Evidence for a simple relationship between natural fluorescence, photosynthesis and chlorophyll in the sea

Abstract

To test the proposal that the natural or solar-induced fluorescence of chlorophyll a in the sea provides a purely optical measure of chlorophyll and the rate of photosynthesis in the sea, we studied the relationship between natural fluorescence and photosynthesis in several environments including the central South Pacific, the western Sargasso Sea, and two sheltered bays. The results of 76 such measurements between 2 and 150 m depth and covering a 1500-fold range in production indicate that photosynthesis is highly correlated ( r > 0.9) with natural fluorescence. Furthermore, a substantial portion of the remaining variability can be explained by an examination of the relationships of the quantum yields of photosynthesis and fluorescence as a function of light level. Specifically, the quantum yield of photosynthesis decreases more rapidly than the quantum yield of natural fluorescence with increasing irradiance. Predicted rates of primary production from measures of natural fluorescence and PAR fall within ±30% for half of the samples, and within ±75% for 90% of the cases (assuming that the carbon fixation measurements were without error). The prediction of chlorophyll from natural fluorescence and PAR exhibit similar correlations. This suggests that natural fluorescence measurements, either as a supplement to direct measurements or as independent optical measurements, provide a new and rapid means of estimating gross photosynthesis in the sea.