Nutrient Sources to Support the Gulf of the Farallones Food Web
- Frances Wilkerson
San Francisco State University
The goal of this ongoing project is to determine whether nutrients from San Francisco Bay impact the rich food web of the Gulf of the Farallones in order to assess if anthropogenic changes in the estuary will have management implications for the coastal ecosystem.
Our scientific objectives are to understand the role of two different sources of nutrients for the lower trophic levels that feed the food web of the Gulf of the Farallones: 1) coastal upwelling and 2) outflow from San Francisco Bay. Each of these sources is likely to be more important at different seasons and under different climate conditions. Upwelling occurs primarily in spring and summer, while major freshwater flow and outflow from San Francisco Bay occurs in winter and early spring during the rainy and snowmelt periods. The additional nutrient source from SF Bay will supplement the more traditionally considered upwelling sources and ensure plentiful nutrients to fuel the Farallones food web.
Besides determining spatial distributions of nutrients in the Gulf of the Farallones, phytoplankton community structure and biomass along with primary productivity and nutrient uptake rates are measured. Grow-out/enclosure experiments are also conducted to track nutrient drawdown, dissolved inorganic carbon changes, primary productivity and NO3 and NH4 uptake by the growing phytoplankton, isolated from grazing pressure and light limitation.
Summary to DateThe data we collected in 2007 are available from either Drs. Dick Dugdale or Frances Wilkerson at the Romberg Tiburon Cenetr, San Francisco State University.
Some of the results were presented at the Estuarine Research Federation meeting in November 2007. See Marchi, A.; Dugdale, R.; Parker, A.; Hogue, V.; Lew, K.; Fuller, J.; Wilkerson, F., Influence of San Francisco Estuary Outflow on the Nearshore Gulf of the Farallones Phytoplankton Communities.
- Chl A
- Other nutrients
- Optical properties
- something else
Study MethodsOVERVIEW: Water samples are collected at 25 stations in a regular sampling grid bounded by the Golden Gate, Farallon Islands and Pt. Reyes. Standard oceanographic techniques are employed with an instrumented (CTD) rosette and Nisken bottles, and Secchi disk to obtain vertical profiles of temperature, salinity, light transmission, nutrient and chlorophyll concentrations, and phytoplankton community structure. Surface samples are analyzed for dissolved inorganic carbon, primary productivity (carbon fixation) and phytoplankton nitrogen uptake. Experimental 20-L grow-out experiments are carried out with surface water sampled from nearshore stations in a north to south transect following the coastline. The enclosures are placed in large tubs where they are mixed gently, maintained at 50% of surface irradiance and maintained at near ambient temperatures with pumped water, and sampled daily for 4 days for inorganic nutrients (NO3, NH4, PO4, Si(OH)4), DIC, size fractionated chlorophyll, cell size spectra using flow cytometry, primary productivity and uptake rate measurements using 15N/13C.
DETAILED METHODS: Primary nutrient concentrations (NO3, NO2, PO4, and Si(OH)4) are analyzed with a Bran and Luebbe AutoAnalyzer II according to the procedures of Whitledge et al. (1981) for all but Si(OH)4 which uses Bran and Luebbe (1999). Separate 25 ml samples are collected for manual colorimetric determination of NH4 (Solorzano 1969). Urea is measured according to Revilla et al. (2005). Dissolved inorganic carbon is measured using a Monterey Bay Research Institute-clone DIC analyzer with acid-sparging and Licor ND infra-red analysis (Friederich et al. 2002).
Phytoplankton biomass is estimated from extracted chlorophyll-a, particulate organic N and C concentrations and by flow cytometry. Size fractionated in vitro chlorophyll (for cells with diameters <5 µm or >5 µm) is determined using the extraction protocol of Arar & Collins (1992) and a Turner Designs Model 10 fluorometer. The size distribution of fluorescent particles and dominant algal type is carried out on board using a CytoSense flow cytometer in 20 ml samples (Dubelaar and Gerritsen, 2000). POC and PON are determined by mass spectrometry. Phytoplankton identification and enumeration is carried out on selected 50 ml samples using the Utermohl method according to Lassiter et al. (2006) and Judah (2002).
Uptake of NO3 and NH4 and C is estimated with dual labeled 15N/13C stable isotope tracer techniques (Slawyk et al., 1977, Legendre and Gosselin, 1996) to yield simultaneous N and C uptake rates from a single sample. Size fractionated uptake is measured to establish uptake of NO3 vs. NH4 by the larger and smaller phytoplankton cells. Water is dispensed into 280 ml incubation bottles and inoculated with either Na13HCO3 and K15NO3, or Na13HCO3 and 15NH4Cl (99 at%). Isotope additions of approximately 10% of ambient concentration are used to avoid substrate enhancement effects. Immediately following inoculation, one sample is filtered onto a precombusted GF/F filter (450ºC, 4-hr) by gentle vacuum to determine the initial PON and POC concentration and isotopic filter blank. Incubations are at ambient temperature and 50% surface light and last 24 hours, and are terminated by gentle vacuum filtration onto precombusted GF/F filters or 5 µm pore-sized silver filters when size fractionated uptake is to be measured. N and C concentration and isotopic composition is determined using a Europa 20/20 mass spectrometer. N and C uptake rates are calculated based on isotopic enrichment according to Dugdale and Wilkerson (1986).