Characterization of the Microcystis Bloom and Its Nitrogen Supply in San Francisco Estuary Using Stable Isotopes

A suite of particulate and dissolved organic and inorganic stable isotopes were needed to determine the source of the nutrients and cells that initiate and sustain the toxic cyanobacteria bloom of Microcystis in San Francisco Estuary. Particulate and dissolved inorganic and organic matter in water and plankton samples were collected biweekly during Microcystis blooms in 2007 and 2008. Stable isotopes for particulate and dissolved organic matter, nitrate, and water (POM-δ¹³C, POM-δ¹₅N, DOC-δ¹³C, C/N ratio, NO₃-δ¹₅N, NO₃-δ¹⁸O, H₂O-δ¹⁸O and H₂O-δ²H) were compared with Microcystis cell abundance, dissolved organic carbon, chlorophyll a, and toxic total microcystins concentration, as well as physical and chemical water quality variables, including streamflow. The isotopic composition of particulate organic matter, nitrate, and water differed for the Sacramento and San Joaquin Rivers and varied along the salinity gradient. The variation of particulate organic matter and water isotopes suggested Microcystis primarily entered the estuary from the San Joaquin and Old Rivers, where it was most abundant. Nitrate isotopes along with streamflow variables indicated that the San Joaquin River was a source of nitrate to the estuary. However, stable isotope comparison of the nitrogen in Microcystis cells with the dissolved inorganic nitrate in the San Joaquin River indicated that nitrate was not the primary source of nitrogen that supported the bloom. Instead, ammonium from the Sacramento River was the likely sole source of the nitrogen for most of the bloom. Selective uptake of ammonium may have further contributed to the magnitude of the Microcystis bloom which increased with the percent of ammonium within the total dissolved inorganic nitrogen pool.