity. Second, NH 4 concentrations must be reduced to a critical range, 1 to 4 μmol L −1 through dilution by precipitation and by phytoplankton uptake. This enables rapid uptake of NO 3 and subsequent increase in chlorophyll. The resulting bloom is due to both the initial uptake of NH 4 and the subsequent uptake of NO 3. The NO 3 uptake step is crucial since it is the larger nitrogen source and uptake occurs at higher rates than that for NH 4 at the concentrations that occur in SFB. Existing models of light-limited, non-nutrient limited productivity in SFB require modification to include the NH 4 inhibition effect. From measured NH 4 uptake rates and initial concentrations, calculations can be made to predict the length of time that favorable irradiance conditions are required for the phytoplankton population to reduce ambient NH 4 concentrations to non-inhibiting concentrations and allow bloom formation to begin. For Suisun Bay, the time required is so long that blooms are unlikely in any season. For San Pablo and Central Bays, these times are too long in summer but sufficiently short in spring to allow bloom development, depending on the ambient NH 4 concentration prior to the productivity season. NH 4 sources to SFB are primarily anthropogenic, from agricultural drainage and sewage treatment plants, and if not sufficiently diluted by runoff and precipitation can prevent development of the spring phytoplankton bloom. Attention should be paid to the form of N making up dissolved inorganic nitrogen (DIN) in nutrient-rich estuaries.