Relationship Between Cross-Sectional Absorption And Chlorophyll Content In Natural Populations Of Marine Phytoplankton

ABSTRACT We have measured the in vivo absorption of light (670 nm) by natural phytoplankton populations collected from a variety of locations in the North Atlantic Ocean. There is a strong correlation between chlorophyll content and absorption. By using a linear regression model the mean value for chlorophyll specific absorption (A*) is 0.0138. The range in this value is about .0100 - .0300. At chlorophyll concentrations less than 0.5 ug/1 the slope differs from the slope at higher concentrations. The scatter of points around the mean increases with increasing chlorophyll.By grouping chlorophyll concentrations a mean and standard error have been determined for each group. This allows the entire data set to be fitted with a power model curve (Y = aX*5 ). The exponent b^ is 0.758(3/4) which argues that A* is a function of cell volume, i.e., A* = .0667 Chl(ug/l)-758. 1. INTRODUCTION There are important reasons for measuring the efficiency of light absorption by phytoplankton and algae. Biological oceanographers, spurred by the success of the technology of satellite colorimetry, now wish to measure global primary production. Several analytical models have been suggested. These techniques require knowing the efficiency at which carbon is fixed per quantum of light energy absorbed by the algae. Given the practicalities associated with remote sensing of chlorophyll pigments, the analytical carbon estimate must assume that the relationship between chlorophyll and cross-sectional absorption is constant, or at least predictable. The experimental evidence, however, shows that phytoplankton1 s specific absorption values, measured in the natural environment or in cultures, have a range of at least a factor of five. This variation appears to be the result of a combination of inter and intra-specific properties of algae, and at first cut, looks unpredictable. With this amount of variation, one can honestly ask whether or not the quantum yield analytical methods are worth it; are they any better than the empirical- statistical estimates that are presently being utilized by the NASA Goddard scientists in preparation of global carbon fixation maps?On the other hand, there are other reasons for studying cellular absorption in natural populations.1 These lie at the very crux of ecological theories which attempt to use cell size as one of the major causes for the diversity of species in natural population.We have used measurements of in vivo specific absorption per unit chlorophyll for water masses in the North Atlantic Ocean. We have lumped data from various depths in the euphotic zone and data taken from frontal regions in warm core rings to coastal and slope waters as well as from the North Central Sargasso Sea. Our goal here is to establish the range of variability between cross-sectional absorption in vivo and the amount of extracted chlorophyll pigment. In this paper, the focus is on the differences between oligotrophic and eutrophic populations. One aim is to develop an algorithm for predicting in vivo absorption by phytoplankton. We also consider the degree of chlorophyll packaging and whether or not packaging is a function of change in cell size of oceanic phytoplankton populations.