Phytoplankton growth rates in a light-limited environment, San Francisco Bay

San Francisco Bay has a high degree of spatial variability in physical properties (e.g.suspended sediment concentrations, water depths, vertical mixing rates) that affect biologcal processes.We used this setting to test the hypothesis that light avdability is the primary control of phytoplankton growth in this turbid nutrient-rich estuary.In situ incubations (24 h), designed to simulate vertical mixing over the water column at 2 rates, were done at 4 sites.The photic depth to mixed depth ratio (G:&,,) at the 4 sites ranged from 0.12 to 1.1.Phytoplankton growth rates were esbmated by 14c assimilation and by changes in cell number.Growth rates were highest (approximately 2 divisions d-l) where the photic depth was large relative to the mixed depth, and small or negative where Zp:& was small.Growth rate increased with total daily light exposure and fit a hyperbolic funcbon that predicts maximum specific growth rate of about 2 &visions d-' and a compensation irradiance of about 1.4 Einst.m-' d-'.