Lance¹, Veronica P., Meredith Armstrong², J. Timothy Pennington³, Zanna Chase³, Kenneth S. Johnson³ and Francisco P. Chavez ³

¹Nicholas School of the Environment and Earth Sciences, Duke University, 135 Duke Marine Lab Road, Beaufort, NC 28516-9721 USA, Tel: 252-504-7631, Fax: 252-504-7648, E-mail: vplance@duke.edu, ²Ocean Sciences Department, University of California, Santa Cruz, CA 95064 USA and ³Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 USA

Quantum yield of photosynthesis in relation to iron in the California coastal upwelling system

Maximum quantum yield of photosynthesis F_m) were determined from productivity vs. irradiance (PvsE) ¹⁴C photosynthetron incubations and phytoplankton spectral absorptions measured along the California coastal upwelling region at 24 stations during two cruises conducted by MBARI in summer 2002. SECRET302 included two cross-shelf transects. MUSEII repeatedly sampled in two nearshore regions: one near Davenport, North of Monterey Bay where the continental shelf is relatively wide and where iron may be entrained in upwelled water; the second near Big Sur, South of Monterey Bay where the continental shelf is much narrower potentially limiting iron entrainment from sediments. Although highly variable, quantum yields in this region can be sorted into a matrix of four zones (I to IV): I - High nitrate, high Fe, cold, salty water where F_m are high (mean of 0.10 +/- 0.02 s.e. mol C mol photon^-1); II – High nitrate, low Fe, slightly warmer and fresher water where F_m are moderate (mean of 0.080 +/- 0.008 s.e. mol C mol photon^-1); III – Low nitrate, high Fe, moderate temperature and salinity water where F_m is high (0.100 mol C mol photon^-1 n=1); IV – Low nitrate, low Fe, warmest and freshest water where F_m are lower (0.075 +/- 0.006 s.e mol C mol photon^-1). Significant overlap of F_m between these zones is evidence that the rate of change in environmental conditions (vertical mixing; upwelling pulses) can exceed the net rate of physiological acclimation of phytoplankton (optical; nutrient availability).