Primary Production Estimates from Recordings of Solar Stimulated Fluorescence in the Equatorial Pacific at 150-Degrees-W

Abstract

Biological, optical, and hydrographical data were collected on the WEC88 cruise along 150-degrees-W and during a 6-day time series station on the equator during February/March 1988. This area was characterized by a subsurface chlorophyll maximum (SCM), located at 50-70 m depth at the equator and descending down to 120-125 m at the north and south end of the transect. Highest primary production rates were near-surface and confined to the equatorial region and stations between 7-degrees and 11-degrees-N. To determine the relationship between solar-stimulated fluorescence (centered at 683 nm wavelength) and primary production, a production-fluorescence model based on phytoplankton physiology and marine optics is described. Results of model calculations predict that there is a linear relation between production and fluorescence. A comparison between morning and midday measurements of the production-fluorescence relation showed that there was some difference between the two, whereas evening measurements, on the other hand, were distinctly different from the morning/midday ones. This seems to suggest that diurnal variations contribute significantly to variability in the quantum yield of photochemical processes. The ratio of the quantum yield of photosynthesis to the quantum yield of fluorescence (PHI(c)/PHI(f)), the parameter which will determine how well production can be estimated from optical recordings, ranged between 0.24 and 0.44 molC Ein-1 (an Einstein equals a mole of photons) for all stations. The highest value for this ratio occurred at the equatorial stations, indicating that interstation (i.e., latitudinal) variability could have an effect on the production-fluorescence relation. Measured (with C-14 incubations) and predicted production compared quite well, although high measured production rates for near-surface samples were underestimated in most cases. Since both production and fluorescence were nonlinear at high irradiance intensities, we recommend in the future that a nonlinear component be incorporated into our model to take this effect into account and thus allow us to refine our estimates of nonlinear data.