The relationship of solar-stimulated natural fluorescence and primary productivity in Mexican Pacific waters

Solar-stimulated natural chlorophyll a fluorescence measured by upwelling radiance in the red spectral band could be a fast and noninvasive method to estimate primary production in aquatic environments if the relationship of primary production to natural fluorescence can be described as a function of easily measured environmental variables. We compared data of natural fluorescence and primary production (C-14 incubation for 2 h) from the California Current and the Gulf of California. The data confirm that the quantum yield ratio of fluorescence to primary production (phi(c):phi(f)) is a function of in situ irradiance, but not of nutrient concentration or temperature, as has been reported in the literature. Published data from the subtropics and tropics and our data yield empirical constants that define the irradiance function of the quantum yield ratio, but variability results from ambiguity of the constant determination caused by high variance of the data. Data from the Antarctic are significantly different from the low latitude data. Below a photosynthetic rate of 300 nmol C m(-3) s(-1) our natural fluorescence data are useful as a proxy of primary production with a correlation coefficient, r(2), of 0.85. Of the unexplained variance (15%), a major portion is due to the C.V. of the primary production method (9.2%). The r(2) value of predicted primary production is similar to other published results, which suggests that without further information about the physiology of the phytoplankton it will be difficult to improve the quality of the primary production estimate.