A. M. Marchi

Phytoplankton blooms and nitrogen productivity in San Francisco Bay

San Francisco Bay has been considered an HNLC or HNLG (high nutrient low chlorophyll or low growth) region with nonlimiting concentrations of inorganic nutrients yet low standing stocks of phytoplankton. Most of the studies leading to this conclusion come from the South Bay and little is known about nutrient processes and phytoplankton productivity in the northern and central parts of the estuary. Data collected over 3 yr (1999–2003) in Suisun, San Pablo, and Central Bays describe the availability of dissolved inorganic nitrogen (DIN), silicate, and phosphate and the seasonal variability in phytoplankton abundance. Rate measurements of fractionated nitrogen productivity provide the relative contributions of different forms of DIN (ammonium and nitrate) and different sized phytoplankton to the development of seasonal phytoplankton blooms. Regional differences in bloom dynamics are observed with Suisun Bay, the least saline, highest nutrient, most turbid region having less phytoplankton biomass and productivity than San Pablo and Central Bays, except in the abnormally wet spring of 2000. Spring blooms in San Francisco Bay are driven primarily by high rates of nitrate uptake by larger phytoplankton cells following a period of increased ammonium uptake that depletes the ambient ammonium. The smaller occasional fall blooms are apparently flueled mostly by ammonium uptake by small sized phytoplankton. The data suggest that the HNLC condition in the northern and central parts of San Francisco Bay is due primarily to light availability modulated by the interaction between ammonium and nitrate, and the relative amounts of the two forms of the DIN pool available to the phytoplankton.

Variation in Water Quality and Phytoplankton Nutritional Status in an Urbanized Lake System (Lake Merced, California, USA)

Abstract Lake chemistry is influenced by land use in the surrounding watershed, particularly in complex urban landscapes, which are commonly subjected to an increase in material loadings. Because land use is rarely uniform, individual ecosystems embedded within the landscape may reflect varying water quality conditions. The Lake Merced system is composed of three lake basins with watersheds (surface area = 13 km2) residing entirely within the city of San Francisco, California. On nine occasions from January-June 1995 we sampled the two main lakes (North and South Lake Merced) within the system to evaluate their relative productivity and nutrient status. On each date, several physical-chemical conditions were measured, and nutrient enrichment bioassays were performed to evaluate the nutritional status of the phytoplankton (nitrogen (N) and phosphorus (P) enrichment in a 2 × 2 factorial experimental design). All the parameters tested except temperature were higher in the north lake compared with the south (pair-wise Wilcoxon Ranked sums), and common trophic state variables were more than 2-fold higher in the north lake. Phytoplankton growth in North Lake Merced was N-limited while growth in South Lake Merced was co-limited by N and P. In sum, our data indicate that differences among these lakes may be explained by simple volume differences between lakes (with the smaller North Lake Merced being more eutrophic), and heavy stocking of fishes to the north lake. Particulate nutrient ratios (N:P) were good indicators of phytoplankton nutritional status throughout the system, while N:P ratios based on total nutrient concentrations gave misleading results.